Change Characteristics On Soil Organic Carbon (Nitrogen) After Conversation From Upland To Paddy Field In Black Soil Region Of Northeast China

Black soil as an important agricultural soil resources in northeast China.There were more areas of upland reclaimed paddy field in recent years.The significant changes would taken place after the conversation from upland to paddy field in black soil,and how the organic carbon(nitrogen)changed in short-term flooding soil enviroment(about 4 to 5 months of the years)was still unclear.Soil organic carbon(nitrogen)as the important indicators of soil fertility,evaluate the dynamic change in the contents of soil organic carbon(nitrogen)after the conversation from upland to paddy field in black soil region scientifically,this results would provide theoretical support for the reasonable utilization and fertilization in black soil region of northeast China.This article chose upland field(soybean planted for over 60 years)and paddy field with different years(3 a,5 a,10 a,17 a,20 a and 25 a,soybean was planted in all the fields before their conversation to paddy field)as research objectives,and employed stable isotope analysis technology and physical and chemical fractionation technologies,the aims were(1)to achieve the change characteristics of soil organic carbon(nitrogen)and particulate organic carbon(nitrogen)and bonding organic carbon,and analyze the stability of soil organic carbon(nitrogen)and particulate organic carbon(nitrogen),(2)to analyze the affecting factors on the changes of soil organic carbon(nitrogen),(3)to reveal the evolving rule of soil organic carbon(nitrogen)after the conversation from upland to paddy field in black soil region of northeast China,the results showed below:(1)Between 25 years of the conversation from upland to paddy field,the change of soil organic carbon(nitrogen)contents and their density in the 0-60 cm soil layers showed the similar change trends of rapid drop between the 3 years,and gradual increase with the extension of plant years between 3 to 25 years of the conversation.The densities of soil organic carbon and total nitrogen were increased by 9.87%-21.48%and 10.2%-19.3%,respectively,in the 0-60 cm soil layers.The close relationship(P<0.01)was found between the changes of soil organic carbon and total nitrogen.Soil 13C and 15N natural abundance value declined gradually with time during the conversation,and they were lower in the paddy filed planted between 5 to 25 years than in the upland field,soil 13C and 15N natural abundance values increased with soil depth in the same years.(2)Between 25 years of the conversation from upland to paddy field,the distribution ratio of soil particulate organic carbon(nitrogen)were 4.96%-12.73%(4.03%-13.07%),compared with upland,they were increased by 6.8%-106.4%(0.7%-224.0%).13C and 15N natural abundance value of soil particulate orgnic matter in the 0-40 cm soil layers were higher in the paddy field than in the upland field in every year,soil particulate 13C increased with soil depth in the same years,but soil particulate 15N natural abundance values decreased with soil depth in the same years.(3)Between 25 years of the conversation from upland to paddy field,the distribution ratio of soil bonding organic carbon were 41.06%-66.48%,they were lower in the paddy filed planted than in the upland field.They were dropped gradually between 3 to 25 years of the conversation,and increased with soil depth in the same years.The change of distribution ratio of Fe(Al)-SOC in the 0-60 cm soil layers and soil bonding organic carbon showed the similar change trends,the distribution ratio of Ca-SOC in the 20-60 cm were gradual increase with the extension of plant years,but Fe(Al)-SOC higher than Ca-SOC obviously in every year,Fe(Al)-SOC was accounted for the main position in the soil bonding organic carbon.(4)Between 25 years of the conversation from upland to paddy field,soil pH,cation exchange capacity,iron-aluminum oxide of different form were gradual increase with the extension of plant years,but the law of change is not consistent.However through correlation analysis and multiple linear stepwise regression analysis found,the important factors affecting soil organic carbon and particulate organic carbon and organic carbon change were cation exchange capacity,exchangeable Ca2+ and complexed iron and aluminum.Soil cation exchange capacity,and exchangeable Ca2+ were played a positive role in soil organic carbon,however complexed iron and aluminum were played a negative effect.In summary,between 25 years of the conversation from upland to paddy field in black soil region,after the conversation from upland to paddy field for more than 5 years,soil organic carbon(nitrogen),particulate organic carbon(nitrogen)were gradual increase with the extension of plant years,soil bonding organic carbon were gradual decrease with the extension of plant years,paddy field showed stronger ability of carbon(nitrogen)sequestration and the accumulation of stable carbon(nitrogen)in the 20-60 cm soil layers.Labile particulate organic carbon showed relative accumulation,its decomposition was relatively active.Therefore after the conversation less than 5 years,more attention should be paid to supplement organic carbon and nitrogen for soil,but after the conversation from upland to paddy field for more than 5 years,more attention should be paid to stable organic carbon and nitrogen for soil in order to maintain and promote the level and stability of soil carbon and nitrogen.