【作者】 付微；

【导师】 黄明斌； 邵明安；

【作者基本信息】 中国科学院研究生院（教育部水土保持与生态环境研究中心） ， 土壤学， 2011， 博士

【摘要】 水蚀风蚀交错带是黄土高原土壤侵蚀最严重地区,尽管提高植被盖度能有效减小土壤侵蚀,但有限的水资源限制了植被盖度的提高。与此同时,该地区脆弱的生态环境表明其水碳循环过程及机制具有一定的独特性。因此,研究植被盖度对土壤水碳循环过程的影响,从而确定最优植被盖度对维持该地区生态环境的可持续发展具有重要意义。本文以该区典型植被——柠条、沙柳为研究对象,通过野外观测土壤CO₂通量排放、植物叶片光合及土壤水分动态变化,系统分析土壤水碳循环过程对植被盖度的响应,利用SHAW模型模拟典型气象年型土壤水分在不同植被盖度下的动态变化,以水量平衡原理为基础,估算了两种植物的最优盖度。取得的主要研究结果如下:1.土壤CO₂通量排放受植被盖度的影响,一般规律是植被盖度越高,土壤CO₂排放通量越大。对同一盖度处理,土壤CO₂排放通量表现出明显的月变化,在整个生育期中,8月份土壤CO₂排放通量达到最大。此外,植被类型对土壤CO₂通量排放产生显著影响,在黄绵土上种植柠条和沙柳,柠条地的土壤CO₂排放通量显著高于沙柳地。2.生物因素和非生物因素调控着土壤CO₂通量排放对植被盖度动态响应。在整个试验期间,生物因素（根系生物量和地上生物量）为主要调控因子,显著影响土壤CO₂通量排放对植被盖度的响应。在非生物因素中,地表温度（0-5 cm）和土壤表层含水量（0-6 cm）及两者交互作用部分地调控着土壤CO₂通量排放对植被盖度的响应。与地表温度（0-5 cm）相比,土壤表层含水量（0-6 cm）与土壤CO₂排放通量动态变化的相关性更大。3.植被盖度对植物叶片光合及植物生长产生显著影响。低盖度植被的叶片净光合速率明显高于高盖度,但其植被冠层光合固碳量却明显低于高盖度植被。植物叶片的光截获能力及植物本身土壤水分利用能力调控着叶片光合过程对植被盖度的响应。与低盖度相比,高盖度处理柠条的株高和茎粗相对较低,与沙柳结果相反。地下土壤水资源竞争（对称竞争）调控柠条植株生长对植被盖度的动态响应,而地上光源竞争（非对称竞争）调控沙柳植株生长对植被盖度的动态响应。4.土壤质地和降雨脉冲对土壤CO₂通量排放及叶片净光合速率有显著影响。柠条和沙柳土壤CO₂通量排放对土壤质地响应截然不同:黄绵土中柠条土壤CO₂排放通量高于风沙土,但黄绵土中沙柳土壤CO₂排放通量低于风沙土。结果表明:柠条异养呼吸和沙柳自养呼吸对土壤质地响应比较敏感。土壤质地对叶片光合速率有显著影响,除了2010年的沙柳生育末期,黄绵土中两种灌木植物叶片净光合速率通常高于风沙土。土壤水分条件是调控植物叶片净光合速率对土壤质地响应的主要因子。土壤CO₂排放通量和叶片光合的月变化趋势表明:叶片光合速率峰值出现时间滞后于土壤CO₂排放通量峰值。当土壤由干变湿,降雨脉冲会对土壤CO₂通量排放产生较大影响。降雨所影响土层深度范围内的土壤水分分布是调控植物叶片净光合速率对降雨与土壤质地交互作用响应的主导因子。5.植被盖度影响土壤水分时空分布。土壤剖面含水量、根层储水量、棵间蒸发（0-15 cm）随植被盖度的增加而降低,不同植被盖度土壤剖面含水量差异显著。0-100 cm土层土壤水分变化幅度较大,对降水、根系吸水及土面蒸发响应敏感。幼龄期,土壤干燥化程度随植被盖度、林龄的增大趋于严重。6.水蚀风蚀交错带土壤侵蚀严重,提高植被盖度是减少土壤侵蚀的有效手段,但是该区有限的降水资源限制了植被盖度的提高,因此基于水量平衡理论确定最优植被盖度是区域生态环境可持续发展的关键。我们利用田间水分观测资料对SHAW模型进行校正和验证,并根据历史气候资料确定了一个代表性气候年型（典型干旱年,出现的概率是10%）,确定了该区柠条和沙柳达到最优植被盖度时的最大叶面积指数分别为1.27和0.70。本研究表明水蚀风蚀交错带不同植被盖度的土壤CO₂通量排放差异主要归因于根呼吸（自养呼吸）的不同。叶片光合及植物生长对植被盖度的响应是光源竞争、土壤水资源竞争和空间竞争相互作用的结果。土壤质地对土壤CO₂通量排放和叶片光合产生显著影响,表明土壤质地空间异质性在研究半干旱生态系统碳循环方面不容忽视。植被盖度对土壤水分时空动态变化的影响与植物生长状况和降雨季节分布特征密切相关。幼龄期,土壤干燥化程度随植被盖度、林龄的增大趋于严重化,因此,以水量平衡为基础最优植被盖度是维持半干旱区生态系统可持续性的关键。研究结果有助于揭示黄土高原水蚀风蚀交错带水碳循环过程,并对该地区植被恢复和重建具有重要指导作用。更多还原

【Abstract】 The transitional belt of wind and water erosion is the center of the intensive soil erosion. Although increasing plant coverage can effectively control soil erosion, limited soil water resources restrict increasing plant coverage. At the same time, the fragile eco-environment of the transitional belt of wind and water erosion indicates that the cycling process and mechanism of soil water and carbon in the region are different from other region of the Loess Plateau. Therefore, it is very important for sustainable development of eco-environment to study the effect of plant coverage on cycling of soil water and carbon to determine the optimal plant coverage. In this study, we analyzed the responses of soil CO₂ efflux, leaf photosynthesis and soil water to plant coverage by field observation. The Simultaneous Heat and Water Transfer （SHAW） model was used to simulate soil water content variations with plant coverage for a representative climatic year to determine the optimal plant coverage for two dominant shrubs （Caragana korshinkii Kom and Salix psammophila） in this area, based on soil water balance. The main results are as follows:1. Plant coverage had effect on soil CO₂ efflux. Generally, plant coverage was higher, soil CO₂ efflux was the larger. For the same plant coverage treatment, monthly variation of soil CO₂ efflux was observed during the growth period: maximum soil CO₂ efflux was achieved in August. In addition, vegetation type had pronounced impact on soil CO₂ efflux, the soil CO₂ efflux of C. korshinkii were significantly greater than those of S. psammophila, growing in silt loam soil.2. The regulation of biotic factors and abiotic factors to the response of soil CO₂ efflux to plant coverage. During the experiment, biotic factors （root biomass and aboveground biomass） were important driving factors mediating the response of soil CO₂ efflux to plant coverage for two shrubs. Among the abiotic factors, the soil water content （0-6 cm）, soil temperature （0-5 cm） and their interaction could partly regulate the response of soil CO₂ efflux to plant coverage. Soil water content （0-6 cm） was more closely correlated with soil CO₂ efflux than soil temperature （0-5 cm）.3. The significant effects of plant coverage on leaf net photosynthesis rate and plant growth. Leaf photosynthesis rate was significantly larger in low plant coverage than in high plant coverage for two shrubs, on the contrary, the canopy photosynthetic carbon fixation was significantly greater in high plant coverage than in low plant coverage. The interaction of light availability and plant water availability could mediate the response of leaf photosynthetic capacity to plant coverage. In C. korshinkii plots, high plant coverage plants had lower plant height and stem diameter compared to low plant coverage plants, which were in contrast to S. psammophila. The response of plant growth to plant coverage may be mainly caused by belowground competition for water （symmetric competition） in C.korshinkii plots, whereas by aboveground competition for light （asymmetric competition） in S. psammophila plots.4. Soil texture and rain pulse significantly affected soil CO₂ efflux and leaf net photosynthetic rate. Soil CO₂ efflux of C. korshinkii was larger growing in silt loam soil than in sandy soil. On the contrary, soil CO₂ efflux of S. psammophila was lower growing in silt loam soil than in sandy soil. This showed that heterotrophic respiration of C. korshinkii was more sensitive to soil texture, but autotrophic respiration of S. psammophila. Soil texture had significant effects on leaf net photosynthesis rate: leaf net photosynthesis rate was significantly larger in silt loam soil than in sandy soil for two shrubs, except that the late growth stage of S. psammophila in 2010. Soil water condition was important driving factors mediating the response of leaf net photosynthesis rate to plant coverage for two shrubs. Significant monthly variation of soil CO₂ efflux and leaf photosynthesis showed that the maximum leaf photosynthesis rates lagged behind the maximum soil CO₂ efflux. When soil became wet, large amount of rainfall could have pronounced impact on soil CO₂ efflux. Soil water distribution from the depth of rainfall infiltration was the main factors mediating the leaf photosynthesis response to the interaction of rainfall event and soil texture.5. The effects of plant coverage on temporal and spatial variation of soil water. The average soil water content, soil water storage in root layer and soil evaporation （0-15 cm） decreased with the increase of plant coverage. There were significant differences in soil water content of different plant coverage. The soil water content in the upper 0-100 cm had larger change range, which was sensitive to rainfall, root water uptake, and soil evaporation. During young-age period, the degree of soil desiccation increased for the shrubs with increasing plant coverages and stand age.6. The transitional belt of wind and water erosion suffered intensive soil erosion. Although increasing plant coverage can effectively control soil erosion, limited soil water resources restrict increasing plant coverage. Therefore, the determination of optimal plant coverage, based on soil water balance, is the key for sustainable development of eco-environment. We used field observation data to calibrate and validate SHAW model. The representative dry year （i.e. 10% probability of a drier year） was determined, according to past climate data. Based on soil water balance, the optimal coverage for the C. korshinkii and S. psammophila shrub species corresponded to a maximum LAI of 1.27 and 0.70, respectively.The results showed that the differences in soil CO₂ efflux among plant coverage could be attributed to the different root respiration （autotrophic respiration） in the transitional belt of wind and water erosion. The responses of leaf photosynthesis and plant growth to plant coverage were caused by interplaying of competition for light, water and space. The effects of soil texture on soil CO₂ efflux and leaf photosynthesis suggests that considering soil texture heterogeneous is important in carbon cycling of semiarid ecosystems. The effects of plant coverage on temporal and spatial variation of soil water were closely related with plant growth and seasonal distribution of rainfall. During young-age period, the degree of soil desiccation increased for the shrubs with increasing plant coverages and stand age. Therefore, based on soil water balance, the optimal coverage is the key to maintain the ecosystem sustainability in semiarid region. The conclusions of this study could help to reveal cycling processes of water and carbon in the transitional belt of wind and water erosion and to guide for vegetation restoration and revegetation.更多还原