Tropical forests across the globe are experiencing novel disturbance regimes with the potential to alter ecosystem processes in ways that could both alleviate or worsen our current climate crisis. Projections of increased frequency and intensity of droughts and hurricanes in tropical forests will likely affect rates of soil carbon (C) and nutrient cycling, with significant implications for the global C cycle. These disturbances can affect the quantity and quality of organic matter inputs to the soil, as well as rates of soil microbial activity, which regulate greenhouse gas (GHG) fluxes, nutrient availability, and ultimately ecosystem productivity. To study the response of soil biogeochemistry to drought in a wet tropical forests, we established a throughfall exclusion experiment (n=5, 12.5-m2 plots located on upper topographic positions) in the Luquillo Experimental Forest in Puerto Rico. Hourly measurements of soil temperature, volumetric moisture, and oxygen concentration at three depths, as well as quarterly soil samplings, have been ongoing since late 2016. Soil GHG fluxes have also been measured biweekly using manual static flux chambers. Throughfall exclusion using transparent plastic shelters began in March 2017, and measurements have continued to the present capturing the impact of two major hurricanes (Irma & María) at our site.
Results/Conclusions
Throughfall exclusion had no effect on soil temperature, but significantly reduced volumetric soil moisture from ~0.5 to ~0.3 m3 m-3 at 0-15 cm, resulting in a significant increase in soil oxygen concentrations. Our treatment was effective in reducing the redox fluctuations that characterize these soils, maintaining surface soil oxygen concentrations between ~18-21%, in contrast to control soils which experienced mean daily values as low as ~12%. The treatment effect on moisture and oxygen was stronger at the soil surface, while responses at depth (15-45 cm) were of smaller magnitude due to the strong influence of subsurface water flow from the surrounding landscape, which received normal rainfall throughout the study period. The observed changes in soil microclimate conditions in response to throughfall exclusion likely affected redox-sensitive biogeochemical processes that drive soil C and nutrient cycling. We will present our measurements of soil GHG fluxes and soil chemistry (C, N, P, and pH) throughout the soil profile, and discuss implications for the future C balance of wet tropical forests. Additionally, we will present preliminary results on the impacts of hurricanes Irma and María on soil biogeochemistry at our site.