Increases of Soil C, N, and P Pools Along an Acacia Tree Density Gradient and Their Effects on Trees and Grasses

Nitrogen (N) fixing trees including many species of Acacia are an important though variable component of savanna ecosystems. It is known that these trees enrich the soil with carbon (C) and N, but their effect on the combined C:N:P stoichiometry in soil is less well understood. Theory suggests that they might reduce available phosphorus (P), creating a shift from more N-limited conditions in grass-dominated to more P-limited conditions in tree-dominated sites, which in turn could feed back negatively on the trees' capacity to fix N. We studied the effects of Acacia zanzibarica tree density upon soil and foliar N:P stoichiometry, and the N2-fixation rates of trees and leguminous herbs in a humid Tanzanian savanna. Foliar N:P ratios and N2-fixation rates of trees remained constant across the density gradient, whereas soil C, N and organic P pools increased. In contrast, the N:P ratio of grasses increased and N2-fixation rates of leguminous herbs decreased with increasing tree density, indicating a shift towards more P-limited conditions for the understory vegetation. These contrasting responses suggest that trees and grasses have access to different sources of N and P, with trees being able to access P from deeper soil layers and perhaps also utilizing organic forms more efficientl

Elephants Mitigate the Effects of Cattle on Wildlife and Other Ecosystem Traits: Experimental Evidence

On rangelands worldwide, cattle interact with many ecosystem components, most obviously with soils, plants, and other large herbivores. Since 1995, we have been manipulating the presence of cattle, mesoherbivores, and megaherbivores (elephants and giraffes) in a series of eighteen 4-ha (10-acre) plots at the Kenya Long-term Exclosure Experiment. We have demonstrated a wide array of cattle effects on this savanna rangeland, including their reduction of grass cover, wildlife use, and soil nitrogen and phosphorus pools, but their increase of primary productivity and termite abundance. Strikingly, we demonstrate that the presence of mega-herbivores (elephants, mainly) reduces the sizes of these cattle. We provide further experimental evidence that this may be because the elephants are reducing the most desirable (N-rich) forage, causing cattle to slow their extraction of (low-N) grasses, while simultaneously reducing tree cover

Nutrient Availability Controls the Impact of Mammalian Herbivores on Soil Carbon and Nitrogen Pools in Grasslands

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

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

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.National Science Foundation Research Coordination Network, Long Term Ecological Research, Institute on the Environment, Strategic Resources of the Netherlands Institute of Ecology, Research Foundation Flanders, VENI grant, NWO-RUBICON grant, NWO-VENI grant, German Centre for Integrative Biodiversity Research, German Research Foundation (FZT 118).http://wileyonlinelibrary.com/journal/gcbpm2021Mammal Research InstituteZoology and Entomolog

Data from: A stoichiometric perspective of the effect of herbivore dung on ecosystem functioning

Ungulate herbivores play a prominent role in maintaining the tree–grass balance in African savannas. Their top-down role through selective feeding on either trees or grasses is well studied, but their bottom-up role through deposition of nutrients in dung and urine has been overlooked. Here, we propose a novel concept of savanna ecosystem functioning in which the balance between trees and grasses is maintained through stoichiometric differences in dung of herbivores that feed on them. We describe a framework in which N2-fixing trees and grasses, as well as ungulate browsing and grazing herbivores, occupy opposite positions in an interconnected cycle of processes. The framework makes the testable assumption that the differences in dung N:P ratio among browsers and grazers are large enough to influence competitive interactions between N2-fixing trees and grasses. Other key elements of our concept are supported with field data from a Kenyan savanna

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

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.This article is published as Sitters, Judith, ER Jasper Wubs, Elisabeth S. Bakker, Thomas W. Crowther, Peter B. Adler, Sumanta Bagchi, Jonathan D. Bakker et al. "Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands." Global Change Biology (2020). doi: 10.1111/gcb.15023.</p

An operational framework for the advancement of a molecule-to-biosphere stoichiometry theory

Biological stoichiometry is an approach that focuses on the balance of elements in biological interactions. It is a theory that has the potential to causally link material processes at all biological levels—from molecules to the biosphere. But the lack of a coherent operational framework has so far restricted progress in this direction. Here, we provide a framework to help infer how a stoichiometric imbalance observed at one level impacts all other biological levels. Our framework enables us to highlight the areas of the theory in need of completion, development and integration at all biological levels. Our hope is that this framework will contribute to the building of a more predictive theory of elemental transfers within the biosphere, and thus, to a better understanding of human-induced perturbations to the global biogeochemical cycles

Nitrogen and phosphorus concentrations in forage and herbivore dung

The data file lists the nitrogen and phosphorus concentrations of the forage and herbivore dung used in Figure 1 in the paper. The data file consists of 3 sheets. Sheet 1 is the meta data explaining the column names in sheet 2 and 3. Sheet 2 contains the forage nutrient data of 4 N-fixing tree species (i.e., Acacia (Senegalia) brevispica, A. mellifera, Acacia (Vachellia) drepanolobium, and A. gerrardii) and 4 C4-grass species (i.e., Brachiaria lachnantha, Pennisetum mezianum, P. stramineum, and Themeda triandra). Sheet 3 contains the dung nutrient data of 5 browser species (i.e., Guenther’s dikdik, eland, giraffe, greater kudu and Thomson’s gazelle) and 4 grazer species (African buffalo, Burchell’s zebra, Grevy’s zebra, and common warthog)