| Wetland ecosystems is one of the most productive ecosystems on the earth. In past few decades, excessive reclamation has lead to reduce the area of natural wetlands, degrade the regulation and storage function, decrease the biodiversity, increase soil pollution and degrade soil quality and so on in the middle and lower reaches of the Yangtze River. After the catastrophic flooding of the Yangtze River in 1998, returning farmland to lake(RFL) is a strategic measure for wetland ecological restoration.Soils from wetlands suffering different historical farming tillage methods (hydroponics and dry farming) and returning under different ways (natural water area with aquaculture-Baidang lake and natural water area without aquaculture-Caizi lake) were sampled to analyze changes of soil physical and chemical properties, and biochemical properties in Anqing Wetland along the Yangtze River, so as to clarify the changes of soil eco-chemical properties, and reveal the impact of different historical farming tillage methods and returning under different ways on the characteristics of the ecological system of wetland soils after RFL, which can provide the scientific basis for the ecological restoration and effective use of wetland soils. The main conclusions are as follows:(1) After returning farmland to lake, soil bulk density all decreased, and soil bulk density under historical farming method of dry farming were significantly higher than those under historical hydroponics. Those suggested that influence of historical farming methods of dry farming on wetland soil bulk density was stronger than that of historical hydroponics methods.(2) In 0-15cm depth, clay content of tested soils is higher than that of 15-30cm, while the changes of silt content and sand content of tested soils are not obvious; in the 0-15cm horizontal soil, clay content decreased in Baidang lake wetlands, while soil clay content increased in Caizi lake wetlands. Under the two different restored ways, the fractal dimension of soil particle has a consistent change with <0.01mm content of soil particle size, so 0.01mm content of soil particle size can be regarded as a critical particle size. Wetland soil particle volume fractal dimension was significantly correlated with soil main nutrient content, which can be regard as a quantitative index of the restoration of wetland soil fertility. Different historical farming tillage methods before returning farmland to lake and the strength of natural processes are main factors accounting for the changes of particle size distribution of tested soil in different restoration models of RFL projects.(3) pH values of tested wetland soils range from 4.8-6.1 with an average of 5.5±0.3. In the 0-15cm horizontal direction, soil pH value decreased in Baidang lake wetlands, while soil pH value increased in Caizi lake wetlands.(4) After returning farmland to lake, in 0-15cm depth, soil organic matter, total N and available N of tested wetland soils were significantly higher than that of 15-30cm; in the 0-15cm horizontal soil, soil organic matter, total N and hydrolyzable N generally decreased in Baidang lake wetlands, while soil organic matter, total N and hydrolyzable N generally increased in Caizi lake wetlands. Both in wetland soils of two lakes, total P generally increased while available P generally decreased after returning farmland to lake. Correlation analysis showed that the test soil clay content and total N had significant positive correlations with soil nutrient content except available P(p<0. 01), the test soil organic matter had significant positive correlations with soil nutrient content except total P(p<0. 01). Soil organic matter and total N content are suitable indicators as soil quality change during wetland ecological restoration.(5) After returning farmland to lake, in 0-15cm depth, soil microbial biomass C and soil enzyme activities of tested wetland soils were significantly higher than that of 15-30cm; in the 0-15cm horizontal soil, soil microbial biomass C and soil phosphatase activity decreased and catalase activity increased in Baidang lake wetlands, while soil microbial biomass C and soil phosphatase activity increased, and catalase activity increased under historical farming method of hydroponics, but no significant changed under historical farming method of dry framing in Caizi lake wetlands. Both in wetland soils of two lakes, soil urease activity increased under historical farming method of dry framing and decreased under historical farming method of hydroponics, soil invertase activity decreased under historical farming method of dry framing and hydroponics after RFL. Through Pearson correlative analysis, there were significant correlation among the soil phosphatase activity, soil urease activity, soil invertase activity and soil microbial biomass C(p<0. 01), and the activities of phosphatase, urease, invertase and soil microbial biomass C were significant related to soil nutrients(p<0. 01), illustrating that the activities of phosphatase, urease, invertase and soil microbial biomass C played a great role in the process of soil C and N transformation, which reflected the nutritional level of wetland soils and can be used as indicators of wetland soil ecological restoration.Those suggested that RFL as natural water area without aquaculture was a better way for wetland restoration than that as natural water area with aquaculture; influence of historical farming methods of hydroponics on wetland soil was weaker than that of historical dry farming methods, which is conducive to the evolution towards a natural wetland ecosystem. Different historical farming tillage methods before returning farmland to lake and the strength of natural processes are main factors accounting for the changes of wetland soil organic matter and other major nutrients, and differences in soil organic matter and water level are main factors accounting for the changes of soil biomass carbon and soil enzyme activities. |
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