内蒙古半干旱草原灌丛化过程中小叶锦鸡儿引起的土壤碳、氮资源空间异质性分布

Spatial heterogeneity in soil carbon and nitrogen resources, caused by Caragana microphylla,in the thicketization of semiarid grassland,Inner Mongolia

过度放牧下内蒙古半干旱草原,由于小叶锦鸡儿多度增加导致植被灌丛化,这已经成为该地草原退化时的普遍现象。草原灌丛化过程中,灌丛内凋落物的累积使得养分循环区域化,草原土壤有机碳(SOC)、土壤全氮(STN)空间异质性增强,导致灌丛沃岛形成。半干旱草原灌丛化过程机制的假说认为:灌丛斑块扩展与其引起的土壤空间异质性的增强之间存在着正反馈作用,正是这种反馈作用促使草地向灌丛地的转变。小叶锦鸡儿通过克隆生长形成不同大小的斑块,它们对应于其发育的不同阶段。因此,将不同大小的灌丛沃岛划分为小灌丛组与大灌丛组,它们代表着灌丛沃岛发育的早期与晚期两个阶段。结果表明:内蒙古半干旱草原灌丛化过程中,小叶锦鸡儿灌丛斑块引起了土壤有机碳(SOC)与土壤全氮(STN)的空间异质性,表现在水平方向上,对于0～5cm,5～10cm,10～20cm3个土壤层次,由灌丛斑块内部向外部SOC与STN均趋于降低;而在垂直方向上,对于灌丛斑块内部、边缘和外部3个位置,由0～10cm到10～20cm再到20～30cm,随着土壤深度增加SOC与STN均趋于降低。随着小叶锦鸡儿灌丛斑块扩展,SOC与STN空间异质性的分布不断增强,表现在灌丛斑块内部相对于外部(或边缘)对SOC与STN富集程度均显著增加。草原灌丛化过程SOC与STN空间异质性变化在0～5cm,5～10cm,10～20cm3个土壤层次中,以表层0～5cm的表现最为强烈。表层0～5cm土壤空间异质性的形成与灌丛自身凋落物的生产以及对灌丛外凋落物的截留有关。由此推断:退化草原灌丛化的过程中,小叶锦鸡儿灌丛斑块引起SOC与STN空间异质性的变化是一个自我增强过程,灌丛斑块扩展与其导致的SOC与STN空间异质性增强之间存在着正反馈作用。因此,以上关于半干旱草原灌丛化机制的假说对我国内蒙古半干旱草原灌丛化也是适用的。

Thicketization of vegetation for the increased abundance of shrub, Caragana microphylla, has been widespread under overgrazing in the semiarid grassland, Inner Mongolia. During this process, the cycling of nutrients is progressively confined to the zones of litter accumulation beneath shrub,and spatial heterogenity of soil organic carbon （SOC） and soil total nitrogen （STN） tends to increase, and results in the formation of the islands of fertility. It is hypothesized that there is the positive feedback between the increase in spatial heterogeneity of SOC and STN and the shrub expansion, which promotes the transition from grassland to shrubland. At our study site, Caragena Microphylla could expand through the clonal growth and form a patch with different size. Shrub patches with the different class of size could be viewed as distinct stages of shrub development. Therefore, different size classes of shrubs （“small”, and “large”） represented corresponding stages of fertile island development （i. e. , “early” and “late” respectively）. Results showed that shrub patches, formed by Caragana microphylla, could induce spatial heterogeneity of SOC and STN during the thicketization of semiarid grassland, Inner Mogonlia. For all of 0-5cm, 5-10cm,10-20cm soil depths, along the horizontal direction from the inside to the edge and to the outside of shrubs, SOC and STN tended to decrease. For all of the inside, edge and outside of shrub patches, along the vertical direction from 0-5cm to 5-10cm and to 10-20cm soil depth, SOC and STN tended to decrease. Spatial heterogeneity of SOC and STN enhanced increasingly as a shrub patch expanded, expressed by SOC and STN accretion, for the inside relative to the edge （or the outside）, it increased significantly in the three soil depths. Spatial heterogeneity of SOC and STN in the surficial 0 - 5cm soil depth changed most significantly among three soil depths, which was associated with the production of litter under shrub patches and the interception of litter from out-shrub areas. It was concluded that spatial heterogeneity in SOC and STN. induced by Caragana microphylla, was self-reinforced, and there was the positive feedback between the increase of spatial heterogeneity in SOC and STN and the expansion of shrub patches. Therefore, the above hypothesis was appropriate to the thicketization of the semiarid grassland. Inner Mongolia.