【作者】 高亚琴；

【导师】 黄高宝； 李玲玲；

【作者基本信息】 甘肃农业大学 ， 生态学， 2009， 硕士

【摘要】 采样分析陇中黄土高原地区农田退耕种植苜蓿3a、5a、8a后0-5、5-10、10-20cm土层土壤碳氮含/储量,并用静态箱—气质联用法对样地土壤系统、土壤-植被系统的CO₂、N₂O排放通量进行了测定,研究农田退耕还草措施对土壤碳氮库及CO₂、N₂O释放通量的影响。主要研究结果如下:1.退耕还草措施能够提高贫瘠土壤总有机碳（TOC）和总氮（TN）含量,且随苜蓿种植年限的延长效果增强。首年弃耕的休闲对照农田土壤TOC、TN基础含量很低,土壤贫瘠,0-5、5-10、10-20cm的TOC含量分别为2.49、2.53和2.46 g·kg-1,0-20cm的平均值为2.49 g·kg-1;TN含量在各层中均为0.23 g·kg-1。退耕后各年限草地土壤0-5、5-10、10-20cm TOC、TN含量均较对照休闲农田有所增加,且随退耕年限的增加呈连续正增长趋势。其中TOC、TN在0-5cm表层的变化最大,在该层退耕3a、5a、8a后TOC分别较对照休闲农田提高16.9%、39.4%和69.5%,TN提高12.5%、33.3%和58.3%,各退耕年限间二者含量均达5%的显著性水平差异。退耕3a、5a、8a的草地0-20cm的TOC平均含量分别为2.68、2.98和3.35 g·kg-1,比对照休闲农田提高7.6%、19.8%和34.5%。0-20cmTN平均含量分别为0.25、0.28和0.31 g·kg-1,比对照增加8.7%、21.7%和34.7%。2.贫瘠农田退耕还草后表现出明显的碳、氮固存效应,有很强碳、氮固存潜力。与未退耕休闲农田相比,退耕3a、5a、8a后的草地0-20cm TOC储量分别提高9.12%、20.18%和34.39%,TOC平均固存率分别为0.17、0.23和0.25Mg·hm-2·a-1。同样,退耕3a、5a、8a后的草地0-20cm TN储量分别较对照提高8.2%、18.0%和29.6%,各退耕年限0-20cm TN平均固存率均为0.2Mg·hm-2·a-1。各退耕年限间0-20cm TOC、TN储量均达5%的显著性差异。3.伴随TOC、TN的增加,退耕还草3a、5a、8a后土壤活性有机碳（SAOC）、硝态氮（NO3-N）含量均有不同程度的增加。SAOC在0-20cm的平均含量分别较对照休闲农田增加9.5%、19.0%和38.1%。其中在5-10cm的增加最明显,分别增加14.3%、33.3%和47.6%。N03-N含量在0-5、5-10、10-20cm都有明显增加,0-20cm的平均含量比对照增加9.5%、20.5%和29.7%。但退耕前后NH4-N含量比较稳定,各年限各土层间无明显差异。4.农田退耕种草后不同生长年限的草地土壤各土层间碳库活度（A）和活度指数（AI）无明显变化,说明在退耕还草初期阶段,退耕对土壤碳的活性影响不大。而碳库指数（CPI）和碳库管理指数（CPMI）的值在各年限各土层都高于对照,总体表现为上层大于下层,且随退耕年限的增加而增大,各年限间0-20cm的值差异显著,说明退耕还草会显著增加土壤表层的总碳库。退耕时间越长,其增加量越大。由于总碳库增加后碳库的活度基本没有发生改变,因此说退耕还草增加的碳以非活性碳为主。5.农田退耕还草后土壤-植被系统的CO₂与N₂O释放通量均较对照有所增加。对照休闲农田各观测日期的CO₂通量平均值为77.66 mg·m-2·h-1,N₂O通量平均值为8.02μg·m-2·h-1。退耕3a、5a、8a的草地土壤-植被系统各观测日期的CO₂通量平均值分别为79.33、86.39和90.15 mg·m-2·h-1,比对照分别增加2.18%、11.24%和16.08%。N₂O通量平均值分别为8.24、8.29和8.65μg·m-2·h-1,比对照分别增加2.74%、3.37%和7.86%。二者排放通量均表现为8a>5a>3a>对照。说明退耕还草措施在引起土壤碳氮固存、表现为大气CO₂吸收“汇”的同时也会加剧CO₂、N₂O的排放,表现出大气CO₂、N₂O的“源”效应。6.土壤CO₂通量与TOC含量、SAOC含量、TN含量、C/N值显著正相关;N₂O通量与SOC含量、矿质氮（NO3-N、NH4-N）含量、C/N值显著正相关。其中CO₂通量与SAOC含量、N₂O通量与NO3-N含量有很高的相关性（R=0.9063,n=15,P=0.01;R=0.936,n=14,P=0.01）。因此说在环境因素相同的条件下,退耕还草措施实施后土壤碳氮各指标含量的增加是引起CO₂、N₂O的排放加剧的重要因素。另外,CO₂、N₂O通量与地上生物量、植物生长以及土壤水分含量之间也有显著的正相关关系,退耕还草后地表大量植物的生长和土壤水文条件的改变也对CO₂、N₂O的排放产生重要影响。更多还原

【Abstract】 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.更多还原