Nitrous oxide emissions increase exponentially with organic N rate from cover crops and applied poultry litter

Best management practices to reduce nitrous oxide (N₂O) emissions following application of organic sources of N to soils are poorly developed. For example, while cover crops are promoted for their conservation benefits, their impact on N₂O emissions varies considerably. Subsurface banding of animal manures is promoted to reduce nitrogen losses through ammonia volatilization, but the impact on soil N₂O emissions is uncertain. To assess the interactive effects of cover crops and subsurface banded poultry litter (SSB PL), we measured annual N₂O emissions for three years in a field trial of corn (Zea mays L.) in Beltsville, MD, following either winter fallow or mulched cover crops (cereal rye [Secale cereale L.], hairy vetch [Vicia villosa Roth.], or a mixture of both species), with four rates of SSB PL (9276 kg plant available N (PAN) ha⁻¹) and selected contrasts with surface banded urea-ammonium nitrate (UAN, 150 kg N ha⁻¹) or broadcast incorporated PL (67 kg PAN ha⁻¹). N₂O emissions increased exponentially with total N input from organic sources (SSB PL + cover crop residue). This relationship differed by cover crop treatment, with lowest emissions following cereal rye, and highest emissions following hairy vetch. The model intercept ranged from 0.3061.371 kg N₂O-N ha⁻¹ (cereal rye < bare ground = mixture < hairy vetch; p = 0.03, 0.24, 0.01), and the exponential coefficient ranged from 0.003000.00603 kg N₂O-N kg⁻¹ N (hairy vetch < cereal rye = bare ground = mixture; p = 0.02, p > 0.10). Corn grain yield was similar (mean, 13.8 Mg ha⁻¹) for all cover crops when PL was applied at 135 kg PAN ha⁻¹. SSB PL increased N₂O emissions relative to tillage-incorporated PL at an equivalent rate following hairy vetch or cover crop mixture (by 76% or 60%, respectively; p < 0.001) while increasing corn yield by 32% or 16%, respectively. SSB PL decreased emissions relative to surface banded UAN at an equivalent rate following the cover crop mixture, but increased emissions following bare ground (by 34% and 45%, respectively; p = 0.002) while having no influence on corn grain yield, which averaged 13.7 Mg ha⁻¹. Our results indicate that grass:legume cover crop mixtures with SSB PL can lower N₂O emissions compared to legume monoculture with SSB PL while maintaining comparable N inputs and corn grain yields.