Landscape Pattern, Soil Quality And Land Carrying Capacity At Qianlianghu Polder Under Converting Polders Back Into Wetlands In Dongting Lake Area

Dongting Lake is the second largest fresh-water and flood channel type lake in China which is situated in the north of Hunan province and connecting four rivers of Xiang, Zi, Yuan and Li with Yangtzer River. For land reclamating from lake by people and natural sediment depositing, however, its natural lake area has decreased form more than 6 000 km2 in Qinhan dynasty to 2 625 km2 now. When lake area decreased sharply, there also resulted in other questions which included capalities of controlling and storaging flood weakening, schistosome japonicum overspreading, biodiversity lossing and environment degradating. In order to improve this conditon, the project of converting polders back into wetlands had been enacted, and original ecological functions had been restored and enhanced to some extent. At the same time, there existed a series of changes and responses of landscape patterns, structures and styles of land use in Dongting Lake. Soil qualities, land carrying capacities and sustainable utilization were effected differently under intensive management especially in non-flooding years.Through comprehensive study methods of oriented researching and random sampling, field surveying and laboratory analyzing, qualitative analyzing and quantitative studying, the effections of converting polders back into wetlands on the landscape pattern changing were studied on the case of Qianlianghu polder, Dongting Lake Area. Based on these results and plot data of poplar forestland soil (Ⅰ1,9a;Ⅰ2,6a;Ⅰ3,4a;Ⅰ4,2a) (Ⅰ), garden-land soil (vegetable soil and fruit soil) (Ⅱ), nonirrigated farmland soil (Ⅲ1, cotton soil;Ⅲ2, sugar cane soil;Ⅲ3, maize soil) (Ⅲ), irrigated farmland soil (rice soil) (Ⅳ) and wasteland soil (Ⅴ), a research had been carried out into soil physical properties, soil nutrient pools, soil microelement availabilities, soil microbe quantities and enzyme activities, distribution, pollution and accumulation of soil heavy metals, soil syntheical qualities in above five typical land use patterns. Combination with the flood safety contructed of Qianlianghu polder, moveover, optimizational measures of land use patterns and sustainable countermeasures were discussed and put forward in order that offerred some suggestions for regulating soil qualities, optimizing land use patterns, utilizing sustainably land resource and planning and designing regional landscape. (1) The dynamic changes of landscape patterns in the years of 1987,1996 and 2008 were analyzed in the patch level and landscape level, respectively. The total changing tendencies of landscape patterns were that diversity, evenness, fragmentation indexes were increasing, and landscape shapes were changing for simplicity to complexity. Through markov transferring matrix and contribution rate of landscpae elements analyzed, it was the main driving forces for landscape pattern changing that were increasing population, constructing urbanization and converting polders back into wetlands.(2) Soil physical properties had been studied and evaluated synthetically through gray incidence analysis in five typical land use patterns. The results were found that soil bulk densities, capillary porosities, non-capillary porosities, total porosities, soil natural water contents, soil capillary moisture capacities, mean weight diameters (MWD) and fractal dimensions (FD) of soil aggregates in 0～50 cm soil layers under different land use patterns were 1.02～1.65 g·cm-3,33.04%～61.70%,2.15%～25.35%,38.40%～64.96%,17.07%～38.33%,23.22%-64.00%,1.76-5.09 mm and 2.2728～2.6388 respectively. Soil physical properties were ordered by>Ⅳ>Ⅰ>Ⅲ>Ⅴ, which showed the soil physical characteristics was the poorest in waste landⅤ(0.6183) and the best in garden landⅡ(0.7763).(3) Base on synthetical index method, the effects of soil nutrient pool in different land use patterns had been studied. The highest contents of TN and the lowest OM occurred in patternⅠ, the highest contents of OM, TK and AP were in patternⅣ, and the highest AN were patternⅢ. However, the contents of TN, TP, TK, AN and AP were the lowest in pattern V. Soil nutrient pool synthetical indexes ranged from 24.33 to 295.93 in five land use patterns, which showed by regularities of IⅣ(231.96)>Im (193.46)>IⅡ(70.90)>IⅠ(59.57)>IⅤ(35.59)(4) Base on root mean square method, single and synthetical available indexes of soil microelements which included Cu,Zn,Fe,Mn,Mo were studied. There existed the regularities of EMo (15.5933)> EZn (4.7767)> EFe (3.1898)> ECu (1.7160)>EMn (1.1913) for single microelements and EⅠ(10.9020)>EⅡ(8.2198)> EⅣ(7.7335) >EⅤ(5.1726)> EⅢ(4.2006) for different land use patterns. Through double sieving stepwise regression analysis, it was found that the maily influence factors of available Cu and Zn were soil bulk density, capillary porosity, water storage in capillary porosity, pH value, organic matter, total N and available K, and for Fe were water storage in capillary porosity and total K. Available Mn and available Mo were mainly affected by available K and total N respectively.(5) The highest quantities of both bacteria and actinomycetes occurred in nonirrigated farmland soil (III), and the highest quantity of funguses in wasteland soil (V). Bacterium was the main category of the total soil microbe with the highest proportion of 44.42%-92.93%, fungus and actinomycete followed with the proportion of 4.89%-42.76% and 1.71%-24.52%, respectively. The lowest activities of phosphataese, urease and dehydrogenase appeared in garden-land soilⅡ, wasteland soil V and irrigated farmland soilⅣ, respectively, and proteinase activity was the lowest in nonirrigated farmland soilⅢ. According to canonical correlation analysis, there was a positive relationship between U and actinomycial number, bacterial number followed, but there was a negative relationship between U and fungus number. There was a positive relationship betweenⅤand urease with the largest regression coefficient, followed by dehydrogenas, but there was a negative relationship between V and phosphataese, as well as V and proteinase.(6) Distribution, pollution and accumulation evaluations of soil heavy metals (Cu, Zn, Mn, Pb, Cd, As and Hg) had been studied. Unfortunately, soils of patternⅠ2,Ⅰ3,Ⅲ2 andⅣhad been polluted Cd, and all heavy metals had been accumulated to some extent in all land use patterns except that As in the whole soil and Zn inⅠ4,Ⅱ,Ⅲ3 andⅤ. The highest integrated pollution index occurred in IV and the lowest one occurred inⅤ, and so for the integrated accumulation index. Through principal component analysis and double sieving stepwise regression analysis, it was found that the maily influence factors of Cu were water storage in capillary porosity and total K, and for Zn were capillary porosity, total N, available P, total K and available K, and for Mn were non-capillary porosity, total N and available K, and for Hg were soil bulk density, total porosity, total P and total K. For Pb, Cd and As, the same factors were non-capillary porosity, water storage in capillary porosity, organic matter, total N, total P, available P and available K.(7) Soil quality evaluation system had been found which contained 28 indexes that belonged to soil physical properties, soil chemical and biological characteristics, pollution and accumulation of soil heavy metals. The comprehensive soil qualities of different land use patterns wereⅣ(0.6479)>Ⅲ(0.5386)>Ⅱ(0.4373)>Ⅰ(0.4364)>V (0.2546) by analytic hierarchy process (AHP). It was proved by index weight vectors that the total porosity, organic matter, bacterium number and integrated pollution index of heavy metals had the closest relationship with soil qualities respectively in four categories indexes.(8) Based on the construction planning of safety area and soil quality results of different land use patterns in Qianlianghu polder, the land and landscape structures were optimized and predicted by using markov model. According with statistical data of population and grain yield during 1987～2008 year, land population carry capacities in the year of 2010,2015,2020,2025 and 2030 were also calculated and predicted under different live consume standards of simply adequate, spaciousness, well-off and prosperity through the tendency extrapolation method. Moreover, sustainable utilization countermeasures of land resources in Qianlianghu polder were put forward.