Nitric oxide emissions from a southern African savanna

Abstract

NO fluxes from soils of a periodically flooded tropical savanna in southern Africa were investigated and modeled. In the laboratory, NO production rates, NO consumption rate constants, NO mixing ratios, relationships between NO emissions and soil temperature and moisture were determined for nutrient-poor, nutrient-rich savanna soils and a floodplain soil. The NO production rate and consumption rate constants of the floodplain soil (1.96 ng N s-1 per kilogram of soil and 2.04 × 10-5 m3 s-1 per kilogram of soil, respectively) were significantly higher than those of the savanna soils (average of 1.28 ng N s-1 per kilogram of soil and 1.47 × 10-5 m3 s-1 per kilogram of soil, respectively), but there were no significant difference between the nutrient-rich and nutrient-poor soils. NO flux rates increased exponentially with soil temperature. NO flux rates increased with soil moisture reaching a maximum near the field capacity (7.5 - 10% and 31.2% gravimetric water content for savanna and floodplain soils, respectively), after which the NO flux rate declined. These laboratory data were used in a model to estimate field NO flux rates, which were compared with actual field NO emission measurements. The NO model was modified to incorporate NO "pulsing" after the first rains of the season. Correlation between the modeled and field NO fluxes from the nutrient-poor savanna, nutrient-rich savanna, and the floodplain soils showed r2 values of 0.91, 0.82, and 0.74, respectively. The NO model was linked with a soil moisture and temperature model to predict annual NO emission estimates from savannas. Annual NO flux from the nutrient-poor and nutrient-rich savannas was estimated to be 0.16 × 10-3 and 0.14 × 10-3 kg N m-2 yr-1, respectively, which agree well with estimates from other savanna studies. Copyright 1999 by the American Geophysical Union.