Increasing Labile Carbon and Nitrogen Pools in Agricultural Soils Requires a Change in System Rather Than Practice.

Labile soil carbon (C) and nitrogen (N) dynamics are key variables in soil health assessments and are critical for soil ecosystem services. Understanding these dynamics will provide insight into how seasonality and farm management practices within forage and dairy cropping systems influence soil C and N. The objective of this research is to study the effects of six different long-term cropping systems on potentially mineralizable nitrogen (PMN), permanganate oxidizable carbon (POxC), and potentially mineralizable carbon (PMC); indicators of labile C and N pools. These labile pools were evaluated using incubation and chemical extraction methods at two soil depths and five times during the growing season at the Wisconsin Integrated Cropping Systems Trial in Leeds, Wisconsin. Preliminary results indicate POxC and PMN were greater in the forage-based systems (FBS) compared to the grain-based systems (GBS), and that FBS had mineralizable N pools steadily replenished throughout the growing season until September. GBS experienced a significant decrease in PMN mid-season (a key point in plant growth), and had significantly lower POxC values in the beginning of the growing season. Total carbon, total nitrogen, and PMC analysis are currently underway. The significant differences in these biological indicators of soil health metrics between GBS and FBS are likely due to reduced soil disturbance, frequent manure additions, and the incorporation of a perennial legume phase in the FBS. Since management practice shifts within a cropping system (GBS/FBS) in the context of this study were not enough to affect labile pools, system changes such as complete tillage reduction combined with annual cover cropping is required.