Plant species control and soil faunal involvement in the processes of above- and below-ground litter decomposition

Background/Question/Methods

Although the litter decomposition is a key process in the terrestrial biogeochemical cycles, roles of soil organisms as a driver of decomposition remain uncovered. The effects of litter quality on decomposition have been considered to be stronger and intensively studied. However, these effects may be indirect through interactions with structure of soil decomposer community, such as presence/absence of soil fauna. Although there is some evidence suggesting the interactive effects of soil fauna and litter quality, most works have exclusively focused on aboveground leaf litter. That is, little is known about relationships between soil fauna and the belowground fine-root litter, which potentially provides the near equivalent to leaf litter in the annual nutrient input into soils. We thus quantified how similarly/differently soil fauna affects the plant species effects on the decomposition of aboveground and belowground litter. Furthermore, we tested whether soil fauna alters the relationship between leaf and root decomposition across species, because more attention has been recently paid to a possible correlation between leaf and root functional traits. Here we used litterbags with different mesh sizes to include/exclude soil mesofauna, and pursued leaf and root decomposition of six major tree species in a Japanese temperate forest for 411 days. 

Results/Conclusions

In both aboveground and belowground litter decomposition processes, we found the plant species effects on litter mass loss rates and microbial respiration rates, regardless of the soil fauna exclusion. Under both situations with and without soil fauna, litter mass loss and microbial respiration showed a significant positive correlation between leaf and root litter decomposition. We found a slower mass loss of root litter than leaf litter regardless of the soil fauna exclusion. These results suggest that the effects of litter quality are robust and minimally affected by the structure of soil decomposer community. Although the presence of soil fauna did not show a positive contribution to both leaf and root mass loss rates, the effects of their presence on microbial activity were different between aboveground and belowground litter decomposition processes. Soil fauna increased microbial respiration rates in the early stage of leaf decomposition, but did not affect root microbial respiration rates through experimental duration for all six species. Our results suggest that soil fauna could drive the divergence in decomposition rates between aboveground and belowground litter, amplifying the slower decomposition of root litter than leaf litter.