| Soil carbon sequestration in agriculture was supposed by the Kyoto Protocol as a ratified and important strategy in carbon sequestration and mitigation. It suggests a win-win effect on sustainning crop production and replying climatic change. Much more attention had been paid on agricultural soil carbon sequestration. It has been understood sufficiently on the capacity of soil carbon sequestration and distribution, translation and stabilization in carbon stock. For the last decades, molecular level of carbon sequestration is being focused on along with the reent developments in the structural characterization of organic matter components. In China, the content variation and distribution of SOC and its components under different agricultural managements have been studied sufficiently. As a unique type of anthropogenic soil, paddy soil has been extensively observed with distinctive trend and large potential of carbon sequestration, it was crucial for replying to climatic change. Thus, it is of vital importance to elucidate the C sequestration fate in paddy sois for developing the win-win technique of C enhancement and crop productivity in rice field and for enhancing the capacity of rice field answering to the climatic change. In this study, the content change of SOC in bulk soil and aggregates of typical paddy soils from South China under long-term agro-ecosystem experiments were conducted with typical and cross-site analysis, particularly, the molecular information on the composition of particulate organic carbon (POC) was elucidated to indicate the relationship of SOC change with environment and crop production, to annotate the physical protection, chemical binding and stabilization involved in soil C sequestration and to enrich the understanding about soil C sequestration fates in paddy soils. The approach and method of soil C sequestration was further explored in order to provide a foundation for the national development of sustanabal rice farming and C sequestration and mitigation technique. The main results are as follows:1. Accumulation of particulate organic carbon sensitively indicated soil C sequestration in paddy soilsVariations of total organic carbon (TOC) and particulate organic carbon (POC) in bulk soil and water-stable aggregates from Tai Lake paddy soil under long-term different fertilizations were conducted. It showed that the depth distribution of both TOC and POC fitted best to power function equations. Different fertilization practices tended to affect not the depth distribution of TOC and POC but the content in topsoil, being the highest under the treatment of chemical fertilizer plus organic fertilizer.In addition, the water stable-aggregates (WSAs) in Tai Lake paddy soil were dominated by the size fraction of>2 mm and 2-0.25 mm. However, the size fraction of>2 mm was increased and that of 2-0.25 mm decreased under fertilization treatments compared to that under no fertilization. POC was found to accumulate in the WSAs of>2 mm fraction and to decrese sharply with the decreasing size. POC in>2 mm fraction was more sensitive to fertilization practices, being significantly higher under chemical fertilization plus straw return, while POC in those of 2-0.25 mm and 0.25-0.053 mm size was slightly higher under chemical fertilization plus pig manure. Regression coefficients showed a possible source of POC in WSAs from increased crop biomass in topsoil (0-5 cm) and from organic matter applied in sub-surface layer (5-15 cm).2. Physical protection in coarse aggregates and chemical binding of ferric oxide contributed to soil C sequestration in paddy soilsIn order to elucidate the physical protection and chemical binding of soil C sequestration, the content of SOC and its binding forms in size fractions of micro-aggregates from purple paddy soil, red paddy soil and Tai Lake paddy soil was discussed with cross-site analysis. It showed that the newly accumulated SOC found its fate mainly in the coarse fraction of 2000-200μm in active response to tillage and fertilization treatments. Of the three studied paddy soils, SOC in red paddy soil exhibited the most intense response to fertilizations, exerting a most prominent trend of SOC accumulation under well management practices. Furthermore, statistic analysis indicated significant parabola relationship between SOC accumulation in bulk soil and in 2000-200μm coarse fractions. Therefore, it is inferred that accumulation of young SOC in topsoil may reach a saturation controlled by the physical protection in the coarse particle-size fraction. However, the maximum protection capacity by the coarse aggregates of the three tested paddy soils had not yet realized, still possessing an obvious potential of carbon sequestration.Calcium bound SOC (Ca-SOC) and iron/aluminum oxyhydrates-bound SOC (Fe(Al)-SOC) in coarse aggregates of 2000-200μm showed the most sensitive response to long- term tillage and fertilization practices. The SOC accumulation in paddy soils under good management practices was represented mostly as increase in Fe(Al)-SOC. Fe/Al oxyhydrates in coarse aggregates from the red paddy soil showed the most intensive chemical protection of SOC. Thus, chemical protection of SOC physically protected in coarse fractions by binding with Fe/Al oxyhydrates during micro-aggregate turnover was supposed as a prevailing mechanism of SOC sequestration in these paddy soils.3. Molecular stabilization of organic matter composition was a fate of particulate organic carbonr sequestration in paddy soilsThe combination of solid state CPMAS 13C-NMR and TMAH thermochemolysis-GC/MS was conducted to investigate the supramolecular structure change of POC from Tai Lake paddy soil and red paddy soil. POC in both soils were composed basically of O-alkyl-C, Alkyl-C and Aromatic-C, with the former being dominating. Fertilization treatments modified the proportion of different C species from the bulk samples as the portion of O-alkyl-C was decreased and that of aromatic-C and phenolic-C increased under both organic fertilization and compound fertilization of organic and inorganic fertilizers, resulting in the high aromaticity and hydrophobicity of POC. It indicated the enhanced chemical recalcitrance and stability of POC under organic amendments. Whereas, increased portion of O-alkyl-C but decreased portion of alkyl-C and aromatic-C was observed under chemical fertilization only and it induced relatively low aromaticity and hydrophobicity and a more readily decomposable nature of POC. The water-stable aggregates also differed in relative proportion of each C species from Tai Lake paddy soils under different fertilization treatments, and it modified the interactive effect of POC on aggregates stability of the paddy soil, whereas those from red paddy soil were not affected by fertilizations.Furthermore, the supramolecular structure of POC from Tai Lake paddy soil showed thermochemolysis products dominated by aliphatic and lignin-derived compounds. The most abundant aliphatic compounds were C8-C30 fatty acid methyl esters (FAMEs) dominated by even-numbered carbon species, in which short chain FAMEs of microbial origin were prevalent. Fertilization systems induced an increased trend of aliphatic compounds and it was mainly attributed to organic matter of microbial origin under chemical fertilizer only and chemical fertilizer plus pig manure, suggesting increased microbial activity in these two treatments; whereas it was primarily attributed to that of higher plant origin, followed by microbial origin under chemical fertilizer plus crop straw. The C stabilization by molecular recalcitrance is supposed as a way to contribute to C sequestration in Chinese paddy soils under good managements.4. Good fertilizations promoted crop carbon assimilation and soil carbon sequestrationTaking an example of a long-term fertilization trial from Tai Lake paddy soil, the effect of fertilization treatments on soil C sequestration and crop C assimilation as indicated by grain yield was analized to elucidate the process of soil-crop ecosystem involved in soil C sequestration. The results were as follows:significant difference in mean rice yield was observed between the treatments, with higher and more stable yield of rice under the combination of organic and inorganic fertilizers. Fertilizations significantly increased the C storage of topsoil and it showed higher C sequestration rate under the combination of organic and inorganic fertilizers than under inorganic fertilizers only. Soil C sequestration rate was in proportional to the total input of crop biomass and organic fertilizer to the soil. This demonstrated a key role of enhanced biomass C input in soil C sequestration of the paddy. Moreover, effect of applied N both on rice C assimilation as indicated by grain yield and soil C sequestration turned stronger under combined fertilizations. This, in turn, evidenced a Win-Win effect of enhancing and stabilizing crop productivity and C sequestration and greenhouse gas mitigation as well by paddies.5. Carbon input contributes to soil carbon sequestration in paddy soils and it varied among soil types with particularly indigenous soil componentIn view of the influence of fertilization on crop C assimilation and soil C sequestration, four sites of long-term soil-agro-ecosystem experiments (purple paddy soil, red paddy soil, Tai lake paddy soil and gley paddy soil) under different fertilizations from South China was further investigated with cross-site analysis to elucidate the soil-crop cooperation involved in soil C sequestration. It showed that fertilization could promote soil C sequestration via increasing the C inpur since it suggested a good linear correlation between annual soil C sequestration and C input. The soil C sequestration efficiency (the slopes of linear correlation between soil C sequestration and C input) differed between different types of paddy soils and it was higher in purple paddy soil and red paddy soil. The saturated C sequestration under good fertilizations was distinctively higher in red paddy soil, followed by gley paddy soil. Statistic analysis indicated negative linear correlation of C sequestration efficiency with initial SOC content, whereas no correlation with the clay content. The saturated C sequestration represented distinctly positive linear correlation with free ferric oxide and annual precipitation in different types of paddy soils. Thus, the abundant ferric oxide promoted the increase of soil C stock, whereas clay exhibited no positive effect on C sequestration in paddy soils. Soil C sequestration also promoted the N efficiency on crop yields. Climate is supposed to be another drive index of soil C stock of paddy soils.6. Ferric oxide could effectively promote agricultural carbon sequestration and mitigationIn order to explore the approach and method of soil C sequestration, the effect of ferric oxide on reducing the mineralization of organic materials (rice straw, pig manure, sewage sludge and granule organic fertilizer) under aerobic incubation and field landfill conditions was explored and elucidated. The results showed that ferric oxide significantly decreased the C mineralization rate of organic materials at each incubation time, it resulted in a significant reduction of the cumulative evolution of CO2 from organic materials during the early incubation period of 23 days, from 10934.45,5426.12,5288.43 and 794.90 mg CO2-C kg-1 to 125.47,1535.15,1473.36 and 498.72 mg CO2-C kg-1 for rice straw, pig manure, sewage sludge and granule organic fertilizer respectively, being more distinct for rice straw. Under the field landfill condition, except for granule organic fertilizer, the organic carbon remained for rice straw, pig manure and sewage sludge, sampled and determined after 30, 60 and 90 days since the beginning of field landfill process, was distinctively higher in organic materials granulated with Fe oxyhydrates than in those granulated with deionized-water. The decomposition rate of organic materials during the entire 90 days of field landfill was reduced by ferric oxide from 34.06%,14.91%and 19.90%to 24.25%,9.45% and 14.24%for rice straw, pig manure and sewage sludge, respectively, being more distinct for rice straw. Both of aerobic incubation and field landfill indicated distinct effect of ferric oxide on reducing the C mineralization and decomposition in organic materials.Thus, the particular fate of soil C sequestration in paddy soils was involved in phycial protection of coarse aggregates, chemical binding of ferric oxides and molecular structure stabilization, while clay protection was not supposed to explain the distinct C accumulation in paddy soils. In addition, soil C accumulation was mainly originated from biological C input and it was different among soil types with particularly indigenous soil component, which could influence the saturation capacity of soil C sequestration. Soil C input and the sequestration fate inside the paddy soil supplemented each other and promoted together the soil C stock and sequestration potential. It still deserves more attention on soil C sequestration fate under large region scale and climate backgfound in order to distinguish the relative importance and contribution of single C sequestration fate in different soil types, and to probe the effect of climate on agricultural C sequestration. The method and approach of C sequestration also deserves more study based on the soil C sequestration fate.
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