Co-Locating Fresh Cover Crop Residues with Manure Triggers Nitrous Oxide Emissions from Organic Cropping Systems.

Reducing nitrous oxide (N₂O) emissions – a potent greenhouse gas – by managing tillage, residue, and manure application can enhance the environmental integrity of organic production systems. Manure is a critical nutrient source in organic systems, but when its application coincides with decomposing cover crop residues, N₂O emissions may increase. We measured N₂O emissions from four corn-soybean-spelt systems with varying tillage, cover crop, and manure management strategies in a Reduced Tillage Organic Systems Experiment at the Penn State Agronomy Farm at Rock Springs, PA throughout 2016. In System 1, a hairy vetch-triticale cover crop mixture was roller-crimped in mid-June prior to no-till corn planting and received manure during fall cover crop establishment. A cover crop of hairy vetch-triticale (System 2) and red clover-timothy (Systems 3 and 4) were terminated in mid-May, followed by liquid dairy manure incorporation by inversion tillage. Measurements indicate that corn-phase N₂O emissions increased following cover crop and manure incorporation in System 2, 3, and 4. Peak emission of 0.6 kg N/ha/d was observed approximately 20-days after manure and cover crop incorporation. The emissions for System 1 were lower and increased slightly after cover crop rolling in mid-June. Cumulative N₂O flux in the corn phase was significantly lower in the no-till corn silage in System 1 when compared with the other three systems that received manure in spring (3.7 vs 13.9 kg N/ha, p<0.05) and indicates a greater risk of N₂O emissions when manure is applied in legume-rich cover crop residues. Using Random Forest, we were able to explain 72% of the variance in N₂O emissions, and found that in addition to the environmental variables soil temperature, moisture, the management variables spring manure application and legume biomass control N₂O emissions. The N₂O emissions sharply increased when legume biomass and volumetric soil moisture content exceed 2.5 Mg/ha and 35%, respectively.