Short-term variability in labile soil phosphorus is positively related to soil moisture in a humid tropical forest in Puerto Rico

Primary productivity in tropical forests is often considered limited by phosphorus (P) availability. Microbial activity is a key regulator of available P through organic matter decomposition (supply) as well as microbial immobilization (depletion). Environmental conditions, such as soil moisture and temperature can fluctuate significantly on hourly to daily time-scales in tropical forested ecosystems. Given the ability of microbes to respond rapidly to changing environmental conditions we would expect concomitant changes in the available soil P pool. Despite the potential for soil P availability to vary on short time-scales, research that investigates hourly to daily changes in the available soil P pool in tropical forests is extremely rare. We quantified diurnal fluctuations in labile soil P and the importance of biotic and abiotic factors in driving these patterns in a wet tropical forest in Puerto Rico. Hourly measurements of Bray-extractable P were made from sunrise to sunset on five separate days along with measurements of soil temperature, moisture, pH, soil respiration, and solar radiation. While we found no significant diurnal variation in labile P, it did, however, vary significantly across the five sample days (2.8–3.8 µg/g). The day-to-day variation in labile P was positively related to soil moisture (R 2 = 0.42, p = 0.009). These findings illustrate the potential for rapid change in the available P pool in response to variable soil moisture status as well as the importance of considering soil moisture conditions when estimating P availability in the humid tropics.