Large seasonal variation of soil respiration in a secondary tropical moist forest in Puerto Rico

Abstract Tropical forests are the largest contributors to global emissions of carbon dioxide (CO2) to the atmosphere via soil respiration (Rs). As such, identifying the main controls on Rs in tropical forests is essential for accurately projecting the consequences of ongoing and future global environmental changes to the global C cycle. We measured hourly Rs in a secondary tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of Rs and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of Rs. Across 3 years of measurements, mean Rs was 7.16 ± 0.02 μmol CO2 m‐2 s‐1 (or 2,710 g C m‐2 year‐1) and showed significant seasonal variation. Despite small month‐to‐month variation in temperature (~4°C), we found significant positive relationships between daily and monthly Rs with both air and soil temperature, highlighting the importance of temperature as a driver of Rs even in warm ecosystems, such as tropical forests. We also found a significant parabolic relationship between mean daily volumetric soil moisture and mean daily Rs, with an optimal moisture value of 0.34 m3 m‐3. Given the relatively consistent climate at this site, the large range in mean monthly Rs (~7 μmol CO2 m‐2 s‐1) was surprising and suggests that even small changes in climate can have large implications for ecosystem respiration. The strong positive relationship of Rs with temperature at monthly timescales particularly stands out, as moisture is usually considered a stronger control of Rs in tropical forests that already experience warm temperatures year‐round. Moreover, our results revealed the strong seasonality of Rs in tropical moist forests, which given its high magnitude, can represent a significant contribution to the seasonal patterns of atmospheric (CO2) globally