晋西北不同年限小叶锦鸡儿灌丛土壤氮矿化和硝化作用

利用PVC管顶盖埋管法研究了晋西北黄土高原区小叶锦鸡儿人工灌丛不同定植年限(5,10,20,30,40a)土壤氮矿化与硝化速率的动态和净矿化与硝化总量。结果表明,⑴小叶锦鸡儿灌丛土壤无机氮主要以NO_-~3-N形式存在,不同生长年限相同月份的土壤硝态氮(NO-3-N)含量分别是铵态氮(NH+4-N)含量的1.5—15.4倍;⑵土壤氮素硝化速率和矿化速率随生长年限延长而加快,30年生时达到高峰,数值达40.2,44.1 mg m~(-2)d~(-1)。从季节性变化看,7—8月份是硝化速率和矿化速率快速增长期,30年生小叶锦鸡儿灌丛土壤硝化速率和矿化速率分别达到86.9,93.1 mg m~(-2)d~(-1),显著高于其它生长年限(P0.05);(3)土壤氮素硝化与矿化总量同样随小叶锦鸡儿生长年限延长而增加,30年生时达到最高,与5年生相比,分别增加了3.7和3.1倍。(4)5—10月份小叶锦鸡儿生长期内,各年限土壤全氮量的2.3%被矿化成无机氮,其中87%最终被转化成NO-3-N形式存在于土体中。

Soil nitrogen mineralization and nitrification under Caragana microphylla shrubs of different ages in the northwestern Shanxi Loess Plateau

Nitrogen (N) is a main limiting factor for plant growth and development in many regions. Soil organic N, accounting for up to 95% of the soil N pool, cannot be used by plants directly, but can be absorbed and utilized by plants after being mineralized into nitrate N (NO₃^--N) and ammonium N (NH₄⁺-N). Therefore, soil net N nitrification (N_nit) and net N mineralization (N_min) rates have been considered two important indicators for evaluating the stability and function of an ecosystem. Caragana microphylla, a leguminous and sand-binding shrub, is a dominant species widely distributed in the Loess Plateau of northwestern Shanxi. Previous studies on soil N dynamics have mainly focused on forest, grassland, and farmland ecosystems to study the effect of environmental factors such as temperature and humidity on soil N mineralization and nitrification. However, few researchers have focused on soil N dynamic changes under shrub lands in arid and semiarid regions. The objective of this study was to explore the content of NO₃^--N and NH₄⁺-N and the rate of soil net N nitrification and net N mineralization under Caragana microphylla shrubs as a function of plant age. We used a closed-top PVC tube in situ incubation method to detect the soil net N mineralization and net N nitrification in Caragana microphylla shrubs of different ages (5, 10, 20, 30, and 40 a) in the northwest Shanxi Loess Plateau. The results indicated that NO₃^--N accounted for a larger proportion of mineral N in Caragana microphylla shrub soil. The NO₃^--N concentration was 1.5-15.4 times higher than NH₄⁺-N in the same month of different years. Soil net N nitrification and net N mineralization rates increased as the Caragana microphylla shrubs'' ages increased, with the greatest average rates from May to October (40.2 and 44.1 mg m^-2 d^-1, respectively) being observed under 30-year-old shrubs. The net N nitrification and mineralization rate increased faster in July and August relative other months during the growing season. The net N nitrification and mineralization rates were 86.9 and 93.1 mg m^-2 d^-1, respectively, at 30-year-old shrubs, which had the highest rates of all age classes (P < 0.05). The total net N nitrification and mineralization also increased with increased age of Caragana microphylla. The total net N nitrification and mineralization from May to October increased by 3.7- and 3.1-fold at 30-year-old shrubs, respectively, relative to 5-year-old shrubs. During the Caragana microphylla growing season from May to October, 2.3% of soil total N was mineralized into inorganic N, 87% of which was eventually transformed into NO₃^--N. The results of this study provide great insight for guiding shrub utility and land management in the establishment and development of shrub ecosystems in the Loess plateau.