Can Managing to Increase Aggregate-Protected and Particulate Organic Matter (POM) Reduce the Need for Nitrogen?.

The way we farm affects the buildup and composition of soil organic matter, but the extent to which crop nutrients can be supplied by soil organic matter is unknown. A better understanding of nitrogen availability as a result of different management practices, and the relationship between management, dynamic soil properties, and need for nitrogen additions will allow for more precise nitrogen recommendations. The objectives of this study were to: (i) evaluate the effect of management on organic matter, (ii) determine an optimum N rate for corn in each cropping system, and (iii) explore the relationship between soil organic matter measurements and the optimum N rate. The study was conducted within the Wisconsin Integrated Cropping Systems Trial, which includes continuous corn, strip-till corn/soybean and an organic corn/soybean/wheat rotation, and three forage-based systems. Nitrogen was applied at different rates to the corn phase of each system to determine an optimum N rate. Soil samples were collected in the spring and analyzed for particulate organic matter. Fall soil samples were fractionated and C and N were measured in each aggregate fraction.  Rotations including perennial crops were significantly enriched in total soil carbon as well as POM. Although POM content did not differ between systems, POM is significantly depleted in C and N under organically managed corn compared to the other two systems. Organic corn has less aggregates by weight but is enriched in N in the silt& clay fractions. Under drought conditions in SW Wisconsin, both the strip-till corn and organic corn out-performed continuous corn in yield, with similar N uptake in each system. These results show how organic management may increase organic matter by providing a large and diverse quantity of inputs through rotation, but lose that advantage to tillage, which leads to increased rates of decomposition and macroaggregate breakdown.