THE RESPONSE OF GREENHOUSE GAS EMISSIONS TO PULSE CHANGES IN MOISTURE CONTENT IN TROPICAL RAINFOREST SOILS

Wet, tropical ecosystems are responsible for emitting a significant proportion of greenhouse gases into the atmosphere, and a large proportion of tropical gas emissions are the resultant byproducts of soil microbial respiration. Microbial respiration rate is directly controlled by environmental factors such as soil moisture, which creates a physical barrier between the soil profile and atmosphere, thus limiting oxygen availability. Given that global environmental change is expected to alter rainfall and precipitation patterns in tropical ecosystems, pulse changes in soil moisture may affect soil microbial community structure and therefore net CH₄ and CO₂ emissions. To test this hypothesis, soils were collected at both a ridgetop and a valley location within the El Yunque rainforest in Puerto Rico. Soils from both locations were manipulated by either drying soils 25% or 50% less, or wetting soils 25% or 50% more, compared to the naturally occurring in situ moisture content. Soil samples were than incubated in jars and CH₄ and CO₂ fluxes were measured daily. In the valley region, wetter soils emitted more CH₄ than control soils, and dry soils emitted no detectable CH₄. CO₂ emissions increased in dried soils relative to controls, and decreased in the wetter soils. Ridetop soils emitted no detectable CH₄ in any treatment. CO₂ emissions were greatest in the wetted treatments, and smallest in the dried treatments. The results of this study indicate that pulse increases or decreases in soil moisture brought on by either increased or decreased frequency and/or intensity of rainfall will alter net CO₂ and CH₄ soil emissions, and that the effect of changes in soil moisture content on gas emissions will differ spatially.