MODEL ESTIMATES OF NITROUS-OXIDE EMISSIONS FROM AGRICULTURAL LANDS INTHE UNITED-STATES

The Denitrification-Decomposition (DNDC) model was used to elucidate t he role of climate, soil properties, and farming practices in determin ing spatial and temporal variations in the production and emission of nitrous oxide (N2O) from agriculture in the United States. Sensitivity studies documented possible causes of annual variability in N2O flux for a simulated Iowa corn-growing soil. The 37 scenarios tested indica ted that soil tillage and nitrate pollution in rainfall may be especia lly significant anthropogenic factors which have increased N2O emissio ns from soils in the United States. Feedbacks to climate change and bi ogeochemical manipulation of agricultural soil reflect complex interac tions between the nitrogen and carbon cycles. A 20% increase in annual average temperature in degrees C produced a 33% increase in N2O emiss ions. Manure applications to Iowa corn crops enhanced carbon storage i n soils, but also increased N2O emissions. A DNDC simulation of annual N2O emissions from all crop and pasture lands in the United States in dicated that the value lies in the range 0.9 - 1.2 TgN. Soil tillage a nd fertilizer use were the most important farming practices contributi ng to enhanced N2O emissions at the national scale. Soil organic matte r and climate variables were the primary determinants of spatial varia bility in N2O emissions. Our results suggest that the United States Go vernment, and possibly the Intergovernmental Panel on Climatic Change (IPCC), have underestimated the importance of agriculture as a nationa l and global source of atmospheric N2O. The coupled nature of the nitr ogen and carbon cycles in soils results in complex feedbacks which com plicate the formulation of strategies to reduce the global warming pot ential of greenhouse gas emissions from agriculture.