Change Characteristics Of Organic Carbon And Nitrogen In Aggregates And Aggregate Fractions After Conversion From Upland To Paddy Field In Black Soil

Soil aggregate is an important carrier of soil organic carbon?nitrogen?sequestration,and the protection of soil organic carbon?nitrogen?by aggregates has the great significance to maintain and improve the level of soil organic carbon?nitrogen?.The black soil in Northeast China is rich in organic matter,and the rice planting is an important way to utilize the black soil.However,what about the evolution of soil aggregates and its fractions organic carbon?nitrogen?changes after the conversion from upland to paddy field in black soil ? It's still unclear.Therefore,in this article,soil samples were collected from upland?soybean planted for over 60 years?in typical black soil and paddy soil with different years(3,5,10,17,20 and 25 years,soybean was planted in all the fields before conversion to paddy soil.The changes of organic carbon?nitrogen?in soil aggregates and fractions were studied by using the technique of wet-sieving method and stable isotope analysis technology.Study the soil aggregates and its fractions organic carbon?nitrogen?changes after the conversion from upland to paddy field of 0 to 25 years,from the micro-scale of soil aggregate fractions,to reveal the dynamic characteristics of carbon?nitrogen?in soil after the conversion from upland to paddy field in black soil.The main results are as follows:?1?In the 0-60 cm soil layers,the 2-0.053 mm aggregates?51.83%-73.32%?were dominated after the conversion from upland to paddy field.Compared with the control treatment,the composition of 2-0.25 mm aggregates in the soil of different rice planting years were decreased,and that of 0.25-0.053 mm and < 0.053 mm aggregates were increased.There is no obvious change in the composition of >2 mm aggregates.?2?In the 0-60 cm soil layers,after the conversion from upland to paddy field,soil organic carbon?nitrogen?showed increased trend with the extension rice planting years,and mainly accumulated in the 2-0.25 mm and 0.25-0.053 mm aggregates.The ?¹³C,?¹⁵N of soil in each size of aggregates were significantly affected by rice cultivation time and soil depth,which increased with the decreasing of aggregate size.The ?¹³C increased with soil depth in the same year,while ?¹⁵N had no significant change.?3?In the 0-20 cm soil layer,the silt and clay ability of organic carbon?nitrogen?sequestration in the paddy soil is powerful.The silt and clay organic carbon?nitrogen?distribution ratio in the macroaggregates?>0.25 mm?and microaggregates?0.25-0.053 mm?were higher than particulates?>0.053 mm?in the paddy soil.The proportion of total silt and clay organic carbon?nitrogen?in soil aggregates were up to 56.66%-70.39%?59.96%-76.25%?.?4?In the 0-20 cm soil layer,the organic carbon?nitrogen?distribution ratio in the occluded microaggregats and free aggregates were up to 8.87%-22.27%?9.16%-24.18%??10.74%-22.86%?11.13%-21.44%?,and they were the key frations for soil organic carbon?nitrogen?sequestration in paddy soil.The coarse particulate and fine particulate organic carbon?nitrogen?content were more changeable,their proportion were increased with the extension of rice planting years,and they were the key fractions to raise the organic carbon?nitrogen?content in paddy soil.In conclusion,during 25 years of the conversion from upland?planted soybean?to paddy field in northeast China balck soil,soil aggregates stability had no significant change,the soil organic carbon?nitrogen?of coarse particulate organic matter in macroaggregates and fine particulate organic matter in microaggregates increased were the key reasons for organic carbon?nitrogen?content enhanced after the conversion from upland to paddy soil,while the physical protection?fine particulate organic matter?in microaggregates and physical-chemical protection?organomineral complexes?were the improtant mechanisms of soil organic carbon?nitrogen?sequestration after the conversion from upland to paddy soil.