Regional-scale evidence that determinants of soil microbial biomass and N mineralization depend on sampling depth and layer on the Mongolian Plateau

Direct evidence on how the determinants of soil microbial biomass and N mineralization differ with sampling depth and layer at a regional scale is lacking. We sampled 132 plots along aridity gradients on the Mongolian Plateau and determined the soil bacterial and fungal biomass and N mineralization rates at four soil sampling depths (0–20, 0–40, 0–60, and 0–100 cm) and layers (0–20, 20–40, 40–60, and 60–100 cm). We found that the determinants of microbial biomass and soil N mineralization differed among the four soil sampling depths or layers. At 0–20 cm, both bacterial and fungal biomasses were directly related to aridity and soil substrate quantity. Bacterial biomass was directly related to aridity and soil substrate quality at 0–100 cm soil depth and was directly related to aridity and plant substrate quantity in the 60–100 cm soil layer. Fungal biomass was directly related to aridity and the soil environment at 0–100 cm soil depth and in the 60–100 cm soil layer. The magnitude of these direct effects on microbial variables differed with soil depth and layer. For example, the direct effects of aridity (negative) and soil substrate quality (positive) on bacterial biomass increased with soil depth but not with soil layer. Soil N mineralization was directly associated with soil the environment and substrates across the four soil sampling depths, but was directly associated with soil substrates and plant quality across the four soil sampling layers. Our results provide the first regional-scale evidence that the determinants associated with soil microbial biomass and N mineralization depend on the sampling depth and layer. These findings indicate that studies based on surface soils may not accurately identify the determinants of microbial communities or ecosystem functions across the entire soil profile of drylands globally.