Assessing in-Situ Sources of Soil N2o Emissions in Response to Manure and Residue Management in Organic Systems.

Assessing in-situ sources of soil N₂O emissions in response to manure and residue management in organic systems

Manure and cover crop management determine the synchrony between crop nitrogen (N) demand and supply in organic systems. In a typical corn-soybean-winter grain rotation in the Mid-Atlantic region, manure is incorporated in spring after cover crop termination before corn planting. This practice creates a short temporal window when carbon (C) and N availability from manure and decomposing cover crops exceeds corn N demand and may favor nitrous oxide (N₂O) emissions from nitrification and denitrification. The initial phase of this study was to assess the effects of manure and cover crop exclusion on N₂O emissions in a corn-soybean-winter grain rotation in the USDA certified Reduced Tillage Organic Systems Experiment at the Penn State Agronomy Farm, Rock Springs, PA. The treatments included 1) both cover crop and manure addition (+M+C), 2) cover crop exclusion (+M-C), 3) manure exclusion (-M+C) and 4) both manure and cover crop exclusion (-M-C). In this rotation, corn receives 274 kg of manure N and a hairy vetch-triticale cover crop mixture biomass of 5 Mg/ha contributes 190 kg N to the total plant available N pool, calculated to be 268 kg N/ha. Preliminary results showed that peak N₂O emission occurred 10 days after manure and cover crop residue incorporation and followed the trend: +M+C = -M+C (0.5 kg N/ha/d) > -M-C (0.4 kg N/ha/d) > +M-C (0.3 kg N/ha/d). Higher N₂O emissions were associated with the presence of cover crop than with manure. Unexpectedly, manure exclusion did not result in lesser emissions when cover crops were present, and cover crop exclusion suppressed emissions when compared with the dual exclusion (-M-C). The next step is to define the pathways by which emissions were generated in each treatment. Soils will be analyzed for molecular markers for nitrification (amoA gene) and denitrification (nosZ genes) and gas samples analyzed for N₂O isotopomers.