Effects of aridity on soil microbial communities and functions across soil depths on the Mongolian Plateau

1. 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. 2. 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. 3. 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). 4. 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. 5. 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 microorganisms 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.

1. 干旱与半干旱草原生态系统约占全球陆地总面积的15%，可提供至关重要的土壤碳（C）与氮（N）固存服务。尽管全球近半数土壤碳与氮储量储存于30厘米以下的深层土壤中，但针对这类干旱区，目前尚无基于区域尺度、贯穿完整土壤剖面的微生物特性及其功能的相关研究。
2. 为探究区域尺度下沿干旱梯度分布的土壤剖面中，微生物特性、碳与氮矿化速率的分布格局及其驱动因子，研究团队在蒙古高原的132个样地中，针对四层土壤（0-20厘米、20-40厘米、40-60厘米及60-100厘米）测定了相关指标。
3. 区域尺度分析显示，土壤微生物特性（微生物生物量及细菌与真菌比值）与碳、氮矿化速率均随土壤深度增加与干旱程度加剧而降低。干旱对土壤微生物特性的抑制作用，在表层土壤（0-20厘米与20-40厘米）中主要源于干旱对地上净初级生产力（aboveground net primary productivity, ANPP）/根系生物量及表层土壤有机碳（soil organic C, SOC）的负面影响；而在深层土壤（40-60厘米与60-100厘米）中，则主要源于干旱对土壤有机碳（SOC）及土壤碳氮比（C/N）的直接与间接负面影响。
4. 干旱引发的土壤碳矿化速率下降，主要由干旱对各土层土壤有机碳及微生物特性的负向间接效应驱动；相较于表层土壤，深层土壤中土壤有机碳的调控作用更弱，而土壤微生物的影响效应更强。干旱诱导的土壤氮矿化速率下降，在60厘米以上的三层土壤中主要源于干旱对土壤有机碳的负向间接作用，在60-100厘米土层中则主要由干旱的直接负面影响所致。
5. 在蒙古高原，干旱通过直接或间接作用，显著调控了贯穿整个土壤剖面的微生物特性与碳、氮矿化速率的分布格局。本研究结果表明，干旱程度加剧可能会引发半干旱草原不同土层深度的土壤微生物群落及其相关功能发生改变；未来全球干旱区的相关模型应考虑不同土层深度中土壤底物与微生物特性之间的动态互作关系。