【作者】 曾昭霞；

【导师】 李凤民；

【作者基本信息】 兰州大学 ， 生态学， 2009， 博士

【摘要】 为了探讨半干旱黄土高原区不同景观位置下不同退耕还草方式对土壤质量的影响，我们选取了四种典型的景观位置，即东北方向上坡位(neu，坡度10—14度)、东北方向下坡位(nel，2—6度)、东南方向中坡位(sem，12—16度)和偏东南方向坡顶(set，坡度4—8度)并在这四个位置上布设农田直接撂荒、农田转化为紫花苜蓿(Medicago sativa)草地、沙打旺(Astragalus adsurgens)草地和草木樨(Melilotus officinalis)草地4种退耕方式，对退耕3年之后不同景观位置和不同退耕方式下它们的土壤有机碳、轻组有机碳、土壤氮素、地上生物量以及土壤水分动态进行了系统的研究。获得了以下主要结果：对于不同景观位置的紫花苜蓿地的分析得出，经过紫花苜蓿3年的生长，东北坡向下坡位的紫花苜蓿样地相对于其他3个位置的来说，具有最高的土壤水分含量、最高的全C、全N含量以及最低的地上生物量、最低的轻组C和矿质N含量以及LFOC／TOC和MN／TN值。坡顶的情况正好与之相反。因此我们得出结论：土壤质量和土壤养分稳定性随坡位的升高而降低，但是土壤养分利用效率则随坡位的升高而增加；不同景观位置撂荒地3年的土壤水分、养分和地上生物量，发现随着撂荒年限的增加，四块样地的土壤水分、地上生物量、TOC含量、TN含量以及LFOC含量都增加，而土壤矿质N含量以及LFOC／TOC和MN／TN值则随时间推移而降低。对于处于坡顶的撂荒地来说，其地上生物量、TOC含量、TN含量和LFOC都要高于其他3个位置。说明在本地区将不同退化程度的农田撂荒是保持土壤质量的一项有效措施，不管对什么坡向、坡位的退化农田，自然撂荒都能显著改善土壤质量；农田转化为紫花苜蓿和农田直接撂荒相比较，农田撂荒之后土壤质量在不同景观位置地块之间差异不显著，东北向下坡位地块的土壤水分、全C、全N含量并未如紫花苜蓿地中的一样表现出优势，相反在农田转化为紫花苜蓿地之后这些指标含量低的坡顶位置在农田撂荒处理中反而具有最高的生物量、全C、全N以及轻组C含量。说明对于土壤质量退化比较严重的陡坡或者坡项来说，自然撂荒比种植苜蓿对土壤质量的恢复更为有利；本文深入分析了坡顶农田退耕并转化为4种不同种类草地后土壤质量的变化，发现这4种转化都能极显著的改善土壤质量。而其中农田直接撂荒的处理对土壤水分的恢复最为有利但其缺点是产量太低。在我们3年的实验期内，农田转化为沙打旺草地之后的土壤水分利用效率最高并分别比紫花苜蓿地、草木樨地和撂荒地高70．02％、42．05％和36．80％。沙打旺地有最高的地上生物量但其TOC含量比草木樨和撂荒地中都低。草木樨地的LFOC含量最高并分别比紫花苜蓿、撂荒和沙打旺地高30．39％、33．56％和15．87％。同时草木樨地的MB-C和TOC含量也比其他3种退耕方式略高。而对于紫花苜蓿地来说，其LFOC和TOC含量最低但其TN含量和TN含量增长率是4种样地中最高的。考虑半干旱黄土高原区脆弱的生态环境，从土壤质量提高和养分稳定的角度，草木樨不失为一种陡坡地或者退化农田退耕的良好选择。更多还原

【Abstract】 This thesis explores the effect of landscape orientation on soil moisture, abovegroundproductivity, and soil C and N pools in alfalfa (Medicago sativa) grassland and fallow landwithin the semi-arid Loess Plateau in China. Four sloping sites with different landscapeorientation were chosen for attained the aims during 2003-2005. The sites were Neu(north-east facing, upper slope), Nel (north-east facing, lower slope), Sem (south-eastfacing, middle slope), and Set (south-east facing, top slope). The study was also conductedto explore the ecological restoration effects of the conversion of cropland to foragelegumes on soil characteristics in the semiarid Loess Plateau of China. Four types oftreatments: (1) fallow (F); (2) alfalfa (Medicago sativa) forage legume (A); (3) sweetclover(Melilotus officinalis) forage legume followed by fallow (SF) and (4) erect milkvetch(AstragaIus adsurgens) forage legume (E) were used to substitute spring wheat, the nativecrop in April 2003 to October 2005. The results indicated that:After 3 years of alfalfa growth (measurements undertaken in April 2005), we foundthat the soil moisture contents within the 5 m soil layer at the Set, Neu, Sere, and Nelslopes were 427.51,543.03, 560.01, and 662.71 mm, respectively. The total forage yieldsat the Set slope in 2005 exceeded those at Neu, Nel, and Sem slopes by 3.1, 29.5, and19.7%, respectively, and the total organic carbon (TOC) content at the Nel slope washigher than that at the Set, Neu, and Sem slopes by 36.8, 23.9, and 38.9%, respectively.The light fraction organic carbon (LFOC) content at the Set slope was higher than that atthe Neu, Sem and Nel slopes by 19.8, 22.9, and 27.4%, respectively. The observed trend ofTN dynamics is similar to that of TOC. The mineral N content at the four sites was Neu≈Sem＞Set＞Nel. In other words, the lower slope (Nel) recorded the highest soil water,TOC, and TN contents, but the lowest aboveground biomass, LFOC, and AvaiN content;the opposite is true for the top slope (Set). We concluded that the conversion of cropland toalfalfa in a semi-arid agro-ecosystem has a positive effect on soil quality for all slope orientations and that the soil quality and stability decrease upslope.After the farmland fallowed for 3 years, we found that the soil moisture, above-groundbiomass, TOC content, TN content and LFOC content in these four slopes were allincreased with time, while the mineral N content in these slopes was decreased with time.The above-ground biomass, TOC content, TN content and LFOC content in the top slopewere higher than those in the other slopes. It means that it will be an effective method toconversion the cropland to fallow for the better soil quality in a semi-arid Loess Plateau.Fallow can improve the soil quality of slopes in every direction, every position and everydegree.Dissimilarly to the situation about the difference in soil quality among differentlandscape position in conversion of cropland to alfalfa grassland, the soil quality has nosignificant difference in every landscape position when 3 years of conversion of croplandto fallow. So fallow must be a more friendly method for improving the soil quality indeteriorated slope than conversion cropland to alfaifa grassland.After the conversion of crops to various forage legumes for 3 years, the highest soilwater content in various soil layers was observed in F treatment during the wholeexperimental period, followed by SF treatments. The light fraction of organic C (LFOC)and increase of soil water content in SF was the highest. The aboveground biomass in SFwas lower than that in E but higher than that in A and F. The total organic carbon (TOC)content in F and SF was higher than that in the other two treatments. But the total nitrogen(TN) content in F and SF was lower than that in A and E. Over the entire experimentalperiod, the water use efficiency (WUE) of E was higher than that of A, SF and F by70.02%, 42.05% and 36.80%, respectively. In conclusion, SF proved to be a better patternfor soil productivity, soil quality and nutrient cycling under semiarid conditions.更多还原