Seasonal changes in soil respiration linked to soil moisture and phosphorus availability along a tropical rainfall gradient

Humid tropical forests contain some of the largest soil carbon (C) stocks on Earth, yet there is uncertainty about how carbon dioxide (CO2) fluxes will respond to climate change in this biome. The magnitude of change in soil respiration over seasonal wetting and drying cycles can provide insight to how CO2 fluxes might respond to precipitation changes. We measured soil respiration in 15 distinct forests across rainfall and soil fertility gradients in lowland Panama to assess spatial variation in seasonal patterns. We predicted that seasonal changes in soil respiration would be related to soil moisture, and that the ratio of wet to dry season CO2 fluxes across landscape gradients would be regulated by soil organic C and nutrient availability. We found that soil respiration during the dry season was relatively stable across the rainfall gradient, averaging 4.3 ± 0.3 µmol CO2 m−2 s−1. In contrast, wet season respiration varied markedly, ranging among sites from a decline of 19% to an increase of 360% in comparison to dry season values. Soil moisture, air temperature, and rainfall were the best predictors of instantaneous soil respiration (R2 = 0.18). Meanwhile, ratios of wet to dry season CO2 fluxes were best predicted by site-scale resin-extractable phosphorus (P; R2 = 0.48). That is, soil respiration in P-rich sites increased more during the wet season relative to respiration in P-poor sites. Also, sites with Wet:Dry season CO2 flux ratios < 1 were all poor in soil P, defined as resin P concentrations < 2 mg P kg−1. Although soil organic C was not related to instantaneous soil respiration rates, forest floor biomass accumulation during the dry season was positively correlated with soil respiration during the wet season (R2 = 0.26), indicating the contribution of microbial decomposition to wet season soil respiration. Overall, nutrient availability regulated soil respiration responses to increased moisture during the wet season, while low soil moisture uniformly suppressed soil respiration across sites during the dry season. Phosphorus availability might therefore regulate feedbacks to climate change among humid tropical forests.