EFFECT OF PASTURE AGE ON SOIL TRACE-GAS EMISSIONS FROM A DEFORESTED AREA OF COSTA-RICA

MEASUREMENTS of the flux of nitrous oxide-an important greenhouse gas-from recently formed pasture in the Amazon basin have shown a threefold increase relative to the flux from the original forest soil1. Based on these measurements, Luizao et al.1 estimated that present rates of conversion from forest to pasture supply up to 1 Tg of N2O-N to the atmosphere each year, corresponding to less-than-or-equal-to 25% of the current imbalance between sources and sinks of this gas. But this estimate assumes that such conversion produces elevated fluxes that remain constant in time. To assess the validity of this assumption, we present measurements of trace-gas fluxes from Costa Rican pastures of varying ages. Nitrogen oxide fluxes peak during the first ten years after conversion, but decline thereafter to values that are even lower than the original forest fluxes. We conclude that previous studies have overestimated the contribution of pastures to the global budget of nitrous oxide, and that accurate predictions of soil-atmosphere trace-gas fluxes will require a detailed knowledge not only of current land use, but also of land-use history.