Variation in abundance of soil fauna trophic groups with depth, season, and litter quality: Implications for the study of decomposition

Background/Question/Methods

It is widely acknowledged that a comprehensive understanding of the fluxes of C and N from leaf litter to the ecosystem is hampered by the currently limited knowledge of soil processes. A specific hiatus in the most recent models of litter decomposition is that, while the microbial component is well represented, they do not explicitly include the soil fauna. The use of litterbags to study decomposition processes has been criticised because they alter microclimate and prevent soil fauna from incorporating fragmented litter into the soil. An alternative approach that enables the study of litter decomposition through leaching, fragmentation and catabolism is the use of stable isotopes to track the fate of litter-derived C and N. However, moving beyond the idea of soil fauna as a “black box” in this process requires a solid understanding of how different trophic groups vary seasonally and along the soil profile and react to the presence and quality of the litter layer. We present results from a field experiment in the tallgrass prairie where nematode and microarthropod trophic group abundances were assessed at three different depths during spring and autumn, in plots that received ¹³C- and ¹⁵N-enriched Andropogon gerardii litter, native litter or no litter.

Results/Conclusions

Soil fauna abundance decreased with depth for all groups except mesostigmatid mites. Seasonal responses were more variable, with a strong increase in numbers of bacterivorous and fungivorous nematodes and a decrease in plant-parasitic nematodes and springtails between May and October. In addition, significant interactive effects between season and depth, season and litter type or depth and litter type were found for several groups. Examples include higher oribatid mite abundances at 5-10 cm depth beneath bare soil than beneath litter or lower abundance of fungivorous nematodes in soils beneath enriched litter compared to native litter in October. Moreover, the fact that bacterivorous, fungivorous and plant-parasitic nematodes reached higher abundances in the enriched than in the native litter may relate to its higher N content. Both ¹³C and ¹⁵N enrichment was detected in all fauna groups from soil beneath enriched litter, at all depths and in both seasons, confirming their contribution to the decomposition of the litter. These findings highlight the importance of taking into account the temporal and spatial heterogeneity of the different groups of soil fauna, as well as the effect of the presence and quality of leaf litter on their distribution, when assessing their ultimate role in decomposition processes.