A Study On Organic Carbon Distribution And Binding In Water-stable Aggregates Of Paddy Soil In The Mollisols Area

Paddy soil developed from Mollisols had been one of the most important soil resource in Mollisols area. Distribution and binding of organic carbon of paddy soil in Mollisols area could largely effect the decomposition, immobilization and stability of soil organic carbon. Evaluate the immobilization and stability characteristics of organic carbon of paddy soil in Mollisols area scientifically was important for reasonable utilization and fertility builtup of black soil. So far, how the organic carbon distributed and bonded was still unclear. Therefore, by using wet-seiving and physical fractionation and stable isotope mass spectrum technology, this study analysed the diatirbution and physical/chemical binding form of organic carbon, factors that affect the chemical-bonded organic carbon, and the δ13C value of soil and aggregates of paddy soil in Mollisols area, the results showed below:(1) 2-0.25 mm aggregate was the dominating aggregate fraction paddy soil in Mollisols area, the distribution rate of 2-0.25 mm aggregate was 43.18%-61.61%, and the distribution rate of this aggregate was significantly negative correlated with soil organic carbon content.(2) Soil organic carbon was rich in 2-0.25 mm aggregate of paddy soil in Mollisols area,>0.25 mm aggregate, especially 2-0.25 mm aggregate could immobilization organic carbon obviously.δ13C value of organic carbon in aggregates increasing with the decreasing of aggregates size, representing that the fresh and active organic carbon was rich in marcoaggregates (>0.25 mm), while the older and stable organic carbon mainly rich in microaggregates (<0.25 mm).(3) Mineral organic carbon content was more than the particulate organic carbon of soil and aggregates of paddy soil in Mollisols area, particulate organic carbon and mineral organic carbon were both rich in 2-0.25 mm aggregate, and the mineral organic carbon was the main pool of organic carbon that immobilized by soil and aggregate. Particulate organic carbon was more sensitive than the total organic carbon, its sensitivity increased with the increasing of aggregate size.(4) Content and distribution rate of Fe(Al)-SOC was obviously higher than Ca-SOC of soil and aggregates of paddy soil in Mollisoils area, both Fe(Al)-SOC and Ca-SOC was rich in 2-0.25 mm aggregate, Fe(Al)-SOC was the main form that soil organic carbon immobilized. Fe(Al)-SOC and Ca-SOC content of soil was both significant correlated with soil organic carbon (P< 0.05 and P< 0.01, respectively). Ca-SOC content of>2 mm aggregate was correlated with the organic carbon content of aggregate (r>2 mm= 0.913, P< 0.05); Fe(Al)-SOC content of all aggregates was significantly positive correlated with the organic carbon of aggregates (P< 0.01).(5) Distribution of cation exchange capacity, exchangeable Ca2+ and iron oxides in aggregates was similar, all showed highest in 2-0.25 mm aggregate, and lowest in<0.053 mm aggregate. Analysis of correlation showed that CEC, exchangeable Ca2+ and iron oxides could not effect the formation of Ca-SOC of soil and aggregates, but in >2 mm and <0.053 mm aggregate, they could significantly effect the formation of Fe(Al)-SOC. In all, CEC was the main contributor to Fe(Al)-SOC of soil and aggregates, the increasing of CEC, Ca2+ and iron oxides content could increase the immobilization of organic carbon of >2 mm and <0.053 mm aggregate.This paper studied the distribution and binding form of paddy soil with different organic carbon level in Mollisols area, and analysed the distributin characteristic of organic carbon, physical and chemical bonded organic carbon of aggregates, investigated the immobilization and stability of soil organic carbon. The results of this study could provide references for clear the immobilization characteristic and mechanism of organic carbon of paddy soil and the theoretic basis for reasonable ultilization black soil and fertility builtup of paddy soil.