Conversion of the coastal Atlantic forest to pasture: Consequences for the nitrogen cycle and soil greenhouse gas emissions

Abstract

We evaluated how land use change affected the nitrogen cycle and greenhouse gas fluxes. We measured soil temperature and water-filled pore space (WFPS), as well as nitrate and ammonium concentrations, mineralization and nitrification rates, and flux of CH 4, CO 2 and N 2O in soils of pasture catchment and in soils of a forest catchment located on the north coast of the State of São Paulo. The main vegetation type in the forest catchment is primary Atlantic Montane Forest, while the pasture is an unfertilized 40-year old area planted with Brachiaria humidicola. Four plots were used for monthly sampling in each land use for an entire year. Soil temperature was always higher in pasture than in forest soils, while WFPS was lower in the pasture in relation to forest soils. Pasture soils were a weak source of CH 4 during the winter months and a less strong sink of methane than the forest soils during the rainy summer months of the year. The annual median CH 4 uptake was −1.8 ± 1.0 mg m −2 d −1 in the forest in contrast to −0.6 ± 0.9 mg m −2 d −1 in pasture soils. CO 2 emissions were similar in the winter, but higher in the summer months in pasture soils in relation to forest soils. The annual median flux in the forest was 4.2 ± 1.5 μmol m −2 s −1 and increased in the pasture to 6.5 ± 2.9 μmol m −2 s −1. In contrast, N 2O fluxes were smaller in the pasture (0.3 ± 0.7 ng cm −2 h −1) than in forest soils (0.5 ± 0.5 ng cm −2 h −1).