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

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.