Emissions Of N₂O From Alfalfa Grassland And Affecting Factors In The Loess Plateau

Alfalfa(Medicago sativa L.) is known as the "Queen of grass" because it has excellent feeding value, and is plays an important role in the protection of grassland ecological security and food safety in our country, however, few studies of the status quo for N2O emission directed to cultivated grassland on the arid areas.In this paper, different aged alfalfa grasslands were studied on the Loess Plateau. N2O emission flux was measured at different phenological stages, using a static-enclosed chamber. Day and seasonal dynamics of N2O emissions were determined using LGR gas analyzers. Abiotic factors and microbial factors were deeply analyzed, combining the test grassland to explore the relationship between N2O emissions and associated microbial activity in a legume-soil system, therefore, which could provide a scientific basis to determine N2O emission for other agricultural systems. The main results showed:1. N2O emission flux increased with increasing growth years on Longdong alfalfa grasslands. The N2O emission flux of3and4and9-year old alfalfa grassland and fallow land ranged from-0.19-0.52mg/(m2.h) through all phonological stages, which was much lower than wetland ecosystems and the international average of emission level0.627mg/(m2■h). Longdong alfalfa grassland in table land was found to be a weaker N2O emission source on the Loess Plateau. The grassland absorbed N2O at re-growth before overwintering and fallow period on3and4-year old alfalfa grassland and leisure land, which became the N2O repository.2. The amounts of nitrite bacteria, nitrate bacteria and denitrifying bacteria had obvious seasonal dynamics and spatial distribution. The surface soil microbial biomass carbon and nitrogen content and microbial quotient increased with increasing growth years on Longdong alfalfa grassland. The change trends of microbial biomass, carbon and nitrogen content and microbial quotient were identical in the0-10cm soil layer, and interaction between the three factors together constituted an important indicator system to measure the micro-environment and soil fertility status.3. Soil moisture, soil temperature and soil ammonium, nitrogen and nitrate content were four major abiotic factors that affected N2O emissions from the alfalfa grassland, however soil moisture played the most important role of the four major abiotic factors. Significant linear correlation was found between N2O emission flux and soil moisture content, soil temperature, soil pH value, soil nitrate content and the amounts of nitrate and denitrifying bacteria, which showed a significant logarithmic function relationship with microbial biomass carbon and nitrogen and microbial quotient.4. N2O emission showed significant day dynamics in the Longdong alfalfa grassland throughout the year, N2O emission flux of the largest peaked at17:00and03:00whether rainy and cloudy or sunny; N2O emission showed significant seasonal dynamic, in which absorption and release peak respectively at freezing and thawing period. Furthermore, N2O emission flux of the dry season was higher than rainy season and the growing season was higher than the non-growing season.