Significant Diel Variation of Soil Respiration Suggests Aboveground and Belowground Controls in a Tropical Moist Forest in Puerto Rico

摘要

Soil respiration in tropical forests represents a major source of carbon dioxide (CO_2) to the atmosphere. The magnitude of this large flux is projected to change in response to climate change, with global implications due to the disproportionate role of tropical forests in the carbon cycle. Evaluating diel patterns of soil respiration concomitantly with biophysical drivers is a valuable approach for elucidating the mechanisms controlling soil respiration. We measured hourly soil respiration rates in a tropical moist forest in Puerto Rico over a 3-year period using automated chambers, as well as soil temperature/moisture, air temperature, relative humidity, and photosynthetically active radiation. Hourly soil respiration exhibited as much as threefold variation on diel time scales (monthly diel amplitude ranged from 1 to 7 μmol CO_2 m~(-2) s~(-1)), and both the magnitude and shape of diel patterns changed significantly from month to month. Soil respiration peaked in the morning and late afternoon with a midday decline that was evident during the warmest summer months. The relationship between soil respiration and soil temperature differed during daytime versus the night, with a nonlinear relationship in the daytime but a significant positive linear relationship at night. These findings suggest factors other than/in addition to temperature could be controlling soil respiration during the day; however, soil respiration did not correlate with any of the other measured biophysical variables. Overall, our results highlight the potential role of aboveground processes as drivers of soil respiration at diel time scales, especially in closed canopy tropical forests with low diel variation in soil temperature/moisture.