Winter cover crops have been shown to promote the accumulation of microbial biomass carbon and nitrogen, enhance nutrient cycling, reduce erosion, improve ecosystem stability, etc. In the black soil area of Northeast China, Triticum aestivum L., Medicago sativa L., Vicia villosa Roth., Triticum aestivum L. and Medicago sativa L. mixed planting, Triticum aestivum L. and Vicia villosa Roth. mixed planting, and winter fallow fields (CK) were selected to investigate the effects of winter cover crops on soil total carbon and nitrogen and microbial biomass carbon and nitrogen. The results showed that (1) after seasonal freeze-thaw, the rate of change in SOC (−2.49~6.50%), TN (−1.54~5.44%), and C/N (−1.18~1.16%) was less than that in SMBC (−80.91~−58.33%), SMBN (−65.03~332.22%), and SMBC/SMBN (−45.52~−90.03%); (2) winter cover crops not only alleviated the negative effects of seasonal freeze-thaw, which reduces SMBC and qMBC, but also increased SMBN and qMBN; (3) there was an extremely significant (p < 0.01) positive correlation between SOC and TN, a significant (p < 0.05) negative correlation between SMBC and SMBN, and there was no significant correlation between SOC and SMBC or between TN and SMBN; (4) alkali-hydrolysable nitrogen had the greatest impact on SOC and TN, while the soil’s saturation degree had the greatest impact on SMBC and SMBN; and (5) the Triticum aestivum L. monoculture was the most effective in conserving soil microbial carbon and nitrogen. In conclusion, winter cover crops can mitigate the reduction in soil microbial biomass carbon caused by seasonal freeze-thaw and also increase the soil microbial nitrogen content in the black soil region of Northeast China, of which Triticum aestivum L. monoculture showed the best performance.

The thinning, leaning, and hardening of arable land in the black soil region of Northeast China has brought serious challenges to the sustainable development of agriculture. It is of great significance to explore suitable conservation tillage for the conservation and sustainable utilization of black soil resources actively. The topsoil of the cropland in the northeastern part of the Songnen Plain with winter fallow (CK), planted alfalfa, and planted winter wheat was used as the research object to analyze the changes in the soil aggregate composition, nutrients, and enzyme activities before and after freeze–thaw, respectively, and to investigate the effect of winter cover crops on the improvement of the quality of the black soil cropland. Compared with the winter fallow field, (1) planting alfalfa significantly increased the mechanical stability of 1–2 mm and 0.25–1 mm particle size aggregates (about 3 times and 25 times over), and planting winter wheat increased the water stability of 0.25–1 mm particle size aggregates significantly (2.7 times over); (2) planting alfalfa and winter wheat increased the soil C/N ratio of >2 mm and 1–2 mm particle size aggregates, and the increment in the C/N ratio in >2 mm particle size aggregates remarkably increased, by 203.6% and 362.7%, respectively; (3) planting alfalfa significantly enhanced the soil invertase activity and urease activity in >2 mm and 0.25–1 mm particle size aggregates, and planting winter wheat significantly enhanced the catalase activity in 0.25–1 mm particle size aggregates. In conclusion, planting winter cover crops during the winter fallow period can maintain and promote the mechanical and water stability of medium and large (0.25–1 mm,1–2 mm) soil aggregates, increase the carbon content and C/N ratio of larger (1–2 mm, >2 mm) aggregates, and enhance the enzyme activity of small and medium (0.25–1 mm, <0.25 mm) aggregates to varying degrees. The results of the study can provide a reference for the promotion of basic research on and technology for winter cover crops in the black soil region.