Soil pH drives the relationship between the vertical distribution of soil microbial biomass and soil organic carbon across terrestrial ecosystems: A global synthesis

The vertical distribution of microbial biomass carbon (MBC) is important for terrestrial soil C cycling. However, we still do not understand what controls the vertical distribution of MBC along the soil profile, which limits our ability to estimate C cycling, especially in deep soils. Here, we conducted a global synthesis of 163 observations from 82 published studies to examine the direct and indirect effects of climate, soil physicochemical properties and ecosystem type on the vertical distribution of MBC. We further explored how climate and soil physicochemical properties influence the relationship between the vertical distributions of MBC and SOC. We used decrease rates of MBC and SOC with soil depth (kMBC and kSOC) to describe the vertical distribution of MBC and SOC, with a greater decrease rate indicating a more surface-skewed distribution. kSOC in our analysis was the main predictor of kMBC. Ecosystem type and clay concentration had indirect positive and negative, respectively, effects on kMBC, through kSOC. Although forests had steeper decrease rates of SOC than agriculture and grasslands, kMBC was invariable across ecosystem types, which we attributed to the effects of kSOC on kMBC being pHdependent. The on average lower pH of forests might have slowed down decrease rates in MBC, notwithstanding the steeper changes in SOC with depth. We show that the vertical distribution of SOC is a robust indicator of the vertical distribution of MBC; however, the vertical distribution of MBC in soils with high pH was, nonetheless, more sensitive to the decrease in SOC with depth, leading to a greater decrease in MBC along the soil profile. Our analysis offers important insights into the spatial patterns of soil microbial communities and their activities and thus improves our understanding of soil C cycling.