| The basic mechanism of organic carbon sequestration in paddy soil under conservation tillage systems had sketched out, but the accumulation, transformation and stability of carbon cycle of paddy soil was still inadequate understanding. Ridge-cultivation and no tillage was beneficial to formation and accumulation of soil organic carbon in paddy fields, and played a more effective role in stabilize and protect active carbon pool compared to other cultivation systems. But the study about tillage systems on soil organic carbon in rice rhizosphere’were short, and lead to the less know about particularity of organic carbon accumulation and transformation. Therefore, the paper based on a long-term field experiment was conducted to investigate the influence of ridge-cultivation and no tillage on soil organic carbon in rice rhizosphere. The results would contribute to comprehend the cycle of soil organic carbon under ridge-cultivation and no tillage, and would provide a scientific management measures to promote carbon sequestration in paddy fields. The field experiment included four tillage treatments:conventional tillage with rotation of rice and winter follow (CT1), ridge-cultivation and no tillage with rotation of rice and winter follow (NT1), conventional tillage with rotation of rice and ripe (CT2), ridge-cultivation and no tillage with rotation of rice and rape (NT2). The main findings were as follows:(1) During the whole growth period of rice, NT1and NT2showed significant advantage of organic carbon accumulation compared with CT1and CT2, especially the treatment of NT2. The order of difference value of total organic carbon(TOC) content (rhizosphere-bulk) was NT2> CT1> NT1> CT2, NT2showed more obvious on the effect of soil organic carbon sequestration by promoting rice root growth and root activity. In terms of labile organic carbon composition, compared with CT1and CT2and other plain-cultivation treatments, labile organic carbon(LOC), dissolved organic carbon(DOC) and particulate organic carbon(POC) content, POC distribution coefficient and mass fraction of particulate soil in rhizosphere soil were higher under NT2, and microbial biomass carbon(MBC) content was higher under NT1than other tillage treatments, which showed that the stability of labile organic carbon of rhizosphere soil under NT2treatment was superior to other tillage treatments.(2) This study found that TOC and LOC, POC, mass fraction of particulate soil were showed a highly significant positive correlation (p<0.01) between rhizosphere and bulk soil. And the coefficient of variation of POC, LOC and mass fraction of particulate soil were significantly greater than TOC. Therefore, it would be more sensitive to detect the short term influence of soil organic carbon under different tillage systems by using the POC, LOC and mass fraction of particulate soil and other labile index. The study also found that LOC was significantly positively correlated to the mass fraction of particulate soil in rhizosphere and bulk soil, and the interaction relationship between them remains to be further study (p<0.05).(3) The order of Kos in rhizosphere and bulk soil was CT2> NT> CT1, but the TOC content in rhizosphere and bulk soil under CT2treatment were significantly lower than other tillage treatments, therefore, it would be better to combined with Kos and TOC to evaluate stability of soil organic carbon, especially the rhizosphere soil. In addition, Kos was gradually increasing with the growth of rice in bulk soil and was higher than rhizosphere soil under the treatment of ridge-cultivation no tillage, and the increasing trend of Kos in bulk soil was especially obvious under NT2, which indicated that NT2would improve the oxidation stability of soil organic carbon of the total soil mass by changing the carbon cycle process of the rice rhizosphere micro domains and may prompting the labile carbon components in rhizosphere micro domains turnover to the bulk domain and sequestration.(4) The variation trend of the daily and cumulative mineralization of SOC between rhizosphere and bulk soil showed good consistency. In addition to CT2and NT2showed no significant difference in rice tillering stage, the cumulative mineralization of SOC and mineralization intensity of rhizosphere and bulk soil under the treatment of NT2were higher than other tillage treatments. Overall, the order of the mineralization rate and cumulative mineralization of SOC and mineralization intensity of rhizosphere and bulk soil under different tillage treatments were as fallowing:NT1> NT2> CT2> CT1. The result indicated that NT2was contribute to promoting mineralization and decomposition of soil organic matter in rice rhizosphere micro-domain.(5) The Kos of soil organic carbon and cumulative mineralization intensity and the intensity of mineralization of rhizosphere and bulk soil under different treatments were highly significant negative correlation (p<0.05). The cumulative mineralization of SOC and the maximum mineralization rate in rhizosphere and bulk soil and LOC showed a significant positive correlation (p<0.05). Therefore, Kos and LOC could be used for the characterization of soil organic carbon mineralization differences under different tillage systems.(6) Soil pH, total phosphorus(TP) and available phosphorus (AP) in the rice rhizosphere were lower than the bulk soil, and NT2did not show obvious rhizosphere effect of P enrichment than conventional tillage. Total nitrogen (TN) and HA-K in the rice rhizosphere soil under different tillage systems were higher than the bulk soil, especially NT2, which showed more obvious effect of TN and HA-K enrichment. In addition, after the correlation analysis between the paddy soil organic carbon dynamics and physical and chemical properties of soil organic carbon, this indicated that there was a significant positive correlation (p<0.05) between soil pH and DOC in the rice rhizosphere soil. TN, TOC, LOC and mass fraction of particulate soil, cumulative mineralization were highly significant positive correlation (p<0.01), and there was also exist in bulk soil. TK and MBC was significantly positive related (p<0.05). TN could be used as a sensitive indicator to characterize the dynamic changes of soil organic carbon.(7) The analysis between soil C and N in paddy rhizosphere and bulk soil reflected that MBC:TOC, MBN:TN and MBC:MBN in rhizosphere soil under the long-term NT2were lower than conventional tillage, but the ratio of C:N in rhizosphere soil was higher than conventional tillage. Furthermore, the ratio of MBC:TOC, MBN:TN and MBC:MBN in rhizosphere soil under the NT2were lower than bulk soil, it was thus clear that, microbial biomass in rhizosphere soil under the NT2did not show higher soil characteristics than bulk soil. The study also found that the intensity of the ratio of C:N and MBC:MBN in rhizosphere soil response to tillage system were higher than bulk soil under different tillage treatments. |
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