中文摘要：

采用土壤有机碳（SOC）物理分组与13C自然丰度相结合技术,研究了稻田长期（19 a）转换为旱地（玉米地）后土壤有机碳及其组分的响应特征,以及不同有机碳组分的周转和更新速率.结果表明,长期种植旱地作物后,农田土壤有机碳和总氮含量显著下降.相同历史背景下,稻田土壤总有机碳（TOC）和总氮（TN）的浓度分别比玉米田高76.7%和47.6%.水稻土包裹态颗粒有机质（oPOM）和矿物结合有机质（MOM）在土壤中的浓度均是玉米田的2倍,但游离态颗粒有机质（fPOM）差异不显著.稻田土壤oPOM和MOM自身碳的浓度均显著高于玉米地,fPOM则相反.特别是oPOM组分,稻田是玉米地的近6倍.表明水稻土团聚体保护碳的能力高于旱地.稻田转换为玉米地19 a后,各组分δ13C值显著升高.fPOM、oPOM和MOM中来自玉米新碳的比例分别达到了54.6%、24.7%和19.0%,平均驻留时间（MRT）依次增大,分别达到24、67和90 a.上述结果进一步证明了稻田土壤比旱地更具固碳潜力,其优势主要体现在土壤中oPOM和MOM组分碳的富积.

英文摘要：

Natural ^13C abundance determination method coupled with physical fractionation of soil organic carbon （SOC） was used to evaluate the responses of SOC and its fractions to long-term land-use conversion from paddy field to upland field （corn cultivation）. Results showed that land-use conversion from paddy field to upland field led to significant decreases in the contents of SOC and total nitrogen （TN）. Concentrations of total organic carbon （TOC） and TN were respectively greater by 76.7% and 47.6 % in the paddy field than those in the corn field. Concentrations of occluded particulate organic matter （oPOM） and mineral-associated organic matter （MOM） on a whole soil basis were two times higher in the paddy field than those in the upland field, while no significant difference was found in free particulate organic matter （fPOM）. Carbon concentrations of oPOM and MOM fractions on their own weight basis were significantly greater in the paddy field than those in the upland field, especially the oPOM fraction, which was 6 times higher in the former than that in the latter. It could be concluded that SOC protection exerted by soil aggregates in paddy soil was greater than that in upland soil. After a 19-year conversion from paddy field to corn field, δ^ 13C values of SOC fractions significantly increased. Maize-derived carbon （C） accounted for 54.6%, 24.7%, and 19.0% in fPOM, oPOM and MOM, respectively. Mean residence time （MRT） of the initial rice-derived C increased in the order fPOM （24 a） 〈 oPOM （67 a） 〈 MOM （90 a）. The above results further indicate that paddy field soil owns greater capability of carbon sequestration than upland soil mainly through increasing the contents of oPOM and MOM in the fractions of SOC.