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 (CO 2 ) to the atmosphere via soil respiration ( R s ). As such, identifying the main controls on R s 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 R s in a secondary tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of R s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R s . Across 3 years of measurements, mean R s was 7.16 ± 0.02 μmol CO 2 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 R s with both air and soil temperature, highlighting the importance of temperature as a driver of R s even in warm ecosystems, such as tropical forests. We also found a significant parabolic relationship between mean daily volumetric soil moisture and mean daily R s , with an optimal moisture value of 0.34 m 3 m ‐3 . Given the relatively consistent climate at this site, the large range in mean monthly R s (~7 μmol CO 2 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 R s with temperature at monthly timescales particularly stands out, as moisture is usually considered a stronger control of R s in tropical forests that already experience warm temperatures year‐round. Moreover, our results revealed the strong seasonality of R s in tropical moist forests, which given its high magnitude, can represent a significant contribution to the seasonal patterns of atmospheric (CO 2 ) globally.