| 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. |
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