No abstract

Belowground communities exert major controls over the carbon and nitrogen balances of terrestrial ecosystems by regulating decomposition and nutrient availability for plants. Yet little is known about the patterns of belowground communities and their relationships with environmental factors, particularly at the regional scale where multiple environmental gradients co‐vary. Here, we describe the patterns of belowground communities (microbes and nematodes) and their relationships with environmental factors based on two parallel studies: a field survey with two regional‐scale transects across the Mongolia plateau and a water‐addition experiment in a typical steppe. In the field survey, soils and plants were collected across two large‐scale transects (a 2000‐km east–west transect and a 900‐km south–north transect). At the regional‐scale, the variations in soil microbes (e.g. bacterial PLFA, fungal PLFA, and F/B ratio) were mainly explained by precipitation and soil factors. In contrast, the variation in soil nematodes (e.g. density of trophic groups and the bacterial‐feeding/fungal‐feeding nematode ratio) were primarily explained by precipitation. These variations of microbe or nematode variables explained by environmental factors at regional scale were derived from different vegetation types. Along the gradient from nutrient‐poor to nutrient‐rich vegetation types, the total variation in soil microbes explained by precipitation increased and that explained by plant and soil decreased, while the opposite was true for soil nematodes. Experimental water addition, which increased rainfall by 30% during the growing season, increased biomass or density of belowground communities, with the nematodes being more responsive than the microbes. The different responses of soil microbial and nematode communities to environmental gradients at the regional scale likely reflect their different adaptations to climate, soil nutrients, and plants. Our findings suggest that the soil nematode and microbial communities are strongly controlled by bottom‐up effects of precipitation alone or in combination with soil conditions.

Summary Among above‐ and below‐ground traits, specific leaf area (SLA, cm2 g−1) and specific root length (SRL, m g−1) are the two key traits reflecting species resource acquisition strategies. However, patterns of variation in SLA and SRL have rarely been examined simultaneously across evolutionary history and environmental gradients, and the SLA–SRL relationship is still controversial on several grounds. We examined the inter‐ and intraspecific variations in SLA and SRL of different root branching orders and the SLA–SRL relationship across 55 species and 21 plant communities of four vegetation types along a 2000‐km transect in the Inner Mongolia grassland. With increasing root branching order, the interspecific variation in SRL increased, but the intraspecific variation in SRL decreased considerably, and the form of SLA–SRL relationship shifted from positive to negative. This indicates that acquisition of soil resources (e.g. water and nutrients) is a fundamental strategy for plant investment to root length. When inter‐ and intraspecific variations in SLA and SRLs were partitioned into alpha (within‐community) and beta (among‐community) components, the alpha component exhibited substantially greater inter‐ and intraspecific variations than the beta component. Across the transect, the evolutionarily late diverged species in phylogenetic tree evolved towards low SLA and SRL‐1 (SRL for first‐order roots) and tended to distribute in resource‐poor conditions along the environmental gradient. The early diverged species, in contrast, had high SLA and SRL‐1 and mostly distributed in resource‐rich conditions. Our findings suggest that patterns of inter‐ and intraspecific variations in SLA and SRL of different root branching orders and the form of SLA–SRL relationship could be well explained by within‐ and among‐community filtering processes and species divergence time. Coordination and trade‐offs between leaves and roots do not mutually exclude but operate simultaneously at different scales and among different root branching orders in arid and semi‐arid grasslands.