Detritivores ameliorate the enhancing effect of plant-based trophic cascades on nitrogen cycling in an old-field system.

Nitrogen (N) cycling is a fundamental process central to numerous ecosystem functions and services. Accumulating evidence suggests that species within detritus- and plant-based food chains can play an instrumental role in regulating this process. However, the effects of each food chain are usually examined in isolation of each other, so it remains uncertain if their effects are equally important or if one chain exerts predominant control. We experimentally manipulated the species composition of detritus-based (isopods and spiders) and plant-based (grasshoppers and spiders) food chains individually and in combination within mesocosms containing plants and microbes from an old-field ecosystem. We tested: (i) their relative impact on N cycling, and (ii) whether interactions between them moderated the influence of one group or the other. We found that spiders in plant-based food chains exerted the only positive effect on N cycling. Detritus-based food chains had no net effects on N cycling but, when combined with plant-based food chains, ameliorated the positive effects of plant-based species. Our results suggest that detritus-based food chains may ultimately limit rates of N cycling by eroding the enhancing effects of plant-based food chains when antagonistic interactions between detritus- and plant-based species exist.