The influence of grazing on grassland ecosystem C:N:P stoichiometry

Grazing alters carbon (C), nitrogen (N) and phosphorus (P) cycles in the plant-microbial-soil continuum of grassland ecosystems. The inconsistent regulation of grazing on the C, N, and P contents of each ecosystem component may lead to the mismatch and imbalance of ecosystem C:N:P stoichiometry. Understanding the response of ecosystem C:N:P stoichiometry to grazing is critical for assessing the impact of human activities on grassland ecosystem functioning and developing sustainable grassland management strategies. However, most recent studies focused either on the aboveground or the belowground ecosystem component in response to grazing. The effects of grazing intensity on C:N:P stoichiometry of different ecosystem components and the linkage between above- and belowground components remain unclear. Furthermore, most grazing experiments are limited to a single site, a regional scale distributed grazing experimental network can lead to high-impact advances that would otherwise be unachievable. Therefore, in this study, based on the integrated database of C, N, and P contents in leaf, root, litter, soil microbial biomass, and soil of the long-term grazing network experimental platform, we analyzed the response of above- and belowground C:N:P relationship to grazing and their regional differences. Specifically, our objectives were: (a) to examine the effects of grazing intensity on C:N:P stoichiometry in each ecosystem component, (b) to quantify the effects of grassland type, climate and grazing intensity on C:N:P stoichiometry in each ecosystem component, (c) to explore the links between grazing effects on above- and belowground C:N:P stoichiometry and its regional differences. The results showed that grazing decreased the C:N:P stoichiometry of leaf, root, litter and soil. The changes of C:N:P stoichiometry in each ecosystem component were different among different grassland types. Grazing effects on C:N:P stoichiometry varied strongly with grazing intensity. Specifically, leaf C:N, root C:N, soil C:N, leaf C:P and litter C:P decreased with increased grazing intensity, whereas microbial biomass C:P (except for alpine meadow) and N:P increased. The effects of grazing intensity on ecosystem C:N:P were related to grassland type and climate. When the influences of grassland type and climate were controlled, the independent effect of grazing intensity on the variations of ecosystem C:N:P were only from 0.4% to 3.7%. Meanwhile, grazing also changed the C:N:P stoichiometric relationships of leaf vs. root and root vs. soil. Positive correlation between the natural log-transformed response ratio (LRR) of leaf C:N and LRR (root C:N) and the negative correlation between LRR (root N:P) and LRR (soil N:P) were more significant in desert steppe. Grazing has changed C:N:P linkages between above-ground and below-ground components, which suggest that the balance of C:N:P stoichiometric relationship in grazing ecosystem should be considered in sustainable grassland management.