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 (+0.69mg 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 KCI led to an additional inhibiting 'salt' effect, which may be more long term than the inhibiting effect of NH4+. (C) 2001 Elsevier Science Ltd. All rights reserved