Differences in the physical protection mechanisms of soil organic carbon with 13C-labeled straw and biochar

Abstract Straw-derived biochar has a greater potential for carbon (C) sequestration than straw that is directly returned to the soils. An incubation experiment was carried out to explore the impact of different straw returning modes (straw and biochar) on soil organic C (SOC) sequestration and their underlying physical protection mechanisms. The 13 C-labeled straw and biochar were applied at rates of 0.5% and 1% (w/w) to three typical upland soils collected from Yuzhong, Yangling, and Changwu (labeled as YZ, YL, and CW, respectively) on the Loess Plateau in China. Soil C fractions and isotopic C were measured, and the pore structure of macroaggregates was quantified using X-ray computed tomography (CT) and verified by 3D visualization images. Compared to the control (CK), the addition of straw biochar increased SOC contents more than straw in three soils, especially at higher rates. Only biochar improved particulate organic C (POC) contents, and the changes of mineral-associated organic C (MOC) contents differed with soil types. Soil potential C mineralization (PCM) and microbial biomass C (MBC) increased with the addition of straw rather than biochar, except for MBC in YZ soil. The contribution percentages and contents of 13 C in SOC, POC, and MOC were higher with biochar addition than with straw addition, especially in YZ and YL soils. The total porosity and pore connectivity increased with straw addition in YL soil and with the 1% biochar application in CW soil. However, only isolated porosity increased in YZ soil. Meanwhile, the macropores increased while the mesopores and micropores showed decreasing trends in both YL and CW soils. Additionally, YL soil exhibited more pronounced positive correlations between pore structure and C fractions compared to the other two soils. These findings suggested that the sequestration of SOC was attributed to the improvements of POC and MOC by affecting soil pore structure in straw- and biochar-amended soils, respectively. Therefore, both the form of imported C and soil properties, such as the fertility and pore structure, should be comprehensively considered to fully optimize the potential for C sequestration in farmland soils. Graphical Abstract