Land use change and biogeochemical controls of nitrogen oxide emissions from soils in eastern Amazonia

The objectives of this study were (1) to determine the effects of land use change on N oxide fluxes from soil in seasonally dry, eastern Amazonia and (2) evaluate the "hole-in-the-pipe" model in a field setting where N availa bility varies among land uses and soil moisture varies among seasons. We me asured N oxide flux from an old-growth forest, a 20-year-old secondary fore st, an active pasture, and a degraded pasture. We also measured soil water content, soil inorganic N stocks, net N mineralization and nitrification po tential. To determine the effects of pasture age on N oxide flux, we measur ed gas fluxes at a chronosequence of pastures (0-13 years). In the land use study, N2O fluxes followed the order: primary forest (2.4 kg N ha(-1) yr(- 1)) > secondary forest (0.9 kg N ha(-1) yr(-1)) > active pasture (0.3 kg N ha(-1) yr(-1)) > degraded pasture (0.1 kg N ha(-1) yr(-1)), and NO fluxes f ollowed the order: primary forest (1.5 kg N ha(-1) yr(-1)) > degraded pastu re (0.7 kg N ha(-1) yr(-1)) 2 active pasture (0.5 kg N ha(-1) yr(-1)) 2 sec ondary forest (0.3 kg N ha(-1) yr(-1)). In the chronosequence study, no tre nd in N oxide emissions with pasture age was apparent, but emissions from p astures were lower than from the forest. Total N oxide flux correlated with a laboratory measure of nitrification potential (r(2) = 0.85). The ratio N 2O:NO correlated with soil water content (r(2) = 0.56). Parameterization of the model accounted for variability in N oxide emissions across land uses and seasons and the model application revealed the importance of studying b oth N oxide gases simultaneously. Model predictions for six independent sit es agreed well with observed fluxes, suggesting that the model may be appli cable at a broader scale. The consistently low annual emissions of N2O esti mated for all of the Amazonian pastures that we studied suggest that conver sions of tropical forests to cattle pastures may not in the long term cause a significant increase in the contribution of soil emissions to atmospheri c N2O or NO.