Introduction
Biodiversity plays an important role in maintaining ecosystem functions including primary productivity (Hooperet al. 2005; Hautier et al. 2015; Delgado-Baquerizoet al. 2020). Understanding the mechanisms underlying positive diversity-productivity relationships is a major goal in ecological research. However, species do not exist in isolation, and plant species interact intimately with microbiota in ways that may mediate biodiversity-productivity relationships (Yanget al. 2021). A number of studies have shown that plant microbiomes are affected by plant diversity (Rottstock et al. 2014; Scheibe et al. 2015; Khlifa et al. 2017; Grossman et al. 2019; Strukelj et al. 2021) and may mediate diversity-productivity relationships (Maron et al. 2011; Schnitzer et al. 2011; Laforest-Lapointe et al. 2017), but the specific mechanisms involved remain unclear.
The positive net diversity effect on productivity (i.e., overyielding) can occur via ‘selection effects’ or ‘complementarity effects’. Positive selection effects occur when communities with more productive species drive the diversity-productivity relationship, while positive complementarity effects occur when more diverse communities exhibit enhanced productivity due to niche partitioning, facilitation, and/or biotic feedbacks (Loreau & Hector 2001; Barry et al. 2019). Soil biota can play a role in complementarity through a variety of mechanisms that involve decomposers, consumers/pathogens, and mutualists (Eisenhauer 2011). For example, soil fungi may influence complementarity if plant species associate with different fungal mutualists that alter resource acquisition strategies to create greater niche separation among host plant species (Koide 2000; Hartet al. 2003; Bever et al. 2010; Afkhami et al.2014; Kazenel et al. 2015; Luo et al. 2018). Additionally, if soil fungal pathogen effects on plant species are density dependent, improved productivity may result when host species are at lower densities in more diverse communities (Maronet al. 2011; van Ruijven et al. 2020). Therefore, soil fungi could play an important role in complementarity effects on productivity, but their effects may be context dependent.
Climate change is expected to have large impacts on plant productivity, for example, by altering soil water availability through shifts in precipitation regimes (Allenet al. 2010; Felton et al. 2021). Positive diversity-productivity relationships are expected to be more common under stressful environmental conditions, i.e., the stress gradient hypothesis, but support for this hypothesis is mixed (Maestre et al. 2009; Wanget al. 2013; Belluau et al. 2021). Soil biota may mediate the influence of soil water availability on diversity-productivity relationships. Soil mutualists such as mycorrhizal fungi can play a role in plant water acquisition and drought tolerance, but water availability can also influence the abundance of pathogens and susceptibility of plants to pathogen attack (Desprez-Loustauet al. 2006; Lehto & Zwiazek 2011). Given the potential of climatic change to alter precipitation regimes, it is important to understand the role of soil water availability on the diversity-productivity relationship and whether soil microbes can modify this relationship (Verheyenet al. 2008).
The objective of this study was to determine whether soil fungal communities influence the diversity-productivity relationship in tree communities exposed to contrasting soil water availability. We hypothesized that (1) soil fungal richness increases with tree diversity and is positively linked to tree community productivity, (2) soil fungal community composition mediates the relationship between tree diversity and productivity, (3) richness of ectomycorrhizal and plant pathogenic fungi influence the diversity-productivity relationship, and (4) soil fungal richness and community composition mediate the effect of water availability on diversity-productivity relationships.