Publication

The study of coupled green and brown food webs has improved our ability to understand how nutrient cycling and plant communities respond to perturbations. Yet, it is still difficult to predict how rapidly and consistently changes in one food web propagate to the other. An area of particular uncertainty is the response of plants to the changes in leaf litter nitrogen release caused by herbivores and detritivores. We combined a field experiment and theoretical analysis to assess how herbivory, fertilization, and detritivory changed leaf litter nitrogen release and plant growth. We produced leaf litter in greenhouse cages with a factorial combination of grasshopper herbivory and additional nitrogen fertilizer. Our experiment used the resulting N-15-labelled leaf litter to measure isopod litter consumption, litter nitrogen release, and plant nitrogen uptake in the field. We then used a dynamical systems model to distinguish whether plant growth relied (a) directly on litter nitrogen release or (b) on other nitrogen pathways so that changes in litter nitrogen release have little immediate impact. Our empirical results show that the effect of nitrogen fertilization on isopod processing of leaf litter was stronger than the effect of herbivory. Regardless, the available soil nitrogen and plant growth were independent of litter nitrogen release. Our dynamical model attributes the independence to other nitrogen sources and sinks, which buffer the plant-available nitrogen pool. Isopods and litter with a history of herbivory reduced above-ground plant growth when we accounted for initial field mesocosm conditions, especially the abundance of the competitively dominant goldenrods. Consequently, the impact of isopods and litter traits do propagate to influence plant growth, but do not have a large enough effect to overcome initial differences in plant biomass and community composition in one year. Synthesis. Our results indicate animals in green and brown food webs can alter plant litter nitrogen release without any significant nutrient recycling effect on plant growth. Considering the availability of external and soil-based nitrogen sources, as well as the current plant community and microbial biomass, may be critical to determining when plant growth will respond to animal-mediated changes in litter decomposition.