Abstract: SOM is crucial for soil fertility and C sequestration, yet we are still far from fully understanding the biological mechanisms that control its formation and persistence. Separating SOM into particulate (POM) and mineral-associated (MAOM) organic matter, can facilitate understanding of the portion of SOM that recycle fertility in the short term (POM) versus the portion of SOM that persists for the long-term (MAOM). Many studies have recognized that soil fauna affect decomposition rates and POM formation, but their impact on highly stable MAOM, primarily composed of microbial necromass, is not well understood. While there is some evidence for the effects of fauna grazing on microbial communities, there are still glaring gaps in our understanding of fauna-microbe interactions and the mechanisms driving microbial necromass stabilization in SOM.
The goal of this project was to explore soil fauna as drivers of microbial necromass production and SOM persistence through their effects on microbial growth, activity, and turnover. To uncover these effects, we conducted a microcosm experiment to test the following hypotheses: (1) The presence of bacterivorous nematodes will reduce microbial biomass but increase soil microbial activity and microbial turnover leading to greater necromass production and subsequent MAOM formation; (2) Introducing higher trophic level predators will release grazing pressure on bacteria, resulting in less necromass production and reduced MAOM formation. To address these hypotheses, we conducted a soil microcosm experiment using isotopically enriched litter to track C and N flows through simplified soil food webs. We manipulated soil food webs by removing and subsequently adding soil nematodes and predatory mites. Preliminary results showed that after 1-month nematodes reduced microbial biomass (-8 μg C soil-1) and soil respiration (-89 μg C soil-1) but increased MAOM formation (+1.7 mg C g soil-1; +0.3 mg N g soil-1). Further data on microbial necromass and community composition will be presented to provide evidence for this phenomenon.
The contribution of soil microbes to SOM formation is relatively well studied, but there are major gaps in our understanding of the contributions of soil fauna. The results of this project provide evidence and replicable experimental design for the effects of soil faunal groups on microbial activity and turnover and advance our understanding of SOM stabilization. Soil biodiversity is important for controlling the formation of soil organic matter (SOM), but until now the essentially mechanical action of soil fauna is often considered separately from the predominantly biochemical action of microorganisms.
