幼苗定居过程中根区水氮对骆驼刺根系生物学特性调节的研究

In arid ecosystems, plant life and distribution is limited by the spatial and temporal availability of water and nutrients. Observation suggests plants in such ecosystems develop, almost exclusively, deep root systems to reach moist soil layers and ground water sources and function essentially as phreatophytes. Alhagi sparsifolia, the dominant and constructive species in the oasis-desert ecotone area of the southern margin of the Tarim Basin, has begun to degrade, and is difficult to regenerate naturally under water and nutrient stress. However, perennial A. sparsifolia is able to reach the groundwater through its deep root system to meet their water and nutrient requirements and makes them independent of soil-supplied water and nutrients. This suggests that A. sparsifolia seedlings require extremely fast root growth for a successful establishment.. As A. sparsifolia occurs at sites with deeper groundwater, we expected the roots of this species to reach down more rapidly under the appropriate dry-wet alternant regulation and low fertilization than that under other soil conditions. Furthermore, we expected seedlings, experiencing moderate irrigation and low fertilization at early growth stage but no irrigation and fertilization during middle and later periods, have deeper reaching roots compared to other irrigation and fertilization couplings. We hypothesized that the possible reasons for the latter may be included (1) there may be no nitrogen (N2)-fixation by plants, therefore, optimum water and nitrogen coupling would stimulate root growth during the early period; (2) the coupling of water stress and low fertilization is beneficial for root reaching to deep and moist soil layers at middle and later growth stages, and at the same time the seedlings meet their nitrogen requirements mainly by the fixation of atmospheric N2. .The pot and field experiments were carried out at the Cele research station, Xingjiang Uyghur Autonomous Region, China. Twelve experimental treatments were conducted: four N regimes × three watering regimes for both pot and field experiments. We used stable isotope 15N labeling, 15N natural abundance method and stratified root excavation methods to investigate the responses of root growth and spatial distribution, root nodulation and biological N2 fixation, nitrogen allocation, and nitrogen use efficiency to root zone water and nitrogen regulation. Based on these results we expectd to test our hypothesis, find out a optimum water and nitrogen coupling for successful ecesis of A. sparsifolia seedlings, and make sure the contribution of biologically fixed nitrogen in vegetation restoration of A. sparsifolia. These research findings can provide theoretical foundation for the development of root biology in arid region, and will offer a good technological support for the restoration and reconstruction of deep root plants.

骆驼刺为塔里木盆地南缘绿洲-沙漠过渡带上的优势建群种植物，已出现退化，在水分和养分共同胁迫下很难实现自然更新；但多年生骆驼刺可通过发达的根系来接触地下水生存。因此，如何通过人工措施调控根系生长，尽快实现骆驼刺幼苗的成功定居是当前期待解决的关键问题。依托策勒国家站长期生态学试验场和人工控制试验小区，结合田间和盆栽试验，通过调节根区水氮条件，采用人工挖掘法、15N标记和15N自然丰度法等，分析骆驼刺幼苗根系生长动态和分布规律、根系结瘤和固氮能力以及植物氮素分配和利用特征对根区水氮条件变化的响应和适应；探明骆驼刺幼苗根系未接触地下水之前，生物固氮对其适应环境胁迫的贡献；查明氮素营养在骆驼刺幼苗根系生长中的作用；阐明水氮组合对其根系生长和分布的影响，证明根区水氮调节是否为骆驼刺植被恢复过程中的有效措施。以期形成骆驼刺人工恢复过程中的水氮调节技术，为干旱区优势植物的修复提供理论依据和技术指导。

在塔里木盆地南缘策勒国家站长期生态学试验地和人工控制试验区，通过设置不同的水氮处理，结合田间和盆栽试验，采用人工挖掘法、15N标记和15N自然丰度法等，研究了骆驼刺幼苗根系生长动态和分布规律、根系结瘤和固氮能力以及植物氮素分配和利用特征对根区水氮条件变化的响应和适应，以期查明骆驼刺幼苗定居过程中的最佳水氮调节措施。结果表明：（1）在幼苗生长早期，干旱低氮处理（土壤含水量4.8-5.6%，施氮12 g•m-2）不仅可显著增加骆驼刺根系生物量累积和干物质分配比例，而且具有较高的细根比根长和氮素利用效率，所以干旱低氮为该生长时期的推荐水氮组合。（2）16月株龄时骆驼刺根系最大扎根深度为220 cm，28月株龄时最大扎根速度为410 cm。除干旱低氮和中水不施氮处理外，其它处理均能促使根系在28月株龄扎到410 cm。（3）水氮交互虽然对扎根深度影响不明显，但对土壤剖面根系生物量累积有显著作用。骆驼刺约有60.2%的根系分布在70-280 cm土层，该土层的粗根和细根生物量分别在高水低氮（土壤含水量12.8-13.6%，施氮12 g•m-2）和中水低氮处理（土壤含水量9.6-10.4%，施氮12 g•m-2）下最高，分别可占整个剖面的67.1%和78%。而在280-410 cm土层均为中水低氮的粗根和细根生物量最高，可占整个剖面的29.3%和29.4%。此外，在中水低氮处理下骆驼刺的成活率也最高（66.7%），因此该处理应为4月株龄以后的推荐水氮组合。（4）在幼苗生长早期，根瘤多着生于<1 mm 细根上，随株龄增加则着生于<2 mm 细根上。水分是影响根系结瘤的主要因子，随土壤水分条件增加根瘤数明显减少。根系结瘤情况并不能如实反映骆驼刺的生物固氮能力。骆驼刺的生物固氮率受到水氮交互作用的显著影响，尤其是在中水条件下施氮后生物固氮效率显著增加，28月株龄时平均为70.5%。16月和28月株龄骆驼刺分别在高水低氮和中水低氮处理下对肥料氮的利用率最高，分别可达48.4%和22.1%。可见，在中水低氮处理下骆驼刺不仅具有庞大的深层根系分布，还具有较高的生物固氮能力和肥料氮利用效率。这也进一步证明了中水低氮处理可作为骆驼刺幼苗定居过程的最佳水氮组合，在今后骆驼刺植被恢复和重建过程中应注重水肥耦合作用的发挥。