Objective Returning straw to the field is a reasonable and effective measure to accelerate the utilization of agricultural waste as resources and to increase the storage of soil organic carbon (C). However, the stabilization and transformation process of straw C in different aggregates fractions of Black soils was not very clear.

Method The ¹³C-labeled straw treatments (as percentages of 1%, 3%, 5% and 10%) were added to low and high fertility levels of Black soil, and then they were incubated in-situ for 360 days. This paper analyzed the dynamic changes of organic C of aggregate fractions, and studied the transportation characteristics of native organic C and straw-C in aggregate fractions, in order to reveal the sequestration process of soil organic C.

Result Compared with the treatment without straw addition, more than 3% of straw addition treatments significantly increased the organic C contents of low and high fertility soils by 21.00% to 81.81% and 12.40% to 44.00%, respectively. Under the high fertility soil, the proportions of macro-aggregate fraction in soil were increased by 10.3 to 21.3% relative to the low fertility soil with the straw addition treatments. At the end of incubation, the average organic C content of macro-aggregate fractions was increased by 11.4% in high fertility soil compared with low fertility soil. Compared with the no straw addition treatment, 10% of straw addition significantly increased the content of organic C in soil micro-aggregate fraction in the low and high fertility soils by 36% and 67%, respectively (P < 0.05). The contribution percentage of straw C to total organic C in macro-aggregate fraction was decreased gradually, while that in microaggregate (53 ~ 250 μm) and sit and clay fraction (< 53μm) were gradually increased in soils with varied amount of straw addition after 360 days of incubation.

Conclusion As the straw decomposition, straw C gradually was transported and accumulated from macro-aggregate fraction to smaller size aggregate fractions. The organic C content of silt and clay fraction in high fertility of Black soil tends to be stable with straw addition. After adding straw to high fertility black soil, the organic C content of clay particles tends to be stabilized.