Straw returning and tillage measures are one of the key measures to improve soil structure and fertilize soil. Different rotation systems and tillage methods (no-tillage or conventional tillage) affect the physical structure and the C sequestration efficiency of soil. Here, we examined the response of soil organic C stocks, soil C fractions and soil structure to straw returning and tillage management based on a 15-year rice-rice-oilseed rape rotation. Our results indicated that the soil carbon stock reached C equivalent within 10 years with straw returning and 5-6 years without straw returning. No-tillage improve the C sequestration efficiency only in the early stage. Soil organic C (SOC) fractions significantly increased after straw returning, that is, dissolved organic carbon (DOC) increased by 10.3%-21.4%, particulate organic carbon (POC) increased by 32.0%-44.2%, light fraction organic carbon (LFOC) increased by 37.9%-61.2%, and microbial biomass carbon (MBC) increased by 7.1%-12.1%. Except for LFOC, no significant difference was observed between no-tillage and tillage for the other SOC fractions. Straw returning significantly improved the proportion of >2 mm aggregates (+40.0%) and the SOC content of >0.25 mm aggregates. Meanwhile, straw returning with conventional tillage (CTS) enhanced the SOC content of <0.25 mm aggregates. The soil structure became irregular (anisotropy increased by 115.0%), more complex (fractal dimension increased by 10.2%) and the number of soil pores increased by 108.5% after straw returning. LFOC and MBC played important roles in promoting the changes in the soil structure and the formation of macro-aggregates. Overall, straw returning was more effective in increasing the SOC, the accumulation of macro-aggregates, and the number of soil macropores, as well as improving the soil structure compared with tillage under the triple upland-paddy rotation system.

Cement-based solidification/stabilization (S/S) techniques have been widely used to produce stable forms of contaminated soils and reduce the mobility of contaminants into the environment. However, information on the long-term performances of S/S under environmental conditions (i.e., variable loading and atmospheric carbon dioxide) remains sparse. In this study, a triaxial test setup was modified to simulate environmental conditions. The permeability and compressive strength of silica sand solidified with portland cement were measured at different stages of four scenarios involving carbonation only, axial strain only, carbonation followed by axial strain, and axial strain followed by carbonation. X-ray computed tomography (CT) was used to characterize the internal structure of the samples. Permeability and compressive strength results indicate that the axial strain accelerated the damage to the S/S specimens and increased their permeability. The deterioration due to the mechanical strain decreased in the presence of carbon dioxide. Consistent changes in microstructure were observed with the CT scan. The results indicate that the influence of stressors on the void size distribution, compressive strength, and permeability is complex and characterized by interactions between the stressors.