Ground beetle trophic interactions alter available nitrogen in forest soil

It is generally held that microbes exert primary control over nitrogen availability in temperate forests. Yet the role of soil and litter-dwelling invertebrates to provide additional control via the breakdown of organic matter is an area of current exploration. Through trophic interactions within soil food webs, predators may indirectly affect prey with cascading effects on litter breakdown and nitrogen availability. The importance of these interactions, however, may be context-dependent, varying with the stage of forest development and associated decomposer species composition given that young and old forests have vast differences in nitrogen availability, vegetation litter, soil properties, and invertebrate functional groups. We examined ground beetle control over soil nitrogen and soil properties using a 68-day mesocosm experiment that manipulated trophic structure (omnivore + predator beetles, predator beetles, and no beetles) in a young and old forest stand in the northeastern United States. In the young forest, net nitrogen mineralization decreased under predator + omnivore and the bulk soil C:N ratio in the old forest. However, we found no response in either forest context to the predator only treatment. Our study demonstrates the potential for ground beetles to strongly impact nitrogen availability and soil properties in forest ecosystems. Therefore, animal trophic interactions and their contexts must be included in our paradigm of nutrient cycles in temperate forests.