Fluxes of nitrous oxide and nitrate from agricultural fields on the Delmarva Peninsula: N biogeochemistry and economics of field management

The effects of two agricultural conservation practices on nitrous oxide (N2O) fluxes to the atmosphere and nitrate (NO3−) fluxes to groundwater were compared to conventional practices. The conservation practices were application of 80% of recommended nitrogen (N) and planting of winter cover crops. N2O fluxes were measured by static chambers, and NO3− fluxes were calculated using measured NO3− concentrations from tile lines and estimated groundwater yields. During the growing season, one of the five 80% N treatments showed significantly reduced N2O fluxes compared to the 100% N control, whereas three of the five 80% N treatments showed significantly reduced NO3− concentrations compared to the 100% N application. The 80% N treatment resulted in reduced crop yields of 5-26% and average economic losses of US$366 ha−1 for corn and US$153 ha−1 for winter wheat. In three winter cover-crop treatments there were two significant reductions in fall N2O fluxes compared to no-cover-crop controls, and tile drain NO3− concentrations were also significantly lower in autumn. The N2O fluxes were a function of soil temperature, moisture, and fertilizer applications (r2 = 0.49, p < 0.001). Integrating N2O and NO3− fluxes to the annual time scale without conservation measures resulted in mean export of 15 ± 8 kg N2O-N ha−1 y−1 and 36 ± 6 kg NO3-N ha−1 y−1. Adding an 80% N conservation treatment reduced N2O fluxes by 79% and NO3− fluxes by 22%, whereas adding cover crops had smaller effects (11% for N2O, 9% for NO3−). However, cover crops were more cost-effective, averaging US$53 (kg N)−1 compared to the 80% fertilizer treatment (US$77 (kg N)−1) due to large economic losses for corn. The state of Maryland (MD) subsidizes cover crops, making the practice even more cost-effective at US$15 (kg N)−1, emphasizing the importance of farmer-friendly policies.