中文摘要：

选取我国南方三种典型水稻土的长期试验田，采集长期不同处理下的未破坏土壤样品，采用低能量超声波分散法分离得到不同粒径的团聚体颗粒组，研究不同处理下这些团聚体颗粒组中的有机碳（Soil organic carbon，SOC）含量及其分配变化，探讨土壤有机碳积累与团聚体物理保护的关系。结果表明：供试三种水稻土团聚体颗粒组的组成以200—20μm和20～2μm粒径为主，分别占22％～43％和27％一44％，微团聚化作用较强。SOC含量以2000—200μm和〈2μm粒组中最高；而易氧化态碳（Labile organic carbon，LOC）主要富集于2000～200μm粗团聚体颗粒组中，其占SOC的比例（LOC／SOC）也是以该粒径中明显最高。直径为2000—200μm的粗团聚体颗粒组作为新增有机碳的主要载体，随不同耕作和施肥等长期处理的变化最为强烈，其中又以红壤性水稻土的SOC和LOC随不同施肥的变化最为强烈，说明其良好管理下的有机碳累积效应最为显著。统计分析表明，全土的有机碳积累量与2000～200μm粗团聚体的有机碳积累量之间的关系可用抛物线拟合（R2=0．95，n=8）。由此看来，长期试验下新固定的有机碳积累及其粗团聚体保护可能存在某种饱和机理。计算表明，供试水稻土的粗团聚体保护在长期试验期内还未达到其饱和限，本研究结果支持了我国学者对于近20年来南方水稻土特别是红壤丘陵区水稻土有机碳固定速率较高的认识。同时，红壤性水稻土的粗团聚体保护作用最强，仍然具有明显的固碳潜力，这也提示土壤中氧化铁对水稻土中有机碳的固定和化学稳定可能有重要贡献，水稻土固碳的团聚体保护作用与团聚体中有机碳的化学结合机制有关。

英文摘要：

In this study, physical protection of soil organic carbon （SOC） in particle size fractions of paddy soils was investigated using cross-site study of topsoil samples from three sites of long term soil-agro-ecosystem experiments from South China. Undisturbed topsoil samples collected at 0 - 15 cm were used for size factions of micro-aggregates by low-energy ultra-sonic dispersion procedure. SOC content and its distribution in particle-size fractions under different fertilization and cultivation treatments were analyzed to address the relationship between SOC accumulation and physical protection of aggregates. The results were as follows ： the distribution of particle-size fractions of three tested paddy soils was dominated by 200 - 20 μm and 20 - 2 μm fractions （22% - 43% and 27% - 44% ） , showing prominent micro-aggregation in these paddy soils. SOC content was higher in fractions of 2 000 - 200 μm and 〈 2 μm than in the other fractions, while labile organic carbon （LOC） content and LOC/SOC ration higher in the coarse fraction of 2 000 -200 μm only. The newly accumulated young SOC found its fate mainly in the coarse fraction of 2 000 ~ 200 μm in active response to tillage and fertilization treatments. Of the three studied paddy soils, SOC and LOC in the red-earth derived paddy soil showed most intense response to fertilization treatments, 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 2 000 - 200 μm coarse fraction. Therefore, it is inferred that accumulation of young SOC of topsoil may reach a saturation controlled by the physical protection in the coarse particle-size fractions. However, as shown by the calculation using the curve obtained, the maximum protection capacity by the coarse micro-aggregates of the three tested paddy soils had not yet realized, still possessing obviously a potential of carbon sequestration. The higher pote