Perennial crops increase soil carbon stocks in the topsoil compared to annuals by modifying enzymatic activities

Perennial crops can be as a sustainable alternative to annual crops owing to plant traits and management practices that improve productivity and may contribute to soil carbon (C) accumulation. However, our understanding of the mechanisms behind the potential differences in C stocks between perennials and annuals is incomplete, especially in terms of how the changers and drivers vary at different soil depths. Based on a 10-year cropping experiment in Denmark with perennials (tall fescue, grass-legume mixture) and annuals (triticale monoculture, triticale in a crop rotation), we investigated soil C stock changes and driving mechanisms at depths of 0-20 cm (topsoil) and 20-50 cm (subsoil). We observed that tall fescue and grass-legume mixture systems increased soil C stock by 6-20% in the topsoil as compared to annual crops. In the subsoil, the tall fescue system even enhanced soil C storage by up to 56%, but there was no difference in soil C stock between grass-legume mixture, triticale, and triticale in a rotation. Most importantly, we found that the major determinants of soil C stock depended on soil depth. In the topsoil, enzymes exerted a dominant effect on soil C stock. Perennials with low C/N for aboveground biomass and high root biomass seemed to depress oxidase (phenol oxidase and peroxidase) activities and stimulated the nutrient-acquiring enzymes (leucine amino peptidase, β-1,4-N-acetylglucosaminidase), thus depressing the decomposition of recalcitrant C and maintaining plant growth, which facilitated soil C storage. In the subsoil, microbial biomass, rather than the balance between functional enzymes, seemed to be controlling the soil C storage. In their entirety, our results highlight that it is feasible to enhance soil C storage in systems with perennials with higher aboveground biomass quality and root biomass. Furthermore, there is a link to biological drivers (i.e., extracellular enzyme activity and microbial biomass), which may play a differential role in topsoil and subsoil. With improved mechanistic understanding, such biological drivers of soil C stock for agricultural systems should be considered in Earth system models to improve the accuracy of predicting agricultural soil C dynamics.