Controls on diurnal variation in labile soil phosphorus of two wet tropical forests

Background/Question/Methods

Tropical forests are generally considered to be phosphorous (P) limited. Variability in soil P in these systems has been shown to affect forest productivity and foliar nutrient concentrations, as well as the rate of soil C loss via effects on decomposition and soil respiration rates. Therefore, the C efflux and long-term stability of C stocks in tropical soils is likely to depend on the availability of P. We quantified short-term (hours to days) variability in labile soil P in wet tropical forest in Puerto Rico and Costa Rica. We evaluated potential environmental controls on available soil P such as soil moisture, soil temperature, solar radiation, precipitation and soil carbon dioxide (CO2) efflux.  In both sites we measured hourly changes in labile P from sunrise to sunset on three to five separate days.

Results/Conclusions

Labile soil P varied significantly across days in both forests. In Puerto Rico, concentrations of labile soil P were two to five times higher than labile soil P in the Costa Rican forest, ranging from 2.75-3.75 µg/g in Puerto Rico and 0.88-2.45 µg/g in Costa Rica. Approximately 30% of the day to day variation in labile P in the Puerto Rican forest was explained by volumetric soil moisture such that labile P increased with increasing soil moisture. In contrast, soil moisture was not an important control on labile soil P in the Costa Rican forest.  While we did find significant diurnal variation in labile soil P in Costa Rica, this was not the case for Puerto Rico, which showed no significant diurnal variation across the five sample days. Together, soil CO2 efflux, soil temperature, and sap flow explained 86% of diurnal variation in labile P in the Costa Rican forest.  Overall, these findings suggest that the labile P pool is highly dynamic on short timescales, but the dominant controls are likely to differ across tropical forest sites.