Soil depth governs microbial community assembly and enzymatic activity in extreme environments

Abstract

Soil microbes have a major impact on microbial function in extreme environments, such as deserts, where environmental factors such as salinity and soil water act as strong filters to community structure. In fact, environmental filtering in these extreme environments can occur over even small gradients in soil depth, which is often overlooked, but important in the functioning of these systems. We leveraged a 200-km desert soil salinity gradient to test the relationships between microbial community structure and function (i.e., enzymatic activity) at different soil depths. Surprisingly, differences in soil bacterial and fungal community structure were not linked to differences in enzymatic activity, implying that soil microbial communities have a high degree of functional redundancy among soil depths. However, we found that the effects of soil depth (at the scale of meters) were just as important as the effects of geographic distance (on the scale of kilometers) on soil bacterial and fungal community structure, while the vertical variability in enzymatic activity along soil depth was largely attributed to the increase in heterogeneity of soil properties (i.e., soil texture, water content, and pH). Although contrasting assembly processes determine community structure and function, these findings suggest that soil depth restructures microbial community structure and function in extreme environments. Our data highlight that soil conservation efforts in arid ecosystems should consider soil depth as a key attribute in the face of ongoing desertification in many ecosystems.