The role of insect carcasses in mediating soil microbial community function and composition in a heathland ecosystem

While land systems have known effects on lake dynamics, lakes can reciprocally provide nutrients to land through insect movement. We have identified an aquatically derived nutrient source that affects soil decomposition rates and soil microbial community composition in a heathland system. In Iceland, aquatic insect carcasses are a high quality and high quantity source of organic input to terrestrial soil during peak midge emergence. To test the hypothesis that insect carcasses positively affect decomposition rates, we deposited midge (Chironomidae) carcasses on 1-m² plots in northeast Iceland. We measured decomposition and the stable isotopic carbon signature of CO₂ respired during lab incubations of both the mineral soil and organic top-layer sampled from midge treatment plots (multi-year addition and single year addition). To test whether insect carcasses affect microbial community structure, we extracted lipids (PLFA) from the soil across all midge treatments. Multi-year (press) midge addition increased overall decomposition rates in the organic top layer, but not in the mineral soil. Initial organic top layer CO₂ flux was highest in the press treatments. Press treatments significantly depleted initial isotopic Δ13C respired from the duff and mineral soil, and altered microbial community abundance. Fungi and bacteria biomass increased in the press treatments, though the F:B was not affected by midge addition. We conclude that insect carcasses are an important source of organic inputs to the soil and positively affect soil decomposition rates, initial CO₂ flux, alter the type of carbon consumed by microbial communities and the structure of microbial communities in heathland ecosystems.