Mechanisms of soil organic matter persistence vary across time and soil depth in long-term cropping systems of the North Central US

Abstract

Soil organic matter (SOM) declines under agricultural production have been well documented, despite efforts to maintain or enhance SOM through practices like rotational diversity and increased carbon (C) input quantity and quality. However, a critical knowledge gap remains in understanding how system management alters the microbial processes that drive C input turnover and stabilization across time and soil depths. This study addresses this gap by leveraging a long-term cropping systems trial to investigate microbial mechanisms of SOM turnover and stabilization across a representative range of cropping systems in the North Central US. We assessed microbial and chemical soil characteristics at two key sampling times throughout the growing season and linked these measurements to indicators of SOM persistence. Particulate organic matter (POM) C:N exhibited significant (P < 0.001) variation across cropping systems, depth, and time reflecting a gradient of system C input quality. POM-C was greatest in pasture systems (P < 0.001) at both time points, suggesting stability of the relationship between POM inputs and decomposition across the growing season. Additionally, microbial growth and respiration were highest in pasture (P > 0.001), which was consistent across time, indicating an active microbial community that facilitates SOM turnover and stabilization. Our findings provide novel insights on the role of rotational and plant input diversity for enhancing microbial turnover and slowing SOM decline through POM substrate quality, particularly in pasture systems, across time and soil depths. This research will serve to inform future cropping system-level soil management strategies aimed at improving SOM persistence.