Soil physico-chemical properties are more important than microbial diversity and enzyme activity in controlling carbon and nitrogen stocks near Sydney, Australia

Soil organic carbon (SOC) and available nitrogen (N) stocks are controlled by the complex interplay of soil physical, chemical, and biological conditions. However, the interrelations of SOC or available N with these drivers as well as their relative importance are rarely evaluated quantitatively. Using investigations of SOC density (SOCD) and available N density (ND) with other detailed soil properties of topsoil (0-10 cm) and subsoil (20-30 cm) from 33 sites under different ecosystems in the vicinity of an eddy flux tower near Sydney, Australia, we investigated the controls of soil physical, chemical, and biological properties (a total of 19 variables) on SOCD and available ND. Structural equation models showed that only physical and chemical properties significantly and directly affected SOCD and available ND. Among these variables, physical and chemical properties were the most influential factors, while the relative influences of microbial biodiversity and enzyme activity were small based on boosted regression tree analysis. In addition, the effects of variables on SOCD and available ND differed between the topsoil and subsoil. In the topsoil, soil physical properties had the highest relative influence followed by chemical properties, enzyme activities, and microbial biodiversity; in the subsoil, however, soil chemical properties had the highest relative importance followed by physical properties, enzyme activities, and microbial biodiversity. This comprehensive soil characterization provides the biogeochemical context for ecosystem carbon cycling being monitored at a nearby eddy flux tower, and demonstrates the importance of including accurate measurements of soil physical and chemical properties to reduce uncertainty in soil C and N predictions in process-based models. However, this is a local-scale study, and large-scale studies are warranted to gain further understanding on this issue.