Background/Question/Methods Climate change is driving altered precipitation patterns across the tropics, with projections of intensifying drought and changing rainfall seasonality in large areas of wet tropical forests such as the Amazon and the Caribbean. Near constant warm, wet conditions in wet tropical forests tend to promote rapid soil carbon (C) and nutrient cycling, but these may slow significantly during extended periods of low soil water availability associated with increasing droughts in the future. In this context, studying the ecological consequences of drought as a major disturbance in these ecosystems is critical, especially due to their important role within the global climate system. To study the response of soil biogeochemistry to drought in a wet tropical forest, we established the Luquillo Throughfall Exclusion Experiment (LTEE; n = 5, 12.5-m2 plots) in the Luquillo Experimental Forest, Puerto Rico. Following a pre-treatment period, throughfall exclusion began in 03/2017 and continued until 11/2018, with the post-treatment period lasting until 01/2019. During this time, we monitored hourly temperature, volumetric moisture, and oxygen concentrations at three depths, conducted quarterly soil samplings for key biogeochemical variables, and measured soil greenhouse gas (GHG) fluxes with manual static flux chambers. Results/Conclusions Throughfall exclusion significantly reduced volumetric moisture from ~0.5 to ~0.3 m3 m-3 at 0-15 cm, and increased oxygen concentrations. Throughfall exclusion 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 (0-15 cm), while responses at depth were buffered due to multiple factors. The observed changes in soil microclimate in the upper soil profile affected redox-sensitive biogeochemical processes. Specifically, we found that drought plots had significantly lower concentrations of labile P at 0-15 cm, and a decrease of almost 30% in soil CO2 efflux (3.41 vs. 2.45 g C/m2/day). Soil CH4 fluxes also responded significantly to the treatment, which enhanced CH4 uptake causing the soil to shift from net source to net sink. Collectively, these results suggest that in wet tropical forests drought disturbances have the potential to significantly alter microclimate conditions in the upper soil profile, with major implications for soil nutrient availability and the magnitude and direction of soil GHG fluxes.
