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

Abstract

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.

1. Introduction

The high-intensity utilization of black soil for more than a hundred years has led to soil erosion and the thinning, leaning, and hardening of the black soil on cropland. The deterioration in soil quality has brought a great challenge to the sustainable development of agriculture [1]. Cultivated soils in the northeastern black soil region are exposed for up to half a year in winter while erosion agents such as hydrodynamic, wind, and freeze–thaw forces alternate or synchronize in time, and stagger or overlap in space. This can result in the coexistence or compounding of soil erosion types [2]. Actively exploring suitable conservation tillage is of great significance to the conservation and sustainable utilization of black soil resources. Cover crops are planted during the gap in agricultural production to reduce or avoid soil exposure in time or space, which is a new strategy to realize sustainable agricultural development [3,4]. Compared with the winter fallow, winter cover crops can form a vegetative cover on the soil surface, which can effectively reduce soil erosion and improve the soil quality [5,6]. As the basic unit of soil’s structure, soil aggregates play an important role in soil’s physical, chemical, and biological processes and functions [7]. Soil enzymes are involved in the processes of soil nutrient cycling and transformation, organic matter decomposition, and sequestration. They are an important reference in evaluating the soil quality and ecological environment. Their activities are also affected by the size of the aggregate particles, nutrients within the aggregate, etc. [8]. Catalase is a type of oxidoreductase that promotes the oxidation of soil’s organic matter and humus formation, and invertase is one of the hydrolytic enzymes associated with the soil carbon cycle and is mainly involved in the decomposition and transformation of organic matter [9]. Urease is able to enzymatically hydrolyze urea into ammonia and is one of the common hydrolytic enzymes involved in the soil nitrogen cycle [10]. Changes in soil enzyme activity can reflect changes in the soil fertility, and can also sensitively reflect changes in environmental factors, land use patterns, and agricultural production activities [11].

In field trials and lab culture studies on non-legume winter-hardy cover crops, it was found that winter cover crops can enhance soil aggregates’ stability, increase the soil’s organic carbon content, and improve surface soil enzyme activities to varying degrees [12,13,14]. Planting different winter cover crops in potato winter fields in Northwest China found that planting alfalfa could reduce the soil bulk density; increase the soil porosity and soil permeability; enhance the activities of soil phosphatase, invertase, urease, and catalase; and promote the growth and reproduction of soil microorganisms to repair the soil ecological environment [15]. Although there have been many studies showing that winter cover crops in temperate environments can be used to replenish winter ecological niches, improve soil quality, and alleviate soil erosion problems, there has been little research in this area in the northeastern black soil region of China. This may be due to the fact that cover crops find it difficult to survive in severe cold regions, and there is insufficient practice. Encouragingly, however, studies in the North American Black Soil Belt, where the hydrothermal conditions are similar, have found that rye, as a winter cover crop, not only germinates and grows rapidly at lower temperatures, but its roots also absorb and accumulate nitrate during growth, reducing nitrogen loss, which then returns into the soil with the plant residue to provide nitrogen for subsequent crops. [16,17]. Post-harvest mixing of rye and oats significantly increased the soil infiltration, accelerated sediment deposition, and thus reduced the incidence of gully erosion [18]. Research on winter cover crops is well established in modern agricultural countries, such as Europe and the United States. Not only are there technological systems that can be directly applied, but they are also incorporated into national policies, with governments taking measures to incentivize farmers to carry out planting winter cover crops. Theoretically, winter cover crops also have a high potential in the sustainable utilization of black soil resources.

In this study, three treatments, alfalfa cover, winter wheat cover, and winter fallow field (control), were set up in the northeastern black soil cropland. The effect of winter cover crops on an improvement in the cropland quality in northeastern black soils was investigated by analyzing the effects of the presence or absence of winter cover crops on the soil aggregate composition, nutrients, and their enzyme activities. It aimed to explore the adaptability and ecological value of winter cover crops in the black soil region of northeastern China, with a view to providing reference for theoretical research and the application of winter cover crops in the black soil region.

2. Materials and Methods

2.1. Experimental Design

The experimental site of this study is located in Keshan Farm in the territory of Keshan County, in the northwestern Heilongjiang Province, which belongs to the Qiqihar Administration of the General Administration of Reclamation of Heilongjiang Province (48°11′–48°24′ N, 125°07′–125°37′ E). The geography is characterized by rolling hills. The climate is a cold temperate continental monsoon climate, with an average annual temperature of 2.4 °C and an annual precipitation of 500 mm.

This experiment was started in September 2022, and the experimental plots were 5 m × 10 m in size, with three treatments of alfalfa cover, winter wheat cover, and winter fallow field (CK), and three replications for each treatment. The plots were randomly arranged within the district group. The test crops were alfalfa-WL298HQ and winter wheat, seeded at 4 g·m⁻² and 60 g·m⁻², respectively. The alfalfa and winter wheat were seeded on 13 September 2022, in conjunction with the actual local cash crop fall harvest schedule. Based on the growth of the cover crop roots before sampling and the intensity of the influence of freeze–thaw alternation, we selected the 0–10 cm soil layer as the research object. Successful overwintering of alfalfa and winter wheat was observed in the spring of 2023, with winter surviving rates of 55% and 82%, respectively. None of the treatments were fertilized, with watering regularly during the experiment.

Temperature and precipitation data were collected throughout the experimental period using small meteorology stations installed in the field. The average monthly temperature and total precipitation for the period of this experiment are shown in Figure 1. The basic physical and chemical properties of the soil in the 0–10 cm soil layer of the test site were as follows: soil bulk weight at 1.01 g cm⁻³, pH at 6.10, organic matter at 44.33 mg g⁻¹, total nitrogen at 2.09 mg g⁻¹. As of the first sampling, the winter wheat and alfalfa surface cover was 36.76% and 7.04%, and their belowground biomass was 17.81 g m⁻² and 2.19 g m⁻² (Figure 2 and Figure 3).

2.2. Soil Sampling and Laboratory Measurements

At the end of October 2022 and the beginning of May 2023, 17 cm × 11 cm × 9 cm of undisturbed soil from the 0–10 cm soil layer of each plot was collected using the “S” sampling method and packed into plastic boxes to avoid damage to the soil structure. After being brought indoors, the soil pieces in the plastic box were gently broken into pieces of about 1 cm in diameter along their natural structure. Stones, plants, and animal debris were removed, and then air-dried under natural conditions for the determination of the soil aggregates.

The dry sieve method was used to sieve four aggregate size fractions: >2 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm [19]. Then, 50 g of the aggregates of each particle size obtained using the dry sieving method was proportionally prepared for wet sieving according to the method of Elliott [20]. Determination of the soil aggregate enzyme activities and soil aggregate nutrients was performed using dry sieved aggregate soil of all grain sizes. The soil’s total carbon and nitrogen content was determined using an elemental analyzer (The Elemental Combustion System 4024, Italy). The soil’s catalase activity was determined using potassium permanganate titration and expressed as milliliters of 0.1 N potassium permanganate consumed by 1 g of soil after 20 min [21]. The soil’s invertase activity was determined using the colorimetric method and using 3,5-dinitrosalicylic acid and expressed as the number of milligrams of glucose that could be hydrolyzed to produce glucose from 1 g of air-dried soil in 24 h [21]. The soil’s urease activity was determined using the indophenol blue colorimetric method and expressed as the milligrams of ammonia nitrogen released from 1 g of air-dried soil over 24 h [21]. The geometric mean (GMea) of the soil enzyme activities can be used to make a comprehensive evaluation of the soil’s enzyme activities [22], calculated as:

$$\mathrm{GMea}=\sqrt[3]{E_{Ure}\times E_{Inv}\times E_{Cat}}$$

(1)

where $E_{ure}$ is the urease activity; $E_{Inv}$ is the invertase activity; $E_{Cat}$ is the catalase activity.

The relative amount of change before and after freeze–thaw of each indicator was used to further analyze the effectiveness of cover crops in responding to a season of freeze–thaw. The relative changes in the soil’s mechanical stability aggregate size distribution (Δ$W_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{a}\mathrm{g}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}}$), soil’s water-stable aggregate size distribution (Δ$W_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}-\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}}$), total nitrogen content (ΔTN), total carbon content (ΔTC), C/N ratio (ΔC/N), catalase activity (Δ$E_{ure}$), invertase activity (Δ$E_{Inv}$), urease activity (Δ$E_{Cat}$) and the geometric mean of soil enzyme activity (ΔGMea) were calculated using the following formulae:

∆_i = (after freezing and thawing − before freezing and thawing)/(before freezing and thawing) × 100%

(2)

2.3. Statistical Analysis

The Microsoft Excel 2019 and SPSS 26.0 software were used to organize and analyze the data, and one-way ANOVA was used to evaluate the significance of the differences between the indicators under different treatments. The criterion used for the statistical significance of the treatment effects was p < 0.05. The Origin 2022 software was used to draw the relevant graphs.

3. Results

3.1. Effect of Winter Cover Crops on the Compositional Characteristics of the Soil Aggregates

The mechanically stable aggregates in all treatments were composed of >2 mm and 1–2 mm particle sizes, and the water-stable aggregates were composed of 0.25–1 mm particle sizes (Figure 4). According to Figure 5, it can be seen that freeze–thaw caused a decrease in the content of mechanically stable aggregates and water-stable aggregates in soils with a >2 mm particle size under each treatment, and an increase in the content of mechanically stable aggregates in soils with a 1–2 mm particle size, as well as in the content of mechanically stable aggregates and water-stable aggregates in soils with a 0.25–1 mm particle size. Compared with CK, the increase in the soil’s mechanical stability aggregates of a 1–2 mm particle size and a 0.25–1 mm particle size under the alfalfa treatment was 2.9 and 24.4 times higher than that of CK, respectively. The effect of the change was significant (p < 0.05). The decrease in water-stable aggregates of a >2 mm particle size under alfalfa treatment was significantly lower than that of winter wheat and CK, and the decrease was 36.1% less than that of CK. The increase in water-stable aggregates of a 0.25–1 mm particle size under the winter wheat treatment was significantly greater than the other two treatments and 2.7 times greater than that of CK. It is evident that planting winter cover crops during the winter fallow period maintains and promotes the mechanical stability and water stability of medium and large (0.25–1 mm, 1–2 mm) soil aggregates.

3.2. Effect of Winter Cover Crops on the Carbon and Nitrogen Content of Soil Aggregates

3.2.1. Soil’s Total Carbon Content

Different treatments had different effects on the total carbon content of the aggregates at each particle level (Figure 6). The soil’s total carbon content was significantly higher with winter wheat and alfalfa than with CK in all but the 1–2 mm particle size aggregates prior to freeze–thaw. After experiencing freeze–thaw, the soil’s total carbon content under the winter wheat treatments was higher than that of the CK at all particle size levels, but the difference was only significant at the <0.25 mm particle size level. As can be seen from Figure 6b, after experiencing freeze–thaw, the total carbon content in the >2 mm particle size aggregates showed an increasing trend under each treatment. The winter wheat and alfalfa increments were significantly higher by 26.8% and 58.7%, respectively, compared to the CK’s increments. Among the <0.25 mm particle size aggregates, only the total carbon content of the aggregates tended to increase under the winter wheat treatment.

3.2.2. Soil’s Total Nitrogen Content

Different treatments had different effects on the total nitrogen content of the aggregates at each particle level (Figure 7). All the treatments before freeze–thaw showed the highest soil total nitrogen content in the 0.25–1 mm particle size aggregates. The soil’s total nitrogen content was higher in aggregates of all particle sizes before and after freeze–thaw under the winter wheat treatment than under the CK. Of these, only the soil’s total nitrogen content in the 1–2 mm particle size aggregates after freeze–thaw was not significantly different from that of the CK. According to Figure 7b, it can be seen that the soil’s total nitrogen content in the >2 mm particle size aggregates under each treatment showed an increasing trend after experiencing freeze–thaw. A decreasing trend was observed in the 1–2 mm particle size aggregates. Freeze–thaw resulted in a significantly greater increase of 14.91% in the total nitrogen content in the <0.25 mm particle size aggregates under the winter wheat treatment than for CK and alfalfa. Among the >2 mm particle size aggregates, the total nitrogen content with CK and alfalfa increased significantly more than with winter wheat, 0.7 and 0.6 times over, respectively.

3.2.3. Soil C/N Ratio

The soil C/N ratios in the 1–2 mm particle size aggregates under the winter wheat treatments before and after freeze–thaw were significantly higher than those of CK. All the treatments exhibited the highest soil C/N ratios in the 0.25–1 mm and <0.25 mm particle size aggregates before freeze-thaw. The highest concentration in the 1–2 mm particle size aggregates was observed after freeze–thaw (Figure 8a). Freeze-thaw reduced the soil C/N ratios in the 0.25–1 mm particle size and <0.25 mm particle size aggregates under all treatments, with the greatest reduction under the two cover crop treatments. In contrast, winter wheat and alfalfa for the >2 mm particle size aggregates increased the soil C/N ratio significantly, by 362.7% and 203.6%, respectively, compared with CK. The enhancement effect was significant. The soil C/N ratio increased in the 1–2 mm particle size aggregates and was higher under both other cover crop treatments than for CK, but the difference was not yet significant (Figure 8b).

3.3. Effect of Winter Cover Crops on the Enzymatic Activity of Soil Aggregates

3.3.1. Soil’s Catalase Activity

As can be seen in Figure 9a, winter cover crops had different effects on the soil’s catalase activity in aggregates of different particle sizes compared to CK. All the treatments showed the highest soil catalase activity in the <0.25 mm particle size aggregates after experiencing freeze–thaw. According to Figure 9b, seasonal freeze–thaw caused a decrease in the catalase activity in the 1–2 mm particle size aggregates under all treatments. A significant decrease of 13.84% was observed under the winter wheat treatment compared to the magnitude of change for CK. The largest increment in the 0.25–1 mm particle size aggregates was seen for the winter wheat treatment, which was 2.5 times higher than the increment for CK, and the effect was significant. The elevated catalase activity in the <0.25 mm particle size aggregates was increased significantly by 78.9% and 68.2% with CK compared to the winter wheat and alfalfa.

3.3.2. Soil’s Invertase Activity

As shown in Figure 10a, the invertase activity in the soil aggregates at all particle size levels under the winter wheat treatment was higher than that using CK and alfalfa before and after freeze–thaw. The highest invertase activity in the soil was demonstrated in the 0.25–1 mm and <0.25 mm particle size aggregates under all treatments. As can be seen in Figure 10b, the seasonal freeze–thaw effect promoted the soil’s invertase activity in the 0.25–1 mm particle size aggregates under each treatment. The change in the soil invertase activity in the >2 mm particle size aggregates was most significant with alfalfa, where the increase was 29.2 times higher than that with CK. The change was significant. Seasonal freeze–thaw promotes increased soil invertase activity in alfalfa-treated aggregates at all particle size levels.

3.3.3. Soil’s Urease Activity

Urease is able to enzymatically hydrolyze urea into ammonia and is one of the common hydrolytic enzymes involved in the soil nitrogen cycle [19]. As can be seen from Figure 11a, in terms of the aggregate size, all the treatments before and after freeze–thaw showed the highest urease activity in the 0.25–1 mm size aggregates. The soil’s urease activity was higher than with the other two treatments in all the particle size aggregates under the winter wheat treatment before freeze–thaw. According to Figure 11b, it can be known that the soil’s urease activity in the 1–2 mm particle size aggregates under each treatment decreased after experiencing freeze–thaw. The lowest decrease was observed with winter wheat, but the differences among the three treatments were not yet significant. The soil’s urease activity in the >2 mm, 0.25–1 mm and <0.25 mm particle size aggregates under alfalfa treatment after experiencing freeze–thaw showed an increasing trend. The most significant difference was in the 0.25–1 mm particle size increment compared to CK. The soil’s urease activity decreased in all the particle size aggregates under the winter wheat treatment.

3.3.4. Soil Geometric Mean Soil Enzyme Activity of Soil Aggregates

As can be seen in Figure 12a, all the treatments before and after freeze–thaw exhibited the highest GMea of the soil’s enzyme activities in the 0.25–1 mm particle size aggregates. Before freeze–thaw, the GMea of the soil’s enzyme activities was significantly higher in all particle size aggregates under the winter wheat treatments than with the CK and alfalfa. According to Figure 12b, it can be seen that seasonal freeze–thaw increased the GMea of the soil’s enzyme activities in soil aggregates of all particle levels under CK and alfalfa treatments. The alfalfa treatment had the largest increase of 22.75% in the >2 mm particle size aggregates, which was a significant increase of almost 50% compared to the change with CK. Using 1 mm as a boundary, freeze–thaw significantly decreased the >1 mm aggregate enzyme activity and increased the <1 mm aggregate enzyme activity in the winter wheat treatments, but the magnitude of the increase was less than that with CK.

4. Discussion

Soil aggregates are the basic units of soil’s structure and also the most important sites for soil nutrient storage and microbial survival [10]. Soil nutrients such as soil carbon and nitrogen and soil enzymes improve the soil fertility while enhancing the cementing capacity of cemented substances and promoting the formation and stabilization of aggregates [23]. The mechanically stable aggregates under each treatment in this study were composed mainly of >1 mm particle sizes, and the water-stable aggregates were composed mainly of <1 mm particle sizes. After seasonal freeze–thaw, the 1–2 mm particle size aggregate content increased significantly under the alfalfa treatment compared to the winter fallow field and winter wheat treatments. This suggests that different cover crops have different effects on the distribution of the aggregate composition. This is similar to the results obtained by Wang, M.L. et al. [24] that demonstrate cover crops enhance the content of mechanically stable aggregates in soils of certain particle sizes. It has also been shown that winter cover crops prevent the decomposition of soil aggregates during the winter and provide a better soil structure after spring plowing compared to bare winter soils [25]. The winter cover crops in this study experienced only one seasonal freeze–thaw and maintained water-stable aggregates of larger particle sizes only before freeze–thaw, which could be attributed to the shorter growth period, the amount of plant roots, as well as the amount of decay restitution, and the fact that physical entanglement and chemical cementation had not yet played a significant role. However, it has been demonstrated an overall trend that winter cover crops can promote the aggregation of medium to large aggregates. The increases in the total carbon content and C/N ratio in the >2 mm particle-size aggregates of the two winter cover crops in this study were significantly higher than those of the winter fallow field, which also testified to the role of winter cover crops in promoting the formation of large aggregates, and enhancing the carbon sink capacity of the black soil cropland by increasing the carbon content of the large particle size aggregates. Zhao, Q. et al. [26] showed that after the use of winter cover crops, the total carbon accumulation in the soil under cover crop treatments was 1.9 to 3.3 times higher than that of the winter fallow field, which is similar to the results of this study. Winter cover crops increase the nutrient content of macroaggregates, which is consistent with the concept of “macroaggregate turnover”. This means that the newly imported organic matter will first form macroaggregates under the action of cementing substances, and the soil’s organic carbon, nitrogen and other nutrients will also be first aggregated in the macroaggregates. With the decomposition of the particulate organic matter, microaggregates will gradually be formed, in which the carbon and nitrogen content will gradually increase [27].

Winter wheat in this study significantly elevated the catalase and urease activities in the <0.25 mm particle size aggregates and invertase activity in the soil aggregates of all particle sizes before freeze–thaw compared to the alfalfa and winter fallow field treatments. This is also in accordance with the results already available. For example, Wang, Y. et al. [28] found that the soil’s enzyme activities in the plots planted with cover crops were higher than those of the control, and the effects of different cover crop types differed from each other. Wei, J. et al. [29] found that winter cover crops increased the soil’s alkaline phosphatase, invertase and urease activities to different degrees when they used winter cover crops to study the fertility and microbiological traits of meadow cinnamon soils. The reason for this may be that cover crop planting improves the soil aeration, aerobic respiration efficiency and soil water content, which promotes an increase in microbial populations, allowing for an increase in the soil’s enzyme activity as well [29]. Moreover, unlike the pattern of winter cover crops first affecting the soil carbon and nitrogen in large particle size aggregates, winter cover crops first improve the soil’s enzyme activity in smaller particle size aggregates. This may be due to the large specific surface area of small particle aggregates, the high number of contact sites with microorganisms and the easy adsorption of soil enzymes by organic–inorganic complexes in small particle aggregates. As well as the fact that most enzymes are produced by bacterial secretion, the high abundance and diversity of bacterial communities in small particle aggregates promotes the accumulation of soil enzymes [30].

Notably, we found that seasonal freeze–thaw had a positive effect on the soil C/N ratio in black soil aggregates of a 1–2 mm particle size but inhibited the catalase activity and urease activity in this particle size. Moreover, after planting winter cover crops, the increase in the C/N ratio in this particle level became larger. Thid may subsequently consume the available nitrogen in the soil and slow down the decomposition rate of organic matter and organic nitrogen, leading to a decrease in the degree of enzymatic and microbial decomposition of organic carbon for this particle size, which ultimately leads to an increase in the accumulation of organic carbon in this grain level [31,32]. Miao, S.J. et al. [33] similarly showed that for typical black soils in Northeast China, two particle size aggregates, 1–2 mm and 0.053–0.25 mm, had the greatest protective effect on the soil’s organic carbon. The 1–2 mm grain size aggregates essentially newly import organic carbon and are rich in particulate organic carbon [34]. The enzyme substrates of invertase involved in the organic carbon cycle (root decay, secretion, etc.) at this grain level showed a tendency to increase after exposure to freeze–thaw, leading to an increase in their activity. The activity of urease, which is involved in the nitrogen cycle, showed a decreasing trend due to the decrease in available nitrogen in the soil at this particle level. The activity of catalase, which reflects the intensity of the soil’s microbial activity, also decreased. This ultimately leads to an increase in the accumulation of organic carbon in the soil in the 1–2 mm particle size aggregates of black soil, which also makes the aggregates not easy to transform into smaller particle sizes. This will maintain and promote the formation of large grain size aggregates, improve the soil quality and enhance the soil erosion resistance. It has been pointed out that 1 mm is the critical aggregate size for the particle loss and enrichment characteristics of agricultural black soil. The migration of typical black soil cultivated soils was also mainly dominated by <1 mm size aggregates, and >1 mm size aggregates did not easily migrate via water erosion [35]. However, it cannot be ignored that this particle size is relatively susceptible to tillage erosion. That is, it is migrated by tillage implements, and, during the migration process, breaks down to enhance mineralization and become a carbon source [36,37,38]. It can be seen that if the planting of winter cover crops can maintain and promote the stability of 1–2 mm particle size aggregates and increase the carbon content of this particle size, it also shows a great potential for the restoration of eroded and degraded black soils. However, the growth time of the winter cover crops in this study is short, the effect of cash crops has not yet been considered and the incremental C/N ratio of this particle level has not yet been found to significantly different from that of winter fallow fields, so further in-depth studies on the complete cycle of cash crops–winter cover crops need to be completed.

5. Conclusions

The cultivation of winter cover crops in the northeastern black soil area can maintain and promote the mechanical stability and water stability of medium and large particle (0.25–1 mm, 1–2 mm) soil aggregates, increase the carbon content and C/N ratio of large particle (1–2 mm, >2 mm) aggregates and enhance the enzyme activity of medium and small particle (0.25–1 mm, <0.25 mm) aggregates to varying degrees. The results demonstrated the great potential of restoring eroded and degraded black soil. Research on cover crops in black soil in Northeast China is still in its initial stage. In order to enrich the choices of protective tillage in this area, and to provide a basis for the sustained utilization of black soil resources and the promotion of the modernization of the agricultural process, it is especially recommended to carry out basic research on winter cover crops and research promoting the technology to use with winter cover crops (winter-hardy, freeze-killed), their varieties (Leguminosae, Gramineae, etc.), the root system configurations (tap root system, fibrous root system, etc.), the crop planting time (pre-harvest, post-harvest) and the planting density (the most suitable degree of coverage before freeze–thaw) for the half-year-long period of exposed freeze–thaw.