Temporal Analysis of Nitrous Oxide Emissions from a Pennsylvania No-till Dairy Cropping System.

Nitrous oxide (N₂O) is a potent greenhouse gas that is released from agroecosystems by the microbial processes of nitrification and denitrification. We investigated how different cropping system practices that include differences in crop residues, N inputs (dairy manure and inorganic fertilizer), timing of N amendment applications, and environmental conditions influence N₂O emissions from no-till soil.  Emissions were measured at the PSU Russell E Larson Agronomy Research Farm in the NESARE Dairy Cropping System experiment that was initiated in 2010 and aims to sustainably produce the forage, feed and fuel for a 65 cow, 240-acre dairy farm in Pennsylvania. We measured N₂O fluxes from soil that was fertilized with dairy manure and planted after each of the following crops: i. alfalfa and orchardgrass, ii. crimsom clover, iii. soybean and iv. rye winter cover crop.  N₂O fluxes were measured biweekly during the corn growing season in 2014 and 2015 using closed chambers (78.5cm x 40.5 cm) in three replications of each crop treatment and in two locations in each treatment-plot. Samples were collected at 10, 20 and 30 minutes after placing the chamber over the soil. For time 0, we sampled the atmospheric air, and we calculated the rate of N₂O emissions [g N₂O ha^-1 day^-1] using linear regression with the four data points. N₂O emissions tended to peak in treatments after cover crops were terminated and manure was applied, prior to rapid corn N uptake. Treatments with low C:N ratio and high biomass contributed to enhanced microbial denitrification. Later in the season, when side-dress fertilizer N was applied to corn, N₂O emissions were lower than when manure was applied earlier. This is likely because fertilizer N was more rapidly taken up by the actively growing corn, allowing a better synchronization between N supply and N demand from corn.