Research On Soil Nitrogen Storage And N₂O Flux Characteristics Of Artificial Alfalfa Grassland With Different Planting Years

In2012, during the period from April to October, by taking samples from artificial alfalfa grasslands which had been planted for1,3,4,5and8ages along the eastern foot of Helan Mountain in Ningxia. Soil nitrogen storage and its vertical distribution characteristics at the depths of0-10,10-20,20-40,40-60,40-60,60-80and80-100cm were analyzed. And the N2O flux at the sampled lands were measured by using static enclosed chamber-GC/MS technique. Upon these measurements, the influence factors of N2O fluxes were discussed which intensed N2O emission by measuring the contents of TN, NO3-N, NH4-N at20cm depth of soil, biological soil environmental factors, weather factors and the aboveground biomass of alfalfa. The main results showed as following:(1) With the planting ages increasing, soil nitrogen content and nitrogen density increased significantly (p<0.05), the order of the content and density of nitrogen in the0-100cm soil layer were4a>3a>5a>la>8a, and both showed significant differences (p<0.05) between the different planting years. Soil profile nitrogen content mainly showed a significant accumulation in the soil depth0to40cm, accounted for63.76%-67.11%from0to100cm, soil profile nitrogen density showed the same accumulation in the soil depth0to40cm, accounted for65.36%～68.05%. The soil vertical distribution characters of concentration and density of nitrogen showed the surface gathering.(2)The soil nitrogen content and density of different sample area at the end of the growing season is higher than than the begining, but the nitrogen fixation rate drops with the increase of growing years. In the whole growing season, the rate of nitrogen conten with alfalfa planted from la to8a are52.37%、30.57%、24.66%、0.15%、-21.60%. It shows when we plant alfalfa to5years, the alfalfa nitrogen-fixing ability began to decline, and if the growing age continues to increase, it could consume nitrogen content of soil.(3) In the growing season, soil nitrate and soil ammonium content of different sample area in the0-20cm soil layer decreased as the soil layer deepen. Soil nitrate content accounted for0.86%～1.14%nitrogen content of soil, soil ammonium content accounted for0.44%～1.03%nitrogen content of soil. The change of soil nitrate conten related to greenhouse gas N2O emissions variation, so it is a major nitrogen component which affects N2O emissions.(4) The N2O fluxes of soil system and soil-vegitation system of alfalfa grassland showed a similar regular peak curve with the maximum in spring and summer and the minimum value in autumn, the followed the pattern with the1year<8year<3year<5year<4year. The seasonal variations of N2O emissions in soil-vegetation system and aboveground biomass have some synchronization.It shows the characteristics of Nitrogen source in general.(5) The average of N2O fluxes of soil-vegetation and soil system in observation was0.031-0.050mg·m-2·h-1and0.022-0.048mg·m-2·h-1, indicating that the plant itself contribute to N2O emissions. After planting for3,4,5,8ages, the average of N2O flux of soil-vegetation system increased by36.08%-58.89%at each observation, and the average N2O flux of soil system increased by 13.88%-114.35%respectively. It shows that soil system is more untable than soil-vegetation, the soil system is easy to affect by exernal factors, lead to the change of N2O emissions, and the soil nitrogen components will change with alfalfa growing.(6) In the growing season, soil-vegitation system and soil system of alfalfa pasture with different planting years have two peaks of N2O flux.First, because when returning green stage is coming in May, soil moisture result from freezing and thawing of soil resolving with rise of temperature increased sharply, the N2O emissions increase. Second, the raising of soil temperature because of direct sunlight enhancement promote N2O emission to increase, in addition to the strogen growth of plants also have some impace.The absorption of N2O at the end of September, is due to the fact that all these factors from the lowering of temperature and soil temperature, the dry environment caused by reducing soil moisture,stopping growing of aboveground biomass and nitrogen fixation of alfalfa affect reciprocally.(7) The flux of N2O was closely significantly positively correlated with the contents of nitrate nitrogen, the flux of N2O was significantly positively correlated with the contents of ammonium nitrogen, but they have no significant correlation of other soil components. It means the N2O flux would be intensified by the increase of nitrate nitrogen and ammonium nitrogen concentrations after planted when environmental factors were similar.(8) In the season range, the N2O flux was closely significantly positively correlated with0-5cm soil temperature, it increases with rising temperature. N2O flux and10cm soil moisture content had a significant positive correlation, in observing period, when soil moisture content reach to45%, the emission of N2O appears peak with hysteresis. So the soil temperature and soil moisture content were very important factors to N2O flux in soil-vegetation system.(9) The flux of N2O was significantly positively correlated with content of aboveground biomass, the correlation coefficient is0.912, the regression equation is y=0.002x+0.225(P<0.05, R2=0.799), meanwhile, the flux of N2O in soil-vegetation system is higher than it in soil system, so, the growth of aboveground biomass was also a very important factor to N2O emission. It is one of the Nitrogen source of N2O emission.To sum up, could come to the conclusion:The alfalfa pasture lands showed significant soil nitrogen, soil nitrate and soil ammonium sequestration and had a great potential of them. Vertical distribution characters of concentration and density of soil nitrogen, soil nitrate and soil ammonium showed the surface gathering. The N2O fluxes of soil-vegitation system and soil system of alfalfa grassland have a same trendency with the maximum in spring and summer and the minimum in autumn, and they followed the same pattern with the la<8a<3a<5a<4a, overall showed Nitrogen source. There were a great many factors such as soil and environment affected N2O emission.