Under the lens: Carbon and energy channels in the soil micro-food web

While carbon flow through soil decomposition channels is well studied, the associated energy fluxes are less considered. In particular, how microbial substrate and energy turnover are linked to higher trophic levels has hardly been investigated to date. Soil nematode communities can serve as a model group to address this knowledge gap. As important microbial grazers nematodes hold a central position in soil food webs. The present study relates the structure and function of the micro-food web to microbial carbon and energy use efficiency. Microbial biomass (phospholipid fatty acids), activity (substrate-induced growth) and energy flow (substrate-induced heat release) are linked with the nematode fauna, i.e. population density, ecological indices and metabolic footprints. Soils from four agricultural sites in central Europe were compared, either long-term unfertilized or fertilized with farmyard manure. Environmental conditions (e.g. soil nutrients, moisture) influenced microbial biomass, nematode population density and decomposition channels more than fertilization. While all arable soils were dominated by bacteria, at sites with moderate nutrient status fungi also contributed to carbon and energy flow. The life strategies of microorganisms and nematodes showed a comparable pattern: nutrient-poor unfertilized soils comprised more K-strategists, characterized by an efficient but slow metabolism. Conversely, nutrient-rich soils represented fast cycle systems, dominated by copiotrophic microorganisms and strong r-strategists among nematodes. Across soils, microbial energy use efficiency was quite balanced compared to carbon use efficiency. Remarkably, nematode functional groups were closely linked to microbial substrate turnover efficiency, suggesting nematode faunal analysis as a useful proxy. The nematode Channel Index, a measure for soil decomposition channel activity, is proposed as a tool for mapping microbial carbon and energy turnover.