Climate models predict an increase in the frequency and severity of drought in Neotropical rainforests with climate change. These environmental conditions are novel for humid tropical forests and are likely to lead to changes in biogeochemical cycling, affecting patterns in carbon storage and greenhouse gas emissions. We report on a research effort to explore the effects of a severe drought in 2015 in the Luquillo Experimental Forest, Puerto Rico. We sampled a pre-existing sensor field that included galvanic oxygen (O2) sensors and time-domain reﬂectometry (for moisture and temperature) along topographic gradients in a lower montane wet tropical forest. Seven sensors of each type were installed at 12 cm depth along a ridge to valley catena; the entire catena transect was replicated five times for a total of 105 sensors. Within the sensor field we also installed nine automated gas flux chambers randomly located in each topographic zone (ridge, slope and valley). A Cavity Ring-Down Spectroscopy (CRDS) gas analyzer was used to measure pseudo-continuous fluxes of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4)). Soil carbon and nitrogen, extractable phosphorus (P) pools, iron (Fe) species, and pH were sampled before and during the drought as indicators of biogeochemical conditions.
Soil moisture and O2 availability followed distinct and robust topographic patterns prior to the drought, with significantly drier and more aerated soils in the upper topographic zones than in the valleys and lower topographic positions. The drought drove a strong threshold-type effect in both moisture and O2 concentrations. Drought resulted in a rapid decline in soil moisture in all topographic positions. For example, ridge soils declined from a mean 45 ± 2% moisture to 13 ± 2% over less than 2 months. Soil O2 concentrations in the valleys doubled in one week from a mean of 5 ± 2% to 11 ± 1%, and increased by 36 % within the first month of the drought. Drought led to higher concentrations of Fe oxides and corresponding declines in soil pH. Drought also led to dramatic and significant declines in inorganic P concentrations in all topographic zones. Emissions of CO2 did not appear to be affected by the drought, while CH4 emissions increased in post-drought soils. Our results suggest that biogeochemical effects of drought in a humid tropical forest are expressed both directly through moisture stress, and indirectly through redox-related effects on P availability and CH4 fluxes.