Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands

Abstract

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature – herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local‐scale herbivory, and its interaction with nutrient enrichment and climate, within global‐scale models to better predict land–atmosphere interactions under future climate change.

Stoichiometric niche, nutrient partitioning and resource allocation in a solitary bee are sex-specific and phosphorous is allocated mainly to the cocoon

“…To date, the majority of studies on nutrient cycling in terrestrial ecosystems highlighted the role of medium-sized or large mammalian herbivores in this process 34 – 37 , but similar effects of invertebrate herbivores are also probable. Indeed, we found considerable amounts of P, Ca, Mg, Zn and Mn built into cocoons, which, together with elements present in larval excreta, are left at nesting sites of bees.…”

“…Overall, our findings indicate that underdeveloped cocoons, which are sometimes observed in O. bicornis 33 , may indicate the scarcity of specific www.nature.com/scientificreports/ elements in food and/or a trade-off in the allocation of elements between the adult body and cocoon, which may have negative fitness implications. To date, the majority of studies on nutrient cycling in terrestrial ecosystems highlighted the role of mediumsized or large mammalian herbivores in this process [34][35][36][37] , but similar effects of invertebrate herbivores are also probable. Indeed, we found considerable amounts of P, Ca, Mg, Zn and Mn built into cocoons, which, together with elements present in larval excreta, are left at nesting sites of bees.…”

Aggregated filter‐feeders govern the flux and stoichiometry of locally available energy and nutrients in rivers

“…For example, aggregating animals across systems as diverse as grasslands and coral reefs promote nutrient and resource heterogeneity through their activities (Allgeier et al., 2014; Coppock et al., 1983). While the terrestrial literature is rich in work illustrating the importance of animal nutrient release in mediating biogeochemical cycling across brown and green food webs (Bohlen et al., 2004; Sitters et al., 2020; Zimmer et al., 2005), animal‐driven nutrient cycling studies in aquatic systems have primarily focused on inorganic N and P influences in green food webs (Atkinson et al., 2017). Therefore, gaps remain in our understanding of aquatic animals as sources of dissolved organic matter (DOM) that may positively influence brown food webs via enhancing microbial production (but see Meyer & O'Hop, 1983; Parr et al., 2019).…”

Herbivore dung stoichiometry drives competition between savanna trees and grasses

“…Hence, the concept appears to be more widely applicable, but there is need for further testing in other grazing ecosystems, particularly under field conditions. Our concept, which shows that connections between N 2 ‐fixing trees and grasses, as well as the herbivores that feed on them, helps maintain the tree–grass balance, illustrates that the influence of these ecosystem components on ecosystem functioning is complex and can easily be disturbed when herbivore species get lost due to anthropogenic interferences (Sitters, Kimuyu, et al., 2020; Sitters, Wubs, et al., 2020).…”