Grazing intensity significantly changes the C:N:P stoichiometry in grassland ecosystems

Aim: Livestock grazing can alter carbon (C), nitrogen (N) and phosphorus (P) cycles, thereby affecting the C:N:P stoichiometry in grasslands. In this study, we aimed to examine the underlying mechanisms for the impacts of grazing intensity on grassland C:N:P stoichiometry, especially for the belowground processes and their linkages with aboveground vegetation properties.

Location: Global.

Time period: 1900-2018.

Major taxa studied: Grassland ecosystems.

Methods: Here, we conducted a meta-analysis based on 129 published studies to synthesize the effects of grazing on the C:N:P stoichiometry of leaves, stems, litter, roots, microbial biomass, and soil in grassland ecosystems.

Results: Grazing significantly affected the C, N and P pools, and then the C:N:P stoichiometry in grassland ecosystems. Grazing effects on C:N:P stoichiometry varied strongly with grazing intensity. Specifically, heavy grazing decreased all C:N:P stoichiometry except litter N:P and root C:N ratios, while light and moderate grazing exhibited the less negative or positive effects. Grazing effects on litter C:N ratio were negatively correlated with grazing effects on soil C:N ratios under light and moderate grazing, but this relationship was positive under heavy grazing. In contrast, the correlation between grazing effect on root C:P and soil C:P was positive under light and moderate grazing but negative under heavy grazing. Importantly, grazing significantly decreased soil N pool by 10.0% but increased P pools by 3.6%, indicating differential mechanisms for grazing impact on N and P cycles in grasslands.

Main conclusions: The divergent effects of light, moderate, and heavy grazing on the C:N:P stoichiometry highlight the importance of grazing intensity in regulating the biogeochemical cycles of C, N, and P by accelerating plant nutrient use efficiency and inducing changes in soil physicochemical processes in grassland ecosystems. Therefore, incorporating grazing intensity into Earth system models may improve predictions of climate-grassland feedbacks in the Anthropocene.

Methods
We searched for peer-reviewed papers published before June 2018 in Web of Science and China Knowledge Resource Integrated Database (CNKI) with the combinations of search terms as: (grazing OR herbivory OR defoliation) and (C:N OR C:P OR N:P OR C:N:P) and (grassland OR pasture OR meadow) to compile the dataset.