Change law of organic carbon in lime concretion black soil aggregates with application of straw by δ13C method

Abstract: Straw application is an essential measure for improving soil organic carbon (SOC) content, promoting soil aggregate formation and improving soil structure. In order to study the effects of application of rice straw with stable carbon isotope (δ13C) on distribution and stability of water-stable aggregates of lime concretion black soil, and explore the dynamic variation and distribution of native soil organic carbon and fresh carbon in soil water-stable aggregates during straw decomposition, the rice straw spiked with the natural abundance of 13C was incorporated with the lime concretion black soil. This experiment was conducted in a constant-temperature incubator indoor for 4 month, which used isotope tracer technique of the natural abundance of 13C. The experiment included 2 treatments: CK (no straw) and Str (added with 1% straw); all samples were separated into 4 aggregate-size classes (>2000, 250-2000, 53->250, <53 μm) by wet sieving in the different incubation period, while organic carbon in bulk soil and soil aggregates in different size fraction were determined. The results showed that the content of microaggregates (<250 μm) in lime concretion black soil without rice straw was the highest, and the concentrations of organic carbon in various aggregates were lower than that with 1% straw. Compared with the control, the application of rice straw in lime concretion black soil not only significantly promoted the formation of >2000 and 250-2000 μm soil water-stable macroaggregates (P<0.05), but also increased the mean weight diameter (MWD), geometric mean diameter (GMD) and macroaggregate content (R0.25) of water-table aggregates. Also, the value of fractal dimension in straw treatments was lower than the control. Specifically, MWD, GMD and R0.25 value of aggregates of the straw treatments increased by 21.5%, 34.3% and 21.3% compared with the CK, respectively. And the fractal dimension value of straw treatments decreased by 2% compared with that of CK. After 120 days of incubation, >2000 and 250-2000 μm soil water-stable macroaggregates increased by 265.5% and 16.0% respectively, while the content of macroaggregates (>250 μm) became the highest, accounting for 63.28%. Consequently, application of rice straw was beneficial to the improvement of soil structure. The concentrations of organic carbon in different levels of aggregates were significantly (P<0.05) increased after additions of rice straw and the organic carbon contents in >2000, 250-2000, 53->250, and <53 μm aggregates were increased by 21.4%, 25.4%, 34.7%, and 50.0% compared with the control after 15 days of incubation. There is a most significant relationship between the GMD, MWD, R0.25 value and the concentrations of organic carbon in 250-2000 and 53->250 μm aggregates (P<0.01), and a significant relationship between the GMD, MWD, R0.25 value and the concentrations of organic carbon of >2000 μm aggregates (P<0.05). The dynamic variation of δ13C in soil water-stable aggregates was significant and the content of δ13C in soil water-stable aggregates improved significantly (P<0.05), which showed that the turnover rate of fresh carbon was faster. The fresh carbon supplied by rice straw was mainly in the 53->250 and <53 μm fraction of soil aggregates, making up 38% and 28% of the total, respectively. The result shows that the addition of rice straw can improve soil structure, and increase soil organic carbon content in all sizes of aggregates, which provide theory basis for soil quality improvement and organic carbon recycle in North China.