Management effects on methane fluxes in humid tropical pasture soils

Tropical ecosystems play an important role in the production and uptake of atmospheric methane (CH4). Our objective was to evaluate the long- and short-term effects of management on the CH4 fluxes in humid tropical pastures in Costa Rica. Using closed chambers, we measured CH4 fluxes on four replicates of three pastures with different management: fertilized, traditional and legume pastures. In experimental fertilizer applications, we also compared the short-term effects of ammonium, nitrate and urea fertilizers. In the course of one year, fertilized pastures showed net CH4 uptake (−0.34 mg CH4 m−2 day−1) while traditional (+0.69 mg CH4 m−2 day−1) and legume pastures (+0.92 mg CH4 m−2 day−1) displayed net CH4 emissions. This difference was probably caused by the combined effect of lower soil water contents in the fertilized pastures and high nitrate concentrations, which may have inhibited production of CH4 in the fertilized pastures. CH4 uptake in the fertilized pasture was only about 25% of CH4 uptake in old-growth forest in the same area. In the fertilizer experiment, CH4 uptake was more reduced by ammonium sulfate (−0.24 mg CH4 m−2 day−1) and urea (−0.26 mg CH4 m−2 day−1) than by calcium nitrate (−0.62 mg CH4 m−2 day−1). We measured a short-term inhibition of CH4 uptake caused by NH4+ that lasted for less than 3 weeks. Addition of KCl led to an additional inhibiting ‘salt’ effect, which may be more long term than the inhibiting effect of NH4+.