Soil Solution Nitrogen Losses During Clearing of Lowland Amazon Forest for Pasture

Abstract

Losses of nitrogen (N) often follow severe disturbance of forest ecosystems. In tropical forests, losses of N associated with the disturbance of clearing may be particularly important because rates of soil N cycling are high and forest clearing now occurs on a large scale. We measured soil solution inorganic N concentrations and fluxes for 1 year in an intact forest in the Brazilian Amazon state of Rondonia and in an adjacent 3-ha forest plot that was cleared for pasture by cutting, burning and planting pasture grass and in established cattle pastures on the same soils that were 5 and 22 years old. The cleared forest had higher soil solution NO 3 − concentrations than the intact forest, but the difference between the cleared and control forests declined with time after the start of the first post-clearing rainy season. Established pastures had much lower solution NH 4 + and NO 3 − concentrations than forest or cleared forest. Estimated annual dissolved inorganic solution N fluxes to below 1 m during the first year after clearing were 2.5 kg ha−1 in forest and 24.4 kg ha−1 in newly cleared forest compared with only 0.5–1.2 kg ha−1 in established pastures. The solution fluxes from cleared forest during the first year after clearing were approximately 7 times greater than gaseous N oxide (N2O+NO) losses estimated for the same time. These results were consistent with the characterization of moist tropical forests on weathered soils as N-rich and likely to respond to disturbances that elevate soil N availability with increased loss to both soil solution and the atmosphere. These results also suggest that the relative increase in N oxide loss is substantially less than the increase solution inorganic N loss.