PS 67-34 - Strong seasonal and diel variation of soil CO2 efflux in a moist subtropical forest in Puerto Rico

Thursday, August 9, 2012
Exhibit Hall, Oregon Convention Center
Omar Gutiérrez del Arroyo1, Tana E. Wood2 and Ariel E. Lugo2, (1)Department of Biology, University of Puerto Rico, San Juan, PR, (2)International Institute of Tropical Forestry, USDA Forest Service, Rio Piedras, PR
Background/Question/Methods

Tropical forest soils are the largest natural source of carbon dioxide (CO2) to the atmosphere. Currently, we have little understanding of how this large C flux will respond to ongoing changes in climate. Identifying climatic controls and natural variability of soil CO2 efflux in these ecosystems could improve our ability to predict how this significant C flux will respond to future climate change. We measured hourly soil CO2 efflux in a secondary, moist subtropical forest with limestone soils in Puerto Rico for a 1-yr period to determine at what time-scale soil CO2 efflux varies and whether this variability can be explained by climatic variables such as air temperature and precipitation. 

Results/Conclusions

Mean annual soil CO2 efflux (±S.E.) was 8.79 ± 0.02 µmol CO2/m2/s. Daily soil CO2 efflux varied significantly over the 1-yr period, and this seasonal variation was significantly positively related to air temperature (R2=0.58, p=0.0001). Accordingly, mean monthly soil CO2 efflux peaked in August (11.43 ± 0.06) and was lowest in March (5.90 ± 0.08 µmol CO2/m2/s). In addition to significant seasonal variation, soil CO2 efflux also varied on a diel time-scale demonstrating a bi-modal pattern with a depression occurring at midday. The diel patterns of soil CO2 efflux were decoupled from temperature with the highest values occurring from 6 to 9 PM. We found seasonal shifts in the amplitude and shape of diel hysteresis effects on soil CO2 efflux. This seasonal shift could indicate other environmental variables, in addition to temperature, are important in regulating daily patterns of soil CO2 efflux. Overall, the strong temporal variability in soil CO2 efflux observed in this study highlights the value of collecting longer-term (months to years) data with high temporal resolution. The significant positive effect of temperature on soil CO2 efflux in this forest, despite low intra-annual variability (<4°C), suggests that soil C loss from moist tropical forests could increase as global temperatures rise.