Ground beetles trophic interactions alter available nitrogen in forest soils

The dominant paradigm is that nitrogen cycles from plants to soil organic matter, being released in mineral form in the soil after organic matter is decomposed by microbes to be taken up again by plants. It is generally held that the process of decomposition is the rate-limiting step in the cycle. Ground dwelling macroinvertebrates may play a large role in ecosystem function by mediating microbe decomposition via predation and could be key links between plant litter and nitrogen availability in soil nutrient cycles. Ground beetles (Carabidae) are an abundant family of soil invertebrates that prey on groups of decomposing invertebrates. The goal of this study was to develop how predation from ground beetles contributes to nitrogen cycling as forests age. We hypothesized that ground beetles in young and old forests would indirectly impact available nitrogen. Our approach to addressing predator impacts on nitrogen cycling in forest soils was an experiment in young and old forest stands at Yale-Myers Forest in the northeastern United States using mesocosm cages stocked with predatory and detritivore ground beetles to create a trophic cascade over 68-days. Both forest sites had five blocks of three clustered treatments (n = 30). Treatments consisted of a control, detritivore, and predator and we took soil cores in each cage to assess available nitrogen. We used standard mesocosm cages that were designed for research on arthropod trophic interactions in ecosystems 1 m², 0.8 m tall cylindrical mesocosms constructed with a scaffolding covered with fine mesh aluminum stocked with live beetles. We conducted a series of linear mixed-effect models in RSudio from the nlme package and lme() function in R Studio to predict the delta nitrogen mineralization rate by treatment in both young and old forests separately. We used ground beetle treatment as a fixed effect and block as a random effect to account for variation in microsite differences. Here we show differences in available nitrogen between forest types (P-value = 0.03). Our hypothesis that predators would impact available nitrogen was supported in young forests. Net nitrogen mineralization (P-value = 0.007) was consistently higher in the predator treatments compared to the control. In conclusion, this study suggests predator top-down control may be important for soil nitrogen availability in temperate forest soil via mediating microbe decomposition. Macroinvertebrates and their food web interactions in the soil should be further investigated and included in soil biogeochemical models.