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 shor t-term effects of management on the CH4 fluxes in humid tropical pastures i n Costa Rica. Using closed chambers, we measured CH4 fluxes on four replica tes of three pastures with different management: fertilized, traditional an d legume pastures. In experimental fertilizer applications, we also compare d 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 pa stures (+0.92 mg CH4 m(-2) day(-1)) displayed net CH4 emissions. This diffe rence was probably caused by the combined effect of lower soil water conten ts in the fertilized pastures and high nitrate concentrations, which may ha ve inhibited production of CH4 in the fertilized pastures. CH4 uptake in th e 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 measu red a short-term inhibition of CH4 uptake caused by NH4+ that lasted for le ss than 3 weeks. Addition of KCI led to an additional inhibiting 'salt' eff ect, which may be more long term than the inhibiting effect of NH4+. (C) 20 01 Elsevier Science Ltd. All rights reserved.