Investigating the Effects of Long-Term Management on Particulate and Aggregate Organic Matter and Their Correlations with Corn Yield.

Mineralization of nitrogen (N) from soil organic matter (SOM) is an important component of crop N supply, but we do not know which pools of SOM are the biggest contributors. This study measured particulate organic matter (POM) and aggregate organic matter after 20 years in three grain cropping systems and explored correlations between yield at 0 kg ha-1 N (0N) and agronomically optimum N rate (AONR). The three systems were chisel-plowed continuous corn (Zea mays L.) (CC), strip-tilled corn-soybean  (Glycine max L.) (CS) and an organically managed corn-soybean-winter wheat (Triticum aestivum L) with green manure (CSW) at the Wisconsin Integrated Cropping Systems Trial (WICST) in southern Wisconsin, U.S.A. When linear plateau models were fit to N rate trial yields, the AONR was 152 kg N ha^-1 in CC, 99.2 kg N ha^-1 in CS, and 113 kg N ha^-1 in CSW. We found higher POM concentration at 0-5 cm in CS, but no difference in POM-C and POM-N. The CSW system had a significantly lower proportion of soil in occluded silt and clay in CSW at 0-5 cm, a lower proportion of soil N and C in free silt and clay at 25-50 cm, and a greater proportion of soil C and N in free microaggregates at 25-50 cm, suggesting that the tillage for weed control in CSW increased the relative abundance of unaggregated C and N in that system. Correlation analysis revealed significant positive correlations between free silt and clay and free microaggregate C and N and yield at 0N and AONR, and significant negative correlations between occluded microaggregate C and N. This suggests that there may be a tradeoff between promoting long term storage of inaccessible carbon and maintaining SOM in a manner which renders it accessible to mineralization and crop uptake on an agronomic timescale.