Tracing nitrous oxide emissions derived from red clover and oat cover crops during the non-growing season and after corn fertilization

Cover crops (CC) are typically used after annual crop harvest for benefits such as improved soil health and uptake of residual fertilizer nitrogen. How much cover crop biomass nitrogen (N) contributes to the emission of nitrous oxide (N2O), a potent greenhouse gas, is not well know due to the difficulty in directly tracing biomass N. Cover crops may impact N2O emissions differently during the non-growing season (NGS), and growing season when N fertilizer is applied to annual crops after CC termination, making it important to trace biomass N into N2O year-round. We conducted a 2-year field study using a randomized complete block design with two cover crop treatments, red clover (Trifolium pratense L.) and oat (Avena sativa L.) used before corn. Aboveground CC biomass (AG) and belowground soil + CC roots (BG) were enriched separately with 15N isotope in autumn, through swapping of AG biomass from two microplots, one receiving 15N and the other not. The objectives were to: i) determine the effect of CC on NGS and annual N2O emissions, and corn yield; ii) quantify the contribution of AG and BG to the N2O emissions by tracing CC 15N into emitted 15N2O, and iii) determine emission factors (proportion of N from CC applied emitted as N2O) for AG and BG. Oat significantly increased NGS N2O emissions by 3.4 times in one of the years compared to red clover. However, red clover consistently increased annual N2O emissions by 1.2–1.9 times compared to oat. The emission difference between CCs was related to the interaction between red clover biomass addition and N fertilization to corn in the growing season. The two-year AG emission factor was 2.3 and 3.6 times higher than BG in oat and red clover, respectively. The two-year average CC annual emission factor was 1.05% for oat and 1.90% for red clover. Results suggests default crop residue emission factors used in IPCC greenhouse gas inventories may underestimate emissions of fresh CC residue.