民勤绿洲边缘退/弃耕地风蚀特征及其对沙尘暴物质的贡献研究

In recent half century, rivers and lakes have dried, groundwater table reduced down and water resource shortaged commonly at the lower reaches of arid inland river water-shed areas in China, it leads the eco-environment deterioration, desertification development and frequent aeolian sand disasters seriously, there have occurred some large areas of abandoned farmland to eco-land along the oasis fringes and it is enlarging continually due to the harsh drought condition, these lands are becoming into new wind-erosion and sand-dust source areas because of bare and soft surface, rich fine particles and strong wind, however, few researches and control steps have been done. The study will select 1a, 5a, 10a,15a,20a typical sample lands and contrasts at abandoned farmland to eco-land in and out of oasis at Minqin oasis fringe in lower reaches of Shiyang watershed area in Hexi,Gansu, some intruments for sand-driven wind at near land surface will be employed, aeolian sand-dust activity(sand-dust vertical/horizental capacity, wind erosion capacity, particle-size，wind speed), vegetation (species, coverage, growth) and soil property (compaction, bulk density, moisture content,organic matter, pH value, salts) in the sample lands will be investigated and analysesed specifically for 3 years, which will combine wind tunnel tests and remote sensing monitoring with long-term ecological data. The correlation between soil wind-erosion and dynamics of vegetation, soil property and aeolian sand-dust activity in space and time from 1a to 22a will be clarified, wind-erosion characteristics and matter contribution for sand-dust storms at abandoned farmland in and out of the oasis will be revealed and regulated under the natural eco-procedure, the theoretical basis on protection and desertification combating of abandoned farmland to eco-land for reducing sand-dust release and aeolian sand disaster in the arid inland river water-shed area will be offered.

近半世纪以来，我国干旱区内陆河流域下游普遍出现了河、湖干涸，地下水位下降，生态退化，沙漠化加强，大面积耕地因干旱缺水和沙化而退/弃耕，且呈扩大趋势。其地表裸露、疏松和干燥且蕴藏着巨量细粒物质，已经和正在演变成新的重要的风蚀地和沙尘源，但目前对其风蚀与风沙活动少有研究与治理。本研究通过在甘肃河西石羊河流域下游民勤绿洲边缘内外1a、5a、10a、15a、20a的退/弃耕地内分别设置典型样地与对照地，安置风沙仪器，开展连续3年近地表风沙活动（沙尘垂直/水平通量、风蚀量、风况等）、植被、土壤观测与沙尘化验、粒度分析等，结合风洞实验验证、遥感与生态资料，进行不同年限（1a-22a）退/弃耕地植被、土壤、风沙活动时空变化与风蚀相关关系及风蚀对沙尘暴物质贡献研究，揭示退/弃耕地自然生态过程中主要风蚀特征，风蚀对沙尘暴影响及物质贡献，为干旱内陆河流域绿洲边缘退/弃耕地保护、治理及风沙灾害防治提供理论依据。

对石羊河流域下游民勤不同类型区绿洲边缘退/弃耕地植被演化、风沙活动与风蚀特征等观测研究得出：1）民勤坝区、泉山区绿洲边缘1-17a退耕地随退耕年限增加植物科数减少，属先增后减，灌木由少变多再减少，草本先增后减，经历了向地带性荒漠植被演变的5个阶段，植被盖度稳定在坝区5-20%、泉山区10-20%；湖区1-50a盐碱和盐渍化退耕地经历了田间杂草、荒漠草本、盐生灌木演化，盖度15-30%；河岸带退耕地经封育封禁灌草修复或造林后，盖度达20-35%；绿洲内部退耕地形成了以造林种为建群种结合荒漠植被演化，盖度20-30%。2）不同退耕类型区年输沙量为坝区、泉山区>湖区>河岸带>绿洲区，同地带为荒漠区>过渡带>绿洲边缘区；在一定退耕期限年输沙量与退耕年限呈二元线性相关关系（y=10.006x2-68.819x +37.926），随退耕年限增加而增加。3）物种数量、丰富度、植被盖度与退耕年限负相关；植被状况对输沙率及高度分布影响明显，随植被高度增加而降低；同一风速下植被生长旺盛期输沙率明显低于生长初期，并与植被盖度、草本植物数量相关；输沙率在不同高度范围内因植被增加而降低，随高度增加降低。4）退弃耕地沙化情况为坝区、泉山区＞湖区＞河岸带区，绿洲内几乎没有，同一地带为荒漠区＞过渡带＞绿洲边缘。5）水平沙尘通量随高度增加而迅速衰减（Qh=2.05 +58.56e×p(-h/15.05)）；估算的民勤退/弃耕地沙尘年降尘/释放量约为1644吨/年，通过PM值分析，80%以上沙尘短距离运移后沉降到地面，而7%的可能到空中较长距离输移。6）随退耕年限增加，土壤有机质、N、P减小，K增加，并随土层深度增加，有机质、N、P、K减小；而浅层土壤水分呈先减再增，深层先增再减。7）根据民勤退耕地植被承载力低的现实，土体紧实，无沙源，盖度25% -20%即可；但在沙地植被盖度<25%，需沙障固沙。