Experimental drought in a wet tropical forest drives increases in soil carbon dioxide losses to the atmosphere

Background/Question/Methods

Declines in precipitation due to both greenhouse gas forcing and biophysical feedbacks resulting from land use change are predicted for many tropical forests, potentially affecting a suite of ecosystem processes. However, while atmospheric observations suggest the potential global-scale consequences of climate variations in the tropics, the mechanisms underlying the effects of precipitation on soil carbon (C) balance remain largely unknown. We conducted a throughfall manipulation experiment in a wet tropical forest in southwestern Costa Rica to explore the effects of potential decreases in total rainfall (i.e., 50% and 75% of ambient precipitation) on soil C and nutrient dynamics. In this extremely wet primary forest (i.e., > 5000 mm/y), high rainfall drives rapid rates of litter mass loss via dissolved organic matter (DOM) leaching from the litter layer to the soil, and inputs of leached DOM have been positively correlated with soil carbon dioxide (CO₂) fluxes. Thus, we hypothesized that decreases in total throughfall would reduce the amount of DOM transported to the soil, thus driving decreases in soil respiration. 
Results/Conclusions

Contrary to our expectations, in response to a 25% decrease in throughfall, soil CO₂ fluxes were not significantly different than those observed in plots receiving ambient throughfall, but a 50% reduction in total throughfall caused a highly significant increase in total soil CO₂ losses. The increase in CO₂ in response to throughfall manipulation were not significantly related to changes in soil moisture, soil nutrient dynamics, microbial biomass or root biomass. However, while the throughfall manipulations effectively reduced the amount of precipitation reaching the soil surface, the quantity of DOM transported through the litter layer in the manipulated plots was much higher than predicted based on throughfall amount alone. Our results show that in this wet forest, increases in the concentration of leached DOM delivered to the soil in response to experimental drought stimulate more rapid and higher overall rates of heterotrophic soil respiration, and suggest that declines in precipitation in wet tropical forests may drive higher losses of soil CO₂ to the atmosphere.