Nitrous oxide emissions from forests and pastures of various ages in the Brazilian Amazon

Abstract

Nitrous oxide emissions from tropical forest soils are thought to account for 2.2–3.7 Tg N yr−1 of the total annual global production of 10–17 Tg N yr−1. Recent research suggests that clearing of tropical forest for pasture can increase N2O emissions but that the period of elevated emissions may be limited and fluxes from older pastures may be lower than from the original forest. Here we report N2O emissions from two land‐use sequences in the Brazilian Amazon's state of Rondônia. Each sequence includes a forest and a set of pastures of different ages. One sequence contains a newly created pasture that we studied intensively through its first 2 years, including forest cutting, burning, and the planting of forage grasses. Emissions from the newly created pasture were about two and one half times the forest emissions during the first 2 years (5.0 kg N2O‐N ha−1 yr−1 versus 1.9 kg N2O‐N ha−1 yr−1). Nitrous oxide fluxes from pastures older than 3 years were on average about one third lower than fluxes from uncut forest (1.4 kg N2O‐N ha−1 yr−1 versus 1.9 kg N2O‐N ha−1 yr−1). The best predictor of N2O flux across the chronosequences was the magnitude of the NO3 pool in the upper 10 cm of soil measured at the time of gas sampling. Using a simple cohort model combined with deforestation rates estimated from satellite images by Brazil's Instituto de Pesquisas Espaciais (INPE) for the period 1978 through 1997, we estimate that for the Brazilian Amazon the basin‐wide flux of N2O‐N from pasture soils was 0.06 Tg in 1997. This is ∼8% of the combined forest plus pasture flux of 0.78 Tg N2O‐N we estimate for the Brazilian part of the basin in 1997. In the absence of any forest‐to‐pasture conversion in the Brazilian part of the basin, we estimate that the basin‐wide flux of N2O‐N would have been only slightly larger: 0.80 Tg in 1997. Through a second modeling analysis we estimate that for the whole of the Amazon Basin, including parts of the basin outside of Brazil, the N2O‐N emissions from forests averaged 1.3 Tg yr−1 over the period 1978–1995.