| With the rapid expansion of China's population and increased use of fertilizer, a large number of lakes, rivers and near sea area have broken out the phenomenon of eutrophication in varying degrees. The Three Gorges Reservoir Area, as an important lake water area and strategic water resource reservoir, has gradually turning into a special deep reservoir that can be artificially controlled and with typical lake and river characteristic. The ecological environment of surrounding area has also changed at the same time. So how to control the fixation and emission of nitrogen in agriculture soil around the reservoir has become a top priority of ensure the water quality of reservoir and the agricultural development of reservoir surrounding. However, domestic and international present research of non-point source pollution was focused on the nitrogen and phosphorus movement of lakes, rivers and other natural landscape. Thus, it is of vital importance to elucidate the movement, transformation mechanism and dynamic process of purple soil nitrogen in soil ecosystem, and also choose a effective controlling technology of nitrogen loss for developing the win-win emission reduction agriculture of environmental protection and agricultural sustainable development, and for enhancing the comprehensive capacity of water and farm systems answering to the climatic change in Three Gorges Reservoir Area. In this study, taking the small catchment of Wangjiagou in Three Gorge Reservoir Area as study object, the effect of topography, land use and soil depth on soil nitrogen distribution was analyzed with the statistical method of geostatistics, combined with the test method of field sampling and laboratory analysis, on this basis, to clarify the basic soil nitrogen condition and soil fertility situation in the target area. Secondly, the mineralization dynamic property of soil organic nitrogen under the influence of multiple environmental factors (soil temperature, moisture, and exogenous nitrogen) was described by means of aerobic incubation method, and the correlation between mineralization process and influencing factors was revealed too, in order to elucidate the transformation mechanism of nitrogen in soil ecosystem. Then, the leaching loss characteristic and dynamic movement feature of nitrogen in vertical profile of different texture soil was annotated through soil column simulation in lab, to enrich the understanding about the fixation mechanism of nitrogen in purple soil. Lastly, based on the knowledge of nitrogen movement and transformation, a field trial was carried out to compare the practical application effect under different control measures in agricultural production in order to select the most appropriate way to control soil nitrogen of Three Gorges Reservoir Area and provide scientific basis to the technology exploitation of non-point source pollution controlling and development of sustainable agriculture. The main results are as follows:1. The spatial distribution of soil nitrogen in Three Gorges Reservoir AreaThe nitrogen spatial distribution and the effect of topography and land use on soil total nitrogen and available nitrogen were studied by taking the small catchment of Wangjiagou as sampling area. It showed that the total nitrogen of central region is higher over the whole small catchment, it decreased gradually as the shape of layer from the center to the surrounding. The highest available nitrogen content area was still distributed in central and southern region and the lowest available nitrogen content area was located in the west and southwest, the available nitrogen content of northeast and east area was at the middle level in the whole small catchment. To the nitrogen distribution of vertical space, the variation of total nitrogen in the vertical profile of 0～100 cm was significant and smooth, with the increasing of soil depth, total nitrogen has showed a trend of decreasing gradually under various land use type, and the difference was not significant (p<0.01). The change law of available nitrogen in the vertical profile of 0-60 cm was very similar with total nitrogen, but the difference between different soil layers was highly significant (p<0.01).The effect of topographic factors on soil nitrogen distribution was very obvious. Within the entire small catchment of each slope gradient, the content of soil total nitrogen was concentrated in the content range of 1.0-1.5 g·kg-1, the distribution condition of soil total nitrogen in lower slope gradient region was stable and moderate, but the distribution condition of soil total nitrogen in higher slope gradient region was generally lower and uneven. The effect of slope gradient on available nitrogen was less than total nitrogen, with the increasing or decreasing of slope gradient, soil available nitrogen content has not showed an obvious change. To the effect of slope aspect on nitrogen distribution, in addition to the lower content distribution of total nitrogen on northeast, the distribution condition of other slope aspects of the whole small catchment was nearly similar which was concentrated in the content range of 1.0-1.5 g·kg-1, only in the southwest slope aspect existed some plots which had higher content of total nitrogen. The highest content region of available nitrogen was appeared in the slope aspect of southwest, and lowest content region of available nitrogen was appeared in the slope aspect of south and southeast, in addition, the content of available nitrogen in other slope aspects was moderate and homogeneous. The effect of slope position on soil nitrogen was varied greatly. In the region of low slope position, the distribution of soil nitrogen was extremely uneven, soil sampling with higher nitrogen content and lower nitrogen content were all appeared. But in the region of high slope position, the content of soil nitrogen was lower, there was no soil sampling with higher nitrogen content. The nitrogen content of middle slope position region was in the range of 80～110 mg-kg-1. The content of soil total nitrogen under different land use types showed a decreasing law of paddy field, forest land, mulberry field, dry land and fallow land, and the distribution of soil available nitrogen follow the decline order of paddy field, forest land, dry land, fallow land and mulberry field. In this part, the impact factors of soil nitrogen distribution have anatomized deeply, and the basic soil nitrogen condition and soil fertility situation has clarified at the same time.2. Nitrogen mineralization characteristics of purple soil in Three Gorges Reservoir AreaThrough the method of aerobic incubation, the mineralization dynamic property of soil organic nitrogen under comprehensive influence of soil temperature, moisture, and exogenous nitrogen was described in detail. It showed that a trend of increasing gradually of net nitrogen mineralization in purple soil with the increasing incubation temperature could be found during each culture stage. The net nitrogen mineralization difference of sandy loam between three temperature levels was highly significant (p<0.01), but the net nitrogen mineralization of loam and clay between 15℃and 35℃as well as 25℃and 35℃both showed a highly significant difference, while the net nitrogen mineralization hadn't showed a significant difference between 15℃and 25℃. The response of purple soil net nitrogen mineralization to the change of moisture was also extremely regular. It showed the trend that the value of net nitrogen mineralization increased gradually with the increasing of incubation moisture, and the difference between three moisture levels was highly significant (p<0.01). In addition, soil temperature and water content has significant interaction effects on nitrogen mineralization of purple soil. When the levels of temperature and moisture were the lowest, the value of net nitrogen mineralization was negative, which indicated that the two factors resulted in the reverse process of mineralization. However, when the levels of temperature and moisture were the highest, the value of net nitrogen mineralization wasn't the highest. The effect of temperature on nitrogen mineralization of purple soil was more than moisture to a certain extent.Exogenous nitrogen showed a significant promoted role on the process of purple soil nitrogen mineralization. The value of net nitrogen mineralization with exogenous nitrogen was obviously higher than the one without exogenous nitrogen during each culture stage under different temperature and moisture levels, and showed a highly significant difference (p<0.01). When the levels of temperature and moisture were the highest, the effectiveness of exogenous nitrogen would increase corresponding, and the mineralization potential of organic nitrogen would be stronger than the biological immobilization of mineral nitrogen, so the value of net nitrogen mineralization would be significantly higher. Nitrogen mineralization of purple soil under different soil texture without exogenous nitrogen showed a decreasing trend of loam, clay and sandy loam, and the nitrogen mineralization with exogenous nitrogen was followed the decline order of sandy loam, clay and loam. Because of the well aeration condition of loam and sandy loam, it had a positive effect on increase in the species and activity of aerobic microbial and further contributed to the occurrence of nitrogen mineralization. This part had elucidated the comprehensive effect of soil texture, temperature, moisture and exogenous nitrogen on the nitrogen mineralization of purple soil, it was help to deeply annotate the transformation mechanism of soil nitrogen.3. The law of soil nitrate leaching in Three Gorges Reservoir AreaSoil leaching regular of different texture based on purple soil in Three Gorges Reservoir Area were conducted and explored combined with soil core lysimeter. It showed that the water leaching quantity of sandy loam was the most in three types of soil texture, and the quantity of clay was the minimum under the same quantity of Irrigation water. The quantity of water leaching was significantly related with the quantity of irrigation water, In the whole leaching period, the maximum of water leaching almost appeared in irrigation prophase (5-10 d). Compared with the real-time leaching concentration of soil nitrogen under different levels of nitrogen fertilizer using, the real-time leaching concentration with high nitrogen input was more than the one with low nitrogen input, and the leaching concentration with no nitrogen input was minimum. The higher value of nitrogen leaching almost appeared in irrigation prophase (5-30d). Because of the higher leaching concentration and the shorter sampling interval, the nitrogen leaching rate of three sorts of soil texture also appeared maximum in the initial stage of irrigation (5-10 d). In the whole irrigation stage, the cumulative leaching concentration showed a trend of increasing first and reducing afterwards under different levels of nitrogen fertilizer using. The maximum of cumulative leaching concentration of nitrate nitrogen appeared in 30 d after irrigation starting. Being different from nitrate nitrogen, the cumulative leaching concentration maximum of ammonium nitrogen showed a different trend which affected by soil texture, the maximum of it in the sandy loam and clay appeared in 20 d after irrigation starting, while the maximum of it in clay appeared in 40 d after irrigation starting.Compared with a variety of nitrogen forms in the leachate, it was could be found that the leaching concentrations of nitrate nitrogen were all greater than ammonium nitrogen under different fertilizer levels, in other words, the movability of nitrate nitrogen was better than ammonium nitrogen in vertical direction. But deal with different soil texture, the vertical movement regulars of nitrate nitrogen and ammonium nitrogen were different. For example, under no nitrogen or low nitrogen fertilizer using, the leaching concentration of nitrate nitrogen of three texture soil were followed the decreasing order of sandy loam, loam and clay, but under high nitrogen fertilizer using, the decreasing order changed into loam, sandy loam and clay. But there were little effect of nitrogen fertilizer amount on the leaching concentration of ammonium nitrogen, the leaching concentration of ammonium nitrogen under three levels of nitrogen fertilizer using were all followed the decreasing order of loam, sandy loam and clay. In this part, the leaching loss characteristic and dynamic movement feature of nitrogen in vertical profile of different texture soil was annotated, and further enriched the understanding about the fixation mechanism of nitrogen in purple soil.4. Loss features and control measures of soil nitrogen in sloping upland of Three Gorges Reservoir AreaBased on the knowledge of spatial distribution and movement, transformation of soil nitrogen in small catchment of Three Gorges Reservoir Area, combined with comparing the practical application effect under different control measures in agricultural production, this part selected the most appropriate way to control soil nitrogen in Three Gorges Reservoir Area. It showed that the rainfall mainly concentrated in June and July in study area. Meanwhile, there were three times of runoff in June, the maximum of runoff up to 21.96 L·m-2. The highest loss of sediment appeared in July, which was 61.85 t·km-2, the loss of runoff and sediment showed a significant positive correlation with the average rainfall intensity in the rainfall period. In the runoff plots which carried out conservation tillage, runoff and soil erosion decreased in various degrees. Among the whole observation period, T3 (no-tillage+downslope cultivation+straw overlay) and T5 (no-tillage+contour cultivation) were the best ways to prevent the loss of runoff, while T4 (downslope cultivation+hedgerow) was the best way to prevent the soil erosion.Runoff and sediment were the two ways which caused the loss of soil nitrogen from sloping uplands. Among them, the nitrogen loss through runoff was relatively less, the lost of ammonium, nitrate, and the total amount of nitrogen were 10.75 mg·m-2,44.55 mg·m-2 and 291.38 mg·m-2 respectively. All the loss maximums of various forms of nitrogen appeared in T1 (conventional downslope cultivation) which had no measure taken. By the contrast, all the loss minimums of ammonium, nitrate, and total nitrogen were 5.63 mg·m-2,24.06 mg·m-2 and 152.62 mg·m-2 respectively, and all appeared in T4. In addition, the nitrogen loss under T2 (downslope cultivation+PAM soil regulators) was relatively less too. Therefore, T2 and T4 were the best two ways to prevent the loss of nitrogen through runoff. The nitrogen loss through sediment was relatively more, especially when the runoff plots were constructed of T1 and T6 (contour cultivation). That is to say, contour cultivation did little contribution to preventing nitrogen by sediment from losing. Compared with T6, T4 and T5 (no-tillage+contour cultivation) did something stable to the nitrogen in sediment, the loss of nitrogen were the least after every rainfall during all the observation period. During the whole observation period, the loss of nitrogen through sediment was far more than which lost through runoff, the loss amount of nitrogen through sediment was 1.83-3.38 times than the one through runoff. However, the loss of nitrogen through sediment was mainly by the kind of NO3--N, and the loss of nitrogen through runoff was mainly by the kind of particulate, the loss of particulate nitrogen account for 53% to 62% of the total nitrogen.In summary, the spatial distribution of soil nitrogen showed as the interaction of multiple external conditions like slope gradient, slope aspect, altitude and land-use patterns in the Three Gorges reservoir area of complicated topography, which constituted the fundamental fertility characteristics of soil nitrogen in small catchment of reservoir area. The soil temperature-oriented nitrogen mineralization dynamics theory was incompetent to express the nitrogen mineralization mechanism which was widely observed in purple soil, the driving force of nitrogen mineralization derived from the interacted function of temperature and water content which, however, could vary along with the change of outside nitrogen and soil texture, further affecting comprehensive response of nitrogen mineralization in purple soil. The leaching concentration and rate of nitrogen in the soil vertical profile largely depends on the proportion of clay particle and the level of nitrogen fertilizer using in soil. The transformative characteristics of soil nitrogen in slopes and the movement regulation of soil nitrogen in vertical profile were complementary, which commonly decided the suitable controlling technology of soil nitrogen loss in small catchment of Three Gorges Reservoir Area, making it more clearly that only under a certain external environment could the control technology adjust the behavior of soil nitrogen. If the change of external environment exceeds a certain threshold level, the function of artificial adjustment was minimal. In the future, the movement and transformation mechanism of soil nitrogen should be conducted under the background of large-scale, whole regional ecosystems and climate zone in Three Gorges Reservoir Area, to know the fundamental storage of soil nitrogen and the potential risks of non-point pollution under different types of agricultural soil in reservoir area. Also, the study on the coupling mechanism of soil nitrogen mineralization dynamics and enzyme function should be enhanced innovatively. Meanwhile, we should build wild observation system of large natural soil columns under the premise of ecological agriculture in order to supply more in-depth, comprehensive basic research data to found the integrated control technology systems of soil nitrogen to a large-scaled area in Three Gorges Reservoir Area, and also to design and integrate the most reasonable and effective technical control system of farm land Nitrogen in Three Gorges Reservoir Area.
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