Carbon Sequestration Mechanism Of Soil Aggregate In Long-term Conservation Tillage Farmland

Changes in agricultural soil carbon pool are closely related to the quality of cultivated land and the emission of greenhouse gases. It would be of significance for grain production and carbon sequestration in China to increase agricultural soil carbon pool and to improve the quality and carbon sequestration potential of cultivated land.Conservation tillage has drawn global attention due to its ability of reducing soil CO2 emission and increasing organic carbon sequestration potential in soils. Some abroad researches have shown that the main reason that conservation tillage can increase soil organic carbon storage is that the physical protection inside aggregates is guaranteed. A large number of studies on carbon sequestration of aggregates in conservation tillage farmland have been conducted, however, the mechanism on carbon sequestration and the microbial response are still in the exploration phase. We studied the exogenous organic carbon conversion and its distribution and disposition in different grain level aggregates in long-term conservation tillage farmlands. The study was conducted based on the long term conservation tillage experiment using stability isotope mark technology. Meanwhile, the study on microbial community structure and number was carried out with modern molecular biology technology, which clarified carbon immobilization process and microbial response mechanism in soils in aggregate level and molecular level. The main conclusions were as follows:1)Effect of different tillage practices on soil aggregates showed that macro-aggregates(>0.25mm)have transformed into micro-aggregates(<0.25mm)under all straw returning tillage(ASRT) and conventional tillage(CK);No-tillage with straw mulching(NTSM) and no-tillage with low stubble(NTLS) were more benefit to the stability of macro-aggregates(>0.25mm).2) Effect of different tillage practices on organic carbon in soil aggregates showed that the distribution of organic carbon in different sizes of soil aggregates remain unchanged under no-tillage with straw mulching(NTSM) and no-tillage with low stubble(NTLS);The proportion of organic carbon in different sizes of soil aggregates had increased in macro-aggregates(>0.25mm)under all straw returning tillage(ASRT) and conventional tillage(CK) measures.3) The distribution of corn-drived SOC in soil aggregates under different tillage practices is revealed, and the dynamics of corn-drived SOC in soil aggregates under under different long-term tillage practices is different. Under the ASRT treatment, the corn-drived SOC firstly appeared in macro-aggregates(>0.25mm), and in micro-aggregates(<0.25mm) under the CK treatment. Under the NTSM treatment, the corn-drived SOC firstly entered into the micro-aggregates and then entered into the macro-aggregates with the increase of time.4) The proportions of corn-drived SOC in soil aggregates-drived SOC were 1.55%—3.68% in all straw returning tillage(ASRT), 1.31%—3.04% in no-tillage with straw mulching(NTSM), 1.60%—3.28% in no-tillage with low stubble(NTLS),1.64%—3.27% in conventional tillage(CK), respectively.5) 40 T-RF fragments could be detected after 16 S r DNA being restricted by MspⅠenzyme in dry lands soil. The dominant species were T-RFs 63 bp and 149 bp in soil layers of 0-5cm, 5-10 cm, and 10-15 cm. Results after bacteria cbb L being restricted by Msp Ⅰ enzyme showed 39 bp fragment population played a key role in carbon sequestration in the treatments of no-tillage with straw mulching and no-tillage with low stubble in the soil layer of 0-10 cm. While 124 bp fragment population played a key role in carbon sequestration under all straw returning tillage(ASRT) and conventional tillage(CK)in the layer of 0-5cm. However, after bacteria cbb L being restricted by Hha I, 31 bp fragment population probably played a key role in carbon sequestration under no-tillage with straw mulching and conventional tillage in the layer of 0-5cm. 62 bp fragment population was the key community in the layer of 5-15 cm in all treatments except the conventional tillage.6) Under different tillage treatments, the abundance in the layer of 0-15 of 16 S r DNA ordered no-tillage with straw mulching(NTSM 1.16×1014 copies·g-1)>no-tillage with low stubble(NTLS 8.81×1013 copies·g-1)>conventional tillage(CK 9.28×1011 copies·g-1)>all straw returning tillage(ASRT 7.35×1010 copies·g-1); and that of gene cbb L ordered conventional tillage(CK 1.75×109 copies·g-1)>no-tillage with low stubble(NTLS 1.49×109 copies·g-1)>all straw returning tillage(ASRT 1.30×109 copies·g-1)>no-tillage with straw mulching(NTSM 4.84×108 copies·g-1) with no significant difference between all straw returning tillage(ASRT) 、 no-tillage with low stubble(NTLS)and conventional tillage(CK). In the case of cbb M, the abundance ordered all straw returning tillage(ASRT 4.54×1011 copies·g-1)>no-tillage with straw mulching(NTSM 4.06×1011 copies·g-1)>conventional tillage(CK 3.44×1011 copies·g-1)> no-tillage with low stubble(NTLS 3.09×1011 copies·g-1)and the differences between treatments were not significant.