Studies on the Effects of grazing intensity on Vwgetation and Soil in Alpine Meadow on the eastern Qinghai-Tibetan Plateau
|Keywords||grazing intensity alpine meadow Qinghai-Tibetan Plateau plantfunctional traits soil physical and chemical properties nitrogenmineralization nitrogen nitrification|
In grassland ecosystem, grazing is one of the most important biotic disturbances, which influences the species composition and biodiversity. Therefore, exploring the response of vegetation to grazing has been identified as a critical research area in grassland ecology. Further-more, grazing significantly changes the content and cycle of soil nutrient, especially carbon, nitrogen and phosphorus, which in turn affects the fuction of grassland ecosystem. Many studies have proved that grazing intensity, time and history are important factors that determine how grazing changes grassland communities, especially grazing intensity. A large number of previous studies used only paired sites (grazed vs. ungrazed), and ignored the effects of different grazing levels. This may have concealed nonlinear responses of plants to increased grazing. Moreover, many studies on the effects of grazing intensity have focused on temperate ecosystems, but there is little information on alpine meadow on Qinghai-Tibetan plateau with the high altitude, cold, and sensitive ecosystems. Therefore, according the residual biomass and earlier studies, our research chose no grazing (GO), light grazing (G1), moderate grazing (G2) and heavy grazing (G3), aimed to investigate the vegetation and soil dymatics under different grazing intensity from2005to2007, which not only complements previous studies on the effects of grazing in alpine area, but can also provide a theoretical framework for the utilization and development of Qinghai-Tibetan plateau.Our study suggest that1). During the experimental period, a total of69species belonging to17families were recorded. Apart from the47plant species occurring in no grazing (GO),9and7new species were observed in light (G1) and moderate grazing (G2), respectively. In heavy grazing (G3),31species were observed. Grazing intensity also negatively affected the total ground cover of plant community. The most pronounced decrease occurred in the third year (2007), when ground cover ranged from99.2±0.5%in GO to52.6±1.2%in G3. Simultaneously, grazing intensity significantly influenced species richness and plant density, which indicated a unimodal response of species richness along the grazing gradients, i.e. a higher species richness and plant density in light and moderate grazing than in none and heavy grazing. These results proved that fencing is an effective way to improve ground cover; however, moderate grazing can help to increase biodiversity of alpine meadow. With the consideration of grazing function, moderate grazing is the solution to resolve the dilemma between sustainable livestock production and development and maintenance of pastoral ecosystems.2). RLQ analysis, based on plant functional traits showed that, increased grazing led to a linear increase of short plants over tall plants, light seeded plants over heavy seeded plants, forbs over grasses, low and medium palatable plants over high palatable plants, annuals over perennials, prostrate over erect plants, rosette and stoloniferous plants over tussock plants. GAM analysis showed that non-linear responses were found in sedges, legumes, unpalatable, clonal, unlonal and leafy stem plants. These results showed that plant functional traits can be used to predicate the grazing response of vegetation composition and that non-linearity should be considered when analyzing functional trait-grazing relationships.3).Canonical correspondence analysis, based on species level showed that the21commone species responded differently to grazing intensity. Specifically, With increasing grazing disturbance, ten species showed reduced ground cover and two species increased ground cover; two species did not show obvious differences among fencing, light and moderate grazing, but significant decrease in heavy grazing; seven species indicated non-linear influences that increased at light and moderate, but decreased at heavy grazing. Analysis based on species levels also showed that species with same single traits exhibited different responses to grazing, which implys that the predictions solely based on single traits would give misleading results for vegetation changes under pasture management. A more thorough functional analysis of the combination of key traits is needed in future studies on the responses of vegetation to grazing.4). The results of three-year study showed that grazing intensity decrease the SOC content of0-15cm soil. The highest decrease occurred in September2007, from57.6±6.1g/kg in no-grazing to36.6±2.2g/kg in heavy grazing intensity. Furthermore, soil organic carbon storage during the plant growing seasons was also significantly reduced by grazing intensity, and heavy grazing caused SOC emission from soil to atmosphere. SOC storage was closely positively related to both aboveground residues and belowground biomass. These results indicates that a grazing-induced reduction in plant biomass productivity and changes in species composition would depress soil carbon storage, and that an increase in grazing pressure can lead to a gradual change of alpine meadow soils from being’carbon sinks’to become’carbon sources’.5). Total nitrogen in the soil was found to decrease with increasing grazing intensity. The highest decrease occurred in May2005, from4.1±0.1g/kg in ungrazed areas to3.3±0.3g/kg in heavily grazed areas. The response of total phosphorus in the soil to grazing was not clear. Soil available nitrogen rose significantly as grazing intensity went up. The highest increase occurred in September2006, from14.9±1.2mg/kg in ungrazed plots to36.4±1.2mg/kg in heavily grazed plots. Different from the response of soil available nitrogen to grazing, both light and moderate grazing increased soil available phosphorus relative to nongrazing, whereas the opposite was true for heavy grazing.6). Cumulative nitrogen mineralized during a60-day period of in situ cultivation increasely progressively with increasing grazing intensity, The cumulative nitrate nitrogen showed a similar response. Nitrogen mineralization and nitrification in alpine meadow of Eastern Qinghai-Tibetan Plateau was negatively related to soil C/N ratio, that is, both mineralized nitrogen and nitrate nitrogen decreased with increasing soil C/N raio. This negative relationship between mineralized nitrogen or nitrate nitrogen and C/N ratio was observed in both2005and2007. Our results show that soil nitrogen mineralization and nitrification are promoted by grazing but negatively affected by soil C/N ratio.