Winter Cover Crops Affect Aggregate-Associated Carbon, Nitrogen and Enzyme Activities from Black Soil Cropland

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.