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在美国俄勒冈州,控制西美圆柏(Juniperus occidentalis)的数量是一种公认的牧场管理措施,该做法有助于恢复蒿草草原(sagebrush steppe)生境,该生境对野生动物和家畜都十分重要.然而,尽管控制木本植物数量会对当地的流域管理和区域性氮库造成重要影响,但砍伐西美圆柏对生态系统中氮元素的影响问题尚未得到很好的解决.本文定量研究了两个相邻流域生态系统中的氮储量,其中一个流域经过处理(流域内的大部分圆柏已被清除掉),而另一个流域未处理(圆柏未被清除).在圆柏被移除13年后,我们测定了两个流域里圆柏树林、灌木丛、草丛和枯枝落叶层的地上氮储量,以及两个流域中两个土层(0-25和25-50 cm)内的地下氮储量(根系和土壤氮储量).研究结果表明,未处理流域的地上氮储量是处理流域的6.9倍,因为未处理流域的地上生物量要大得多.然而,由于砍伐圆柏导致林下植被的根系生物量增加,所以处理流域的根系氮储量是未处理流域的3.1倍.0-25和25-50 cm土层氮储量没有受到圆柏砍伐的影响.总体而言,生态系统总氮储量在处理流域(9536 kg N ha-1)和未处理流域(9456 kg N ha-1)之间并没有显著差异.在两个流域里,生态系统中最大的氮积累量(至少95％的生态系统总氮量)均存在于地下(0-25 cm深的土壤以及根系).这项研究证明,清除圆柏并不会显著影响生态系统的储氮能力.“,”Aims In the Oregon of USA,the control of western juniper (Juniperus occidentalis) is an accepted rangeland management practice to restore sagebrush steppe habitats of importance to wildlife and livestock.The effects of juniper cutting on ecosystem nitrogen,however,have not been well addressed although woody plant control has important implications for local watershed management and regional nitrogen pools.Methods We quantified ecosystem nitrogen stocks in two adjacent watersheds,comprised of a treated watershed(most juniper removed) and an untreated watershed (juniper not removed).Thirteen years after juniper removal,we measured aboveground nitrogen stocks for juniper trees,shrubs,grasses and litter in both watersheds.We also measured belowground nitrogen stocks (roots and soil) in both watersheds at two soil depths (0-25 and 25-50 cm).Important Findings Aboveground nitrogen stocks were 6.9 times greater in the untreated than in the treated watershed considering the much larger aboveground biomass.However,root nitrogen stocks were 3.1 times greater in the treated one due to the gain of understory root biomass associated with juniper cutting.Soil nitrogen stocks at both 0-25 and 25-50 cm depths were not affected by juniper removal.Overall,total ecosystem nitrogen stocks did not differ between the treated (9536 kg N ha-1) and untreated (9456 kg N ha-1) watersheds.The greatest ecosystem nitrogen accumulation (at least 95％ total ecosystem nitrogen) resided belowground (soil 0-50 cm and roots) in both watersheds.This study provides evidence that the benefits of juniper removal can be attained without significantly affecting the capacity of ecosystem nitrogen storage.