| The Qinghai-Tibetan Plateau is a geographically expansive and unique region with a potentially large impact on global greenhouse gas budgets. It remains largely unknown how livestock grazing affects the soil-to-atmosphere flux of carbon dioxide (CO2) and methane (CH4) in such arid alpine regions.In this study we quantified the diurnal and seasonal pattern of CO2 and CH4 fluxes in the high-altitude Qinghai-Tibetan Plateau for ungrazed pasture and for pastures subjected to low or moderate sheep stocking rates.In this study, we quantified the diurnal and seasonal patterns of both CO2andCH4 fluxes of the grazed meadow by Tibetan sheep in the Tibetan Plateau. Our measurements were conducted based on three gredients of grazing rate:0,8, and 16 sheep/ha, three seasons:warm season (July-August) in 2010 and 2011, cold season (November-Decemeber) in 2010 and 2011, and spring (April-May) in 2011 and 2012. The main results were listed as follows.At warm seasons, soil absorbed methane. Absorption trend of all treatments were increased firstly and then reduced. The maximum and minimum absorption effluxes appeared in 4:00 am and 14:00. Under 16 sheep/ha, soil methane absorption was the highest. The pasture also absorbed metnane in each treatment, the maximum and minimum absorption appeared in 0:00: 00-4:00 and 12:00-14:00, and the grazing improved methane absorption. In cold seasons, soil and pasture emitted methane at daytime and absorbed at night under 0 and 8 sheep/ha treatment. The maximum and minimum absorption flux of soil appeared in 0:00-4:00 and 8:00-10:00. Soil still presented methane absorption in cold season, and grazing increased the absorption flux. Pasture methane absorption under 16 sheep/ha was greater than others. Maximum absorption appreared at 0:00. In spring, soil and pasture emitted methane at daytime and absorbed at night. Under 16 sheep /ha, soil presented methane absorption. Gross efflux presented methane absorption under 16 sheep /ha.For carbon dioxide flux, the max and min emission of soil in warm season appears in 4:00 am and 14:00. Pasture got the max emission under 16 sheep/ha, the max and min emission flux appears in 14:00 and 0:00-4:00. In cold season, the maximum and minimum emission flux of soil appears in 14:00 and 4:00, pasture missions was 16> 0> 8 sheep/ha, the max and min emission flux appears in 14:00-16:00 and 4:00. In spring, soil daily cumulative emissions flux was 16> 8> 0 sheep/ha, the max and min emission flux appears in 12:00-14:00 and midnight.There is no significant relationship between soil, pasture, vegetation methane fluxes and 5 cm soil temperature in different seasons. But for CO2, soil presented a positive relationship with 5cm soil temperature and air temperature in warm season; in the cold season, only significant with air temperature; in spring, significantly positive correlation was observed between 5cm soil temperature and flux, and grazing increased the correlation, all of the treatments showed a positive correlation with air temperature.In warm season, methane fluxes had a negatively correlated with 0-5 cm soil microbial carbon, nitrogen content, but had a positive correlation with 5-10 cm indexes. In cold season, Pasture methane flux had a positively correlated with 0-5 cm and 5-10 cm soil microbial carbon content, and a negatively correlation with 0-5 cm soil microbial nitrogen content, soil methane flux was negatively correlated with 0-5 cm,5-10 cm soil microbial carbon content and positively correlated with 0-5 cm,5-10 cm soil microbial nitrogen content. In spring, pasture methane flux was positively correlated with 0-5 cm soil microbial carbon and nitrogen content, while soil microbial methane flux negatively correlated with 0-5 cm carbon content, and positively correlated with 0-5 cm soil microbial nitrogen content. Both of pasture and soil methane fluxes were negatively correlated with 5-10 cm soil microbial carbon and nitrogen content.In warm season, pasture carbon dioxide fluxes was negatively correlated with 0-5 cm soil microbial carbon content, and positively correlated with 5-10 cm soil microbial nitrogen content while soil flux was positively correlated with 0-5 cm soil microbial carbon content, and negatively correlated with 5-10 cm soil microbial nitrogen content. Both pasture and soil were negatively correlated with 5-10 cm soil microbial carbon content, and positively correlated with 0-5 cm soil microbial nitrogen content. In cold season, pasture was positively correlated with 0-5 cm,5-10 cm soil microbial carbon content, and 0-5 cm soil microbial nitrogen content. Soil was negatively correlated with 0-5 cm,5-10 cm soil microbial carbon content, and negatively correlated with 0-5 cm soil microbial nitrogen content. Both pasture and soil were positively correlated with 5-10 cm soil microbial nitrogen content. In spring, pasture was negatively correlated with 0-5 cm soil microbial carbon content, positively correlated with 0-5 cm carbon content and negatively correlated with 5-10 cm soil microbial nitrogen content, while soil was positively correlated with 0-5 cm soil microbial carbon and nitrogen content, and positive correlation with 5-10 cm soil microbial nitrogen content. Both pasture and soil were positively correlated with 5-10 cm soil microbial carbon content.Gas fluxes were near zero in sub-zero temperatures in winter. In spring, soil and herbage respiration was lower than at similar temperatures in summer. Most statistically significant effects were detected in summer when gas fluxes were higher. In summer, grazing significantly reduced soil and herbage respiration, and herbage emission of CH4 was confirmed. Soil respiration was more strongly related to air temperature than to soil temperature, while for soil CH4 oxidation the reverse was true. The combined total for soil and herbage respiration ranged from 28 t CO2 ha-1 yr-1 on ungrazed plots to 33 t CO2 ha-1 yr-1 at the higher sheep stocking rate. By comparison, net herbage production at the experimental site is estimated to be about 3.0 t DM ha-1 yr-1. This suggests that warming temperatures in the region may be triggering oxidation of soil organic carbon (SOC) but this point requires further study. Direct emissions by grazing sheep of CO2 and CH4 were estimated from published relationships based on animal bodyweight, and it is concluded that the questions that will determine appropriate future management of Tibetan grasslands are similar to those for more productive lowland grasslands. Future research focus should be moved towards understanding factors affecting soil carbon sequestration in Tibetan grasslands. |
PDF Full Text Request