Nitrogen deposition in tropical forests from savanna and deforestation fires

摘要

We used satellite-derived estimates of global fire emissions and a chemical transport model to estimate atmospheric nitrogen (N) fluxes from savanna and deforestation fires in tropical ecosystems. N emissions and reactive N deposition led to a net transport of N equatorward, from savannas and areas undergoing deforestation to tropical forests. Deposition of fire-emitted N in savannas was only 26% of emissions - indicating a net export from this biome. On average, net N loss from fires (the sum of emissions and deposition) was equivalent to approximately 22% of biological N fixation (BNF) in savannas (4.0 kg N ha-1 yr-1) and 38% of BNF in ecosystems at the deforestation frontier (9.3 kg N ha-1 yr-1). Net N gains from fires occurred in interior tropical forests at a rate equivalent to 3% of their BNF (0.8 kg N ha-1 yr-1). This percentage was highest for African tropical forests in the Congo Basin (15%; 3.4 kg N ha-1 yr-1) owing to equatorward transport from frequently burning savannas north and south of the basin. These results provide evidence for cross-biome atmospheric fluxes of N that may help to sustain productivity in some tropical forest ecosystems on millennial timescales. Anthropogenic fires associated with slash and burn agriculture and deforestation in the southern part of the Amazon Basin and across Southeast Asia have substantially increased N deposition in these regions in recent decades and may contribute to increased rates of carbon accumulation in secondary forests and other N-limited ecosystems.