Studies On Main Factors That Influence Organic Carbon In Purple Paddy Soils

Purple paddy soil is an important resource in Southwest China. Many studies have been concerned with soil fertility of these soils in relation to fertilization, farming systems and so on. Based on former researches, this paper studied changes and sequestrations of organic carbon in different factors, including temperatures, water contents, soil types and tillage managements in purple paddy soils. By analyzing contents and fractions of organic carbon, effecting of environmental factors and human activities were illustrated. The main results are as follows:1 The carbon mineralization of purple paddy soils was increased when temperature got higher. During 62 days incubation at the reference moisture content, the cumulative carbon mineralized amounts were 66.79 g/kg and 86.99g/kg in 28°C and 40°C, respectively. The accumulative mineralized amount of CO₂-C in the 40°C was significantly higher than those in the 28°C treatments. Carbon mineralized amount was highest during the first 40 days of incubation, but it decreased afterwards. The data of carbon mineralization in purple paddy soils were fitted with the van't Hoff equation. The results showed that the values of the temperature coefficient Q₁₀ was between 1.1 and 1.2, and the effect of temperature was not significant. The mineralization rate of plant residues could be quantitatively described by the first-order kinetics equation. The decomposition rate constants (k) of plant residues was in the order of Vicia faba residues > Oryza sativa L.resides >Zeamays L. residues in the 28°C treatments, and Oryza sativa L..resides > Vicia faba residues > Zeamays L. residues in the40°C treatments.2 The mineralization rate of organic C in the purple paddy soil maintained at steady levels after 30d incubation period. The accumulative mineralized amount of CO₂-C in the aerobic treatments was significantly higher than those in the submergence treatments (p<0.01). The mineralization rate of plant residues could be quantitatively described by the first-order kinetics equation. The decomposition rate constants (k) of plant residues was in the order of Vicia faba residues > Zeamays L. residues > Oryza sativa L.resides in the aerobic treatments, and Oryza sativa L. resides > Vicia faba residues > Zeamays L. residues in the submergence treatments. In summary, significant differences occurred in soil water content and chemical composition of plant residues affecting C dynamics, and therefore accumulative mineralized amount of CO₂-C.3 Organic carbon content was smaller down to the profiles in these four types of purple paddy soils. Soil organic carbon contents were decreasing in following order at 0～40cm depth: Gleyic paddy soil>percogenic paddy soil>submergic paddy soil>waterlogged paddy soil. The contents of soil microbial biomass carbon and readily oxidized organic carbon were smaller down to the profiles. The content of water-soluble organic carbon was highest in different type of those purple paddy soils at 0～20cm depth. The content of water-soluble organic carbon was decreased smoothly down to the profiles in percogenic paddy soil, submergic paddy soil and Gleyic paddy soil, and was decreased firstly but increased later in waterlogged paddy soils. Total soil organic carbon content has significantly positive correlations with readily oxidized organic carbon, but has no correlation with the soil microbial biomass carbon and water-soluble organic carbon. There were significantly positive correlations among the soil microbial biomass carbon, readily oxidized organic carbon and water-soluble organic carbon. Aggregates in size of 0.25～2mm was highest in submergic paddy soil and >2mm was highest in other type paddy soils, accounting for over 40%. Different type of purple paddy soils has different soil organic carbon content in aggregates because of different types and different depths of soil. Soil organic carbon content has significantly positive correlation with aggregates in size of >2mm and significantly negative correlation with aggregates in size of <0.25mm.4 Organic carbon content was smaller down to the profiles in purple paddy soils in different tillage managements, and different treatments had different organic carbon content. Tillage managements influenced the distribution of soil aggregates. Aggregates in size of <0.053mm was highest in each treatment, and in size of 0.053～0.02mm and >0.25mm secondly. The soil organic carbon content in aggregates of every treatment showed as a single-peak distribution. There was the highest content of soil organic carbon in aggregates in size of 0.25～2mm (except at 40～60cm depth). The soil microbial biomass carbon was smaller down to the profiles in each treatment. No-till and reduced till promoted the content of soil microbial biomass in purple paddy soil. Readily oxidized organic carbon was smaller down to the profile in no-till and ridge culture and plough and ridge. Readily oxidized organic carbon was down to the profile in conventional culture at 0～40cm depth, but a little increase at depth of >40cm. Readily oxidized organic carbon was increased firstly and decreased later, and the maximum was at 10～20cm depth. Water-soluble organic carbon were decreasing in following order at 0～30cm depth: plough and ridge> no-tillage and ridge culture> conventional culture>rotation of paddy and upland, and highest content was in conventional culture treatment at depth of >30cm. Total organic carbon content was significantly positive correlation with each active organic carbon fractions, and there was significantly positive correlation among each active organic carbon fractions.