由于气候的改变、过度放牧、火烧以及幼树与草本植物的竞争，灌丛入侵已成为全球干旱、半干旱草原的普遍现象。小叶锦鸡儿（Caragana microphylla）是一种典型的草原旱生豆科灌木，它通过克隆生长形成不同大小的灌丛斑块，广泛的分布于我国内蒙古温带草原。尽管现在草原灌丛化非常普遍，但是很少研究灌草之间对氮的竞争，也不清楚灌草的氮获取策略是什么。而氮是限制植物生长的至关重要的营养元素。基于此本文采用15N同位素技术，在北京师范大学太仆寺旗灌草竞争固定围封样地（2007年建立于内蒙古太仆寺旗黑山庙），探究内蒙古温带草原小叶锦鸡儿及其伴生草本植物在不同养分条件下的氮吸收以及氮在植物与土壤系统中的运移分配的策略。主要得出以下结论： （1）内蒙古温带草原小叶锦鸡儿和草本植物均能够直接吸收土壤中的有机氮，但是它们仍以吸收无机氮硝态氮为主。小叶锦鸡儿和草本植物硝态氮的贡献率分别是81.3 %和82.5 %。总体来说，小叶锦鸡儿的氮吸收能力大于草本植物。草本植物主要吸收利用土壤表层的氮，小叶锦鸡儿的氮吸收利用比较灵活，既可以利用表层土的氮也可以利用深层土的氮。小叶锦鸡儿和草本植物具有不同的氮利用策略（物种 × 氮形态，P < 0.05），表明了它们氮利用形态上的生态位的分化。尽管小叶锦鸡儿和草本植物在坡上没有表现出较多的利用有机氮，但是表层土与深层土比较，可以发现小叶锦鸡儿和草本植物在深层土利用较多的有机氮，即土壤养分条件差氮受限的条件下，植物较多的利用有机氮。 （2）短期内（4小时），微生物对氮的竞争是优于植物的，且与标记的氮形态无关。就微生物与植物的氮竞争效率（微生物15N回收率与植物15N回收率比值）而言，表层土的硝态氮的氮竞争效率最低，深层土的竞争效率在不同的坡位表现不同，坡上，坡中位置甘氨酸的竞争效率最低，坡下位置铵态氮的竞争效率最低。对比不同土壤养分条件下的微生物与植物的氮竞争效率，硝态氮和铵态氮均是坡下的微生物竞争效率最低，且深层土的竞争效率大于表层土的；甘氨酸在表层土是坡下的微生物竞争效率最低，而在深层土时坡下的微生物竞争效率最高，且甘氨酸坡下的深层土的竞争效率大于表层土的，其它坡位则相反。土壤养分条件越差，微生物与植物对无机氮的竞争效率越高。 （3）内蒙古温带草原小叶锦鸡儿的固氮率（%Ndfa）大约是79.52 %，大约能固定空气的氮量是29.91 g N m-2 。草本植物从固氮植物小叶锦鸡处转移的氮率约是43.51%，相当于8.14 g N m-2。小叶锦鸡儿转移给草本植物的氮量约占到其固定大气氮量的27.21 %。小叶锦鸡儿灌丛对灌丛边缘草本植物叶、根氮含量，土壤可溶性有机碳和总氮的影响很大。灌丛边缘草本植物叶、根的氮含量显著大于灌丛外部。小叶锦鸡儿灌丛对灌丛边缘草本植物影响范围能达到5 m，甚至更远。 （4）通过分析硝态氮15N和铵态氮15N在植物与土壤系统的运移与分配规律，发现短期内硝态氮15N主要存在土壤可溶性氮库，而土壤微生物氮库是铵态氮的主要氮库，也是硝态氮的第二大氮库。长期来看，土壤有机质是硝态氮和铵态氮15N主要的氮库（> 10%），尤其在深层土中（> 30%）。对比硝态氮和铵态氮15N不同氮形态的移动特点，当注射深度为5 cm时，植物的硝态氮的回收率大于铵态氮的回收率，而土壤的回收率则是铵态氮大于硝态氮，植物与土壤系统总的回收率表现为铵态氮大于硝态氮。当注射深度为20 cm时，植物的铵态氮回收率大于硝态氮，土壤的硝态氮回收率大于铵态氮的，植物与土壤系统总的回收率表现为硝态氮大于铵态氮，表明硝态氮和铵态氮15N在植物与土壤系统的不同生态效应。

Shrub encroachment is a global phenomenon in arid and semi-arid grasslands as an ecosystem responses to climate change, overgrazing, fire management practices, and competition between grasses and seedlings. Caragana microphylla is a typical drought-enduring legume shrub, which forms shrub patches of various sizes by cloning growth, and is widely distributed in temperate grasslands of Inner Mongolia. As a result, the shrub encroachment is widespread in globally. In spite of the fact that Nitrogen (N) is a limited and indispensable nutrient for plant growth few studies have explicitly investigated the competition for N to document the characteristics of N absorption strategies of grasses and the shrub. A better understanding of plant N strategies is critical for predicting mechanisms underlying species coexistence and managing ecosystem nutrient cycling. In this study based on field experiment stations of Beijing Normal University in Inner Mongolia. we used 15N isotope technology to explore the strategies of nitrogen acquisition of C. microphylla and accompanied grasses and the N allocation in the plant - soil ecosystem. The main results are as follows: (1)Although the shrub and grasses were able to take up organic N after four hours injection, the NO3- was the dominant N source, with average 81.3 % and 82.5 % contribution to total N absorption, respectively. The shrub took up available N more effectively than grasses. The grasses prefer to using N in topsoil, while the N absorbed by shrub has a variable profile from the topsoil to subsoil. Species differ in their preference for different forms of N (species × N form , P < 0.05), and thus there was an evidence of niche differentiation. The contribution of organic N to total N absorption for shrub and grasses in upper slope was not higher than in lower slope. In fact, compared to topsoil, shrub and grasses took up more organic N in subsoil, and suggested that organic N may be an important N source when other forms of N are not available. (2)The soil microbes were more effective than plants in competing for N in a short time (4 hours), irrespective of N form. The ratio of 15N-NO3- recovery by microbial biomass to 15N-NO3- recovery by plants was the least for topsoil (0-10 cm) in all slope positions. However, the ratio of 15N-glycine in upper slope and middle slope, and the ratio of 15N-NH4+ in lower slope were the lowest for subsoil (10-30 cm). In term of the effect of slope positions, the least ratio of 15N-NO3- and 15N-NH4+ were at the lower slope in two soil layer, the same as the ratio of 15N-glycine in topsoil. The effective of subsoil were higher than topsoil for 15N-NO3- and 15N-NH4+. Thus, the ratio of inorganic N recovery by microbial biomass to inorganic N recovery by plants may be higher when more N are not available. (3)The N derived from the atmosphere (%Ndfa) by C. microphylla was about 79.52 %, and the amount of N fixed by C. microphylla was about 29.91 g N m-2. The amount of N in grasses transferred from C. microphylla was about 43.51 %, or 8.14 g N m-2. The amount of N transfer accounted for about 27.21 % of the amount of N fixation. The setter of shrub in grassland could affect the the N content of roots and leaves in grasses, the soil dissolved organic carbon and total N of edge-shrub. The N content of roots and leaves in grasses of edge-shrub were higher than that of out-shrub. Moreover, the distant of C. microphylla positive effects was up to 500 cm, or further. (4)The aim of this portion was to follow the fate of 15N-NO3- and 15N-NH4+ in the soil-plant system of temperate grassland. Initially (i.e., 4h), the largest 15N recovery of NO3- and NH4+ was soil dissolved N pool and microbial N pool, respectively. After labeling 45 days, the main fates of the 15N-NO3- and 15N-NH4+ were in soil organic matter (> 10%), especially in the subsoil (> 30%). The recovery of 15N-NO3- was larger than that of 15N-NH4+ in plant, but was less than that of 15N-NH4+ in soil for injection depth of 5 cm. The total recovery of 15N-NH4+ in plant and soil was larger than that of 15N-NO3- . However, for injection depth of 20 cm, the recovery of 15N-NO3- was larger than that 15N-NH4+ in soil, but was less than that 15N-NH4+ in plant. The total recovery of 15N-NO3- in plant and soil was larger than that of 15N-NH4+ . This indicated that there was great different ecological effect between the fates of NH4+ and NO3-.