民勤衰败固沙林下土壤结皮发育对红砂定居的影响及作用机制

The Minqin Oasis is located in the northwest Hexi Corridor, and surrounded by the Tengger and Badain Jaran Deserts. To manage sand and prevent sandstorms, large tracts of sand-fixing plantations have been planted on the fixed and semi-fixed sand dunes of the oasis fringe. At present, A “Green Great Wall” sand-fixing belt arounding the oasis periphery, more than 330 kilometers, formed by sand-fixation plantations, and plays a great role in protecting industrial and agricultural normal production. With gradually improving of the vegetation coverage in the sand-fixing belt, original flow or semi-fixed dune gradually was stabilized, and different types of soil crust had formed and developed on the dune surface gradually. However, there was a phenomenon showed that the partial sand-fixation plantations with soil crust was being declined due to multiple biological factors. At the same time, a kind of woody species named as Reaumuria songarica can naturally invade and successfully reproduce once soil crust has formed and developed on soil surface of the sand-fixing forest land. Nowadays, there are no reports about influence and its mechanism of soil crust formation and development on R. songarica settlement in the degraded dune-fixing forest. In the project, a series of experiments were to conducted using a method of spatial sequence instead of time successional sequence, including the evolvement rules of soil ecological factors and the seed settlement, seed germination and plant growth of R. songarica in decline dune-fixing woodland as soil crust development. The expected results will demonstrate the relationship between the evolutions of soil ecological factors and the parameters of R. songarica settlement, and also reveal the influence mechanism of soil crust development on R. songarica settlement. Meanwhile, it will construct a technical system of R. songarica artificial population planting, which provides a theoretical and practical guidance for renovating the degraded dune-fixing plantation and constructing sustainable wind break and sand fixation forest in the arid deserts area of North-west China.

民勤绿洲-荒漠过渡带的人工固沙林在其绿洲外围形成长达330多公里的“绿色长城”固沙带，对保障绿洲内部正常工农业生产发挥着举足轻重的作用。随着固沙林植被盖度增大，原有的流动或半固定沙丘逐渐趋于稳定，沙丘表面生物土壤结皮不断形成。近年来，受多因素影响，地表已形成土壤结皮的固沙林已出现更新受限现象，部分固沙林甚至开始衰败；然而，荒漠灌木红砂却能自然侵入，并能成功繁殖更新。但目前有关土壤结皮形成发育对红砂定居影响及作用机制方面的研究未见报道。为此，本项目以民勤衰败固沙林下的土壤结皮为研究对象，以时空互代的方法研究土壤结皮发育过程中土壤生态因子演变规律以及红砂种子捕获、萌发、植株生长及结实等相关参数动态变化，耦合土壤生态因子与红砂定居相关参数间的关系，揭示土壤结皮发育影响红砂定居的作用机制，构建干旱荒漠区红砂人工种群建植技术体系，为民勤绿洲边缘衰败固沙林修复以及可持续防风固沙林重建提供理论依据。

民勤绿洲边缘的人工固沙林已在其外围形成长达330多公里的“绿色长城”固沙带，从根本上改变了民勤的生态环境，极大保障了当地社会、经济的全面发展。然后，人工固沙林下一旦固定并有生物土壤结皮形成后，原有灌木逐渐出现形态及生理特性的衰退现象，其幼苗或根蘖苗成功定居率极低，但红砂种群不仅可以自然侵入并成功定居，还能实现其种群繁殖与扩散。为了探清民勤固沙林下生物土壤结皮发育对红砂种群定居的影响及作用机制，本项目以民勤绿洲边缘固沙林下不同发育阶段的生物土壤结皮为试验场所，围绕生物土壤结皮发育过程中地表微形态、土壤特性、土壤水文过程的演变及不同发育阶段土壤结皮对红砂种子吸水、发芽、幼苗存活、植株生长的影响为科学问题开展研究。结果显示：（1）在红砂种子定居阶段，藻类结皮通过增强地表稳定性来降低了枯落物的流失量，进而促进种子定居宿存；地衣结皮层龟裂并翻卷导致地表出现大量宽而深的裂缝（≥3mm）及微坑（深、口径≥5mm），进而通过阻碍种子移动或直接捕获种子来促进种子定居；苔藓生长发育后，部分藓株间形成了大量空隙，一定大小的空隙（≥3mm）降低了种子迁移速度并促进其宿存。（2）在红砂种子萌发及幼苗存活阶段，物理-藻类、地衣和藓类初期结皮通过减缓雨水的及时入渗来缩短种子饱和吸水时间，从而缩短强降雨后种子发芽高峰期出现的时间，并有效提高短时间内幼苗的成活率；但在藓类后期，长藓使种子处于一个悬置状态，进而导致种子萌发率及幼苗保苗率减小。（3）在植株生长阶段，相对于物理-藻类结皮，地衣和藓类结皮不仅阻碍了干旱沙区强降雨的及时入渗，还能显著增大蒸发量，从而导致植株根系可利用水分减少，所以不同类型结皮上的红砂形态特征及生物量出现明显差异。因此认为，在干旱沙区绿洲-荒漠过渡带结皮前期至地衣结皮林地可以开展红砂人工种群建植工作，并建议在流沙到藻类-地衣结皮区域以红砂实生苗栽植的方式为主，地衣结皮以补充红砂种子源的方式为主。