| Basin is a natural unit with exuberant vitality surrounded by all kinds of water bodies, which provides rich resource of water for agriculture production and daily life. It has become the base of regional economic development and obtained more and more attention of scholars at home and abroad. The Four-lake Watershed is an important production base of grain, cotton and edible oil, which plays a key role in the economic development of Hubei Province, especially in the growth of agricultural production. But with the increasingly acute of contradiction between people and land in the past couple of years:reclamation of land from the lake, unconscionable irrigation lead to waterlogged elimination of stains and decline of soil productivity, eventually become a bottleneck impeding the economic prosperity of Four-lake Watershed. Thus, investigation of the fertility levels of cultivated land of the Four-lake Watershed is important for establishing policy of land exploiting and reform.Six catchments in middle and lower reaches of the Four-lake Watershed in Jianghan Plain are objects to be studied in this paper.13indicators of pH, available nutrients, total nutrients and trace elements were selected to be evaluation factors of soil fertility. Kringing of Geostatistics Analysis based on GIS was used to calculate spatial distribution of soil fertility. Analytic Hierarchy Process (AHP) was used to determine the weight of factors, and Fuzzy Comprehensive Evaluation was used to calculate the grade of membership. Land of study areas was divided into six categories based on Integrated Fertility Index (IFI). The main results are as follows:(1) According to actual fertility level of soil in, pH was divided into six levels, the available nutrient was divided into five grades, the total nutrient was divided into four levels and trace elements was divided into two levels. Results concluded from semivariance analysis based on Geostatistics showed that: pH, organic matter, total nitrogen and total phosphorus had a highly spatial correlation; available nitrogen, available phosphorus, available potassium and water-soluble boron had moderate spatial correlation. Interpolation showed that pH of the middle and lower reaches of the Four-lake Watershed soil was6.5-7.5, generally suitable for crop growth. Content of available nutrient was medium-low mostly, organic matter and available nitrogen were lack, sum to3806.71km2and3928.26km2land area. Arable lands whose content of available phosphorus was5mg/kg-10mg/kg and (or) content of available potassium was50mg/kg-100mg/kg were most widely, accounted for42.83%and60.34%of the total area studied separately. Content of total nutrient was mainly rich, distribution of which was different from available nutrient, area of arable lands with rich total nitrogen, phosphorus and potassium sum to3171.40km2,2856.49km2and3281.74km2respectively. Few arable lands has suitable level of trace elements content, except for available Cu, content of which beyond0.2mg/kg occupied3969.67km2area, distributed73.07%of the middle and lower reaches of the Four-lake Watershed. Arable lands with remaining trace elements content below suitable level sum to more than90%area of the studied region.(2) IFI of the middle and lower reaches of the Four-lake Watershed land was0.316-0.864, fertility of cultivated land in this region was divided into six levels based on actual situation of the studied area. From the perspective of the entire watershed, the first-class land to the six-class land makes the Main Channel as its axis, opens to both wings in turn along the direction of the northwest-southeast, the adjacent grades gradually transmitted. Area of the fourth-class land and fifth-class land sum to1867.52km2and2009.90km2distributed most widely, accounted for29.66%and48.37%of total area respectively, larger than the second-class land (area,1149.38km2) and the third-class land (area,386.59km2). The first-class land and the sixth-class land were the smallest; add up to no more than10%of the region investigated.Land quality of catchment north of the Main Channel and catchment south of the Main Channel was better than other regions, area of the first-third class land sum to625.95km2and906.01km2, accounted for99.23%of the total. Besides, there was no first-third class land in Honghu catchment, Luoshan catchment or catchment south of lower reaches;no first-class or second-class land in catchment north of lower reaches, the third-class land of which only11.88km2area, accounted for3.33%of entire catchment.The study found that long-term waterlogging stress and eluviations of water were the main reasons for the quality decline of cultivated land. Overly rely on the chemical fertilizers not only increased the background value of soil, but also reduced the crop absorption and utilization of nutrient and caused serious pollution to the environment. Some reassures and suggestions were proposed under the condition of the actual situation of the study basin, eager to provide some reference to the comprehensive improvement of the Four-lake Watershed and sustainable development between human and environment. |
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