Extreme Emission of N(2)O from Tropical Wetland Soil (Pantanal, South America)

Nitrous oxide (N(2)O) is an important greenhouse gas and ozone depleter, but the global budget of N(2)O remains unbalanced. Currently, ∼25% of the global N(2)O emission is ascribed to uncultivated tropical soils, but the exact locations and controlling mechanisms are not clear. Here we present the first study of soil N(2)O emission from the Pantanal indicating that this South American wetland may be a significant natural source of N(2)O. At three sites, we repeatedly measured in situ fluxes of N(2)O and sampled porewater nitrate [Formula: see text] during the low water season in 2008 and 2009. In 2010, 10 sites were screened for in situ fluxes of N(2)O and soil [Formula: see text] content. The in situ fluxes of N(2)O were comparable to fluxes from heavily fertilized forests or agricultural soils. An important parameter affecting N(2)O emission rate was precipitation, inducing peak emissions of >3 mmol N(2)O m(−2) day(−1), while the mean daily flux was 0.43 ± 0.03 mmol N(2)O m(−2) day(−1). Over 170 days of the drained period, we estimated non-wetted drained soil to contribute 70.0 mmol N(2)O m(−2), while rain-induced peak events contributed 9.2 mmol N(2)O m(−2), resulting in a total N(2)O emission of 79.2 mmol N(2)O m(−2). At the sites of repeated sampling, the pool of porewater nitrate varied [Formula: see text] with higher concentrations of [Formula: see text] (p < 0.05) found in drained soil than in water-logged soil, indicating dynamic shifts between nitrification and denitrification. In the field, O(2) penetrated the upper 60 cm of drained soil, but was depleted in response to precipitation. Upon experimental wetting the soil showed rapid O(2) depletion followed by N(2)O accumulation and a peak emission of N(2)O [Formula: see text] Assuming that the observed emission of N(2)O from these wetland soils is generally representative to the Pantanal, we suggest that this undisturbed tropical wetland potentially contributes ∼1.7% to the global N(2)O emission budget, a significant single source of N(2)O.