Study On Carbon And Nitrogen Distribution Pattern And Cycling Process In An Alpine Meadow Ecosystem Under Different Grazing Intensity

Long-term overgrazing has resulted in considerable deterioration in alpine meadow of the northwest Sichan Province. In order to explore management strategies for the sustainability of these alpine meadows, we selected four grasslands with different grazing intensity (no grazing-NG: 0, light grazing-LG: 1.2, moderate grazing-MG: 2.0, and heavy grazing-HG: 2.9 yaks ha^-1) to evaluate carbon, nitrogen pools and cycling processes within the plant-soil system in Waqie Village, Hongyuan County, Sichuan Province.1. Grazing obviously changed the plant species composition, especially on dominant plant species. Total number of species is 22, 23, 26, and 20 for NG, LG, MG and HG, respectively. Vegetation coverage under different grazing intensity ranked in the order of 96.2% for HG＞93.6% for MG＞89.7% for LG＞73.6% for NG. The dominator of HG community shifted from grasses-Roegneria nutans and Deschampsia caespitosa dominated in the NG and LG sites into sedges-Kobresia pygmaea and K. setchwanensis. At the same time, with the increase of grazing intensity, the numbers of forbs, such as Ranunculus brotherusii, Stellera chamaejasme, Potentilla anserine and Plantago depressa, increased with grazing intensity.2. Over the growing season, aboveground and belowground biomass showed a single peak pattern with the highest biomass in August. For each month, aboveground biomass usually was the highest in the NG site and lowest in the HG site. Belowground biomass showed a trend of increase as grazing intensity increased and it was significantly higher in the HG and MG site than in the NG and LG sites. Total plant biomass averaged over the growing season is 1543, 1622, 2295 and 2449 g m^-2 for NG, LG, MG and HG, respectively. The proportion of biomass to total plant biomass for NG, LG, MG and HG is 88%, 82%, 76% and 69%, respectively. Higher allocation ratio for is an adaptive response of plant to grazing.3. Carbon and nitrogen storage in plant components followed the similar seasonal patterns as their biomass under different grazing intensities. Generally, heavy grazing significantly decreases aboveground biomass carbon and nitrogen compared to no grazing. Carbon and nitrogen storage in root tended to increase as grazing increased and they are significantly higher in the HG and MG sites compared to the LG and NG site. Total Carbon storage in plant system averaged over the growing season is 547, 586, 847 and 909 g m^-2 for NG, LG, MG and HG, respectively, while 17, 17, 23 and 26 g m^-2 for nitrogen. The proportion of carbon storage in root to total plant carbon for NG, LG, MG and HG is 88%, 82%, 76%, 69%, respectively, while 65%, 71%, 70% and 79% for nitrogen.4. Carbon storage in soil (0-30cm) decreased slightly in July, then increased in August and peaked in September. Nitrogen storage in soil tended to increase with season and grazing intensity. Total Carbon storage in soil averaged over the growing season is 9.72, 10.36, 10.62 and 11.74 kg m^-2 for NG, LG, MG and HG, respectively, while 1.45, 1.56, 1.66 and 1.83 for nitrogen. The proportion of carbon (nitrogen) storage in soil to plant-soil system carbon (nitrogen) storage for NG, LG, MG and HG is more than 90%, which is not markedly different among different grazing intensities.5. Gross nitrification, denitrification, CO₂ and N₂O flux rates in soil increased from June to July and then declined until September, all of which tended to increase with the increase of grazing intensity. Generally, heavy and moderate grazing intensity significantly enhanced these process compared to no and light grazing intensity.6. After decomposing in situ for a year, relative weight, carbon and nitrogen loss in the litter of Roegneria nutans and Kobresia setchwanensis tended to increase as grazing intensity increased. Under the same grazing intensity, relative weight, carbon and nitrogen loss in the litter of Kobresia setchwanensis were higher than these in the litter of Roegneria nutans.7. Although heavy grazing intensity resulted in higher levels of carbon and nitrogen in plant and soil, it decreased vegetation coverage and aboveground biomass, which are undesirable for livestock production and sustainable grassland development. What is more, heavy grazing could also introduce potential carbon and nitrogen loss via increasing CO₂ and N₂O emission into the atmosphere. Grazing at moderate intensity resulted in a plant community dominated by forage grasses with high aboveground biomass productivity and N content. The alpine meadow ecosystems in Tibetan Plateau are very fragile and evolve under increasing grazing intensity by large herbivores; therefore, deterioration of the plant-soil system, and possible declines in soil C and N, are potential without proper management in the future.