中文摘要：

【目的】分析东北黑土区旱田改稻田后土壤有机碳、全氮含量及其密度和~（13）C、~（15）N自然丰度值的动态变化,探讨旱田改稻田后土壤有机碳（氮）的固定能力及其稳定性,揭示旱田改稻田后土壤有机碳（氮）的演变规律,为东北黑土的合理利用和培肥提供理论依据。【方法】结合野外实地调查,选择典型黑土区旱田土壤（种植大豆年限大于60年）和改种不同年限的稻田土壤（3、5、10、17、20和25年,旱田改稻田前种植历史基本相同,均为大豆）,利用稳定同位素分析技术,研究旱田改稻田后土壤有机碳、全氮的动态变化特征。【结果】旱田改稻田25年间,在0—60 cm土层,土壤有机碳和全氮含量的变化趋势均表现为：在改种的前3年迅速下降,降幅分别为13.60%—43.27%和10.40%—40.60%,在3—25年间随改种年限延长呈逐渐增加的趋势,且在20—60 cm土层出现累积,但在3—5年期间增加幅度较大,在5—25年期间增加较为缓慢,在改种17—25年期间,稻田土壤有机碳和全氮含量均高于旱田土壤;0—60 cm土层土壤有机碳和全氮密度的变化趋势与其含量的变化趋势大致相同,在改种的3年间0—60 cm土层土壤有机碳和全氮密度分别降低了26.53%和21.89%,在改种5—25年间0—60 cm土层稻田土壤有机碳和全氮密度均大于旱田土壤,增幅分别为9.87%—21.48%和10.2%—19.3%;旱田改稻田后,土壤全氮与有机碳的变化密切相关,土壤全氮与有机碳的含量、密度之间均呈显著线性正相关关系（P〈0.01）。在0—60 cm土层,土壤δ~（13）C值在改种的3年间明显上升,在3—25年间随改种年限延长呈逐渐下降趋势,且大于5年的稻田土壤δ~（13）C值均低于旱田土壤,而土壤δ~（15）N值在改种的25年间随改种年限延长呈逐年下降趋势,各年限稻田土壤δ~（15）N值均低于旱田土壤,相同年限土壤的δ~（13）C值和δ~（15）N值均随着土?

英文摘要：

[ Objective ] The aims of this article were to analyze the dynamic changes in the contents of soil organic carbon and total nitrogen and their densities, and in the values of 13C and 15N natural abundance, explore the sequestration and stability of soil organic carbon and total nitrogen, and to reveal the evolving rule of soil organic carbon （nitrogen） after conversation from upland to paddy field in black soil region of Northeast China. The results would provide a theoretical basis for the reasonable utilization and fertilization in black soil. [Method] Combined with field investigation, the upland field （soybean planted for over 60 years） and paddy field with different years （3, 5, 10, 17, 20 and 25 a, soybean was planted in all the fields before their conversation to paddy field） were chosen as research objectives, and stable isotope analysis technology was employed to study the changes of the characteristics of soil organic carbon and total nitrogen after conversation from upland to paddy field. [Result] In 25 years of conversation from upland to paddy field, the changes of soil organic carbon and total nitrogen contents in the 0-60 cm soil layers showed the similar change trends of rapid drop in the first 3 years, the soil organic carbon and total nitrogen contents were decreased by 13.60%-43.27% and 10.40%-40.60%, then gradually increased with the extension of plant years between 3 to 25 years of conversation, and showed accumulation in the 20-60 cm soil layers, but increased faster between 3 to 5 years of conversation, increased slower between 5 to 25 years of conversation, between 17 to 25 years of conversation, the soil organic carbon and nitrogen contents in the paddy field all higher than in the upland field; the densities of soil organic carbon and total nitrogen in the 0-60 cm soil layers showed the similar change trends with their contents, they were decreased by 26.53% and 21.89% in the first 3 years, and their densities in the paddy field all higher than in the upland field between 5 t