| Soil carbon cycle is a systematic process of geosphere which is closely related to the change of worldwide climate. The research on soil carbon cycle has an important meaning for promotion of carbon sequestration in terrestrial ecosystem and improving its stability, reducing greenhouse gas emissions and mitigating climate change. Conservation tillage is a reliable measure for soil resources sustainable development, to get public attention for keeping water, soil, fertilizer and environment, and its effect on increasing agricultural soil has been proved by many scholars. No-tillage and ridge culture is a prominent representative of conservation tillage in the southern China paddies. Strengthening the research on soil carbon cycle under no-tillage and ridge culture, and comprehensive understanding the soil organic carbon accumulation, particularity of changes and its internal mechanisms of no-tillage and ridge culture, these2things could provide an important reference for building paddy farming systems which have both environmental benefits and economic benefits. A20years experiment was conducted to study the effects of different tillage systems on soil organic carbon accumulation, organic carbon stability and organic carbon mineralization in purple paddy soil. The experiment included four tillage treatments:conventional tillage with rotation of rice and winter fallow system (CRWF), no-tillage and ridge culture with rotation of rice and winter fallow system (NRWF), no-tillage and ridge culture with rotation of rice and rape system (NRR), and conventional tillage with rotation of rice and rape system (RRR).The main research results were listed as follows:The carbon content in the surface soil was obvious different after the long-term tillage for20years, the four tillage treatments have increased in varying degrees, and no-tillage and ridge culture increased the most.The order of organic carbon content in the surface soil was:NRR> CRWF> NRWF> RRR. The correlation analysis of soil organic carbon content and soil depth reflected that there was significant negative linear correlation between them(R2=-0.938, P<0.001). The organic carbon content of four tillage treatments decreased with soil depth increasing. The order of organic carbon content in20-40cm layer soil was NRR> NRWF> CRWF> RRR, and there were obviously differences among our tillage treatments (P<0.05), it showed that conservation tillage no-tillage ridge improved the accumulation of organic carbon, especially no-tillage and ridge culture with rotation of rice and rape system.With particle size increasing, organic carbon content of soil organo-mineral complex reduced. Compared with conservation tillage, there were less organic carbon in the large size fraction complex, and more organic carbon in the small size fraction complex with conventional tillage. The labile organic carbon was relatively high in the small size fraction complex (<2μm and2-10μm) under conventional tillage. Compared with conventional tillage, No-tillage and ridge culture could promote the soil quality and fertility.There was obvious difference between the amount of labile organic carbon and the oxidation stability coefficient under different tillage systems. The oxidation stability coefficient of organic carbon under long-term no-tillage and ridge culture was higher than that under conventional culture. The Kos was relatively small in the small size fraction complex (<2μm and2-10μm) with conventional tillage, it might because tillage destroyed macro aggregate in soil, then micro-aggregate number increased, and it made the stability of organic carbon in the small size fraction complex lower. The Kos was higher in complex (10-50μm and>50μm) under no-tillage and ridge culture, it might because that conservation tillage was conducive to the formation of macro-aggregate, aggregate made the organic carbon in soil organo-mineral complex more stable due to the protection and stabilization of the organic carbon, and it could not easily be broken down or mineralized. So from the part of improving soil quality and fertility, no-tillage and ridge culture is better than conventional tillage.Particulate organic carbon was positively related to soil organic carbon content, POC content in soil decreased obviously after tillage. Both the order of POC content and POC concentrations under different tillage systems were as following:NRR> CRWF> NRWF> RRR. From the soil profile point of view, the POC content and POC concentrations of four kinds tillage systems decreased with soil depth increasing. The proportion of POC to total organic carbon is general not high, but it is important to soil fertility maintenance and soil carbon storage change. Compared with the content of POC, it is generally believed that the percentage of POC to organic carbon reflects the soil carbon pool status and soil organic carbon quality more accurately. Compared with conventional treatment, the DOC content was obviously increased in every soil layers under no-tillage and ridge culture, it could increase the effective supply of nutrients for crops and made crop yield. The microbial biomass carbon content was in a decreasing order of NRR> NRWF> CRWF> RRR. The microbial biomass carbon content under no-tillage and ridge culture was obviously higher than it under conventional tillage. The manifestation laws of carbon pool activity, activity index, carbon pool index and carbon pool management index were similar with the laws of soil total organic carbon and labile organic carbon. Carbon pool management index in every soil layer were more than100under the no-tillage and ridge culture, it indicated that conservation tillage was a proper farming system, it could maintained even improved the soil fertility, made carbon pool in a good situation to maintained sustainable utilization of soil resource. This paper also analyzed the correlation between soil total organic carbon and labile organic carbon, the results showed that there were high significant positive correlations among the of soil total organic carbon, easily oxidized organic carbon, particulate organic carbon, dissolved organic carbon and microbial biomass carbon.The mineralization rate of soil organic carbon reached the maximum at the first day under all tillage treatments, the maximum mineralization rate was in a decreasing order of CRWF> RRR>NRWF>NRR. Mineralization rates of organic carbon and pH were in significantly positive correlation.There were obvious differences on the mineralization rate of soil organic carbon under different tillage, and the differences gradually reduced and reached a steady state with time going. There was highly significant positive correlation between cumulative mineralization and incubation time (P<0.01). The mineralization law of the different treatments showed good consistency, cumulative mineralization content rapidly increased in the prophase of cultivation, and then the growth rate gradually slowed. The descending order of cumulative mineralization of soil organic carbon under different tillage system was CRWF> RRR> NRWF> NRR. After the regression analysis of mineralization of soil organic carbon and the oxidative stability of soil organic carbon, this indicated that there was very significant positive correlation between cumulative mineralization content and the oxidative stability of organic carbon (R2=0.6708, n=12, p<0.01). No-tillage and ridge culturewas an efficient tillage practice for organic carbon sequestration. This study showed that no-tillage and ridge culture improved the capacity of the management system into promoting soil quality in Sichuan Basin of China. In order to maintain soil fertility or improve soil quality, conservation tillage should be chosen to keep the soil carbon pool in a healthy state, and to ultimately maintain the sustainable use of soil resources. |
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