Quantifying nitrous oxide fluxes on multiple spatial scales in the Upper Midwest, USA

This study seeks to quantify the roles of soybean and corn plants and the cropland ecosystem in the regional N₂O budget of the Upper Midwest, USA. The N₂O flux was measured at three scales (plant, the soil–plant ecosystem, and region) using newly designed steady-state flow-through plant chambers, a flux-gradient micrometeorological tower, and continuous tall-tower observatories. Results indicate that the following. (1) N₂O fluxes from unfertilized soybean (0.03 ± 0.05 nmol m⁻² s⁻¹) and fertilized corn plants (−0.01 ± 0.04 nmol m⁻² s⁻¹) were about one magnitude lower than N₂O emissions from the soil–plant ecosystem (0.26 nmol m⁻² s⁻¹ for soybean and 0.95 nmol m⁻² s⁻¹ for corn), confirming that cropland N₂O emissions were mainly from the soil. (2) Fertilization increased the corn plant flux for a short period (about 20 days), and late-season fertilization dramatically increased the soybean plant emissions. (3) The direct N₂O emission from cropland accounted for less than 20 % of the regional flux, suggesting a significant influence by other sources and indirect emissions, in the regional N₂O budget.