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.

研究目的：放牧可改变碳（C）、氮（N）与磷（P）循环过程，进而影响草地生态系统的C:N:P化学计量比（stoichiometry）。本研究旨在探究放牧强度对草地C:N:P化学计量比产生影响的潜在机制，重点关注地下生态过程及其与地上植被特征的关联。
研究区域：全球范围。
时间跨度：1900年—2018年。
研究对象类群：草地生态系统（grassland ecosystems）。
研究方法：本研究基于129项已发表的研究开展元分析（meta-analysis），综合整合放牧对草地生态系统中叶片、茎秆、枯落物、根系、微生物生物量以及土壤的C:N:P化学计量比的影响效应。
研究结果：放牧显著改变了草地生态系统的C、N、P库，进而改变其C:N:P化学计量比；放牧对C:N:P化学计量比的影响随放牧强度差异呈现显著分化。具体而言，重度放牧会降低所有C:N:P化学计量比指标，仅枯落物N:P比与根系C:N比除外；而轻度与中度放牧则表现出更弱的负效应或正效应。在轻度与中度放牧条件下，放牧对枯落物C:N比的影响与对土壤C:N比的影响呈负相关，但在重度放牧条件下二者呈正相关。与之相反，放牧对根系C:P比的影响与土壤C:P比的影响在轻度与中度放牧条件下呈正相关，而在重度放牧条件下呈负相关。值得注意的是，放牧使土壤N库显著降低10.0%，但使土壤P库提升3.6%，这表明放牧对草地N、P循环的影响机制存在差异。
主要结论：轻度、中度与重度放牧对C:N:P化学计量比的差异化影响，凸显了放牧强度在调控草地C、N、P生物地球化学循环中的重要性——其通过提升植物养分利用效率，以及诱导土壤物理化学过程改变来实现调控。因此，将放牧强度纳入地球系统模型（Earth system models），可改善人类世（Anthropocene）背景下气候-草地反馈关系的预测精度。