川西北高寒草地沙化过程中土壤氮素变化特征

Changes in soil nitrogen characteristics during grassland desertification in Northwest Sichuan

草地沙化是我国最严重的环境问题之一,但关于草地沙化过程中氮素变化特征的研究报道多集中于干旱半干旱地区,而半湿润地区的相关报道还比较缺乏。通过野外调查,研究了川西北半湿润地区高寒沙质草地沙化过程中土壤氮素变化特征。结果表明,草地沙化对0—100cm土层土壤氮素具有显著影响,全氮、碱解氮、铵态氮(NH_4^+-N)、硝态氮(NO_3^--N)和微生物量氮(MBN)均呈现极显著下降的变化特征,极度沙化阶段较未沙化阶段分别减少了73.95%、77.72%、76.75%、79.77%和84.12%。其中,0—20cm土层变化最显著,全氮、碱解氮、NH_4^+-N、NO_3^--N和MBN含量分别减少了86.43%、83.52%、82.11%、88.82%和91.77%。随着土层深度增加,不同程度沙化草地土壤氮素含量及其变化量逐渐减少;草地沙化过程中,不同沙化阶段土壤氮素损失数量不尽相同,其中,以轻度沙化阶段氮素损失最严重,全氮、碱解氮、NH_4^+-N、NO-3-N和MBN含量分别降低了41.18%、35.17%、46.74%、43.46%和46.88%。草地沙化过程中,土壤全氮、碱解氮、NH_4^+-N、NO_3^--N和MBN含量与土壤粉粒、粘粒含量和植被群落盖度均呈极显著正相关特征,与土壤沙粒含量呈极显著负相关特征。研究区土壤氮素损失与风蚀选择性吹蚀土壤粉粒、粘粒及地表植物覆盖状况逐渐变差密切相关,因此该区域治沙的关键是采取措施降低风蚀对地表土壤吹蚀作用,提高沙化草地地表植被覆盖。同时,还应及时对沙化前期阶段及潜在沙化的草地进行生态治理,从而避免草地沙化继续恶化。

Grassland desertification is one of the most serious environmental problems in China. However, many studies examining changes in nitrogen characteristics during grassland desertification have focused on arid or semi-arid regions, and few studies have considered semi-humid regions, hi this study, the soil nitrogen content during the process of desertification in a northwest Sichuan alpine area, a semi-humid region, was analyzed. Desertification had a significant impact on soil nitrogen in the 0-100 cm soil layer, and total nitrogen, available nitrogen, NH4＋-N, NO3--N, and microbial biomass nitrogen （MBN） decrased significantly during the process of grassland desertification by 73.95%, 77.72%, 76.75%, 79.77%, and 84.12%, respectively）. Total nitrogen, available nitrogen, NH4＋-N, NO3--N, and MBN decrased most strongly in the 0-20 cm soil layer among all layers within 0-100 cm by 86.43%, 83.52%, 82.11%, 88.82%, and 91.77%, respectively. As gradually decreased. The desertification. During the the soil depth increased, both the content and degree of change during grassland desertification extent of the decrase in soil nitrogen content varied during the progression of grassland early degradation stage, soil nitrogen decreased the most; The total nitrogen, available nitrogen,NHn-N, NO^-N and MBN decrased 41.18%, 35.17%, 46.74%, 43.46%, and 46.88%, respectively. Based on these results, the changes in soil nitrogen observed as the degree of desertification increased in semi-humid regions were similar to those observed in previous studies for arid and semi-arid regions. Plants are important for the sequestration of soil nitrogen, not only because they act as nitrogen sources, but also because they protect soil containing nitrogen from blowing away. A correlation analysis showed that total nitrogen, available nitrogen, NH4-N, NO3-N and MBN were all significantly positively correlated with community coverage （P〈0.01）. In addition, total nitrogen, available nitrogen, NH4-N, NO3-N and MBN were all significantly positively correlated with silt and clay, which have high nitrogen contents （P〈0.01）. In contrast, coarse sand lacking nitrogen was significantly negatively correlated with total nitrogen, available nitrogen, NHa-N, NO3-N and MBN （P〈0.01）. Based on these results, we inferred that soil nitrogen loss was caused by a gradual decrease in community coverage during grassland desertification, and the loss of silt and clay via wind erosion. Consequently, efficient measures should be taken to protect silt and clay from wind erosion, and to improve community coverage during the desertification of grassland. In addition, potential grassland desertification, particularly during the early stage, should be managed using ecological methods to avoid the negative effects of increasing grassland desertification.