【作者】 白宏兵；

【导师】 吴发启；

【作者基本信息】 西北农林科技大学 ， 水土保持与荒漠化防治， 2013， 博士

【摘要】 我国黄土高原地区是世界上水土流失最为严重的地区,同时还面临着气候干旱、土壤养分贫瘠、植被稀疏等严峻的生态环境问题。植被在防治水土流失和改善生态环境方面具有重要作用,然而受水肥条件的限制植被在自然状态下恢复十分缓慢,不利于植被的生态效益与经济效益的充分发挥,因此,探索加快植被恢复的人工干预或调控措施显得十分必要和迫切。本研究针对黄土丘陵沟壑区土壤氮素缺乏,将施氮作为一种重要的人工干预措施,通过田间施肥试验,采用野外调查、采样与室内分析相结合的方法,研究了该区退耕地不同恢复年限的草本植物群落以及土壤养分对施氮干预的响应过程,分析和比较了不同施氮水平对不同群落类型的组成与结构、生物量分配、细根形态及分布特征、植物叶片与根系的氮分布、土壤养分变化等的影响,以明确土壤氮素对植被恢复的限制状况以及生态适应性,为黄土高原丘陵沟壑区的植被恢复与重建提供科学依据,以促进我国西部的生态环境建设和退耕还林还草工程的顺利实施。主要取得了如下研究结论：(1)随施氮量的不断增加,不同恢复阶段的群落中个体密度和物种丰富度均有不同程度地下降,表明施氮肥降低了物种的多样性。不同退耕年限或演替阶段的植物群落物种组成及生物多样性特征对施氮后的响应有所不同,施氮肥对演替初期植被的调控效果更佳,而演替后期的群落对氮肥施用表现出有很强的适应性和稳定性。(2)施氮同时提高了不同植物群落的地上、地下生物量,但二者在不同施氮水平下的平均提高幅度具有明显差异。随着施氮量的增加,生物量总体呈增加趋势,表明施氮肥的对群落生物量具有明显的提高作用。不同群落地下生物量主要分布在0-20cm土层中,占65%以上,施肥可促进地下根系生物量积累,有利于增加土壤碳储量,进而提高土壤质量。相关分析结果表明,地上与地下生物量之间呈极显著的正相关关系,施氮对群落地上部生长的促进作用大于根系,从而导致随着施氮量的不断增加地下生物量／地上生物量降低。(3)施氮不同程度地提高了各植物群落0-60cm土壤中的细根生物量、根长密度、细根表面积,但并不改变它们随在土壤剖面上随土壤深度的增加而急剧降低的垂直分布特征,细根的根长密度、表面积以及生物量呈现出相似的变化规律,同时施氮主要对直径<0.5mm、0.5-1mm细根根长密度、根表面积影响较大,氮肥施用后明显提高了直径较小的细根根长,相应地增加了细根表面积。(4)不同植物群落均表现为随施氮量增加,叶片中氮含量显著增加,并且在N5、N10及N15水平下增幅相对较大,在N20和N30水平则达到稳定或略有下降。细根中氮含量同叶片中氮含量表现出相似的规律,但明显低于叶片中的氮含量。从生长季节的变化来看,不同月份群落叶片与细根中氮含量差异明显,二者变化规律相似,均表现为5月份大于7月和9月,9月略高于7月。相关分析结果表明,不同植物群落细根中氮含量与叶片中氮含量之间存在显著的正相关关系。(5)施氮对土壤养分化学性质具有不同程度的影响,施氮肥可显著降低0-60cm剖面土壤的pH值,特别是表层0-20cm层土壤。施氮肥可增加0-20cm、20-40cm土层有机质和全氮含量,而对40-60cm层二者的影响相对较小。总体来看施氮对0-60cm各层土壤N03--N含量的影响大于NH4--N,随施氮量的不断增加,各层土壤N03--N含量均显著增加,且以0-20cm土层提高幅度最大,而各层土壤NH4+-N含量随施氮量增加变化无明显规律。随施氮量增加,各土层速效磷含量的变化趋势不同,其中0-20cm先增加后显著降低,而20-40cm土层总体呈不断降低趋势,施氮对土壤速效钾含量的影响则表现出与速效磷相一致的变化规律。整体来看,施氮确实可以在一定程度上促进植被恢复和演替以及改善土壤养分,同时也可能会带来一定的生态风险和环境问题,如何把施氮作为调控植被恢复一种有效手段还有许多的科学问题亟待解决,今后应注重开展长期性、系统性的研究以更好地揭示施氮对生态系统的影响机理,更好地为我国黄土高原地区生态环境建设和社会经济发展服务。更多还原

【Abstract】 The Loess Plateau was one of the most serious soil erosion regions in the world. This area faced many ecological environmental problems, such as dry climate, poor soil and sparse vegetation. The vegetations had important functions on soil erosion prevention and improving the ecological environment in this region. However, due to the limitation of soil water and soil fertility, Vegetation restoration was very slow in the natural state, which was not conducive to enhancing the ecological benefit and economic benefit of vegetation. So, exploring manual intervention or controlling measures for accelerating vegetation recovery speed was very necessary and urgent. The soil nitrogen content was commonly low in the loess hill and gully area. This research took the nitrogen utilization as an controlling measure, adopting the combined method of fertilization methods, field investigation, sampling and soil or plant analysis to study the response process of nitrogen fertilizer on different plant communities in abandoned lands, including the species composition and structure in plant communities, biomass, fine root’s morphology and distribution characteristics, nitrogen content in leaf and in root, and soil nutrient contents. The objective of this study was to determine the restriction and ecological suitability of soil nitrogen on vegetation recovery. It could provide science proofs for vegetation restoration and reconstruction, improving environment construction in the western part of our country, and also making sure the project of returning farmland to forest and grassland would be smoothly implemented. The main results and conclusions were as follows:(1) With the increase of nitrogen application amounts, the individual densities and species richness index of the different community in abandoned land declined to some extent, which indicated that nitrogen application reduced the species diversity. The responding characteristics of species composition and the biological diversity of different community on nitrogen utilization were different. The regulative effect of nitrogen fertilizer was more obvious in the early stage of plant succession, but the communities in the latter stages had the better adaptability and stabilization. (2) Nitrogen utilization at the same time increased the below ground biomass and above ground biomass, but the increasing effects were different between them in the same nitrogen treatment. With the increase of nitrogen application, the total biomass of community was all increased on the whole. The underground biomass of different community was mainly distributed in0-20cm soil layer, which accounted for above65%. Nitrogen utilization could help to promote the accumulation of the under-ground biomass, and improve the soil quality. Correlation analysis showed that there were significant positive correlations between above ground biomass and below ground biomass. Nitrogen utilization had better increasing effect on the above ground biomass than the underground part of community, so the proportion of below ground biomass and above ground biomass increased with the nitrogen utilization level.(3) Nitrogen fertilizer raised the fine root biomass, fine root length density and root surface area in the0-60cm soil profile. The vertical distribution characters of fine root biomass, fine root length density and root surface area in the soil profile were not changed significantly under different nitrogen treatments, and the changes appeared similarity. Nitrogen fertilizer had great effect on length density and root surface area of fine root with diameter<0.5mm and0.5～1mm. After nitrogen application, fine root length density with smaller diameters and increased the fine root surface area.(4) The nitrogen content in leaf of different community increased significantly with nitrogen application increase. As a whole, nitrogen content in leaf increased considerably higher in the N5、N10and N15treatments than in the N20and N30treatments, which the leaf nitrogen content began steady or reduced slightly. Although the changes of nitrogen content in fine root were similar to that in leaf, the content in root was significantly lower than that in leaf. In different growth seasons, there were obvious differences between the nitrogen contents in leaf and in fine root, which had the similar variation patterns. The plant nitrogen content in August was greater than that both in July and in September, and the content in September slightly exceeded in July. The correlation analysis showed there was a positive correlation between the nitrogen content of different communities in leaf and fine root.(5) The nitrogen utilization could improve the soil nutrient content on several levels. The soil pH in0-60cm soil profile could be reduced significantly with the use of nitrogen fertilizer, especially in0-20cm soil layer. With the increase of nitrogen fertilizer amount, the soil organic matter ant total nitrogen in0-20cm and20-40cm soil layer increased, but that in40-60cm soil layer was not obviously changed. On the whole, the influence of nitrogen utilization on soil NO3--N content in0-60cm soil was greater than the NH4+-N content. With the increase of nitrogen fertilizer, the NO3--N contents in each soil layer increased significantly, and the content in0-20cm soil layer was increased greatly. However, changes of the NH4+-N contents in each soil layer were not obvious. Besides, with the increase of nitrogen fertilizer, soil available P content in0-20cm soil layer were increased at the beginning, and then rapidly decreased. In20-40cm soil layer, soil available P content tended to decrease gradually. The effect of nitrogen fertilizer on soil available K content was similar to soil available P.As a whole, nitrogen application could hasten vegetation restoration and succession, and improve soil nutrients. However, the nitrogen fertilizer might also bring some Ecology risks and environment problem. There are still many critical problems urging to be solved in using nitrogen addition as an efficient way of intervening and regulating vegetation restoration. In the future, we should pay more attention to the long-term and systemic consequences of nitrogen application in soil and plant system in order to reveal the mechanism of nitrogen fertilization affecting ecosystem, and better guide the construction of ecological environment and social and economic development on the Loess Plateau.更多还原