Water availability regulates negative effects of species mixture on soil microbial biomass in boreal forests

Journal Pre-proof Water availability regulates negative effects of species mixture on soil microbial biomass in boreal forests Xinli Chen, Han Y.H. Chen, Chen Chen, Sai Peng PII:

S0038-0717(19)30298-6

DOI:

https://doi.org/10.1016/j.soilbio.2019.107634

Reference:

SBB 107634

To appear in:

Soil Biology and Biochemistry

Received Date: 3 May 2019 Revised Date:

13 October 2019

Accepted Date: 19 October 2019

Please cite this article as: Chen, X., Chen, H.Y.H., Chen, C., Peng, S., Water availability regulates negative effects of species mixture on soil microbial biomass in boreal forests, Soil Biology and Biochemistry (2019), doi: https://doi.org/10.1016/j.soilbio.2019.107634. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Water availability regulates negative effects of species mixture on soil microbial biomass in

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boreal forests

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Authors: Xinli Chen1, Han Y. H. Chen1,2*, Chen Chen1, Sai Peng1

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Affiliation:

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Bay, Ontario P7B 5E1, Canada

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Education, Institute of Geography, Fujian Normal University, Fuzhou, 350007, China

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*Corresponding author: Han Y. H. Chen, phone: (807) 343-8342, fax: (807) 343-8116, email:

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Faculty of Natural Resources Management, Lakehead University, 955 Oliver Road, Thunder

Key Laboratory for Humid Subtropical Eco-Geographical Processes of the Ministry of

[email protected]

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Abstract

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Soil microorganisms are critical for the maintenance of terrestrial biodiversity and

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ecosystem functions. Both plant diversity and water availability are individually known to

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influence soil microorganisms; however, their interactive effects remain largely unknown. Here,

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we investigated whether the effects of tree species mixtures on microbial biomass and

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composition were altered by water availability. This was accomplished by sampling soils in the

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growing season from stands that were dominated by Populus tremuloides and Pinus banksiana,

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respectively, and their relatively even mixtures under reduced (-25% throughfall), ambient, and

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added (+25% throughfall) water. Microbial community biomass and composition were

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determined by phospholipid fatty acid analysis. We found that water addition increased soil total

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microbial biomass and by individual groups, whereas water reduction had no effect. Under

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ambient water conditions, soil total microbial biomass, arbuscular mycorrhizal fungal, bacterial,

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gram-positive (GP) bacterial, and gram-negative (GN) bacterial biomass were significantly lower

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in mixtures than from those of constituent monocultures, but saprotrophic fungal biomass and the

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ratios of fungal/bacterial and GP/GN bacteria were not significantly affected by tree species

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mixtures. Water reduction increased species mixture effects on total and individual group

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microbial biomass from negative to neutral, while water addition only increased mixture effects

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on arbuscular mycorrhizal fungal and GP bacterial biomass. Across all water treatments, soil

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total and individual group microbial biomass significantly increased with the abundance of

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broadleaved trees, but only weakly with species richness. Further, microbial community

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compositions differed significantly with both overstory type and water treatment. Microbial

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community compositions exhibited strong associations with tree species richness, soil moisture,

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soil pH, and litterfall production, whereas microbial biomass did not. Our results suggest that

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higher species diversity is not always of benefit for soil microorganisms; however, mixed tree

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species have the potential to regulate ecosystem responses to climate change.

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Key words: plant diversity; water availability; PLFA; microbial biomass; microbial composition

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Introduction Forest soils harbour extremely rich microbial communities (Bahram et al., 2018), which

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are essential for many ecosystem functions that underpin a suite of services in terrestrial

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ecosystems (Lange et al., 2015; Li et al., 2019). Soil microbial biomass plays a major role in

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maintaining primary aboveground production, greenhouse emissions, and nutrient recycling (van

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der Heijen, 2008; Lange et al., 2015; Walker et al., 2018; Li et al., 2019). Furthermore, shifts in

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relative microbial abundance and composition drive biogeochemical cycles in soil. For example,

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a higher fungal to bacterial biomass ratio (F/B) is associated with higher decomposition

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efficiency and greater soil carbon (C) storage potential (Malik et al., 2016), while a lower ratio of

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gram-positive: gram-negative (GP/GN) bacteria indicates higher C availability in the soil (Fanin

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et al., 2019). Biodiversity loss and water availability change are two significant stressors on

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ecosystem productivity, element cycling, and other ecological processes (Cardinale, 2012;

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Hisano et al., 2018; Zhou et al., 2018). Recent advances have been made in our understanding of

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the respective effects of water availability and plant diversity on soil microbial biomass and

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composition (Zhou et al., 2018; Chen et al., 2019b). However, it remains unclear whether species

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mixture effects on soil microbial biomass and composition change with water availability. In the

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face of the anthropogenic environmental changes(IPCC, 2014), it is important to better

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understand how the plant diversity effects on soil microbes may change with water availability

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so that effective forest management and conservation strategies can be developed accordingly.

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Concurrent changes in terrestrial water availability and plant diversity, driven by

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anthropogenic activities are expected to interact and alter soil microorganisms via changes in

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plant-derived litter inputs and the soil environment (Hicks et al., 2018; Valencia et al., 2018;

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Chen and Chen, 2019). Terrestrial water availability, driven by changes of precipitation and

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evapotranspiration, also affects soil microbial biomass, as the availability of soil water is

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essential for the growth of both soil microorganisms and vegetation (Manzoni et al., 2012;

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Valencia et al., 2018). Decreased water availability is anticipated to reduce soil microbial

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biomass, whereas an increase has the opposite effect (Zhou et al., 2018). Meanwhile, a rapid loss

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in biological diversity has been recognized as a critical threat to ecosystem functioning

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(Cardinale et al., 2012). For example, species mixtures yield greater above- and belowground

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productivity in diverse ecosystem types, over monocultures (Ma and Chen, 2016; Duffy et al.,

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2017). Recent meta-analyses of manipulated experiments revealed that plant mixtures could

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promote soil microbial biomass via a higher quantity and multiplicity of plant-derived food

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resources (e.g., litterfall, roots) that enter the soil, in contrast to monocultures (Thakur et al.,

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2015; Chen et al., 2019b; Chen and Chen, 2019). Additionally, soils in diverse forests may have

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higher moisture and pH than those of monocultures (Zapater et al., 2011; Dawud et al., 2016),

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which could favour microbial growth (Rousk et al., 2010a; Rousk et al., 2010b). Furthermore,

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the effects of plant diversity may be contingent on how plant species in mixtures are

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phylogenetically and/or functionally dissimilar (Cadotte, 2017; Chen et al., 2019a).

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Phylogenetically dissimilar broadleaved and coniferous trees provide distinct and diverse

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qualities and quantities of food resources and living habitats for soil microorganisms, and their

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mixture effects may also depend on the proportion of species from either group in mixtures

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(Dawud et al., 2016; Tedersoo et al., 2016). Broadleaved forests tend to have higher soil

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microbial biomass, as their litters contain more labile compounds, nitrogen, and less phenol than

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coniferous forests (Liu et al., 2012).

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The compositions of microbial communities are also sensitive to changes in the configuration of plant species diversity and water availability. An increased quantity of plant

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inputs with plant diversity could enhance the soil F/B ratio, as fungi might benefit more from

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increased litter inputs than bacteria (Malik et al., 2016; Chen et al., 2019b). Within bacterial

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communities, the GP/GN bacteria ratio may decrease with plant diversity due to their higher

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dependence of GN than GP bacteria for plant litter inputs (Fanin et al., 2019). Additionally,

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differences in litter chemistry, root exudates, and soil environment between different plant

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community types can also regulate the composition of soil microbial communities (Wan et al.,

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2015). Further, the relative abundance of bacteria to fungi would increase with soil pH (Rousk et

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al., 2010a), which has a strong positive association with plant diversity (Dawud et al., 2016).

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Additionally, fungi are expected to have a greater capacity to tolerate water stress than bacteria

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due to their ability to accumulate osmoregulatory solutes and hyphal/mycelial growth forms

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(Manzoni et al., 2012). Within bacterial communities, GP bacteria have stronger and thicker cell

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walls; thus, they are inherently more tolerant to drought than GN bacteria (Schimel et al., 2007;

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Zhou et al., 2018). Although reduction of water availability also decrease the plant inputs into

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soils, and may decrease soil F/B ratio, we still expected water reduction could enhance both the

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soil F/B ratio and GP/GN bacteria ratio, whereas water addition has the opposite effect, due to

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the critical role of water availability for microbial community (Schimel et al., 2007; Manzoni et

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al., 2012; Zhou et al., 2018). Moreover, previous grassland studies suggest that the effects of

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plant diversity on soil microbial biomass and composition are dependent on water availability

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(Wagner et al., 2015; Valencia et al., 2018).

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Mixed forests may serve as a buffer against the negative impacts of reduced water

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availability on plant production, since (i) the growth of some species, which increases under

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drought conditions, may compensate for others with reduced growth; (ii) competition for water

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in mixtures may decrease due to complementary resource use, (iii) and denser canopies might

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reduce soil water loss (O'Brien et al., 2017; Ammer, 2019). Given the strong positive link

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between plant-derived C and soil microbes (Chen and Chen, 2019), the positive effect of plant

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diversity on productivity may be propagated to soil microbial communities via plant litter inputs

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and improved microenvironments for microbial growth. We also expected that the effects of

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mixed forests on soil microbial biomass could be enhanced by mild water addition, since they

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contain additional species with unique water requirements and water acquisition strategies;

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therefore, they more readily exploit increased water resources (Wright et al., 2015; Fischer et al.,

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2016).

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To the best of our knowledge, no previous study has evaluated whether water availability

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may regulate the effects of species mixture on soil microbial biomass and composition in forests.

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Here, we examined how soil microbial biomass and composition would respond to water

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addition and reduction in mixed forests and corresponding monocultures. We tested four

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hypotheses as follows: (1) water reduction decreases overall and individual microbial group

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biomass, but increases the soil F/B ratio and GP/GN bacteria ratio, whereas the addition of water

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has the opposite effects; (2) mixed forests have a higher total and individual group microbial

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biomass, F/B ratio, and a lower GP/GN bacteria ratio, on average, than those in corresponding

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monocultures, alternatively, these microbial attributes increase with species richness except

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GP/GN bacteria ratio, which showed the opposite pattern; (3) these mixed forest effects and

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species richness effects are more pronounced under both water reduction and addition conditions;

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(4) the proportion of broadleaf trees has a positive effect on microbial biomass and influences

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microbial composition; (5) the composition of microbial communities differs between overstory

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types and water treatments, and is concurrently altered by the diversity and composition of tree

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species, plant-derived litter inputs, and soil environments.

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Methods

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Site description

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This study was carried out in the central boreal forests of Canada, located north of Lake Superior

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and west of Lake Nipigon (49°27' N–49°38' N, 89°29' W–89°54' W). The study area falls within

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the Moist Mid-Boreal (MBX) ecoclimatic region, and is characterized by warm summers and

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cold, snowy winters (Ecoregions Working Group, 1989). The mean annual temperature is 2.5 °C,

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and annual precipitation is 712 mm. The soils of the upland sites are relatively deep glacial tills

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of the Brunisolic order (Soil Classification Working Group et al., 1998). The study area has an

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extensive history of stand-replacing fire, with an average fire return interval of approximately

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100 years for the past century (Senici et al., 2010). In young forests, the dominant overstory tree

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species included jack pine (Pinus banksiana Lamb.) and trembling aspen (Populus tremuloides

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Michx.). Common understorey shrub species in the area include mountain maple (Acer spicatum

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Lam.), alder (Alnus spp.), and beaked hazel (Corylus cornuta Marsh.).

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Experimental design

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In July 2016, we established the experiment in young stands (11 years old) from three overstory

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types: single-species stands dominated by Populus tremuloides Michx. (Populus), dominated by

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Pinus banksiana Lamb. (Pinus), and their relatively even mixtures (Populus+Pinus) (Table 1).

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Each of the overstorey types was replicated in triplicate. The resulting nine stands were allocated

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with a distance of > 1 km between each, to minimize spatial autocorrelation. Within each stand, a

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circular plot (400 m2) located at least 50 m from the forest edge was randomly established.

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Within each plot we applied three split-plot level water availability treatments: ambient, 25%

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growing season (May to October) throughfall reduction under tree overstory, and 25%

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throughfall addition, which comprised the median changeability of expected water availability

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during the 21st century for Canada’s boreal forests (IPCC, 2014). Each of the 27 treatment split

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plots consisted of an area of 6 × 6 m (36 m2), with a mean tree density of 98 stems per plot

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(range = 33 to 416 stems), which was similar to those observed in a mature forest plot area of 0.2

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ha (Chen and Luo, 2015). For each of the nine throughfall reduction treatment plots, we

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constructed rain shelters under canopies that consisted of four shelters (3 × 3 m), which were

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held in place by metal stakes and wires. Each metal frame supported 4 U-shaped clear acrylic

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troughs (3 m long × 20 cm wide) that were spaced at 35 cm. The U-shaped troughs were oriented

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at a 10° angle, with the high end positioned at 1.8 m and the low end at 1.35 m above ground

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level (Fig. S1). The rain shelters funnelled water into eight 8-cm (inner diameter) polyvinyl

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chloride (PVC) pipes, each with six different-size holes (diameters: 0.64, 1.91, 3.18, 4.45, 5.72,

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and 6.99 cm), arranged at intervals of 46 cm, to distribute the collected water evenly over the

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adjacent throughfall addition plot. A ≥ 5 m buffer zone was established between the treatment

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plots (Fig. S1).

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Field measurements

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To explore how the changes in vegetation and soil environments between canopy types and

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water treatments were linked to variations in soil microbial composition, we identified all tree

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species and measured the diameter at breast height (DBH; 1.3 m above the root collar) of all

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trees in each split plot, at the conclusion of the 2018 growing season. Within each split plot, three

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0.322 m2 litterfall traps were randomly located to collect litterfall (Chen et al., 2017). We placed

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litter traps in May 2017 and collected litterfall every four weeks during the snow-free period,

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until late October 2018. The litterfall samples were dried at 65ºC in a convection oven until a

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constant mass was achieved, which was generally less than 48 hours. The total oven-dry biomass

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of annual litterfall production was calculated as Mg ha-1y-1 by summing all litterfall collections

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for the entire calendar year, from July 2017 to August 2018. We employed the minirhizotron

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method to monitor root length with a specific scanner (CI-600 Root Scanner, CID Inc., camas,

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WA, USA) (Mommer et al., 2015). Two tubes (Ø63.5 mm x 105-cm long) per treatment plot

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were installed in May 2017, and digital images were continuously obtained monthly by Rootsnap!

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Software (CID Inc., camas, WA, USA), from May 2018. We simultaneously estimated the

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“standing root length” for each subplot as the sum of individual root lengths in each image. The

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volumetric soil water content was measured biweekly, using a Decagon soil moisture sensor, at a

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depth of 5 cm below the soil surface during the growing season.

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Soil collection and pH analysis

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Composite soil samples were acquired by mixing soil samples (0–10 cm) that were collected

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from five random points in each split plot with a soil corer (Ø3.5 cm) in August 2018. The soil

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samples were immediately placed in polyethylene bags, stored in a cooler, and transported to the

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laboratory. Following sieving (<2 mm) and the removal of visible plant material, the soil pH was

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determined using a soil to water ratio suspension of 1:2·5. The remaining soil samples were

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stored in a refrigerator at −20°C until they were analyzed for microbial community structure and

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composition. Soil microbial analyses assays were performed within two weeks of sample

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collection.

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Phospholipid fatty acid (PLFA) analysis

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The microbial PLFA composition was determined using 3.0 g freeze-dried soil samples,

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following the protocol described in Quideau et al. (2016) and Frostegard and Baath (1996). A

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surrogate standard (19:0; 1,2-dinonadecanoyl-sn-glycero-3-phosphocholine, Avanti Polar Lipids

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Inc, Alabaster, USA) was added prior to the initial extraction. An instrument standard (10:0Me;

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methyl decanoate, Aldrich, St. Louis, USA) was added prior to GC analysis. Fatty acid methyl

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ester analysis was conducted using an Agilent 6890 Series capillary gas chromatograph (GC;

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Agilent Technologies, Santa Clara, USA), which was fitted with a 25 m Ultra 2 column

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(Crosslinked 5 % PhMeSilicone), and a FID detector. The Sherlock Microbial Identification

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System Version 6.3 software (MIDI, Inc., Newark, USA), was employed to identify and quantify

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FAMES, and the microbial species were estimated using the MICSOIL3 method. The groups of

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lipid biomarkers used in the calculations of PLFA biomass and ratios are detailed in Table S1.

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Data analysis

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Initially, we tested the effects of overstory type (T) and water treatment (W) on microbial

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biomass, F/B ratio, and GP/GN bacteria ratio using the following linear mixed-effect model, and

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post-hoc comparisons via the TukeyHSD test:

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Yijkl = Ti + Wj(k) + Ti × Wj(k) + πk + εl(ijk)

(1)

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where Yijkl is the soil microbial attribute of interest (total microbial biomass, saprotrophic fungal

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biomass, AM_fungal biomass, bacterial biomass, Gram-positive bacterial biomass, Gram-

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negative bacterial biomass, F/B ratio, or GP/GN bacteria ratio); Ti (i = 1, 2, 3) is the overstory

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type (broadleaf, conifer, and mixed wood); Wj(k) is the water treatment (j = 25% throughfall

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reduction, ambient, or 25% throughfall addition) nested in each whole plot k (k =1, 2,…9); πk is

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the random effect of plot; εl(ijk) is the sampling error. We conducted the analysis using the

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restricted maximum likelihood estimation with the package lme4 (Bates et al., 2017).

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To test the species mixture effect, the response ratio (RR) was employed to quantify the effects of species mixtures on soil microbial biomass, F/B ratio, and GP/GN bacteria ratio: RRsm=Xobserved/Xexpected

(2)

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where Xobserved and Xexpected were the observed and expected values of total and individual group

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microbial biomass, F/B ratio, and GP/GN bacteria ratio in mixtures. We calculated the expected

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weighted values of the component species in monocultures following Loreau and Hector (2001):

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Xexpected = ∑ (Vi ×pi)

(3)

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where Vi is the observed value of total and individual group microbial biomass, ratios of F/B and

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GP/GN bacteria in the monoculture of species i, and Pi is the proportion of species i basal area

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(m2 ha-1) in the corresponding mixture per split plot. To test whether the effects of species

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mixtures on microbial biomass and composition differed with water availability, we employed

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the following linear mixed-effect model:

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RRijk = Wi + πj + εk(ij)

(4)

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where Wi is the water treatment (i = throughfall addition, ambient, throughfall reduction); πj is

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the random effect of the overall plot; εk(ij) is the sampling error. The mixed species effects were

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significant at α = 0.05 if the 95% CIs of estimated RR did not cover 1. Values above or below 1

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indicate and quantify over- and underyielding, respectively. The difference between groups was

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significant if 95% CIs of their coefficients did not overlap the other’s mean.

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To simultaneously examine and identify species and diversity effects on soil microbial

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attributes, we also tested the effects of broadleaved tree proportions and tree species richness on

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soil microbial biomass, the F/B ratio, and GP/GN bacteria ratio, and whether the effects of tree

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species richness differed with water availability using the following linear mixed-effect model:

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Yijkl = Bj + Ri + Wk(l) + Bj × Wk(l) + Ri × Wk(l) + πl + εm(ijkl)

(5)

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where Yijkl is the soil microbial attribute of interest and soil water content, soil pH, litterfall

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production, standing root length used; Ri is the tree species richness; Bj is the broadleaved tree

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proportion; Wk(l) is the water treatment (j = 25% throughfall reduction, ambient, or 25%

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throughfall addition) nested in each sample plot l (l =1, 2,…9); πl is the random effect of plot;

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εm(ijkl) is the sampling error.

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To examine the effects of overstory type and water treatment on microbial community

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composition, we conducted permutational multivariate analysis of variance (perMANOVA) and

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included plot identity as a random factor in the model, implemented in the ‘adonis’ function

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from the ‘vegan’ package. The microbial community composition as a whole was analyzed using

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biomass data from major microbial groups of PLFAs, as well as individual PLFAs (Table S1). In

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perMANOVA, we employed the Bray-Curtis dissimilarity matrix to summarize species

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composition and used 999 permutations to determine statistical significance. We visualized the

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compositional data using nonmetric multidimensional scaling with the Bray-Curtis dissimilarity

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measure. To gain insights into the effects of mixed forests and variable water availability on

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microbial communities, we also tested the effects of overstory type (T), water treatment (W), and

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species mixture effects on soil water content, soil pH, litterfall production, and standing root

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length, using eqns. 1, 3&4, respectively. The Pearson correlation was used to examine the

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associations between soil water content, soil pH, litterfall production, standing root length, total

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microbial biomass, saprotrophic fungal biomass, AM_fungal biomass, bacterial biomass, Gram-

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positive bacterial biomass, Gram-negative bacterial biomass, F/B ratio, and GP/GN bacteria ratio.

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Subsequently, we examined the association between the PLFAs of soil microorganisms and

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overstory tree richness, broadleaved tree proportion, total stand basal area, annual litterfall

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productions, standing root length, and soil environment (soil water content, soil pH) across

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overstory types and water treatments, and used the envfit function of the vegan package

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(Oksanen et al., 2017). Assumptions of normality and homogeneous variance were examined by

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Shapiro-Wilk’s and Leven’s tests, respectively. All statistical analyses were performed in R 3.5.3

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(R Core Team, 2018).

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Results

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We found that the soil water content during the growing season differed between water

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treatments, but not among overstory types (Table. 2). Across all stand types, compared with

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ambient water conditions, throughfall addition, on average, increased soil moisture, while

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throughfall reduction had no significant effect (Fig. S2). Total soil microbial biomass,

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saprotrophic fungal biomass, arbuscular mycorrhizal fungal biomass, bacterial biomass, GP

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bacterial biomass, and GN bacterial biomass differed between overstory types and water

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treatments, while the ratios of F/B and GP/GN bacteria did not (Table 2, Fig. 1a & 2a). The

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significant interactive effect of overstory type and water treatment was found only for

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saprotrophic fungal biomass (Table 2). Across all water treatments, the overall and individual

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group microbial biomass were higher in the Populus stands than Pinus and Pinus+Populus stands,

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except for saprotrophic fungal biomass (Fig. 1a). Across all stand types, additional water, on

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average, increased the biomass of all microbial attributes, except for saprotrophic fungal biomass

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(Fig. 1a). Saprotrophic fungal biomass was higher under additional water in Pinus stands, but not

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in the Pinus+Populus and Populus stands (Table 2, Fig. S3). In contrast to ambient water

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conditions, water reduction did not significantly change the total soil microbial biomass,

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saprotrophic fungal biomass, arbuscular mycorrhizal fungal biomass, bacterial biomass, GP

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bacterial biomass, or GN bacterial biomass (Fig. 1a).

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Total soil and individual group microbial biomass were significantly lower (P<0.05) in

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mixtures than expected, from those of constituent monocultures in ambient water sites, except

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for saprotrophic fungal biomass (P = 0.273, Fig. 1b). However, compared with ambient water

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treatment, water reduction significantly (P < 0.05) or marginally significantly (P < 0.10)

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increased species mixture effects on soil total microbial biomass, saprotrophic fungal biomass,

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arbuscular mycorrhizal fungal biomass, bacterial biomass, and GP bacterial biomass, from

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negative to neutral, whereas water addition increased species mixture effects on arbuscular

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mycorrhizal fungal biomass and GP bacterial biomass, from negative to neutral (Fig. 1b, Table

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S2). The ratios of F/B and GP/GN bacteria were not significantly different between the mixed

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stands and the average of constituent monocultures (Fig. 2b). Neither water reduction nor

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addition affected species mixture effects on the F/B and GP/GN bacteria ratios (Fig. 2b, Table

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S2).

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Compared with species richness, broadleaved tree proportions explained a greater

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proportion of the variation in microbial attributes, except for GP/GN (Fig. 3, Table S3). Soil total

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microbial biomass, saprotrophic fungal biomass, arbuscular mycorrhizal fungal biomass,

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bacterial biomass, and GP bacterial biomass was not altered by species richness; however, they

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were significantly or marginally increased with the proportion of broadleaved trees (Fig. 3, Table

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S3). The GN bacterial biomass increased with both species richness and the proportion of

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broadleaved trees (P = 0.046 and 0.002, respectively, Fig. 3, Table S3). The GP/GN bacteria

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ratio decreased marginally with species richness (P = 0.030), whereas the F/B ratio was not

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affected by species richness nor the proportion of broadleaved trees (Fig. 3, Table S3). There was

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no significant interactive effect of species richness and water treatment on any microbial

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attributes (Table S3).

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The soil water content and standing root length differed between water treatments;

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however, soil pH and litterfall production did not change with water treatments, while soil water

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content, pH, litterfall production, and standing root length were similar among overstory types

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(Table 2). Across all stand types, water addition on average increased soil water content, whereas

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water reduction did not affect soil water content, primarily due to the early stage null effect (May

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to July); however, water reduction led to significant negative effects on water content at the late

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stage (Figs. 4a, S2). On average, water reduction decreased, but water addition did not affect the

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standing root length (Fig. 4a). The soil water content was generally lower in mixtures than

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expected from those of constituent monocultures, with a significant negative species mixture

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effect under water reduction treatment (Fig. 4b). The soil pH and litterfall production were not

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significantly different between the mixed stands and the average of constituent monocultures

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across all water treatments, and species mixture effects on these elements were not impacted by

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altered water availability (Fig. 4b). The stand root length was significantly lower in mixtures

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than anticipated from those of constituent monocultures at ambient water sites; however, water

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reduction and addition significantly increased the species mixture effects from negative to

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neutral and positive, respectively (Fig. 4b).

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The soil pH was negatively correlated with the GP/GN ratio, whereas the litterfall

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production was positively correlated with saprotrophic fungal biomass and the F/B ratio (Fig. S4).

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The soil water content and standing root length were not correlated with any microbial attributes

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(Fig. S4). The perMANOVA analysis revealed that microbial community compositions differed

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significantly with both overstory type and water treatment, for both individual PLFAs and

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microbial group levels (Fig. 5). Between overstory types, the broadleaf was distinct from the

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others (Fig. 5). The soil microbial community compositions revealed strong associations with

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species richness, soil moisture, soil pH, and litterfall production on individual PLFAs levels (Fig.

332

5a). At the microbial group level, soil microbial community compositions showed strong

333

associations with species richness only (Fig. 5b).

16

334

Discussion

335

We demonstrated that both species mixture and water availability influenced soil microbial

336

community biomass and composition. Our results provided experimental evidence that the

337

effects of water availability on soil microbial biomass and composition were generally consistent

338

across overstory types. Importantly, significant underyielding effects in total soil and individual

339

group microbial biomass were observed, and these underyielding effects could be increased by

340

water reduction and addition. Our results were in contrast to previous studies, which reported

341

mostly positive relationships between plant diversity and microbial biomass (Chen et al., 2019b),

342

and also indicated the crucial role of water availability in shaping biodiversity and ecosystem

343

functioning relationships. Additionally, although the broadleaved proportion was a more critical

344

driver of soil microbial biomass, tree species richness explained more of the variation in

345

microbial composition.

346

We discovered that water addition generally increased soil total and individual group

347

microbial biomass, whereas water reduction had no impact, tracking the variation in soil water

348

content, reinforcing the critical role of soil water availability in controlling soil microbial

349

biomass (Manzoni et al., 2012; Zhou et al., 2018). The negligible effect of water reduction on

350

soil moisture content might be partly attributable to the low proportion of the decrease in water

351

availability (25%), and significant water inputs from snow melt in the spring. Additionally,

352

plants may acclimatize to reduced water availability and decreased water uptake from the soil, as

353

indicated by decreased standing root length under water reduction (Brunner et al., 2015). Our

354

analysis also revealed that neither water addition or reduction affected the F/B or GP/GN

355

bacteria ratios. The lack of water reduction effects on the F/B or GP/GN bacteria ratios was

356

likely due to the limited changes in soil moisture, while the lack of water addition effects can be

17

357

partly attributed to the adaptation of microbial community composition (Huang et al., 2015;

358

Yang et al., 2017). Soil microbial community composition may acclimate mesic environments

359

and sustained a relatively stable structure (Huang et al., 2015; Yang et al., 2017). The

360

simultaneous increases in individual group microbial biomass indicate a common limitation of

361

water to whole microbial community growth in boreal forests.

362

In contrast to previous experimental studies and meta-analysis, where positive species

363

mixture effects on microbial biomass were found (Thakur et al., 2015; Chen et al., 2019b), our

364

analysis revealed lower soil total and individual group microbial biomass in mixed forests than

365

the average of corresponding monocultures. This underyielding of microbial biomass might have

366

been the result of reduced root and root-exudation inputs, which were indicated by negative

367

species mixture effects on standing root lengths. Species mixtures may increase the efficiency of

368

water use; thus, reducing the investment required for roots (Canarini et al., 2016). Species

369

mixture had no significant effect on litterfall biomass, which might be partly attributable to that

370

both Populus tremuloides and Pinus banksiana are shade intolerant (Ma et al., 2019). Shade

371

tolerance heterogeneity among constituent species is the main driver for positive plant diversity

372

effect in aboveground production (Zhang et al., 2012). Alternatively, the negative species

373

mixture effect is possibly a result of lower understory productivity in overstory species-rich plots

374

(Zhang et al., 2017). The overstorey tree diversity tends to have a negative effect on understory

375

above-ground biomass in boreal forests via resource filtering (Zhang et al., 2017), and thus,

376

decreased microbial biomass due to the critical role of understory vegetation on soil microbes

377

(Yang et al., 2018; Lyu et al., 2019).

378 379

As expected, we found that water reduction increased species mixture effects on soil total and individual group microbial biomass from negative to neutral. This was likely due to a 25%

18

380

decrease in throughfall, which led mixed forests to allocate additional production partitioning

381

belowground to support greater water and nutrient demands for higher ecosystem productivity,

382

via more horizontal and vertical soil volume filling (Brassard et al., 2013; Ammer, 2019).

383

Despite a positive species mixture effect on standing root lengths with water addition, which is

384

possibly a result of the additional production partitioning belowground to exploit increased water

385

resources (Wright et al., 2015; Fischer et al., 2016),we found that water addition did not impact

386

species mixture effect on total soil microbial biomass and individual microbial group biomass

387

except arbuscular mycorrhizal fungi and GP bacteria. This was likely due to the short

388

experimental duration of our study (two years) and the delayed response of microbes to changes

389

in plant inputs (Chen and Chen, 2018). Arbuscular mycorrhizal fungi establish symbioses with

390

plants from which they receive a substantial fraction of readily available carbon sources in

391

exchange for nutrients; which might facilitate a more rapid response to changes in root growth

392

(Drigo et al., 2010). Typically, GP bacteria interact synergistically with arbuscular mycorrhizal

393

fungi, which might benefit from the increase in arbuscular mycorrhizal fungi (Artursson et al.,

394

2006). Our results revealed that the responses of microbial biomass to species mixture could be

395

primarily root-derived, which reinforced the critical role of roots in the coupling of plant and

396

microbial productivity (Paterson, 2003; Lange et al., 2015).

397

We found that neither F/B nor GP/GN bacteria ratio changed with tree species mixture or

398

richness, as previously reported in temperate forests (Khlifa et al., 2017). One possible

399

interpretation might be that species mixtures affect microbial composition primarily by altering

400

the abiotic soil environment (e.g., soil pH, moisture) and litterfall biomass, as indicated by our

401

correlation analysis. However, for our study, soil pH and moisture did not differ between mixed

402

forests and monocultures (Rousk et al., 2010a; Lange et al., 2014; Leloup et al., 2018). Across all

19

403

water treatments, total soil and individual group microbial biomass increased significantly with

404

the abundance of broadleaved trees, but only weakly with species richness. This was in

405

agreement with previous forest diversity-manipulated experiments (Gunina et al., 2017), which

406

indicated that tree species composition had a more potent effect than species number on

407

microbial biomass.

408

The microbial community of the mixed stands possessed more similarities to the conifer

409

stands, over that of the broadleaved stands. Our results revealed that coniferous trees had a

410

dominating effect on the composition of microbial communities; thus, they could be considered

411

as the keystone species with regard to plant-microbial interactions. Interestingly, we found that

412

tree species richness was the main explanatory factor for microbial composition at both the

413

individual PLFA and microbial group levels. This indicated its critical role in plant diversity on

414

soil microbial communities, as was previously observed in grassland diversity experiments,

415

which was likely due to a diversity of litters and root exudates (Steinauer et al., 2016; Leloup et

416

al., 2018). Similar to previous studies (Rousk et al., 2010b; Huang et al., 2014; Leloup et al.,

417

2018), we observed that soil pH, moisture, and litterfall biomass affected microbial composition;

418

however, they had a negligible or no impact on microbial biomass, which warrants further

419

investigations to unveil the specific underlying mechanisms. Additionally, soil physicochemical

420

properties, such as soil C/N and soil texture also play determinant roles in controlling soil

421

microbial community structure (Bach et al., 2010; Wan et al., 2015). Thus, future measurements

422

of our experiment should explicitly include soil physicochemical properties to evaluate potential

423

mechanisms.

424 425

In conclusion, we found that the mild addition of water served to increase influence on microbial biomass, whereas the mild water reduction did not. Additionally, our study revealed a

20

426

lower soil microbial biomass in mixed forests over monocultures under ambient precipitation,

427

where tree identity effects dominated over tree diversity effects on the microbial biomass.

428

Further, we found that the effects of tree species mixtures on microbial biomass increased

429

following the modification of water availability, which indicates that interactions between plants

430

and soil microbes have the potential to regulate ecosystem responses to climate change.

431

Therefore, mixed forests may have the capacity to attenuate the impacts of changing water

432

availability.

433

Acknowledgments

434

This study was funded by the Natural Sciences and Engineering Research Council of Canada

435

(RGPIN-2014-04181, STPGP428641, RTI-2017-00358, and STPGP506284). X.C. wishes to

436

thank the Government of Ontario for an Ontario Trillium Scholarship. We thank Dr. Eric Searle

437

for his helpful editorial comments

438

21

439

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30

1

Table 1. Characteristics (mean and 1 s.e.m., n = 3) of the study stands in Northwestern Ontario,

2

Canada. Stand types are single-species Pinus banksiana dominated (Pinus), single-

3

species Populus tremuloides dominated (Populus), and their mixtures (Populus+Pinus). Stand type Populus 2 -1 Stand basal area (m ha ) 1.55 ± 0.35 -1 Stand density (trees ha ) 5933 ± 1790 Tree species richness 2.67 ± 0.33 Tree species composition (% of stand basal area) Pinus banksiana 3±2 Populus tremuloides 92 ± 2 Betula papyrifera 3±2 Other broadleaf species 2±1 -1 Soil carbon content (g kg ) 20.41 ± 1.65 1.20 ± 0.12 Soil nitrogen content (g kg-1)

4

Pinus 0.93 ± 0.33 11600 ± 4148 2.67 ± 0.33

Populus+Pinus 1.39 ± 0.24 9200 ± 1301 4.33 ± 0.33

98 ± 1 1±1 1±1 1±0 14.24 ± 2.39 0.93 ± 0.12

48 ± 7 28 ± 3 15 ± 4 4±1 17.15 ± 3.86 1.06 ± 0.21

5

Table 2. Effects (P values) of overstory type (T), water treatments (W), and their interactions on

6

soil microbial biomass, F/B ratio, GP/GN bateria ratio, soil water content, soil pH, litterfall

7

production, and standing root length. W

T

T×W

Attribute df

F

P

df

F

P

df

F

P

SMB

2,6

6.32

0.033

2,12

9.22

0.004

4,12

2.06

0.150

Saprotrophic fungi

2,6

6.12

0.036

2,12

5.21

0.024

4,12

4.07

0.026

AM-Fungi

2,6

10.05

0.012

2,12

5.01

0.026

4,12

0.57

0.691

Bacteria

2,6

6.44

0.032

2,12

7.84

0.007

4,12

1.67

0.221

GP

2,6

5.48

0.044

2,12

5.94

0.016

4,12

2.03

0.154

GN

2,6

5.75

0.040

2,12

10.07

0.003

4,12

1.36

0.306

F/B

2,6

0.89

0.459

2,12

1.70

0.224

4,12

2.46

0.102

GP/GN

2,6

0.47

0.648

2,12

0.26

0.777

4,12

1.36

0.304

Soil water content

2,6

0.87

0.465

2,12

3.96

0.048

4,12

1.09

0.404

Soil pH

2,6

0.01

0.991

2,12

0.34

0.722

4,12

1.56

0.247

Litterfall biomass

2,6

0.20

0.826

2,12

1.22

0.330

4,12

1.31

0.321

2,6

0.44

0.662

2,12

4.75

0.030

4,12

2.88

0.069

Standing root length 8 9 10 11 12 13 14

Note: Linear mixed-effects model fit tests used Satterthwaite approximations for denominator degrees of freedom (df). P are significance of the model and P < 0.05 are highlighted in bold. SMB, Saprotrophic fungi, AM-Fungi, Bacteria, GP, GN, F/B, and GP/GN are total soil microbial biomass, saprotrophic fungal biomass, AM-fungal biomass, bacterial biomass, Gram-positive bacteria biomass, Gram-negative bacteria biomass, fungal: bacterial ratio, and Gram-positive: Gram-negative bacterial ratio, respectively.

1 2

Figure 1. The total and individual group microbial biomass in relation to forest types and

3

water treatments and the effects of species mixtures. a) Total soil microbial biomass (SMB),

4

soil saprotrophic fungal biomass, AM-fungal biomass, bacterial biomass, Gram-positive bacterial

5

biomass (GP), and Gram-negative bacterial biomass (GN) in relation to overstory types and

6

water treatments. Different capital letters indicate a significant difference between overstory

7

types and different lower-case letters indicate a significant difference between water treatments

8

(α = 0.05). b) Effects of species mixtures on SMB, soil saprotrophic fungal biomass, AM-fungal

9

biomass, bacterial biomass, GP, and GN. The effects represent the response ratio of a given

10

microbial attribute compared between mixtures and the corresponding monocultures (see

11

Methods). Values are mean ± 95% confidence intervals (CIs). The mixed species effects were

12

significant at α = 0.05 if the 95% CIs did not cover 1. The difference between groups was

13

significant if 95% CIs of their coefficients did not overlap the other’s mean.

14

15 16

Figure 2. The ratio of fungal to bacterial biomass and gram-positive to gram-negative

17

bacteria in relation to forest types and water treatments and the effects of species mixtures.

18

a) Ratio of fungal to bacterial biomass (F/B) and gram-positive to gram-negative bacteria

19

(GP/GN) in relation to overstory types and water treatments. Different capital letters indicate a

20

significant difference between overstory types and different lower-case letters indicate a

21

significant difference between water treatments (α = 0.05). b) Effects of species mixtures on the

22

ratio of F/B and GP/GN. The effects represent the response ratio of a given microbial attribute

23

compared between mixtures and the corresponding monocultures (see Methods). Values are

24

mean ± 95% confidence intervals (CIs). The mixed species effects were significant at α = 0.05 if

25

the 95% CIs did not cover 1. The difference between groups was significant if 95% CIs of their

26

coefficients did not overlap the other’s mean.

27

28

29

Figure 3. The total and individual group microbial biomass and the ratio of fungal to

30

bacterial biomass and gram-positive to gram-negative bacteria in relation to tree species

31

richness and proportion of broadleaved trees. a) Total soil microbial biomass (SMB), soil

32

fungal biomass, AM-fungal biomass, bacterial biomass, Gram-positive bacterial biomass (GP),

33

Gram-negative bacterial biomass (GN) and ratio of fungal to bacterial biomass (F/B) and gram-

34

positive to gram-negative bacteria (GP/GN) in relation to overstory tree richness. b) SMB, soil

35

fungal biomass, AM-fungal biomass, bacterial biomass, GP, GN, F/B and GP/GN for different

36

overstory types as relates to the proportion of broadleaved trees. The red line and grey shaded

37

areas represent the fitted regression and its bootstrapped 95% confidence intervals. The

38

significance (P) is presented for each term tested.

39

40

41

Figure 4. The soil properties and litter inputs in relation to forest types and water

42

treatments and the effects of species mixtures. a) Soil volumetric water content, soil pH,

43

litterfall production and standing root length in relation to forest types and water treatments.

44

Different capital letters indicate a significant difference between overstory types and different

45

lower-case letters indicate a significant difference between water treatments (α = 0.05). b) Effects

46

of species mixtures on soil volumetric water content, soil pH, litterfall production, and standing

47

root length. The effects represent the response ratio of a given microbial attribute compared to

48

the monocultures and the mixtures (see Methods). Values are mean ± 95% confidence intervals

49

(CIs). The mixed species effects were significant at α = 0.05 if the 95% CIs did not cover 1. The

50

difference between groups was significant if 95% CIs of their coefficients did not overlap the

51

other’s mean.

52 53

Figure 5. Nonmetric multidimensional scaling ordination of soil microbial communities as

54

individual PLFAs (a) and microbial groups (b) for different overstory types and water treatment

55

combinations in relation to aboveground characteristics and soil moisture. Ellipses represent

56

standard errors of the weighted averages of scores of corresponding to overstory types. Predictor

57

variables included overstory tree richness (R), proportion of broadleaved trees (BL), total stand

58

basal area (BA), soil water content (SW), pH, annual litterfall production (LP), and standing root

59

length (RL). The vector lengths represent correlation (r), and red and blue vectors indicate

60

significance P ≤ 0.05 and > 0.05, respectively.

61

Water addition increased soil microbial biomass, but water reduction had no effect; Species mixture decreased soil total and individual group microbial biomass; Water reduction increased species mixture effects on soil microbial biomass; Soil microbial biomass increased with the abundance of broadleaved trees; Microbial compositions differed with both overstory type and water treatment.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: