Controls on diel variation of soil respiration in a moist subtropical forest in Puerto Rico

Background/Question/Methods

Moist tropical forests have the highest soil respiration rates (R_s) of any terrestrial ecosystem and account for approximately one third of the world’s soil carbon (C) pool. Small increases in the magnitude of R_s in these ecosystems can result in high rates of soil C loss, with significant consequences for global climate change. Identifying the climatic controls of R_s in moist subtropical forests will improve our ability to predict how this large C flux will respond to climate change. Our objectives were (1) to determine whether R_s varies on diel timescales, (2) whether diel R_s patterns vary seasonally, and (3) identify biophysical drivers of this temporal variation. We measured hourly R_s in a secondary, moist subtropical forest in Puerto Rico for a 3-year period using an automated soil respiration system (LI-COR 8100/8150 with six chambers). Concomitant with R_s we measured hourly variation in several climatic drivers (air/soil temperature, soil moisture, relative humidity, and photosynthetically active radiation). The high temporal resolution of our data set allowed us to detect temporal variation of R_s at multiple time-scales, contributing to the growing number of studies that use automated soil respiration systems to improve our understanding of the processes that drive R_s.

Results/Conclusions

Soil respiration showed significant diel variation, with the magnitude, amplitude, and shape of these curves varying throughout the year. Overall, diel R_s peaked in the late afternoon and reached a minimum in the early morning. Diel amplitudes ranged from 1.5 to 5 μmol CO₂ m^-2 s^-1, with larger amplitudes occurring in warmer months that also have higher rates of R_s. In warmer months R_s exhibited a strong bimodal pattern, and a narrower diel range with a single peak in cooler and drier months. Diel R_s was positively correlated with soil temperature, but this relationship was non-linear during the day and linear at night (i.e., hysteresis). The bimodal pattern of R_s may be due to a mid-day depression of photosynthesis when humidity is low and air temperature is high, thereby reducing transport of photosynthate to the roots and decreasing rhizospheric respiration. The hysteresis between R_s and temperature suggests multiple controls on R_s on diel time-scales. Research that partitions R_s into its components could provide insight into their respective sensitivities to different climatic drivers, improving our capacity to understand the effects of climate change on the tropical forest C cycle.