Influences of tropical tree diversity on soil carbon dynamics: Results from a laboratory incubation

Tropical forests dominate land-atmosphere exchanges of carbon dioxide (CO₂), and rates of carbon (C) uptake are often limited by soil nutrient availability. Plant species effects on soil nutrient availability are relatively well documented; however, species effects on soil C cycling are less well understood, especially in species-rich ecosystems of the humid tropics. Here we investigate how tree species differences in leached dissolved organic C (DOC) and nutrients affect soil CO₂ fluxes in a series of laboratory incubations. Our field site in SW Costa Rica has 100-200 tree species per hectare and receives approximately 5000 mm of precipitation annually. Previous research indicates that leaching of dissolved organic matter (DOM) from the litter layer accounts for a significant proportion of litter mass loss during decomposition. Other research indicates wide inter-specific variation in foliar phosphorus (P) availability across the tropics. We hypothesized that: species differences in litter chemistry may exert significant control over the amount and fate of DOM; and soil respiration would be strongly related to P availability. In this study we found eight-fold differences in soluble DOC concentrations and 30-fold differences in dissolved organic phosphorus. Subsequent laboratory incubations revealed soil respiration rates that varied by as much as an order of magnitude between species. Overall, we observed four-fold differences in total soil CO₂ efflux. Differences in CO₂ fluxes were significantly correlated with sorption to soil particles, DOM nutrient content, and C-chemistry. Notably, variation in C:P ratio accounted for 51% of the variation in total CO₂ flux between species. These results indicate that tropical tree diversity likely influences soil C storage and mineralization via inter-specific variation in litter chemistry.