风险评价 Soil nutrient，Agricultural nonpoint source pollution, Nitrogen
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Abstract随着农地的过度开发利用，土壤中养分亏缺成为制约农业可持续发展的关键因子。大量使用化肥和农药成为保持农田高产的主要手段，但是如何提高土壤中养分的利用效率，降低氮、磷流失的风险成为一个新的问题。研究农业区土壤氮磷的时空动态变化特征、土壤氮磷储量与盈亏平衡及其环境因子的关系，对于客观评价农业非点源污染氮磷流失的风险，制定科学合理的农业非点源污染控制措施具有重要意义。 本论文以北京市延庆盆地农业区作为研究区域，针对不同的土地利用类型果园、蔬菜地、玉米地，进行野外土壤布点采样分析，分五层（0-10cm、10-25cm、25-40cm、40-70cm和70-100cm采集土壤样品），共计38个土壤采样点，分别于2004年4月、6月、8月、10月，2005年4月和10月采集土壤样品。与此同时，针对采样样地进行农田管理措施方面的问卷调查。在此基础上，探讨了不同农田生态系统作用下，氮磷养分元素的时空变异特征、储量年度变化特征和土壤氮磷盈亏状态，分析了农业非点源污染氮流失的风险。获得如下结论： （1）在一个生长周期内，不同的农田土地利用，其氮磷养分在土壤剖面上具有显著的时空变异性。不同土地利用影响下土壤全氮全磷含量均表现为：从剖面自上向下呈现逐渐降低的趋势，并且各层次之间差异显著。对于玉米地和蔬菜地来说，0-10cm和10-25cm是氮素变化的活跃层，25-40cm是氮素变化的缓冲层，40-70和70-100cm是氮素淋失层。蔬菜地全磷含量在0-40cm土层变化剧烈，在40-70cm土层以下开始变化平稳；果园全磷含量在整个剖面上变化都很平稳。 6月份到8月份这段时间是玉米地发生氮磷淋失的高风险期，蔬菜地和果园在全年的生长季中都存在氮磷的淋失风险。 （2）经过土地利用管理和植被的作用，玉米地和果园的全氮储量在1m土层内，均表现出了增加的趋势；蔬菜地在40-70cm和70-100cm土层表现出大量的氮素淋失。对于玉米地来说，从40cm开始向下，养分已经开始表现出大量积累；对于蔬菜地来说，从40cm开始，氮素出现大量淋失；对于果园来说，在整个1m剖面上都存在发生氮素淋失的风险；在70cm以下，氮素积累显著增加，对地下水体具有巨大威胁。研究区内春季的风蚀和冬季的降雪也是引起土壤养分流失的重要因素。研究区内土壤磷储量不断增加，具有不断向下淋溶流失的趋势，其对水体富营养化威胁也越来越大。 （3）经过2005年4月和2005年10月两期土壤中氮素和磷素的盈亏分析，结果表明土地利用和地貌类型对于土壤养分的盈亏具有显著的影响。三种主要农业用地类型氮素积累的大小关系为：果园 > 蔬菜地 > 玉米地。淋溶是土壤磷素流失的一个重要途径，蔬菜地土壤中积累的磷素是最高的，远高于果园和玉米地；果园土壤中的磷素积累在40-70cm土层出现大量亏损，在70-100cm的大量积累。对于农民追求经济利益的最大化，在政策上要应给予积极引导，不能盲目地将玉米地等相对稳定的土地利用类型，转化成为蔬菜地和果园，同时要争取在一定区域内合理配置土地利用方式，利用不同土地利用方式下土壤养分盈亏的差异，实现既提高肥料的利用效率，降低经济投入，又能减少土壤养分的流失，降低农业非点源污染发生的几率，减小对水体富营养化的压力。 （4）对于我国农业的农户分散经营方式而言，地块尺度氮分级方案可以很好地识别出影响氮流失的关键因子以及具有高流失风险的地块，可以对其实行有效的管理措施。
With the overexploitation of agricultural land, soil nutrient deficit has become the key factor that affects agricultural sustainable development A high crop yield of farmland was kept by large use of fertilizer and pesticide during crop production. But how to improve use efficiency of soil nutrient, decrease the risk of nitrogen and phosphorus losses, have become a new problem. Studying the characteristics of spatio-temporal changes of soil nitrogen and phosphorus, the relationship between soil nitrogen and phosphorus pool and breakeven, and its environmental factors, has highly significance in evaluating the risk of nitrogen and phosphorus losses through nonpoint source pollution objectively, making the policy of scientific and reasonable management practices of solving nonpoint source pollution. The study area is located in Yanqing basin, northwestern Beijing. In this study, 38 sampling sites were choosed randomly based on the typical land use types and topography in the river basin. We examined soils in three typical agro ecosystem management patterns such as orchard, vegetable, corn field. Questionnaires were conducted in each sampling site to record the management practices In April, June, August and October of 2004, April and October of 2005, soil samples were collected from 38 soil profiles within 100 cm soil depth which represents the three land use types and management practices. For each sampling site, 5 samples were collected within a 100 cm radius and pooled for soil nutrient analysis. Samples were collected with a 100 cm deep soil auger and sectioned into 0-10 cm, 10-25 cm, and 25-40 cm, 40-70 cm and 70-100 cm increments. Soil physical, chemical properties were analyzed. Then, spatio-temporal changes of soil nitrogen and phosphorus, changes of annual storage of soil nitrogen and phosphorus, and breakeven of soil nutrient, are discussed. The risk of soil nitrogen and phosphorus losses was analyzed. The following conclusions were derived: (1) Significance differences were shown on soil total N (TN) and total P (TP) in different land use types and management practices within the soil profiles. The contents of these nutrients decreased from upper layers downward and significant differences occur in each two layers. 0-10cm and 10-25cm soil intervals are active layer for nitrogen uptake and utilization, and 25-40cm soil intervals is a buffer layer for nitrogen uptake and utilization, and 40-70cm and 70-100cm soil intervals are leaching layer for nitrogen under vegetable and corn field. There is a strong change in 0-40 soil intervals for soil phosphorus under vegetable field, but changed slowly in 40-70cm and 70-100cm soil intervals. It is important period that nutrient loss from June to August in the study area under corn field, but all year under vegetable and orchard field. (2) After interaction of management practices and growing, TN and TP storage was increasing in 1m soil radius for all the land use types. Nitrogen leaching occurs in 40-70cm and 70-100cm soil intervals in vegetable field. Nitrogen accumulation occurs from 40-70cm soil intervals downward in the corn field. As for vegetable field, leaching begins from 40-70cm soil intervals in soil profile. A large risk present in 1m soil radius in orchard field, there is nitrogen accumulation from 70-100cm soil intervals in soil profile. Wind erosion and snowfall are the important reasons for the nonpoint source pollution in study area. The storage of soil phosphorus continuously increases, and has a trend of leaching in deep soil, and show great threat to water eutrophication. (3) Based on the storage of nitrogen and phosphorus in soil profile between April, 2005 and October, 2005, it was found that land use and landforms affect the balance of soil nutrient strongly. Annual of nitrogen accumulation is orchard field > vegetable field > core field. Leaching is an important means of phosphorus loss. As phosphorus accumulation, it is the highest in vegetable field, higher in vegetable field than corn field. Phosphorus loss is showed from 40-70cm soil intervals downward in orchard field. Nitrogen accumulation is showed from 70-100cm soil intervals downward in orchard field. Some education should be given to farmers to help them to protect environment and reduce the fertilizer application. Using the difference of breakeven of soil nutrient within different land use types, it is can be solved that improving soil nutrient efficiently utilization, decreasing economic input, reducing soil nutrient losses, decreasing the probability of nonpoint source pollution, reducing the pressure of water eutrophication. (4) Nitrogen loss risk assessment on the 38 typical fields at field scale shows that the revised field scale NRS not only takes the important factors that affect the nitrogen loss into account, but also reflects the field with high risk of nutrient losses, and some management practices should be to do for these fields.