Soil fauna, microbes and plant litter decomposition in tropical and subalpine forests
PhD: University of Colorado Boulder., 1999.
Studies of 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. This work addresses the effects of soil organisms on litter decomposition along a wide climatic gradient: in dry and wet tropical forests and north- and south-facing subalpine forests.
I quantified decomposition rates and N fluxes in control and fauna-excluded treatments to test whether soil fauna exhibit different effects under diverse climate and/or substrates. Litterbags contained two different plant species varying in lignin content. All three factors (climate, substrate quality, and soil fauna) independently influenced the decomposition rate of plant litter in the tropical and subalpine forests. The faunal effects on decay rates and N mineralization of both litter species were higher in the tropical wet forest than the tropical dry and subalpine forests, and no significant interaction of fauna and litter species on the annual decay rates was found. There was a higher density of soil fauna per gram of litter in the tropical wet forest as compared to all sites.
Little consensus has been reached on the extent to which soil fauna can regulate microbial activities. This uncertainty is due to the complexity of the functional roles of fauna and their effects on microbes. To test whether soil fauna exhibit different effects on microbes under diverse climatic conditions, I quantified soil microbial biomass and maximum growth rates in control and defaunated treatments across the study sites. It was found that exclusion of soil fauna resulted in a higher total microbial biomass and lower maximum growth rate in the subalpine forests. No effect of fauna exclusion on microbes was found in the tropics.
This study indicates differential mechanisms by which soil fauna affect soil ecosystem processes in tropical and temperate systems. In the subalpine forest, soil fauna play a more significant role in ecosystem processes through selective interactions with the microbial community. In the wet tropics, soil fauna have a direct effect on ecosystem processes through comminution of organic matter and the modification of the soil environment.