Effects Of Cropland To Grassland Conversion To Soil C, N Pool And CO₂, N₂O Flux In Loess Plateau Area

By taking samples from forage grasslands (alfalfa) which had been converted from croplands for 3, 5, 8 years in Loess Plateau area in middle of Gansu, changes of soil total organic carbon(TOC), total nitrogen (TN) contents/storage at the three depths of 0-5,5-10 and 10-20cm were analyzed. And the fluxes of CO₂ and N₂O at the sampled lands were measured by using static enclosed chamber– GC/MS technique. Upon these measurements, the effects of cropland to grassland conversion to soil C, N pool and CO₂, N₂O fluxes were discussed. The main results showed as following:1. Before converting to alfalfa grassland, croplands had very low TOC and TN contents, the contents of TOC at 0-5, 5-10, 10-20cm soil layers were 2.49, 2.53, 2.46 g·kg-1 respectively,the average contents at 0-20cm was2.49 g·kg-1.The contents of TN at the 3 depths were all 0.23 g·kg-1. After converting , TOC and TN contents of converted grassland at 0-5,5-10,10-20cm were both increased comparing to that of un-converted croplands,and the increment increased when the conversion years prlonged. The greatest change of TOC and TN contents occurred at the 0-5cm layer, with increase of 16.9%,39.4% ,69.5% of TOC and 12.5%,33.3% ,58.3% of TN comparing to croplands after converting for 3,5 and 8 years.The contents of TOC and TN of different conversion years were up to 5% sinificant level.After converting for 3, 5 and 8 years, the average content of TOC at 0-20cm were 2.68, 2.98 and 3.35 g·kg-1, with increase of 7.6%, 19.8% and 34.5% comparing to that of croplands.Meanwhile, the average content of TN at 0-20cm were 0.25,0.28 and 0.31 g·kg-1, with increase of 8.7%, 21.7% and 34.7% respectively.2. The infertile sampled lands showed significant TOC, TN sequestration and had a great potential of it. After conversion of annually crop to perennial alfalfa for 3,5 and 8 years ,the storage of TOC at 0-20 cm increased by 9.12%,20.18%,34.39% comparing to cropland, and TOC sequestration rates were estimated to be on average of 0.17,0.23,0.25 Mg·hm-2·a-1 respectively. The storage of TN at 0-20 cm increased by 8.2%,18.0% and 29.6% , TN sequestration rates of different conversion years were all estimated to be on average of 0.2 Mg·hm-2·a-1.3. Following with increase of TOC and TN, the contents of SAOC and NO3-N increased with somewhat extent. After converting for 3, 5 and 8 years, the average content of SAOC at 0-20cm were increased by 9.5%, 19.0% and 38.1%. The greatest change of SAOC occurred at the 5-10cm, with increase of14.3%, 33.3% and 47.6% comparing to that of croplands. But the chang of NH4-N contents of different conversion years were not significant.4. Soil carbon activity (A)and activity index (AI) of converted soil at different sampled layers showed no significant change after converting,which meant the effect of convertion to soil active carbon were not great at the initial convertion stage. But carbon pool index(CPI)and carbon pool management index (CPMI) of different convertion year grasslands at different sampled layers were both higher than that of cropland, and that of upper layers were higher than lower layers , the discrepancy of different converted soil at 0-20 cm were sinificant. The result meant the measurement of convertion would increase corbon pool of surface soil. For the activity nearly not changed when the total carbon pool increased, so it can be concluded that the un-active corbon took a greater part of the increased carbon.5. After converting, the fluxes of CO₂ and N₂O were both increased comparing to that of croplands. The average fluxes of CO₂ and N₂O of croplands during observing date were 77.66 mg·m-2·h-1 and 8.02μg·m-2·h-1,After converting for 3,5 and 8 years,the average fluxes of CO₂ were increased to 79.33, 86.39 and 90.15 mg·m-2·h-1respectively,with increase of 2.18%,11.24% and 16.08% comparing to that of croplands.and the average fluxes of N₂O were increased to 8.24,8.29,8.65μg·m-2·h-1,with increase of 2.74%, 3.37% and 7.86%. The two fluxes were both showed as"8a>5a>3a>contrast"as a whole. The result meant the measurement of cropland to grassland conversion would intensify CO₂ and N₂O emission, which displayed as a"source"of CO₂ and N₂O to atmosphere except it displayed as a"sink"of atmosphere CO₂ caused by C sequestration.6. The fluxes of CO₂ were significantly positively correlated with the contents of TOC, SAOC, TN and C/N ratio. Also, the fluxes of N₂O were significantly positively correlated with the contents of TOC, TN, NO3-N, NH4-N and C/N ratio. Of that, the fluxes of CO₂ were closely positively correlated with the contents of SAOC, the fluxes of N₂O were closely positively correlated with the contents of NO3-N.(R=0.906 ,n=15,P=0.01;R=0.936, n=14,P=0.01). So it can be concluded that the increase of soil corbon and nitrogen contents caused by conversion were very important factors to intensifying CO₂ and N₂O emission when environmental factors were similar. In addition, fluxes of CO₂ and N₂O were significantly positively correlated with content of up-ground biomass, growth of vegetation and content of soil water. So, the growth of generous up-ground vegetation and the change of soi moisture caused by convertion were also very important factors to CO₂ and N₂O emission.