Forests have been recognized as carbon sinks. However, they can influence the concentration of atmospheric greenhouse gases (GHG), particularly through methane (CH4) emissions from wetlands, CH4 capture in soils under aerobic conditions, and rapid decomposition of soil organic matter under suitable moisture and temperature producing N2O (nitrous oxide) and CO2 (carbon dioxide). Understanding natural fluxes of GHGs is thus crucial to understand mechanisms behind climate change. In this study we investigated seasonal drivers of CO2, N2O and CH4 emissions in a seasonally dry tropical forest (SDTF) in Costa Rica during 2015 (an El Niño year) and 2016. We periodically monitored N2O and CH4, using a static chamber methodology and a Li-8100 to measure soil respiration. We worked in 8 plots on two soil types (Vertisols and Entisols), encompassing a variation in forest structure (mature or young) in order to address the effect of precipitation (soil moisture), soil properties and forest composition on the fluxes. We recorded changes in soil available nitrogen (ammonia -NH4+- and nitrate -NO3--) and we determined carbon, nitrogen and phosphorus in soil microbial biomass. We hypothesized that GHGs emissions would be mostly driven by soil water content but also that emissions would be influenced by forest composition and soil type as these factors determine nutrient and organic matter inputs, temperature, and air/water movement respectively.
Results/Conclusions
Our results show that CO2 and N2O fluxes increased with the onset of the rainy season, by up to 300% relative to dry season values . However, fluxes were lower than those observed for rain forest. Additionally, the soil behaved as a CH4 sink for most of the year, except for when the soil moisture exceeded 70% (m3/ m3). Increment in N2O during the rainy season was mirrored by concentrations of available N, but the overall relationship between soil available N and N2O was weak except in November 2015 when the fluxes peaked (R2=0.61). As 2016 was wetter than 2015, we saw an average 10 fold increment on N2O emissions compared to the previous year, due to the role of soil moisture on N mineralization/C availability. The relationship between GHG fluxes and forest composition is currently under analysis. How GHG fluxes are affected by precipitation and site conditions is important, as their dynamics could respond to forest management, land use change and to a warmer and drier scenario, as predicted for the region.