Temporal and spatial variability of greenhouse gas fluxes from soil in an undisturbed forest in the Brazilian Amazon

Methane (CH₄) exchange between the soil and atmosphere is the net result of the balance between production, oxidation and transport. Nitrous oxide (N₂O) is produced in soil during by nitrification and denitrification processes. Tropical soils are an important source of N₂O) to the atmosphere and may be either a source or sink of CH₄. Soil temperature and moisture exert strong controls on the soil-atmosphere exchange of both greenhouse gases. We measured the soil-atmosphere flux of CH₄ and N₂O with an automated chamber system that was installed in April 2001 in the Tapajos National Forest, Para, Brazil, a mature forest on a clay Oxisol. The mean annual temperature is 25C with a diel range often exceeding the variability of the annual daily means. The mean annual precipitation is ca. 2000 mm per year with a distinct dry season from July to December. Eighteen aluminum chambers were installed in a 0.5 ha area close to the flux tower at the km 67 Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) site. Eight of these chambers are sampled about 5 times per day (closed 7% daily) and the other 10 chambers are sampled individually approximately once per day (closed 1.5% daily). As expected, the drained upland soils were most often a weak CH₄ sink but during wet periods episodic positive fluxes of CH₄ were observed. N₂O fluxes ranged between 0.8 and 2.8 ng N- N₂O cm^-2 h^-1 during the wet season and 0.02 to 0.7 ng N- N₂O cm^-2 h^-1 during the dry season. Fluxes of CH₄ showed no correlation with soil temperature or moisture on the daily time scale while N₂O emissions correlated with 5 cm soil temperature over the diel cycle only during the wet season.