Gross, Background, and Net Anthropogenic Soil Nitrous Oxide Emissions from Soybean, Corn, and Wheat Croplands.

Agricultural soils are the largest single source of NO emissions globally. However, soils left uncultivated would still release NO. Distinguishing anthropogenic from natural emissions (i.e., background emissions) in crops is important if we want to assess the net effect of human activity. This study aimed to characterize NO emissions from croplands and unmanaged grasslands to estimate the net anthropogenic emissions and to gain a better insight into their main drivers. We established a replicated manipulative field experiment in the Pampas Region of Argentina to quantify soil NO emissions from corn ( L.), wheat ( L.), and soybean [ (L.) Merr.] crops, and from adjacent unmanaged grassland plots for 1 yr. We also analyzed the main controls of NO emissions and the correlation between the normalized difference vegetation index (NDVI) and NO fluxes. Background emissions represented between 21 and 32% of total emissions from croplands, depending on crop type. No differences were detected in NO emissions between total and background during winter and peak crop growing season. NDVI showed a significant correlation with NO fluxes which was positive in grasslands and negative in growing season of soybean crops. Our results showed that NO emissions from croplands were higher than background emissions, but also that background represented an important fraction of cropland emissions. Higher emissions in croplands occurred during pre-seeding, after harvest, and after N fertilization in fertilized crops. In addition, our study informs about NO emissions from crops and unmanaged systems in South America where field data are very scarce.