Study On Transformation Of Different Forms Of Carbon And The Coupling Relationships With Iron In Momoge Wetland Soil Of Songnen Plain

As one of the major carbon pools in all terrestrial ecosystems, the exchanges of soil organic carbon in wetlands play important roles in global carbon cycle and carbon balance. Soil iron plays special roles in biological and geochemical cycles in wetlands, because there is a process for coupling between soil iron and carbon by microbe under anaerobic environment. Migration and transformation of soil iron can greatly affect the soil organic carbon cycle. It is crucial to study the interactions between soil iron and carbon in wetlands in order to understand the biological and geochemical processes in wetland ecosystem as well as the important effects on global climate change. Experiments were set up in Momoge National Nature Reserve, which is an inland salt marsh in Wetlands of Songnen Plain. Three types of soil, Phragmite australis marsh （PA）, Phragmites-Leymus chinensis meadow （P-L） and Leymus chinensis meadow（LC） in Momoge wetlands, were taken as objects of study and the indexes of soil carbon, soil iron, soil pH and soil enzyme activities were detected and the temporal-spatial changes and interactions of soil iron and carbon were studied in the three types of soil. Samples of soil were collected in 4 layers of soil at 0-90cm depth by churn drill （Eijkelkamp Agrisearch Equipment, the Netherlands） in May, July and October 2010. Soil respiration, which is helpful to analyze the dynamic balance of soil carbon in wetlands, is not only one of the main pathways to transfer soil carbon to atmosphere but also the largest fluxes in the global carbon cycle. Test sites were in Phragmites australis marsh （PA） and Leymus chinensis meadow （LC）, of which the seasonal and diel variations of soil respiration as well as the environmental factors such as temperature, soil moisture and photosynthetically active radiation were measured by ACE automatic soil CO2 exchange system from April 2010 to May 2011. The main results are summarized as follows:（1） The changing pattern of soil respiration in Leymus chinensis meadow （LC） for diel variation was a unimodal curve, and that in Phragmites australis marsh （PA） was a multimodal curve. The value of soil respiration rates in growing seasons （from May to October） was significantly higher than that in non-growing seasons （from November to April of the next year）. With the increase of temperature, there was a uptrend in the value of soil respiration rates. There was a maximum correlation between soil respiration of Leymus chinensis meadow （LC） and the surface air temperature; there was a maximum correlation between soil respiration of Phragmites australis marsh （PA） and the soil temperature of 5cm depth; there was significantly correlation between soil respiration and soil moisture, especially that in the surface layer, by which 43%-59% of soil respiration rates can be explained; there was great correlation between soil temperature and the surface soil moisture of Leymus chinensis meadow and Phragmites australis marsh, by which 72%-68% of soil respiration can be explained.（2） Differen values of total organic carbon （TOC） and microbial biomass carbon （MBC） in the three types of soil were reordered as Phragmites australis marsh （PA） Leymus chinensis meadow （LC） and Phragmites-Leymus chinensis meadow （P-L） from high to low. In the three types of soil, the values of total organic carbon （TOC）, dissolved organic carbon （DOC） and microbial biomass carbon （MBC） distributing with vertical profile descended from the surface layer to the deep layers. The values of total organic carbon （TOC） in the three types of soil and microbial biomass carbon （MBC） in Leymus chinensis meadow （LC） and Phragmites-Leymus chinensis meadow （P-L） were reordered as in October, September and May from high to low; the values of microbial biomass carbon （MBC） in Phragmites australis marsh （PA） were reordered as in May, October and September from high to low; the values of dissolved organic carbon （DOC） in Phragmites australis marsh （PA） and Leymus chinensis meadow （LC） were reordered as in July, October and May from high to low, while those in Phragmites-Leymus chinensis meadow （P-L） were reordered as in May, July and October from high to low. According to the results of correlation analysis, there was positive correlation between total organic carbon （TOC） and microbial biomass carbon （MBC） in Phragmites australis marsh （PA） and Leymus chinensis meadow （LC）, while there was positive correlation between total organic carbon （TOC） and dissolved organic carbon （DOC） in Phragmites australis marsh （PA）.（3） Differen mass fraction of total iron, amorphous iron and complex iron in the three types of soil were reordered as Phragmites australis marsh （PA）, Leymus chinensis meadow （LC） and Phragmites-Leymus chinensis meadow （P-L） from high to low, while that of free iron in the three types of soil were reordered as Leymus chinensis meadow （LC）, Phragmites australis marsh （PA） and Phragmites-Leymus chinensis meadow （P-L） from high to low. The mass fraction of amorphous iron and complex iron distributing with vertical profile descended from the surface layer to the deep layers; while the mass fraction of free iron descended from the surface layer to the deep layers and rose again at the bottom; there was no significant change in the mass fraction of total iron distributing with vertical profile. From the point of views of the changing seasons, the mass fraction of free iron was reordered as in May, July and October from high to low; the mass fraction of complex iron was reordered as in May, October and July from high to low; the minimums of mass fraction of amorphous iron and total iron were in May, while the maximum of amorphous iron and total iron were in different seasons in the three types of soil.（4） Compared with those of the other two types of soil, there was a minimum of degree of ionization as well as maximums of degree of activation and degree of complexation and there were the most significant changes of these three indexes with vertical profile in the soil of Phragmites australis marsh （PA）. From the point of view of the iron distribution, the degree of ionization decreased from the surface soil to deep and began to increase on 60-90cm depth; the degree of ionization was reordered as in May, July and October from high to low. In the soil of Phragmites australis marsh （PA） and Phragmites-Leymus chinensis meadow （P-L）, the degree of activation and degree of complexation decreased with the increasing of soil depth; in the soil of Leymus chinensis meadow （LC）, the degree of activation and degree of complexation increased with the depth of soil and the minimum appeared in the bottom. The minimums of degree of activation in the three types of soil appeared in May; the maximum of degree of activation in Phragmites australis marsh （PA） appeared in July and the maximum in Phragmites-Leymus chinensis meadow （P-L） and in Leymus chinensis meadow （LC） appeared in October; the maximums of degree of complexation in the three types of soil were in October; the minimum of degree of complexation appeared in October, the minimum of degree in the three soil appeared in May in Phragmites australis marsh （PA） and those appeared in July in Phragmites-Leymus chinensis meadow （P-L） and Leymus chinensis meadow （LC）.（5） Different soil enzyme activities in the three types of soil decreased from the surface soil to deep with vertical profile. The enzyme activities of Fe₂O₃ reductase and phosphatase were reordered in Phragmites australis marsh （PA）, Leymus chinensis meadow （LC） and Phragmites-Leymus chinensis meadow （P-L）. The pH values in the soil of Phragmites australis marsh （PA）, Leymus chinensis meadow （LC） decreased significantly from the surface soil to deep; the pH values in Phragmites-Leymus chinensis meadow （P-L） increased significantly from the surface soil to deep. The pH in the three types of soil with seasonal change were reordered in May, July and October from the smallest amount to the largest amount, the minimum of pH appeared in soil of Phragmites australis marsh （PA）.（6） There were obvious correlations among Fe₂O₃ reductase and TOC, MBC, complex iron, amorphous iron and total iron; there were significant correlations among sucrase, phosphatase and TOC, MBC, free iron and amorphous iron. There was negative correlation among pH and TOC, MBC; there was positive correlation between pH and DOC; there was negative correlation among pH and free iron, complex iron.（7） In the soil of Phragmites australis marsh （PA）, there was positive correlation among amorphous iron and TOC, DOC and MBC; there was significantly correlation among complex iron and TOC, MBC. In the soil of Phragmites-Leymus chinensis meadow （P-L）, there was significantly negative correlation among amorphous iron, total iron and DOC, there was significantly positive correlation among free iron, complex iron and DOC. In the soil of Leymus chinensis meadow （LC）, there was significantly positive correlation between amorphous iron and DOC. There was positive correlation among the degree of ionization and free iron, complex iron and negative correlation among the degree of ionization and amorphous iron, total iron. There was positive correlation among the degree of activation and amorphous iron, total iron; there was negative correlation among the degree of activation and free iron, complex iron; there was correlation among the degree of ionization and activation with DOC; there was positive correlation among degree of complexation and complex iron, TOC; there was negative correlation between the degree of ionization and the degree of activation; there was no correlation between the degree of ionization and the degree of complexation.（8） There was certain relationship between soil respiration rates and soil organic carbon, especially the change of DOC; the amorphous iron and complex iron can effect the soil respiration rates significantly.