Print PageSoil moisture and temperature are key drivers of biogeochemical processes in terrestrial ecosystems, strongly impacting carbon and nutrient cycling. Increased frequency of drought and warmer temperatures are predicted for humid tropical forests, which are a significant global source of carbon dioxide (CO2) from soil respiration. Consequently, changes in the climate regime of these ecosystems could feed back to future climate change. We used small, well-replicated, partial rainfall exclusion shelters to evaluate soil moisture and temperature controls on soil carbon dioxide (CO2) emissions from a humid tropical forest in Puerto Rico. We measured hourly changes in soil respiration, temperature and moisture in both control and exclusion plots (n=6) for 6-months. The variance of each time series (e.g, temperature, moisture, respiration) was decomposed on a scale-by-scale basis using orthonormal wavelet transformation. The correlation among variables was explored using Haar-wavelet coherence.
Results/Conclusions
Soil respiration in control plots showed high coherence with soil moisture for a broad range of time scales, with a peak correlation corresponding to a two-day periodicity. Further analyses of the time series revealed a decline in soil respiration following large rainfall events. These events have previously been shown to lower soil redox potential, and suggest the possibility for a strong control of oxygen availability on the production and diffusion of CO2 from soils. Although temperature varies diurnally, there was no coherence between temperature and soil respiration at this time-scale. There was, however, an evident relationship between soil respiration and temperature, with peak respiration occurring during the period of highest temperatures as expected. A longer time-series (1 year) is needed to confirm whether the observed coherence at this time-scale is significant. Partial rainfall exclusion resulted in a reduced coherence between soil respiration and temperature, as well as soil moisture. The reduced CO2 response to temperature in the exclusion plots suggests that the positive effect of temperature on the CO2 efflux is constrained by soil moisture availability. Overall, a reduction in rainfall could lead to additional CO2 release from humid tropical soils. Our results support the expectation that increased temperatures will increase soil respiration rates; however, more frequent or severe droughts in humid tropical forests could significantly alter this relationship.
