OOS 27-5 - Do resource subsidies alter energy flows through soil food webs? Implications for C and N cycling in subarctic Iceland

Background/Question/Methods
The flow of insects from aquatic to terrestrial systems can serve as important resource subsidies to recipient habitats, with the capacity to restructure plant composition and consumer communities. Although the impacts of resource subsidies are well known, few studies have described how they change soil food web structure and associated energy flows. Because insect subsidies are rich in nutrients—such as carbon (C) and nitrogen (N)—we hypothesize that adding insect subsidies will increase the amount of biomass and energy moving through soil food webs. In this study, we investigated how insect subsidies affect a subarctic soil food web near Lake Mývatn, Iceland, where midge (Diptera: Chironomidae) deposition to land is high. We simulated insect subsidies by adding midge carcasses to 1-m² heathland plots for two years (2017 and 2018) and measured the responses of soil arthropods in plots with and without midge addition. We then analyzed the food web structure and flow of C and N using energetic food web modeling.

Results/Conclusions
Our results indicate that adding midge carcasses shifted the composition of arthropod communities to a more detritivore-based community by the end of the experiment, with increases in springtails (Collembola) and oribatid mites (Acari: Oribatida). We also found that midge addition strongly increased the biomass of arthropods in the food web, elevating arthropod biomass by 11x. Finally, the addition of midge carcasses signficantly enhanced the flow of C and N, indicating the midge subsidies boosted the amount of energy flow through the subarctic food web. These findings highlight how insect subsidies can restructure and increase the flow of energy in soil food webs. Thus, as global change threatens to decouple nutrient flows between habitats, this study suggests that recipient soil webs that rely on resource subsidies are likely to break down and inhibit the flow of nutrients through terrestrial ecosystems.