Objectives Microbial residual carbon is an important composition for the stability of soil carbon pool. Study on the effects of freeze-thaw intensity on the accumulation of fungal and bacterial residual C and their contributions to soil organic carbon (SOC), as well as the impact of corn straw incorporation on these processes, will deepen the knowledge on the microbial regulatory mechanism of SOC sequestration, and provide a theoretical support for the improvement of soil fertility in Northeast China.

Methods Indoor incubation method was used in the research, with maize straw and black soil as experimental materials. Freeze-thaw intensity treatments of weak (melting temperature/freezing temperature was 5℃/−4℃) and strong (melting temperature/freezing temperature was 5℃/−9℃) were setup, whereas constant 5℃ room temperature served as control. Each freeze-thaw intensity treatment was conducted under straw and non-straw conditions. The soil samples were incubated at 5℃ for 24 h, then decreased to the treated freezing temperature for 48 h, and then rose to 5℃ for 24 h to complete a freeze-thaw cycle. The experiment included 16 times freeze-thaw cycle and lasted 65 days. After the 0, 3, 8, 12, and 16 times freeze-thaw cycle, soil samples were collected to determine the glucosamine (fungal residue biomarker) and muramic acid (bacterial residue biomarker) contents, and the accumulation of microbial residue C and its contribution to SOC were analyzed.

Results In the non-straw control, the strong freeze-thaw treatment significantly increased the fungal and bacterial residue C contents and their contributions to SOC, and decreased the fungal/bacterial residue C (F/B) compared with the constant temperature treatment in the early stage, while the relevant indicators in the weak freeze-thaw treatment did not change significantly compared with the constant temperature treatment. After 16 times cycle, strong freeze-thaw treatment significantly reduced the soil microbial residue C and its contribution to SOC, and increased the F/B value. At constant temperature and weak freeze-thaw treatments, straw addition promoted the accumulation of fungal and bacterial residue C at the early stage, but after the 16th cycle, the contents of fungal and bacterial residue C were reduced. For the strong freeze-thaw treatment, straw addition significantly increased the content of bacterial residue C and decreased the F/B value at the end of the experiment. At the end of the experiment, the freeze-thaw intensity had little effect on the contribution of microbial residue C to SOC after straw incorporation.

Conclusions Repetitive and intensive freeze-thaw reduces the microbial residue C, especially bacterial residue C accumulation. When maize straw was added into soil, the fungal residue C was not influenced, but the accumulation of bacterial residual C was increased significantly, which led to higher ratio of fungi to bacterial C and higher contribution of microbial C to the whole soil organic carbon storage. So, straw addition could help the stability of soil organic carbon pool to a certain extent.