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.