The overlooked dimension: soil depth governs microbial community assembly and enzymatic activity in extreme environments

Soil microbes have a major impact on ecosystem functions in extreme environments, such as deserts, where environmental factors such as salinity and soil moisture act as strong filters in shaping community structure. In fact, environmental filtering in extreme environments can occur at a small scale along soil depths, which is often overlooked, but it is important for an explicit understanding of how microbial community structure relates to microbial functions. Here, we leveraged a 200-km desert salinity gradient to evaluate microbial community structure-function relationships in extreme environments, and to further assess the prime ecological processes controlling the assembly of microbial communities and the activity of carbon, nitrogen, and phosphorus cycling-relevant enzymes along soil depths. We found that 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. We further found that soil depth (at the scale of meters) was just as important as geographic distance (at the scale of kilometers) in shaping 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, such as soil texture, water content, and pH. Our results suggest that dispersal limitation is particularly important to soil microbial community assembly, while environmental filtering is important to enzymatic activity. Such a mismatch in ecological processes could have disproportionate influences on soil microbial community structure and enzymatic activity, leading to uncoupled structure-function relationships. Our results further suggest that soil depth governs the assembly and enzymatic activity of soil microbial communities. This work highlights that soil conservation efforts should consider soil depth as a key attribute in the face of ongoing desertification in arid ecosystems.