Effects Of Grazing Intensity On Foliar C,N,P Stoichiometry And Nutrient Resorption In Alpine Meadow Plants

The alpine grasslands on the Tibetan Plateau are degrading due to the impacts of global climate change and overgrazing.As a result,several severe consequences have arisen.Carbon(C),nitrogen(N)and phosphorus(P)play crucial role in sustaining grassland ecosystems.Previous studies have shown that plants can enhance nutrient use efficiency,including N and P,through resorption.However,limited research has been conducted on how different grazing intensities affect plant foliar elements.Thus,this study was conducted at the grazing experimental platform of the Haibei Station on the Tibetan Plateau to investigate the response of foliar C,N and P stoichiometry of 13 species to different yak grazing intensities,including no grazing(CK),light grazing(LG),medium grazing(MG)and heavy grazing(HG).Carbon,nitrogen and phosphorus content were determined using an elemental analysis and changes in their stoichiometric characteristics and foliar N and P resorption were analyzed under different grazing intensities.This study aimed to gain an in–depth understanding of the nutrient use strategies of different species,explore the response and mechanisms of plant ecological stoichiometry and resorption to grazing intensity at three levels: species,functional groups and communities.This research is of great significance in revealing the process and mechanism of grassland degradation caused by grazing.The key results of this study are:1.At the species level,the mature leaf carbon concentration of Saussurea pulchra increased with grazing intensity and was significantly higher under heavy grazing compared to the control,while the carbon concentration of senescent leaves of Oxytropis kansuensis,Gentiana straminea and Saussurea nigrescens were significantly decreased in heavy grazing.The nitrogen concentration of mature leaves of Kobresia humilis,Festuca rubra,Deyeuxia flavens,G.straminea,S.pulchra and S.nigrescens increased significantly under treatment.Additionally,nitrogen concentration of senescent leaf of S.nigrescens increased significantly under grazing treatments.Meanwhile,from the perspective of species leaf stoichiometric characteristics,grazing significantly reduced the carbon and nitrogen ratios of mature leaves of K.humilis,D.flavens,G.straminea,S.pulchra and S.nigrescens.light and heavy grazing increased the carbon and phosphorus ratios of mature leaves of K.humilis and Stipa aliena,but medium grazing decreased the carbon and phosphorus ratios.In addition,for nitrogen resorption efficiency,the nitrogen resorption efficiency of F.rubra under medium grazing was significantly higher than that of heavy;while S.nigrescens was significantly lower than that of heavy.Nitrogen resorption efficiency of D.flavens was lower under light grazing than medium and heavy grazing,and nitrogen resorption efficiency of G.straminea was significantly higher under heavy grazing than light grazing;Morina chinensis was significantly higher under heavy grazing than other treatments.For phosphorus resorption efficiency,K.humilis was significantly higher under medium grazing than heavy grazing;Phosphorus resorption efficiency of Carex przewalskii and D.flavens under medium grazing was significantly higher than that of light;In contrast,M.chinensis showed significantly higher grazing under heavy grazing than control.2.At the functional group level,grazing did not have a significant effect on the carbon and phosphorus concentrations in mature and senescent leaves of functional groups,but it did have a significantly effect on the nitrogen concentrations in functional group leaves.Specifically,the treatment significantly reduced mature and senescent leaves of legume,while senescent leaves of grass showed a significant increase in light grazing.Regarding functional group leaf C:N ratios,grazing significantly reduced the mature and senescent leaf C:N ratios of sedge plants,and the mature leaf C:N ratios of grass were significantly reduced under heavy grazing.However,there were no significant differences in the functional group C:P and N:P ratios between treatments.In addition,grazing has no significant impact on the nutrient resorption efficiency at the functional group level.3.At the community level,grazing did not have a significant effect on the carbon concentrations in plant community leaves,but it significantly increased nitrogen concentrations in both mature and senescent plant leaves.Mature and senescent leaves of plant communities were significantly increased in the medium and heavy grazing treatments compare to the control and light grazing.The carbon to nitrogen ratio of mature and senescent leaves of the plant community decreased with increasing grazing intensity,and the carbon to phosphorus ratio of mature leaves decreased significantly in medium grazing.However,the nitrogen to phosphorus ratio of mature and senescent leaves increased significantly in light and heavy grazing.Besides,grazing has no significant impact on the nutrient resorption efficiency at community level.In conclusion,grazing disturbance affects plant growth by altering plant leaf element concentrations and plant nutrient use efficiency.Therefore,investigating the effects of grazing on the leaf metric characteristics and nutrient resorption of grassland plants provides a theoretical basis for plant recovery after grazing.