Emissions of N2O From Grass Stands: Effects of Historical Nutrient Applications and Stand Termination Methods.

Nitrogen (N) losses from grass stands tend to be low due to extensive root systems which usually result in little free nitrate in the soil. However, mineral N may accumulate in the soil during stand renovation due to the breakdown of accumulated root, plant residues and manure added over the life of the stand, and remain in the soil until the new crop is well established. This study examined emissions of nitrous oxide (N₂O) before and after termination of a stand of tall fescue. Termination was carried by tillage or application of glyphosate herbicide. The stand had received annual applications of mineral fertilizer or dairy slurry at 400 kg total N ha^-1, or no fertilizer, over 15 years and the grass was harvested four times annually. Dairy slurry was applied by surface banding to minimize ammonia-N loss. Emissions from the undisturbed stands remained low over the 54-day test period regardless of historical nutrient applications (< 0.15 kg N₂O-N ha^-1). Emissions increased sharply within two days of tillage for all nutrient treatments. Emissions increase almost as soon but more gradually after spraying than after tillage but emissions from the sprayed plots exceeded tilled plots about 20 days after treatment. Total N₂O-N emissions after spraying were about 1 and 2 kg ha^-1 for the fertilized and manured plots, respectively, and emissions were about half those values, respectively, after tillage. More nitrate-N was released by tillage than by spraying so the higher emissions for the sprayed plots were due to other factors such as higher moisture in untilled soil. Higher emissions after manure than after fertilizer were not associated with greater release of nitrate.  This study showed that emission factors for N application on forages must account for emissions during stand renovation and tillage can be used to reduce emissions.