Tracking assimilation of microbial biomass, leaf litter and artificially created soil organic matter by soil fauna using multi-resource stable isotope labelling

Abstract

Understanding the resource utilization of soil invertebrates is essential for elucidating nutrient cycling and energy flow in terrestrial ecosystems. The soil environment offers a wide range of resources to its consumers, including microbes, soil organic matter (SOM), and plant litter. Soil invertebrates are small and cryptic, and typically rely on diverse arrays of basal resources, making the determination of its specific contributions to the diets of distinct animal groups challenging. This study investigated the carbon and nitrogen flows from different organic resources to soil meso- and macrofauna using stable isotope dual-labelling (¹³C and ¹⁵N) in an experimental temperate forest mesocosm over a one-month period. Studied resources included bacterial and fungal biomass, leaf litter and artificial mineral-associated organic matter ‘a-MaOM’ made of microbial necromass and vermiculite – a newly developed substrate produced in-house to experimentally mimic stabilized SOM. Our findings indicate that mesofauna incorporated the isotopic label from bacteria and fungi within three days after label introduction, demonstrating their role as primary microbial consumers in soil food webs and highlighting the importance of microbial biomass as nutrient sources for soil mesofauna. In contrast, macrofauna showed no detectable label uptake during the entire experimental period, suggesting either dietary preferences for other, unidentified resources or reflecting physiological factors such as lower metabolic turnover and slower assimilation of labelled materials. No detectable label was found in the studied invertebrate groups in the a-MaOM and litter treatments during the short experimental period, indicating that its consumption by soil fauna was negligible in comparison to microbial biomass. Beyond feeding ecology, our study introduces a methodological innovation by producing stable ¹³C- and ¹⁵N-labelled a-MaOM, offering a new tool for experimentally tracking stabilized SOM pathways in soil food webs while its bioavailability to soil organisms remains to be studied. Overall, our results reveal distinct feeding strategies among soil invertebrates, emphasizing the importance of mesofauna-microorganism interactions in soil nutrient cycling and the differentiated feeding modes of meso- and macrofauna in temperate forest ecosystems.