The detritus-based microbial-invertebrate food web contributes disproportionately to carbon and nitrogen cycling in the Arctic

Abstract

The Arctic is the world’s largest reservoir of soil organic carbon and understanding biogeochemical cycling in this region is critical due to the potential feedbacks on climate. However, our knowledge of carbon (C) and nitrogen (N) cycling in the Arctic is incomplete, as studies have focused on plants, detritus, and microbes but largely ignored their consumers. Here we construct a comprehensive Arctic food web based on functional groups of microbes (e.g., bacteria and fungi), protozoa, and invertebrates (community hereafter referred to as the invertebrate food web) residing in the soil, on the soil surface and within the plant canopy from an area of moist acidic tundra in northern Alaska. We used an energetic food web modeling framework to estimate C flow through the food web and group-specific rates of C and N cycling. We found that 99.6% of C processed by the invertebrate food web is derived from detrital resources (aka ‘brown’ energy channel), while 0.06% comes from the consumption of live plants (aka ‘green’ energy channel). This pattern is primarily driven by fungi, fungivorous invertebrates, and their predators within the soil and surface-dwelling communities (aka the fungal energy channel). Similarly, >99% of direct invertebrate contributions to C and N cycling originate from soil- and surface-dwelling microbes and their immediate consumers. Our findings demonstrate that invertebrates from within the fungal energy channel are major drivers of C and N cycling and that changes to their structure and composition are likely to impact nutrient dynamics within tundra ecosystems.