Light availability and plant shade tolerance modify plant-microbial interactions and feedbacks in subtropical trees.

Plant-soil feedbacks (PSFs) are an important mechanism of species coexistence in forest communities. However, evidence remains limited for how light availability regulates PSFs in species with different shade tolerance via changes in plant-microbial interactions. Here we tested in a glasshouse experiment how PSFs changed as a function of light availability and tree shade tolerance. Soil bacterial and fungal communities were profiled using the 16S rRNA and ITS2 gene sequencing, respectively. Under low light, individual PSFs were positively related to shade tolerance, while the least shade-tolerant species produced the most positive PSFs under high light. Pairwise PSFs between species with contrasting shade tolerance were strongly positive under high light but negative under low light, thereby promoting the dominance of less shade-tolerant species in forest gaps and species coexistence under closed canopy, respectively. Under high light, PSFs were related to soil microbial composition and diversity, with the relative abundance of arbuscular mycorrhizal fungi being the primary driver of PSFs. Under low light, none of soil microbial properties were significantly related to PSFs. These findings indicate PSFs and plant shade tolerance interact to promote species coexistence and improve our understanding of how soil microbes contribute to variation in PSFs.

Drought soil legacy alters drivers of plant diversity-productivity relationships in oldfield systems.

Ecosystem functions are threatened by both recurrent droughts and declines in biodiversity at a global scale, but the drought dependency of diversity-productivity relationships remains poorly understood. Here, we use a two-phase mesocosm experiment with simulated drought and model oldfield communities (360 experimental mesocosms/plant communities) to examine drought-induced changes in soil microbial communities along a plant species richness gradient and to assess interactions between past drought (soil legacies) and subsequent drought on plant diversity-productivity relationships. We show that (i) drought decreases bacterial and fungal richness and modifies relationships between plant species richness and microbial groups; (ii) drought soil legacy increases net biodiversity effects, but responses of net biodiversity effects to plant species richness are unaffected; and (iii) linkages between plant species richness and complementarity/selection effects vary depending on past and subsequent drought. These results provide mechanistic insight into biodiversity-productivity relationships in a changing environment, with implications for the stability of ecosystem function under climate change.

Precipitation Changes Regulate Plant and Soil Microbial Biomass Via Plasticity in Plant Biomass Allocation in Grasslands: A Meta-Analysis.

In theory, changes in the amount of rainfall can change plant biomass allocation and subsequently influence coupled plant-soil microbial processes. However, testing patterns of combined responses of plants and soils remains a knowledge gap for terrestrial ecosystems. We carried out a comprehensive review of the available literature and conducted a meta-analysis to explore combined plant and soil microbial responses in grasslands exposed to experimental precipitation changes. We measured the effects of experimental precipitation changes on plant biomass, biomass allocation, and soil microbial biomass and tested for trade-offs between plant and soil responses to altered precipitation. We found that aboveground and belowground plant biomass responded asynchronically to precipitation changes, thereby leading to shifts in plant biomass allocation. Belowground plant biomass did not change under precipitation changes, but aboveground plant biomass decreased in precipitation reduction and increased in precipitation addition. There was a trade-off between responses of aboveground plant biomass and belowground plant biomass to precipitation reduction, but correlation wasn't found for precipitation addition. Microbial biomass carbon (C) did not change under the treatments of precipitation reduction. Increased root allocation may buffer drought stress for soil microbes through root exudations and neutralize microbial responses to precipitation reduction. However, precipitation addition increased microbial biomass C, potentially reflecting the removal of water limitation for soil microbial growth. We found that there were positive correlations between responses of aboveground plant biomass and microbial biomass C to precipitation addition, indicating that increased shoot growth probably promoted microbial responses via litter inputs. In sum, our study suggested that aboveground, belowground plant biomass and soil microbial biomass can respond asynchronically to precipitation changes, and emphasizes that testing the plant-soil system as a whole is necessary for forecasting the effects of precipitation changes on grassland systems.

Limiting similarity shapes the functional and phylogenetic structure of root neighborhoods in a subtropical forest.

Environmental filtering and limiting similarity mechanisms can simultaneously structure community assemblages. However, how they shape the functional and phylogenetic structure of root neighborhoods remains unclear, hindering the understanding of belowground community assembly processes and diversity maintenance. In a 50-ha plot in a subtropical forest, China, we randomly sampled > 2700 root clusters from 625 soil samples. Focusing on 10 root functional traits measured on 76 woody species, we examined the functional and phylogenetic structure of root neighborhoods and linked their distributions with environmental cues. Functional overdispersion was pervasive among individual root traits (50% of the traits) and accentuated when different traits were combined. Functional clustering (20% of the traits) seemed to be associated with a soil nutrient gradient with thick roots dominating fertile areas whereas thin roots dominated infertile soils. Nevertheless, such traits also were sorted along other environmental cues, showing multidimensional adaptive trait syndromes. Species relatedness also was an important factor defining root neighborhoods, resulting in significant phylogenetic overdispersion. These results suggest that limiting similarity may drive niche differentiation of coexisting species to reduce competition, and that alternative root strategies could be crucial in promoting root neighborhood resource use and species coexistence.

Species Identity and Initial Size Rather Than Neighborhood Interactions Influence Survival in a Response-Surface Examination of Competition.

To measure intraspecific and interspecific interaction coefficients among tree species is the key to explore the underlying mechanisms for species coexistence and biodiversity maintenance in forests. Through the response surface experimental design, we established a long-term field experiment by planting 27,300 seedlings of four tree species (Erythrophleum fordii, Pinus massoniana, Castanopsis fissa, and Castanopsis carlesii) in 504 plots in different species combinations (six pairwise combinations of four species), abundance proportions (five abundance proportions of two species, i.e. A: B = 1:0, 3:1, 1:1, 1:3, 0:1), and stand densities (25, 36, 64, and 100 seedlings per plot). In this initial report, we aimed to quantify the relative importance of biotic and abiotic factors on seedling survival at the early stage of growth, which is a critical period for seedling establishment. We found that plot-level seedling survival rate was determined by species combination and their abundance proportion rather than stand density. At the individual level, individual survival probability was mainly explained by species identity, initial seedling size, and soil conditions rather than neighborhood competition. Our study highlights that the seedling intrinsic properties may be the key factors in determining seedling survival rate, while neighborhood effects were not yet prominent at the seedling life stage.

Soil microbes alter seedling performance and biotic interactions under plant competition and contrasting light conditions.

BACKGROUND AND AIMS: Growing evidence suggests that the net effect of soil microbes on plants depends on both abiotic and biotic conditions, but the context-dependency of soil feedback effects remains poorly understood. Here we test for interactions between the presence of conspecific soil microbes, plant competition and light availability on tree seedling performance. METHODS: Seedlings of two congeneric tropical tree species, Bauhinia brachycarpa and Bauhinia variegata, were grown in either sterilized soil or soil conditioned by conspecific soil microorganisms in a two-phase greenhouse feedback experiment. We examined the interactive effects of soil treatment (live, sterilized), light availability (low, high) and plant competition (no competition, intraspecific and interspecific competition) on tree seedling biomass. We also investigated the linkages between the outcomes of soil feedback effects and soil microbial community structure. KEY RESULTS: The outcomes of soil feedback effects on seedling biomass varied depending on both competition treatment and light availability. Under low light conditions, soil feedback effects were neutral irrespective of competition treatment and plant species. Soil feedback effects were negative in high light for seedlings with interspecific competition, but positive for seedlings growing alone or with intraspecific competition. Soil feedback effects for seedlings were driven by variation in the Gram-positive:Gram-negative bacteria ratio. Light and conspecific soil microbes had interactive effects on the competitive environment experienced by tree species; in low light the presence of conspecific soil microbes decreased plant competition intensity, whereas in high light both the intensity and the importance of competition increased for seedlings in the presence of soil microbes, irrespective of plant species. CONCLUSIONS: Our findings underline the importance of light and plant competition for the outcomes of soil feedback effects on young tree seedlings, and suggest that reduced light availability may reduce the influence of conspecific soil microbes on plant-plant interactions.

Species richness alters spatial nutrient heterogeneity effects on above-ground plant biomass.

Previous studies have suggested that spatial nutrient heterogeneity promotes plant nutrient capture and growth. However, little is known about how spatial nutrient heterogeneity interacts with key community attributes to affect plant community production. We conducted a meta-analysis to investigate how nitrogen heterogeneity effects vary with species richness and plant density. Effect size was calculated using the natural log of the ratio in plant biomass between heterogeneous and homogeneous conditions. Effect sizes were significantly above zero, reflecting positive effects of spatial nutrient heterogeneity on community production. However, species richness decreased the magnitude of heterogeneity effects on above-ground biomass. The magnitude of heterogeneity effects on below-ground biomass did not vary with species richness. Moreover, we detected no modification in heterogeneity effects with plant density. Our results highlight the importance of species richness for ecosystem function. Asynchrony between above- and below-ground responses to spatial nutrient heterogeneity and species richness could have significant implications for biotic interactions and biogeochemical cycling in the long term.

Plant community responses to precipitation and spatial pattern of nitrogen supply in an experimental grassland ecosystem.

Recent work suggests that soil nutrient heterogeneity may modulate plant responses to drivers of global change, but interactions between N heterogeneity and changes in rainfall regime remain poorly understood. We used a model grassland system to investigate the interactive effects of N application pattern (homogeneous, heterogeneous) and precipitation-magnitude manipulation during the growing season (control, +50 % rainfall, -50 % rainfall) on aboveground biomass and plant community dominance patterns. Our study resulted in four major findings: patchy N addition increased within-plot variability in plant size structure at the species level, but did not alter total aboveground biomass; patchy N addition increased community dominance and caused a shift in the ranking of subordinate plant species; unlike community-level biomass, plant species differed in their biomass response to the rainfall treatments; and neither aboveground biomass nor community dominance showed significant interactions between N pattern and rainfall manipulation, suggesting that grassland responses to patchy N inputs are insensitive to water addition or rainfall reduction in our temperate study system. Overall, our results indicate that the spatial pattern of N inputs has greater effects on species biomass variability and community dominance than on aboveground production. These short-term changes in plant community structure may have significant implications for longer-term patterns of vegetation dynamics and plant-soil feedbacks. Moreover our results suggest that the magnitude of precipitation during the growing season plays a limited role in grassland responses to heterogeneous organic N inputs, emphasizing the need to consider other components of precipitation change in future heterogeneity studies.