| In ecosystem, the study of soil greenhouse gas fluxes and characteristics of carbon sink and source in global climate change is the hot spot. In order to ascertain the characteristics of soil carbon and nitrogen storage, and reveal the rule of soil greenhouse gas fluxes emission in alfalfa pasture, and provide basis of assessing the ecological effect on alfalfa pasture accurately in ecologically fragile regions, and provide theory evidence and scientific support about rational use on alfalfa pasture system. Taking a chronosequence of alfalfa plantations at1,3,4,5, and8age as the subject, this paper had the studies that planting year and soil depth had the effects on soil bulk density, soil nutrients, soil carbon and nitrogen storage characteristics, and soil CO2、N2O and CH4fluxes characteristics, and the main factors affected soil greenhouse gas fluxes emission by combining with field trial and laboratory experiments. The results were as followed as:1. Along with the increased age, the soil bulk density changed irregularly with relative high values and tight soil structure. Soil organic matter increased with the increased age, while decreased significantly along the soil profiles. Soil organic matter of five years increased was the most obvious. It reached the minimum at the age of eight. It showed surface accumulation for each sites and the accumulation emerged after some planting age. Soil nitrogen content indicated a strong shift along the chronosequence of this plantation, but it decreased significantly with the soil profiles. Soil nitrogen content reached the peak at the age of four. It reached the minimum at the age of eight. And soil nitrogen content mainly focused on the soil depth of0to40cm. It suggested there was a correlation of soil nitrogen content with the growth of alfalfa that improved the fixation of soil nitrogen in the soil surface. Soil nitrate nitrogen content indicated first increases and then decreases along the chronosequence of this plantation, but it decreased significantly with the soil profile. Higher soil nitrate nitrogen content was found in the surface soil at the depth of0and5cm. There was a significant correlation of soil nitrate nitrogen content with the growth of alfalfa. While soil ammonium nitrogen changed little in the soil profile. There was no significant correlation of soil ammonium nitrogen content with the growth of alfalfa. Soil total phosphorus content also indicated first increases and then decreases, and then increases along the chronosequence of this plantation, it decreased significantly with the soil profiles. And soil phosphorus content mainly focused on the soil depth of0to40cm. The surface accumulation feature was obviously after the age of planting alfalfa from one to three. The ages of three and eight planting year were the enrichment of phosphorus period, and the ages of four and five planting year were the consumption of phosphorus period. The three kinds of soil enzyme activity decreased along with the chronosequence and soil profiles with the decrease, especially in the surface and subsurface soils. Urease activity, invertase activity and catalase activity were mainly accumulated on the soil surface.2. Along with the chronosequence, soil carbon and carbon storage increased significantly and indicated the significant difference in different years. The overall performance was that the maximum of soil carbon storage appeared in the five years alfalfa land and the minimum was in the age of eight. SOC and SAOC indicated strong effect of carbon intercept and potential of carbon storage. The soil carbon and carbon storage generally had the surface accumulation feature in soil vertical profiles. It suggested a sensitivity of SAOC storage the time and a key index of soil organic matter relative to SOC storage. It showed that the change of soil nitrogen content and storage were the same as the soil carbon and storage.3. The changing trend of CO2emission fluxes indicated the similarity between soil-vegetation systems and soil systems with the order of4a>5a>8a>3a>1a and the seasonal variation regularity was that summer was greater than autumn. CO2content from soil systems was lower than soil-vegetation system. The changing trend of N2O emission fluxes also indicated the similarity between soil-vegetation systems and soil systems with the order of4a>5a>3a>8a>1a and the seasonal variation regularity was that spring and summer were greater than autumn. And N2O emission fluxes were the peak in mid-May and mid-August, and there was a "source" effect at this time. The changing trend of CH4emission fluxes indicated the similarity between soil-vegetation systems and soil systems with spring higher. The plant respiration contributed much to CO2and N2O emission fluxes and had no contribution on CH4emission fluxes. CO2and N2O emission fluxes were affected by standing ages, the maximum of two soil greenhouse gas fluxes were all in the four year alfalfa land and the minimum of two soil greenhouse gas fluxes were all in the age of one year. But standing ages had no effect on CH4emission fluxes.4. There was a quadratic correlation of CO2emission fluxes with soil organic carbon content, the ratio of carbon to nitrogen, and soil active organic carbon in the soil systems at the significance of p<0.05, and with soil total nitrogen, air temperature and0-5cm soil temperature at the significance of p<0.01. There was a positive correlation of CO2emission fluxes with aboveground biomass in the soil-vegetation systems at the significance of p<0.05. There was a positive correlation of N2O emission fluxes with soil organic carbon content, the ratio of carbon to nitrogen, and soil ammonium nitrogen content in the soil systems at the significance of p<0.05, and with soil total nitrogen, soil nitrate nitrogen content at the significance of p<0.01. There was a negative correlation of N2O emission fluxes with0-10cm soil water content at the significance of p<0.01. There was a positive correlation of N2O emission fluxes with aboveground biomass in the soil-vegetation systems at the significance of p<0.01. There was a positive correlation of CH4emission fluxes with soil total nitrogen and soil ammonium nitrogen content in the soil systems at the significance of p<0.01.In conclusion, can be considered:Alfalfa planting ages and soil depth had generally effects on soil organic matter, soil total nitrogen, soil ammonium nitrogen, soil total phosphorus content and three kinds of soil enzyme activity, even soil carbon and nitrogen storage and generally had the surface accumulation feature in soil vertical profiles. At the same time, alfalfa planting ages had different effects on CO2and N2O emission fluxes, but no effect on CH4emission fluxes. Soil total nitrogen, air temperature and0-5cm soil temperature were the limiting factors of CO2emission fluxes. Soil total nitrogen, soil nitrate nitrogen content and0-10cm soil water content, aboveground biomass were the limiting factors of N2O emission fluxes. Soil total nitrogen and soil ammonium nitrogen content were the limiting factors of CH4emission fluxes. And the further study will discuss how to have the best soil carbon and nitrogen storage and the least greenhouse gas emission through better alfalfa pasture management measure. |
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