Nitrogen recycling in coupled green and brown food webs: Weak effects of herbivory and detritivory when nitrogen passes through soil

Abstract

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 15N‐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.

Microbial and animal nutrient limitation change the distribution of nitrogen within coupled green and brown food chains

“…S2 and S3). Our results are therefore consistent with empirical assessments that brown food web dynamics may play an underappreciated role in green-brown coupled N cycling (Holtkamp et al 2011, Kostenko et al 2012, Laungani et al 2012, Buchkowski et al 2018.…”

Invertebrate functional traits and terrestrial nutrient cycling: Insights from a global meta‐analysis

“…Furthermore, predator effects will depend on whether they are mediated by herbivores in the plant-based food chain, by detritivores in the detritusbased food chain, or by a cascading chain of effects among both prey groups. For example, predator-prey interactions can determine herbivore effects on plant biomass and chemistry via changes in litter quality, thereby altering detritivore impacts on soil nutrient availability (Buchkowski et al, 2019). Incorporating such a food-web framework will likely uncover important predator effects on terrestrial nutrient cycling.…”

Do litter-feeding macroarthropods disrupt cascading effects of land use on microbial decomposer activity?

“…These drastic changes in microbial activity following litter consumption by G. marginata raise an important issue: the impact of soil macro-detritivores on the coupling between above-and belowground components of ecosystems. Only few studies that have hitherto examined the influence of aboveground processes on plant litter decomposition considered the role of macroarthropods (Buchkowski, Schmitz & Bradford, 2019). For example, studies in fine-mesh litter bags excluding macrofauna have shown that intraspecific changes in litter quality in response to herbivory affect litter decomposition rates (Olofsson & Oksanen, 2002;Semmartin & Ghersa, 2006;Wang et al, 2018).…”