黄土高原沟壑区森林带不同植物群落土壤氮素含量及其转化

为了探讨在黄土高原退耕还林还草过程中植物群落对土壤氮素含量及形态分布的影响,本文选择退耕历史较长的黄土高原沟壑区——安塞县洞子沟流域8种典型植物群落下0-10cm和10-20cm的土壤为对象,测定了土壤中有机氮、矿化氮、微生物量氮、硝态氮和铵态氮的含量。结果表明,从草本群落到乔灌草群落,土壤各形态氮素含量均增加,整体表现为乔灌草群落>灌草群落>草本群落。然而人工刺槐林的土壤氮素水平远低于自然恢复的乔灌草群落,甚至低于灌草群落。0-10cm 土层各形态氮素均高于10-20cm 土层。硝态氮对植物群落的变化最为敏感,可作为土壤氮素水平的敏感指标。土壤有机质、pH、容重与氮素含量极显著相关,各种氮素间极显著正相关。各种氮素占总氮的比例对总氮的变化有着不同的响应,有机氮、可矿化氮和微生物量氮占总氮的比例相对稳定,硝态氮占总氮的比例随总氮含量的增加而增加,铵态氮占总氮的比例随总氮含量的增加而降低。

Soil nitrogen concentrations and transformations under different vegetation types in forested zones of the Loess Gully Region

The "Grain for Green" project is essential for controlling soil erosion in the Gully Region of the Loess Plateau, which has one of the most serious water and soil loss problems in the world. Different vegetation types change the soil environment to different extents, influencing the biogeochemical cycles of materials, such as nitrogen (N). At present, a growing number of researchers are investigating the relationship between vegetation and N forms in natural ecological systems to provide a theoretical basis for ecological restoration in these areas. In this paper, the effect of vegetation types on the concentration and transformation of various soil N types (soil total N, organic N, N mineralizable within 14 d, microbial biomass N, nitrate N and ammonium N) in the Dongzigou Basin, Ansai, Shaanxi Province was investigated. Soil samples were collected from the 0-10 and 10-20 cm layers under eight typical vegetation types, including Quercus liaotungensis, Acer buergerianum Miq, Platycladus orientalis, Robinia pseudoacacia, Syzygium aromaticum, Sophora davidii, Artemisia giraldii Pamp. and Artemisia sacrorum Ledeb. The results showed that the six types of soil N as a whole gradually increased as vegetative succession proceeded from grass communities to more complex mixtures of trees, bushes and grasses. Overall, the maximal N concentration occurred under Q. liaotungensis, while the minimum occurred under A. sacrorum Ledeb. Soil total N ranged from 0.35 to 3.22g/kg at 0-10 cm, the maximum value being 9.2 times the minimal. For organic N, N mineralizable within 14 d, microbial biomass N, nitrate N and ammonium N, the maximum value was 9.3, 8.7, 6.4, 9.0 and 2.3 times the minimum, respectively. Soil N concentrations under artificial R. pseudoacacia forests were lower than those under natural successional vegetation. The various soil N concentrations all were higher at 0-10 cm than 10-20 cm. Nitrate N changed more quickly than the other N fractions; therefore, nitrate N could be used as a sensitive indicator of early changes in soil N during natural succession. Soil organic matter, pH, and bulk density were closely related to soil N. There was a significant positive correlation among the six N fractions; however, the ratio of each N fraction to total N responded differently to changes in total N. Specifically, the ratio of organic N, N mineralizable within 14 days, and microbial biomass N to total N did not change, regardless of the total soil N concentration. In contrast, the ratio of nitrate N to total soil N increased as the total soil N concentration increased, whereas the ratio of ammonium N to total N decreased. The ratio of nitrate N to total N was positively correlated with total N (r=0.58), while the ratio of ammonium N to total N was significantly negatively correlated with total N (r=-0.83). Further research is still required to explore the processes of vegetation change and N transformation.