Rates and controls of nitrous oxide and nitric oxide emissions following conversion of forest to pasture in Rondonia

Tropical soils are important sources of nitrous oxide (N2O) and nitric oxide (NO) emissions from the Earth's terrestrial ecosystems. Clearing of tropical rainforest for pasture has the potential to alter N2O and NO emissions from soils by altering moisture, nitrogen supply or other factors that control N oxide production. In this review we report annual rates of N2O and NO emissions from forest and pastures of different ages in the western Brazilian Amazon state of Rondonia and examine how forest clearing alters the major controls of N oxide production. Forests had annual N2O emissions of 1.7 to 4.3 kg N ha(-1) y(-1) and annual NO emissions of 1.4 kg N ha(-1) y(-1). Young pastures of 1-3 years old had higher N2O emissions than the original forest (3.1-5.1 kg N ha(-1) y(-1)) but older pastures of 6 years or more had lower emissions (0.1 to 0.4 kg N ha(-1) y(-1)). Both soil moisture and indices of soil N cycling were relatively poor predictors of N2O, NO and combined N2O + NO emissions. In forest, high N2O emissions occurred at soil moistures above 30% water-filled pore space, while NO emissions occurred at all measured soil moistures (18-43%). In pastures, low N availability led to low N2O and NO emissions across the entire range of soil moistures. Based on these patterns and results of field fertilization experiments, we concluded that: (1) nitrification was the source of NO from forest soils, (2) denitrification was not a major source of N2O production from forest soils or was not limited by NO3- supply, (3) denitrification was a major source of N2O production from pasture soils but only when NO3- was available, and (4) nitrification was not a major source of NO production in pasture soils. Pulse wettings after prolonged dry periods increased N2O and NO emissions for only short periods and not enough to appreciably affect annual emission rates. We project that Basin-wide, the effect of clearing for pasture in the future will be a small reduction in total N2O emissions if the extensive pastures of the Amazon continue to be managed in a way similar to current practices. In the future, both N2O and NO fluxes could increase if uses of pastures change to include greater use of N fertilizers or N-fixing crops. Predicting the consequences of these changes for N oxide production will require an understanding of how the processes of nitrification and denitrification interact with soil type and regional moisture regimes to control N2O and NO production from these new anthropogenic N sources.