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

Abstract

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.