Soil Fauna and Plant Litter Decomposition in Tropical and Subalpine Forests

The decomposition of plant residues is influenced by their chemical composition, the physical–chemical environment, and the decomposer organisms. Most studies interested in latitudinal gradients of decomposition have focused on substrate quality and climate effects on decomposition, and have excluded explicit recognition of the soil organisms involved in the process. To test whether soil fauna exhibit different effects under diverse climates or substrates, we quantified decomposition rates and N fluxes in control and fauna-excluded treatments with litterbags containing relatively high-quality (Quercus gambelii) and low-quality (Cecropia scheberiana) litter in sites representing large differences in climate as measured by AET (actual evapotranspiration). Two subtropical sites included a wet and a dry forest, and two temperate sites included north- and south-facing subalpine forests. We found that: (1) all three factors (climate, substrate quality, and soil fauna) independently influenced the decomposition rate of plant litter in the tropical and subalpine forests; (2) faunal effects on decomposition rates and N mineralization of Q. gambelii and C. scheberiana were consistently higher in the tropical wet forest than in the tropical dry and subalpine forests; (3) there was no significant interaction of fauna and litter species on the annual decay rates; and (4) the density (numbers per gram of dry litter) of total fauna was highest in the tropical wet forest, intermediate in the subalpine forests, and lowest in the tropical dry forest. Our results indicate that soil fauna have a disproportionately larger effect on litter decomposition in a tropical wet forest than in a tropical dry or a subalpine forest. The annual decay rates of Q. gambelii and C. scheberiana are not influenced differentially by the soil fauna in the tropical and subalpine forests.