|Other Abstract||In order to solve the grass-livestock contradiction on the Tibetan plateau, the ecological livestock cooperative business model has been developed faster on the Tibetan plateau in recent years with the support of national policies. Grassland animal husbandry in ecological livestock cooperatives has the characteristics of ecological intensification, then sustainable nutrient management needs to be incorporated into the ecological livestock cooperative system. For most alpine grassland livestock systems, nitrogen is one of the most important factors limiting productivity. Therefore, studying the dynamics and changes of nitrogen storage in "soil-grass-livestock-water", which are operated by ecological livestock cooperatives, is important for the rational use of grassland, scientific fertilization, and replenishment of grassland nitrogen in ecological livestock cooperatives. It is an important scientific basis for the sustainable operation of ecological livestock cooperatives, which is to maintain the nitrogen balance of grassland and maintain the sustainability of grassland productivity.
In this study, the soil, plants, livestock manure, and surrounding water bodies in the grassland and livestock systems of three typical ecological livestock cooperatives and their control herders in The Three-River Source Region of the Qinghai-Tibet Plateau were selected as the research objects, and the total nitrogen content and stable nitrogen isotope δ15N values in these components were measured. The aim is to reveal the dynamic changes of nitrogen storage through the operation and circulation process of nitrogen in each link of "soil-grass-animal-water"and to trace the source and flow direction of nitrogen in different links by using 15N natural isotope technology. The main results are as follows. The main results of the study are as follows.
(1) The seasonal dynamics of total nitrogen, total soluble nitrogen and soluble inorganic nitrogen (ammonium nitrogen and nitrate nitrogen) in grassland soils showed inverted "V"changes, while the seasonal dynamics of soluble organic nitrogen storage showed opposite changes.
(2) The intensive management of alpine grassland animal husbandry significantly increased the soil nitrogen storage of natural grassland (up to 9421.9 kg/ha), and the balance between vegetation nitrogen uptake and soil inorganic nitrogen could be maintained throughout the growing season, and inorganic nitrogen was mainly NH4+-N, which improved the nitrogen utilization.Both degradation of ecological livestock cooperatives'grasslands and reclaimed cultivation will lead to a decrease in soil nitrogen storage and reduce grassland productivity. Although the vegetation nitrogen uptake and soil inorganic N supply of degraded grassland can basically reach a balance between supply and demand, it can be clearly reflected from the experimental data that soluble inorganic N accounts for the vast majority of total soluble N (82.32% and 72.61% of total soluble inorganic N in the soil of cooperatives and herders'single camps, respectively), making the fixed inorganic N in grassland soil gradually increase and become ineffective consumption . Although the nitrogen uptake and soil inorganic N supply of the restored grassland after artificial planting can also basically reach the balance of supply and demand, it can be seen by comparing the ratio of two kinds of inorganic N storage to total soluble N. The NO3--N (49.50%) of the artificial grassland in the pastoralist single-camp model is significantly higher than NH4+-N (8.12%), and the grassland is poorly permeable;the cooperative's artificially planted oatland is not well permeable from late July to Before harvesting in late July to late September, the supply of soil inorganic N to plant N uptake was significantly insufficient.
(3) Livestock manure was the main source of NO3-in the four rivers around its grassland. For rivers flowing through fertilized artificial grassland, NO3-in fertilizer (or NO3-produced by mineralization of NH4+) was the second largest source of NO3-in the water body;while for the non-fertilized grassland, NO3-produced by organic nitrogen in the soil through mineralization and nitrification and other reactions was the The second largest source of NO3-in the water body.
(4) In the grassland ecosystem underthe two management modes, the δ15N enrichment factor of pasture-soil stage ranged from 0.549‰to 2.932‰;the δ15N enrichment factor of livestock-soil stage ranged from 1.128‰to 2.293‰;and the δ15N enrichment factor of water-soil stage ranged from 3.674‰to 4.340‰. In contrast, the enrichment in the process of rising pasture-livestock trophic level showed diametrically opposite effects between two different grassland types, degraded grassland and natural grassland: the stable N isotopes in the pasture-livestock stage of degraded grassland showed extremely weak or even negative enrichment, while the livestock in natural grassland enriched the N stable isotopes from pasture.
(5) By comparing the nitrogen balance accounts of three grassland types (nearly undegraded natural grassland, artificial grassland and degraded natural grassland) under the cooperative model and the herding monoculture model, it can be seen that there are large differences in nitrogen input and output per unit area among different grassland types under the cooperative model, especially the nitrogen utilization rate of the rangeland is lower and the overall state of nitrogen surplus is shown;while the nitrogen utilization rate of the three grassland types under the herding monoculture model, although In contrast, although the nitrogen utilization rate of the three grassland types under the herding monoculture model was higher than that of the cooperatives, the overall state of nitrogen deficit was observed, and the productivity of the grassland was lower.
In summary, the development of ecological livestock cooperatives in The Three-River Source Region not only realized the integration of human, grass and livestock to a certain extent, which made the local grassland livestock industry develop in the direction of scale and intensification, increased the income structure of herders and drove the development of the local economy, but also improved the nutrient storage capacity of natural grassland through the measure of "returning grazing to grass", and optimized the nitrogen storage capacity of natural grassland. It also improves the nutrient storage capacity of natural grassland through the measure of "returning pasture to grass", optimizes the nitrogen balance account, and improves the productivity of grassland. In the process of future cooperative scale development, we should pay attention to the nutrient management of degraded grassland and the improvement of the comprehensive level of grassland, optimize the management of artificial grassland fertilization and livestock manure, attract more herding households to join the cooperative, and strengthen the monitoring of nutrients in the surrounding river water bodies to prevent the potential environmental risks caused by excessive intensive animal husbandry, and promote the ecological livestock cooperative to realize ecological intensive management, so as to realize The sustainable development of the cooperative in The Three-River Source Region of the Tibetan plateau.|