Change Characteristics Of Organic Carbon In Soil Aggregates And Aggregate Fractions Of Reclamation For Rice In Mollisol

Soil organic carbon?SOC?is an important indicator affecting soil quality.Physical or physicochemical protection of SOC by aggregates is of great significance to SOC sequestration.Mollisol has high natural fertility and rich organic matter,and suits for cultivation in Northeast China.Reclamation for rice has become the main utilization of Mollisol.However,what about the evolution of organic C in aggregates and its fractions after reclamation for rice in Mollisol?It's not clear yet.Therefore,The natural meadow?0 years,as the control?of typical Mollisol in Northeast China,and paddy soil reclamation from natural meadow in 12,35,62,85 years were chosen as research objects.Both the wet-sieving method and density flotation were applied to extract aggregates and fractions,and the organic C content and?¹³C value were analyzed with elemental-stable isotope mass spectrometry.Study on historical changes in organic C accumulations of macro-aggregates and micro-aggregates in the paddy soil over a long-term of 12 to 85 years reclamation from natural meadow in Mollisol.To reveal the sequestration and stabilization mechanism of organic C after reclamation for rice in typical Mollisol of Northeast China in the micro-scale of fractions in soil macro-aggregate and micro-aggregate.The main results are as follows:?1?The SOC and ¹³C stock of 0-60 cm soil layer increased firstly and then decreased with reclamation years.Among them,the stock of 0-20 cm soil layer increased significantly during 12 years of reclamation for rice?P<0.05?,while the stock of 20-40 cm and 40-60 cm soil layer increased significantly during 35 years of reclamation for rice.The?¹³C value in all soil layers increased year by year after 12 years of reclamation for rice,which increased the stability of SOC year by year.?2?In 0-20 cm soil layer,the macro-aggregates?>0.25 mm?were dominated in paddy soil?59.43%to 75.59%?,and the organic C was mainly concentrated in macro-aggregates.Soil macro-aggregate composition and its organic C stock,and ¹³C stock,increased during 12years of reclamation for rice and then decreased year by year.There was a significant positive correlation between the macro-aggregates organic C stock and SOC stock?P<0.01?.The?¹³C value in aggregates of all sizes increased with the decrease of aggregate size,and the?¹³C value in aggregates decreased during 12 years of reclamation for rice and then increased year by year.?3?In 0-20 cm soil layer,the macro-aggregates of meadow soil and paddy soil were dominated by coarse particulate organic C?inter-m M c POC?and mineral-associated organic C?MOC,the sum of inter-m M MOC and intra-m M MOC?,the distribution ratio of inter-m M c POC and MOC were 14.93%to 44.69%and 27.08%to 50.59%,respectively.The changes of inter-m M c POC and MOC influence each other and make they showed a waxing and waning trend.The inter-m M c POC stock was greatly affected by the reclamation years,and inter-m M c POC stock increased during 12 years of reclamation for rice and then decreased year by year.The changes of inter-m M c POC stock had significant effects on the macro-aggregates composition and its organic C stock,SOC stock?P<0.05?.?4?In 0-20 cm soil layer,the stock and distribution ratio of particulate organic C?intra-m POC?and mineral-associated organic C?intra-m MOC?in free micro-aggregates in meadow soil and paddy soil decreased during 12 years of reclamation for rice and then increased year by year,but they were less affected by the reclamation years.?5?In 0-20 cm soil layer,the distribution ratio of POC and MOC in each size aggregates of meadow soil and paddy soil were 31.18%to 55.92%and 44.08%to 68.82%respectively,and they showed a waxing and waning trend.POC was mainly enclosed in macro-aggregates in the form of inter-m M c POC,and organic C activity was stronger.The distribution ratio of MOC enclosed in micro-aggregates in macro-aggregates was significantly higher than that in free micro-aggregates,and organic C was more stable.In conclusion,during 85 years of reclamation for rice in typical Mollisol natural meadow,The amount of SOC was decreasing,but the stability of SOC was increasing.The decrease of inter-m M c POC stock under the physical protection of macro-aggregates in surface soil?0-20cm?was the key fraction to lead to the decrease of SOC.The increase of MOC stock under the physicochemical protection of micro-aggregates in macro-aggregates and free micro-aggregates was important reason to improve the stability of SOC.