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.

Arid and semi‐arid grassland ecosystems cover about 15% of the global land surface and provide vital soil carbon (C) and nitrogen (N) sequestration. Although half of the soil C and N is stored in deep soils (below 30 cm), no regional‐scale study of microbial properties and their functions through the soil profile has been conducted in these drylands. To explore the distribution and determinants of microbial properties and C and N mineralization rates through soil profile along aridity gradient at a regional scale, we investigated these variables for four soil layers (0–20, 20–40, 40–60 and 60–100 cm) in 132 plots on the Mongolia Plateau. Soil microbial properties (biomass and bacteria:fungi ratio) and C and N mineralization rates decreased with increasing soil depth and aridity at the regional scale. Aridity‐induced declines in soil microbial properties mainly resulted from the negative effects of aridity on ANPP/root biomass and soil organic C (SOC) in the surface soil layers (0–20 and 20–40 cm) but from the direct and indirect (via SOC and soil C/N) negative effects of aridity in the deep soil layers (40–60 and 60–100 cm). Aridity‐induced declines in soil C mineralization rates mainly resulted from the negative indirect effect of aridity on SOC and microbial properties in each soil layer, with weaker effects of SOC and stronger effects of soil microbes in the deep soil layers. Aridity‐induced declines in soil N mineralization rates mainly resulted from the negative indirect effect of aridity on SOC in the three soil layers above 60 cm and mainly resulted from the negative direct effect of aridity in the 60–100 cm soil layer. Aridity via direct or indirect effects strongly determined the patterns of soil microbial properties and C and N mineralization throughout soil profiles on the Mongolian Plateau. These findings suggest that the increases in aridity are likely to induce changes in soil micro‐organisms and their associated functions across soil depths of semi‐arid grasslands, and future models should consider the dynamic interactions between substrates and microbial properties across soil depths in global drylands. A plain language summary is available for this article.

No abstract

Aim To assess how the diversity of above‐ and below‐ground organisms changes along environmental gradients at the regional scale and whether the effects of diversity changes on biomass are similar for above‐ and below‐ground organisms. Location Semi‐arid grasslands of the Mongolian Plateau. Methods We investigated diversity (α‐, β‐ and γ‐) and biomass of plant and soil nematodes as well as environmental factors (climate, soil environment, and soil resource) at 44 field sites (220 plots) along a 2000‐km east–west transect and a 900‐km south–north transect across grasslands on the Mongolian Plateau. Regression was used to examine the relationships between diversity components and biomass of plants and nematodes. Hierarchical structural equation modelling (SEM) was performed to analyse the effects of environmental factors on diversity components and their linkages to biomass. Results The biomass of plants and nematodes correlated positively with biodiversity measures for plants and nematodes except that nematode biomass decreased as nematode β‐diversity increased. The relationship between plant and soil nematodes was positive for biomass and for α‐ and γ‐diversity, but it was negative for β‐diversity. When considering the environmental factors, hierarchical SEM indicated that variation in plant or nematode γ‐diversity was associated with changes in climate, soil environment, and soil resources. Variation in plant or nematode α‐diversity was mainly associated with changes in γ‐diversity, while variation in the plant or nematode β‐diversity was mainly associated with changes in γ‐diversity and climate. The climate and soil resources explained most of the variation in plant biomass, whereas climate and α‐ and γ‐diversity explained most of the variation in nematode biomass. Surprisingly, plant biomass or diversity was only weakly related to soil nematodes when considering the environmental factors. Main conclusions Diversity and biomass patterns of nematodes and perhaps of other below‐ground organisms are different from those of plants, and this difference is highly climate dependent. These findings suggest that a more complete understanding of diversity–biomass relationships will require further examination of more taxa across a broader range of environmental gradients.

A rapid selection system was used to screen Glycyrrhiza uralensis plants containing high concentrations of glycyrrhizin (GC) by Eastern blotting using anti-GC monoclonal antibody (MAb). Chromatographic fingerprinting by Eastern blotting correlated with the GC concentration analyzed by enzyme-linked immunosorbent assay (ELISA). The roots of wild G. uralensis growing in Mongolia were analyzed by Eastern blotting to identify plants containing high concentrations of GC, and the GC concentration was confirmed by ELISA. G. uralensis plants cultivated in the greenhouse were also analyzed in the same manner. GC concentrations in wild G. uralensis roots and cultivated plants varied widely: between 0.06 and 9.36 percent dry weight (dw%). To confirm the homogeneity of GC concentrations in the cultivated plants, we monitored GC concentrations in the plants over 2 years. Although GC concentrations changed in two plants, they remained comparatively constant in the other five plants, suggesting that GC concentrations are genetically determined. To identify high GC-producing plants, 1025 plants were analyzed, and the highest concentration of GC was 5.36 dw%.