Tillage direction and mulching improve aggregate stability, carbon and nitrogen

Direção do preparo e cobertura melhoram estabilidade de agregados, carbono e nitrogênio

ABSTRACT

Sloping croplands in the black soil region of Northeast China are experiencing decreases in aggregate stability and nutrient retention. To address this issue, a field experiment with four tillage-cover management treatments, i.e., downslope no-tillage with straw mulch (SM), cross-slope no-tillage with straw mulch (CM), downslope no-tillage with cover crop (SC), and conventional tillage (CK), was conducted to investigate the dynamics and patterns of water-stable aggregate size distribution and stability indices (water-stable aggregates > 0.25 mm (WR_0.25), mean weight diameter (MWD), and geometric mean diameter (GMD)), as well as aggregate-associated soil organic carbon (SOC) and total nitrogen (TN) in the 0-60 cm soil profile across tillage directions and mulching measures. All the mulching-tillage treatments increased the WR_0.25, MWD, GMD, aggregate-associated SOC and TN contents, and the contribution of > 0.25 mm aggregates to total SOC and TN; however, these improvements generally decreased with increasing soil depth. The aggregate stability indices were positively correlated with the SOC and TN stored in the aggregates, with the strongest relationships observed in the 0-20 cm layer. Overall, cross-slope no-tillage with straw mulch (CM) treatment was the most effective treatment for enhancing aggregate stability and promoting carbon and nitrogen accumulation in relatively larger aggregates.

Key words:
sloping cropland; straw mulching; conservation management; soil structure

HIGHLIGHTS:

Cross-slope no-tillage with straw mulch treatment was the most effective treatment for improving soil aggregate stability.

Soil aggregates of 0.25-2.00 mm particle size exhibit the highest proportion among all soil aggregates.

Soil aggregate stability was positively associated with carbon and nitrogen contents in aggregates.

RESUMO

As lavouras em declive da região de solos negros do Nordeste da China vêm apresentando redução da estabilidade dos agregados e da retenção de nutrientes. Para enfrentar esse problema, foi conduzido um experimento de campo com quatro manejos de preparo do solo e cobertura: plantio direto no sentido do declive com palhada (SM), plantio direto em contorno com palhada (CM), plantio direto no sentido do declive com cultura de cobertura (SC) e preparo convencional (CK). O objetivo foi avaliar a distribuição dos agregados estáveis em água, os índices de estabilidade dos agregados - agregados estáveis em água > 0,25 mm (WR_0,25), diâmetro médio ponderado (DMP) e diâmetro médio geométrico (DMG), bem como os teores de carbono orgânico do solo (COS) e nitrogênio total (NT) associados aos agregados no perfil de 0-60 cm, sob diferentes direções de preparo e práticas de cobertura. Todos os manejos com cobertura aumentaram WR_0,25, DMP, DMG, os teores de COS e NT associados aos agregados e a contribuição dos agregados > 0,25 mm para o COS e NT totais; contudo, esses efeitos diminuíram, em geral, com o aumento da profundidade do solo. Os índices de estabilidade dos agregados correlacionaram-se positivamente com o COS e o NT armazenados nos agregados, com relações mais fortes na camada de 0-20 cm. De modo geral, o manejo CM foi o mais eficiente para aumentar a estabilidade dos agregados e promover o acúmulo de carbono e nitrogênio em agregados de maior tamanho.

Palavras-chave:
terras agrícolas inclinadas; cobertura morta com palha; manejo conservacionista; estrutura do solo

INTRODUCTION

The black soil croplands in Northeast China constitute approximately 27% of the nation’s cultivated land and account for approximately 25% of its grain output (Zhao et al., 2021). However, approximately 60% of these croplands are sloping lands characterized by long gentle slopes and alternating hills and valleys. Owing to long-term intensive cultivation and unsustainable management, sloping black soil farmlands are prone to severe soil and water loss. Soil aggregates, the fundamental units of soil structure, play a critical role in maintaining soil fertility by sequestering organic carbon and nitrogen (Lin et al., 2022; Liu et al., 2022a). Stable aggregates physically protect soil organic carbon (SOC) and total nitrogen (TN), enhancing soil structure, whereas the loss of aggregates leads to decreased carbon sequestration and decreased fertility (Wang et al., 2022). Therefore, the interactions between aggregate stability and soil carbon-nitrogen contents in sloping croplands are elucidated and crucial for sustaining soil health and productivity in black soil regions.

Tillage and mulching measures significantly influence soil aggregate stability and associated carbon-nitrogen (Kasper et al., 2009). Numerous studies have shown that compared with conventional tillage, conservation measures such as cross-slope tillage, no-till straw mulching, and cover cropping can improve macroaggregate formation and soil nutrient retention. For instance, cross-slope tillage increases the proportion of macroaggregates and soil nitrogen in sloping farmlands (Yan et al., 2024), whereas traditional downslope tillage tends to create runoff channels that accelerate aggregate breakdown and nutrient loss (Wang et al., 2024). Similarly, incorporating straw residues or planting cover crops generally increases the SOC content and aggregate stability, although the magnitude of improvement can vary with soil type and conditions (Zhang et al., 2019; Chang et al., 2024). These findings highlight the potential for optimizing the tillage direction and mulching treatments to maintain soil structure and fertility on vulnerable sloping lands.

Despite these advances, the combined effects and associated patterns of different mulching-tillage treatments on soil aggregate stability and carbon-nitrogen contents in Northeast China’s sloping black soils remain unclear. Few studies have simultaneously examined how the combination of tillage direction and mulching strategy affects aggregate-associated carbon and nitrogen across soil profiles. Therefore, a field experiment was conducted on sloping cropland in the Northeast China black soil region, and multiple tillage-plus-mulch treatments were evaluated. The objectives were to (i) determine how different mulching-tillage treatments influence the distribution and stability of soil aggregates and the contents and contribution rates of aggregate-associated SOC and TN within those aggregates at various depths, and (ii) explore the relationship between soil aggregate stability and aggregate-level SOC and TN under these treatments.

MATERIAL AND METHODS

The experiments were conducted from May to October in 2022, 2023, and 2024. The experimental site is located in Guangrong village, Qianjin town, Hailun city, Heilongjiang Province (47° 22′ 38″ N, 126° 51′ 04″ E), which is located in the central part of the typical black soil region in Northeast China and is a rolling hill area prone to soil and water loss. The average altitude of the site is 210 m, with the gradient of sloping cropland ranging from 0.5-7°, and sloping cropland accounts for more than 90% of the total land area. The dominant soil types in the area are black soil and meadow soil, among which black soil accounts for 63.4% of the total land area, with a thickness of approximately 30 cm. The chemical and physical characteristics of the soil at the experimental site before the experiment are shown in Table 1. The experimental site has a cold temperate continental climate, characterized by cold and dry winters, hot and rainy summers, and coincident rainfall and hot periods. The average annual temperature is 2 ℃, and the average annual rainfall ranges from 500 to 600 mm. Specifically, the rainfall during the crop growing seasons in 2022, 2023, and 2024 were 337.22, 691.19, and 467.12 mm, respectively.

Table 1
Chemical and physical attributes of the soil in the experimental area

Attributes	pH (H2O) 1:2.5	TN g kg⁻1	TP g kg⁻1	TK g kg⁻1	SOC g kg⁻1	BD g cm⁻3	Water-stable soil aggregates (%)
							> 2 mm	0.25-2 mm	0.053-0.25 mm	< 0.053 mm
0-20 cm	7.27	1.33	14.78	0.67	21.29	1.43	4.09	38.95	33.21	23.75

For this study, sloping cropland with an average slope gradient of 5° was selected, and 12 experimental plots were established, each with a size of 20 m (length) × 5 m (width). A maize-soybean rotation system was adopted for crop cultivation in all the plots. The field experiment involved four tillage-cover management treatments: downslope no-tillage with straw mulch (SM), cross-slope no-tillage with straw mulch (CM), downslope no-tillage with cover crop (SC), and conventional tillage (CK), each with three replicates. For the SM and CM treatments, after no-tillage sowing in spring and the emergence of seedlings, the straw was shredded and applied as mulch; the shredded straw length ranged from 15-20 cm, the application rate was 7,500 kg ha⁻¹, and the mulch thickness was approximately 5 cm. The cover crop was the leguminous species Melilotus officinalis. Seeds were broadcast uniformly by hand between crop rows at a seeding rate of 100 kg ha⁻¹. The experimental treatments are summarized in Table 2.

Table 2
Experimental treatments

Treatment	Tillage measures	Mulching measures
CK	downslope tillage, rotary tillage ridging in autumn	no straw mulching
SM	downslope tillage, spring no-tillage sowing	straw uniform coverage
SC	downslope tillage, spring no-tillage sowing	planting cover crops
CM	cross-slope tillage, spring no-tillage sowing	straw uniform coverage

The experimental site operated under a maize-soybean rotation: maize in 2022, soybean in 2023, and maize in 2024. The maize cultivar used was “Dika 1563,” with a planting density of 56,700 plants ha⁻¹; the soybean cultivar was “Huajiang 12,” with a planting density of 340,000 plants ha⁻¹, sown in double rows. The fertilization regimes were identical across all the plots and on the basis of local farming practices, the basal fertilizer rates were 125 kg ha⁻¹ N, 90 kg ha⁻¹ P, and 120 kg ha⁻¹ K. An additional top-dressing of N (125 kg ha⁻¹) was applied at the maize jointing stage, whereas no top-dressing was applied in the soybean plots.

After harvest, the five-point method was used to collect soil samples and loose soil samples from 0-20 cm, 20-40 cm, and 40-60 cm soil layers. The contents of SOC and TN in the soil and granular aggregates were determined by the total organic carbon analyzer and the Kjeldahl nitrogen analyzer. To determine the water-stable aggregate content, the wet-sieving method was employed to fractionate the aggregates into different size classes, resulting in the separation of macroaggregates (> 2 mm), mesoaggregates (0.25-2 mm), microaggregates (0.053-0.25 mm), and silt-clay fractions (< 0.053 mm) (Zhang et al., 2024).

MWD, Eq. 1, WR_0.25, Eq. 2, and GMD, Eq. 3 (Xing et al., 2015) were calculated as follows:

(1) $$\mathrm{MWD}=\sum _{\mathrm{i}=1}^{\mathrm{N}}\left({\bar{\mathrm{X}}}_{\mathrm{i}}{\mathrm{W}}_{\mathrm{i}}\right)$$

(2) $${\mathrm{WR}}_{0.25}=\sum _{\mathrm{i}}^{\mathrm{N}}{\mathrm{W}}_{\mathrm{i}}$$

(3) $$\mathrm{GMD}=\exp [\sum _{\mathrm{i}=1}^{\mathrm{N}}\log {\bar{\mathrm{X}}}_{\mathrm{i}}{\mathrm{W}}_{\mathrm{i}}/\mathrm{m}]$$

Where:

${\bar{\mathrm{X}}}_{\mathrm{i}}$ - the average diameter of soil aggregates with different particle sizes (mm);

W_i - the percentage of soil aggregates with different particle sizes in the soil samples (%); and,

n - the number of particle size levels greater than 0.25 mm.

The contribution rate of aggregates SOC to soil SOC, Eq. 4 and the contribution rate of aggregates TN to soil TN, Eq. 5 (Zhou et al., 2021) were calculated as follows:

(4) $${\mathrm{R}}_{\mathrm{C}}=\frac{100({\mathrm{AC}}_{\mathrm{i}}\times {\mathrm{W}}_{\mathrm{i}})}{\mathrm{TC}}$$

(5) $${\mathrm{R}}_{\mathrm{N}}=\frac{100({\mathrm{AN}}_{\mathrm{i}}\times {\mathrm{W}}_{\mathrm{i}})}{\mathrm{TN}}$$

Where:

R_C, R_N - contribution rate (%);

AC_i, AN_i - SOC and TN content of aggregates of different particle sizes (g kg^-1);

TC - soil SOC content (g kg^-1); and,

TN - soil TN content (g kg^-1).

All data were analyzed using IBM SPSS Statistics 27 (IBM Corp., Armonk, NY, USA). One-way analysis of variance was performed to assess the significant differences among different mulching-tillage treatments. The overall significance of treatment effects was first tested using the F-test. Fisher’s least significant difference (LSD) test was applied as a post-hoc multiple comparison test to determine differences among treatments. Pearson’s correlation analysis was performed to examine the relationships between aggregate stability and the carbon and nitrogen contents in aggregates, with the significance determined by the t-test. All figures were generated using Origin 2024 (OriginLab Corporation, Northampton, MA, USA).

RESULTS AND DISCUSSION

Table 3 presents the contents of water-stable aggregates across different size classes. In the 0-60 cm soil profile of sloping cropland in the black soil region of Northeast China, water-stable aggregates were dominated by fractions of size > 0.25 mm. Mesoaggregates accounted for the greatest proportion (35.77-60.29%), whereas macroaggregates accounted for the smallest proportion. Compared with the control, the different mulching-tillage treatments significantly increased the contents of macroaggregates and mesoaggregates throughout the 0-60 cm profile while reducing the contents of microaggregates and the silt-clay fraction. With increasing soil depth, the contents of macroaggregates and mesoaggregates gradually decreased, whereas the contents of microaggregates and the silt-clay fraction continuously increased. This trend contrasts with the findings of Ren et al. (2025) reported that the proportion of macroaggregates gradually increased with increasing soil depth. The discrepancy may be attributed to differences in field tillage operations. In general, tillage results in greater disturbance to the topsoil than to the subsoil, which can promote the fragmentation of large aggregates in the surface layer.

Table 3
Particle size distribution of water-stable aggregates under different mulching-tillage patterns

Years	Soil depth	Treatment	Water-stable soil aggregates (%)
			> 2 mm	0.25-2 mm	0.053-0.25 mm	< 0.053 mm
2022	0-20 cm	SM	5.92 ± 0.12 b	50.20 ± 2.02 ab	26.61 ± 3.07 b	17.27 ± 0.97 ab
		SC	5.19 ± 0.02 c	47.17 ± 0.25 b	29.18 ± 2.29 ab	18.45 ± 2.19 ab
		CM	6.49 ± 0.18 a	52.35 ± 1.68 a	21.87 ± 1.83 c	19.28 ± 3.55 b
		CK	5.03 ± 0.02 c	41.09 ± 2.47 c	30.79 ± 0.94 a	23.10 ± 1.56 a
	20-40 cm	SM	5.22 ± 0.12 b	46.59 ± 0.32 ab	28.40 ± 2.35 b	19.79 ± 2.27 ab
		SC	5.03 ± 0.02 c	44.97 ± 0.58 b	34.92 ± 1.17 a	15.09 ± 0.60 b
		CM	5.69 ± 0.09 a	48.08 ± 0.18 a	26.96 ± 1.68 b	19.27 ± 1.78 ab
		CK	4.68 ± 0.03 d	37.32 ± 1.78 c	36.88 ± 1.67 a	21.13 ± 3.34 a
	40-60 cm	SM	4.59 ± 0.12 a	43.77 ± 0.18 b	29.46 ± 1.54 b	22.18 ± 1.55 a
		SC	4.13 ± 0.06 b	41.71 ± 0.34 c	36.77 ± 0.06 a	17.39 ± 0.31 b
		CM	4.64 ± 0.03 a	44.23 ± 0.19 a	29.91 ± 1.36 b	21.22 ± 1.34 a
		CK	4.05 ± 0.07 b	35.77 ± 0.18 d	37.27 ± 0.06 a	22.91 ± 0.12 a
2023	0-20 cm	SM	6.10 ± 0.17 b	53.35 ± 1.49 a	22.93 ± 2.48 ab	17.63 ± 1.42 b
		SC	5.34 ± 0.12 c	48.31 ± 2.36 b	24.98 ± 2.29 a	21.3 ± 1.73 ab
		CM	6.59 ± 0.06 a	56.58 ± 0.76 a	19.07 ± 1.96 b	17.76 ± 1.99 b
		CK	5.09 ± 0.06 d	42.66 ± 1.54 c	26.76 ± 1.37 a	25.49 ± 2.85 a
	20-40 cm	SM	5.77 ± 0.10 b	48.22 ± 2.85 ab	24.98 ± 2.29 ab	21.03 ± 2.38 b
		SC	5.18 ± 0.03 c	45.87 ± 1.41 b	29.01 ± 2.12 bc	19.94 ± 0.73 b
		CM	6.03 ± 0.20 a	50.52 ± 1.69 a	23.22 ± 1.72 a	20.23 ± 0.30 b
		CK	4.95 ± 0.07 d	38.52 ± 1.23 c	31.11 ± 1.82 c	25.43 ± 2.96 a
	40-60 cm	SM	5.26 ± 0.12 a	46.42 ± 1.05 b	25.46 ± 1.01 c	22.85 ± 0.18 c
		SC	4.80 ± 0.06 b	42.60 ± 0.61 c	27.72 ± 0.41 b	24.88 ± 0.32 b
		CM	5.30 ± 0.03 a	48.63 ± 0.96 a	23.64 ± 0.97 d	22.43 ± 0.16 d
		CK	4.71 ± 0.07 b	36.65 ± 1.23 d	32.41 ± 1.08 a	26.24 ± 0.17 a
2024	0-20 cm	SM	6.30 ± 0.14 b	55.17 ± 1.78 b	28.19 ± 2.32 ab	10.35 ± 1.11 b
		SC	5.76 ± 0.20 c	51.27 ± 2.70 b	31.24 ± 4.14 ab	11.73 ± 1.45 b
		CM	6.70 ± 0.20 a	60.29 ± 2.11 a	25.31 ± 1.27 b	7.71 ± 2.74 b
		CK	5.16 ± 0.14 d	44.90 ± 1.50 c	33.05 ± 1.14 a	16.89 ± 2.77 a
	20-40 cm	SM	5.92 ± 0.08 b	51.84 ± 1.57 a	30.61 ± 1.75 bc	11.63 ± 3.20 b
		SC	5.60 ± 0.10 c	46.13 ± 1.48 b	35.21 ± 1.46 ab	13.07 ± 0.84 b
		CM	6.57 ± 0.05 a	54.37 ± 1.01 a	27.19 ± 2.82 c	11.88 ± 1.94 b
		CK	4.84 ± 0.13 d	39.50 ± 2.08 c	38.28 ± 2.05 a	17.38 ± 0.13 a
	40-60 cm	SM	5.35 ± 0.09 b	47.91 ± 0.02 b	31.15 ± 1.33 c	15.60 ± 1.32 ab
		SC	5.24 ± 0.05 b	44.47 ± 0.33 c	35.92 ± 0.57 b	14.38 ± 0.86 b
		CM	6.07 ± 0.14 a	51.69 ± 0.04 a	27.58 ± 0.74 d	14.67 ± 0.58 b
		CK	4.77 ± 0.15 c	37.29 ± 1.53 d	39.73 ± 1.32 a	18.22 ± 2.64 a

Overall, mulching-tillage treatments increased the proportion of macroaggregates and mesoaggregates by 1.85-35.62% and 13.25-38.62%, respectively; the CM treatment had the most pronounced effect (p < 0.05), with increases of 12.46-35.62% and 23.65-38.62%, respectively. Conversely, these treatments decreased the contents of microaggregates and the silt-clay fraction by 1.33-30.57% and 3.22-54.39%, respectively; the CM treatment resulted in the greatest reduction (p < 0.05), with decrease of 19.73-30.57% and 7.39-54.39%, respectively. These results indicate that mulching-tillage measures favor the formation of macroaggregates and mesoaggregates and improve soil structural stability primarily by increasing the proportion of larger aggregates.

Among all mulching-tillage treatments, the CM treatment was the most effective at increasing the content of > 0.25 mm aggregates and decreasing the content of < 0.25 mm aggregates. The underlying mechanisms may include the following: (i) polysaccharides produced during the decomposition of straw mulch increase the interparticle bonding strength, thereby increasing soil cohesiveness (Liu et al., 2023); (ii) the clay fractions have high specific surface area and cation exchange capacity (Ellerbrock et al., 2018), therefore they can adsorb organic matter and microbial exudates, promoting macroaggregate formation, inhibiting microaggregate generation, and enhancing aggregate stability; and (iii) the mulch layer provides physical protection by dissipating raindrop kinetic energy and thus reducing the risk of aggregate breakdown due to raindrop impact.

With increasing soil depth, the contents of macroaggregates and mesoaggregates decreased, whereas those of microaggregates and the silt-clay fraction increased, indicating that the effects of mulching-tillage on the aggregate size distribution are more pronounced in the topsoil. This pattern may be associated with greater root exudation and higher microbial activity in the surface soils, which stimulate the production of binding agents (Ma et al., 2022). Moreover, mulching increases organic matter input; organic matter can bind mineral particles through adsorption, facilitating the formation of organo-mineral complexes, which then combine with binding agents to form larger aggregates (Tang et al., 2024).

The changes in MWD, GMD, and WR_0.25 across mulching-tillage treatments as shown in Figure 1. Compared with the control (CK), all the mulching-tillage treatments significantly increased the MWD, GMD, and WR_0.25 in each soil layer (p < 0.05). For all the treatments, these indices decreased with increasing soil depth, indicating that the effects of the mulching-tillage treatments on improving soil structural stability were more pronounced in the topsoil.

Figure 1
Trends of MWD (A), GMD (B) and WR_0.25 (C) under different mulching-tillage patterns

For the same mulching practice, compared with the SM treatment, the CM treatment resulted in higher MWD, GMD, and WR_0.25 across the 0-60 cm profile, with increases of 0.99-7.54%, 1.61-8.20%, and 1.05-8.99%, respectively. These findings suggest that tillage direction is closely linked to erosion control and aggregate stabilization on sloping cropland, as a cross-slope arrangement can intercept runoff, reduce rill development, limit the lateral transport of organic matter, and increase straw-soil contact, helping preserve aggregates (Mao et al., 2022). Under the same tillage practice, SM consistently outperformed SC across soil layers, increasing the MWD, GMD, and WR_0.25 by 2.19-9.56%, 2.26-7.19%, and 3.62-11.67%, respectively. These findings indicate that compared with cover-crop mulching, straw mulching has a stronger effect on enhancing aggregate stability on sloping land, possibly because straw-derived phenolic compounds and decomposition products promote the formation and stabilization of macroaggregates and mesoaggregates (Tan et al., 2023). In addition, the nitrogen released during straw decomposition may further facilitate aggregate formation and stabilization (Wang et al., 2019). Collectively, these findings indicate that mulching and tillage measures can increase the proportion of macroaggregates and enhance aggregate stability, effectively improving the structural stability of sloping black soils.

The variations in SOC and TN contents among aggregate size classes under different mulching-tillage treatments (Figure 2) indicated that, compared with the control (CK), all the treatments increased the SOC and TN contents across the aggregate fractions at all the depths, with the SOC content increasing by 0.77-58.00% and the TN content increasing by 7.13-104.88%. In general, the SOC and TN contents decreased with decreasing aggregate size, suggesting that larger aggregates tend to contain more organic matter, which may reflect their stronger capacity to retain C and N. However, Cheng et al. (2023) reported a nonmonotonic pattern in which aggregate SOC first increased but then decreased as particle size decreased; such discrepancies may be attributed to differences in soil structure or land management measures. In addition, the SOC and TN contents within the aggregates decreased with increasing soil depth, which is consistent with the typical pattern of organic matter enrichment in surface layers.

Figure 2
The organic carbon content in soil aggregates at soil depths of 0-20 cm (A), 20-40 cm (B), and 40-60 cm (C), respectively, while the total nitrogen content in soil aggregates at soil depths of 0-20 cm (D), 20-40 cm (E), and 40-60 cm (F), respectively

Over the three years in which the experiments were conducted, compared with the CK treatment, all the mulching-tillage treatments generally increased the aggregate-associated SOC and TN contents, and the overall treatment ranking was largely stable; the cross-slope no-tillage with straw mulch (CM) treatment resulted in the greatest enhancement in all aggregate size classes and soil depths.

For the same mulching treatment, compared with downslope tillage, cross-slope tillage increased SOC by 9.77-31.19% and TN by 0.58-39.80%. Under the same tillage treatment (except for macroaggregates in 2024), the SOC and TN contents in all aggregate size classes in the straw mulching (SM) treatment were greater than those in the cover-crop (SC) treatment, with the SOC content increasing by 0.62-20.05% and the TN content increasing by 2.23-49.15%. These results suggest that cross-slope cultivation and straw mulching can better enhance soil structure and promote the transformation of smaller aggregates into larger aggregates, thereby increasing C and N retention in sloping croplands.

This finding is consistent with previous findings (Du et al., 2022; Wang et al., 2023). Mechanistically, cross-slope cultivation results in a natural contour barrier through crop rows, thereby reducing surface runoff and associated N losses (Chen et al., 2020; Wang et al., 2021), and simultaneously decreasing SOC and dissolved organic carbon (DOC) export. Straw mulching reduces raindrop impact, suppresses soil evaporation and organic matter loss, and further promotes organic matter accumulation and microbial activity; the association between organic matter and mineral particles facilitates macroaggregate formation, thereby strengthening the capacity of macroaggregates to protect C and N (Xie et al., 2022). Therefore, straw mulching combined with cross-slope cultivation can increase soil C and N retention in sloping farmlands primarily by suppressing runoff generation and runoff-induced losses.

Across the mulching-tillage treatments, the SOC (Figure 3A) and TN (Figure 3B) contribution rates of the aggregate size classes indicate that the mesoaggregate fraction contributed the most, ranging from 39.38-60.24% for SOC and 38.53-62.67% for TN, and suggesting that mesoaggregates constitute the primary reservoir of SOC and TN in sloping cropland of the black soil region. Compared with the CK treatment, the mulching-tillage treatments increased the SOC and TN contribution rates of macroaggregates and mesoaggregates by 4.77-31.53% and 13.57-39.72% (SOC) and 0.10-40.54% and 5.67-38.92% (TN), respectively, while their rates of contribution to the microaggregates and the silt-clay fraction decreased by 6.00-33.26% and 6.68-47.42% for SOC and 1.71-34.99% and 5.61-54.94% for TN, respectively. These results suggest that mulching-tillage measures promote the transfer of SOC and TN from smaller aggregates to larger aggregates, increase the capacity of larger aggregates to retain SOC and TN, and consequently increase SOC and TN in sloping black soils and the increase on mesoaggregates and contribution of organic carbon rate in Figure 3 for 20-40 and 40-60 cm depths over the years mainly for CM seems to be fast just because straw.

Figure 3
Contribution rate of organic carbon from aggregates with different particle sizes (A), Total nitrogen contribution rate of aggregates with different particle sizes (B)

With increasing soil depth, the SOC and TN contribution rates of macroaggregates and mesoaggregates decreased, whereas those of microaggregates and the silt-clay fraction increased, indicating that the response of SOC and TN allocation to mulching-tillage measures is most pronounced in the topsoil.

For the same mulching practice, compared with SM, CM treatment increased the SOC and TN contribution rates of macroaggregates and mesoaggregates by 0.40-14.28% and 2.28-8.99% (SOC) and 0.95-12.59% and 3.56-9.78% (TN), respectively, while the contribution rates of microaggregates and the silt-clay fraction decreased by 3.29-17.59% and 1.08-22.51% (SOC) and 6.42-19.19% and 1.35-28.73%(TN), respectively. These findings imply that cross-slope tillage is more conducive to transferring SOC and TN from smaller aggregates to larger aggregates on sloping cropland. When the tillage measures were the same, the SOC and TN contribution rates of the macroaggregates and mesoaggregates were greater in the SM treatment than in the SC treatment, with increases of 2.40-11.54% and 0.75-9.05% for SOC and 2.21-19.33% and 2.88-11.60% for TN, respectively. Moreover, the contribution rates of SOC and TN in microaggregates and the silt-clay fraction decreased by 3.22-20.43% and 2.36-16.85% (SOC) and 9.07-17.80% and 3.42-17.05% (TN), respectively.

Aggregate stability was closely associated with aggregate-associated C and N storage, particularly in the 0-20 cm layer, which is consistent with a physical protection mechanism, in which larger, more stable aggregates preferentially retain SOC and TN (Figure 4). Across soil layers, the SOC and TN contents in aggregates of all size classes were positively correlated with WR_0.25, MWD, and GMD. Specifically, the correlations between the aggregate SOC and TN contents and these stability indices were stronger in the 0-20 cm layer than in the 20-40 cm and 40-60 cm layers. Other than the TN content in the < 0.053 mm aggregates, the TN content in the various size classes was significantly correlated (p ≤ 0.05), whereas the SOC content in aggregates of all the size classes was highly significantly correlated (p ≤ 0.01 or p ≤ 0.001). These results indicate that increases in aggregate SOC and TN in the 0-20 cm layer are more favorable for improving soil stability. With increasing soil depth, the correlations between aggregate TN and WR_0.25, MWD, and GMD gradually weakened, whereas the correlations between aggregate SOC and these indices remained highly significant (p ≤ 0.01 or p ≤ 0.001).

Figure 4
Pearson correlation between aggregate stability and soil aggregate carbon and nitrogen composition and contribution rate

Further correlation analyses revealed that the SOC and TN contribution rates of aggregates > 0.25 mm were positively correlated with WR_0.25, MWD, and GMD, whereas the SOC and TN contribution rates of aggregates < 0.25 mm were negatively correlated with these stability indices. Notably, these correlations were extremely significant (p < 0.001) in the 0-20 cm layer. With increasing soil depth, the correlation strength between the TN contribution rate of > 2 mm aggregates and the stability indices gradually weakened, whereas the correlation strength between the SOC contribution rate and these stability indices remained extremely significant (p < 0.001).

Moreover, the trends in the SOC and TN contribution rates were consistent with the aggregate size distribution, and the aggregate stability indices were positively correlated with the SOC and TN contents. These results suggest that aggregate size distribution and stability regulate the allocation of C and N, highlighting the importance of physical protection mechanisms. This underlying mechanism may be explained as follows: mulching-tillage measures can effectively increase aggregate stability, and due to the high C/N ratio of mulch materials, they provide carbon sources in the soil (Pang et al., 2020), which not only increases soil organic matter content but also supplies binding materials for large aggregates, thereby promoting macroaggregate formation and stabilization. In addition, stable large aggregates are characterized by dense pore structures and high mechanical stability and can physically encapsulate organic carbon within their internal microenvironments to generate a physical isolation effect (Rong et al., 2022), thereby reducing the mineralization rate of organic matter and limiting microbial decomposition of C and N (Liu et al., 2022b).

CONCLUSIONS

1. Mulching-tillage management significantly affected soil aggregate stability and aggregate size distribution in sloping black soil. Cross-slope no-tillage with straw mulch (CM) showed the most consistent improvement in the proportion of water-stable macroaggregates (> 0.25 mm) and in aggregate stability indices, especially in the 0-20 cm soil layer.

2. Aggregate-associated soil organic carbon (SOC) and total nitrogen (TN) varied among aggregate fractions and management treatments. Higher SOC and TN contents, as well as higher contribution rates, were generally observed in the mesoaggregates, particularly under CM, indicating that this treatment favored a more stable distribution of carbon and nitrogen within the soil structure.

3. The relationships between aggregate stability and aggregate-associated SOC and TN were strongest in surface soil. These results suggest that conservation-oriented management, especially cross-slope no-tillage with straw mulch, is a promising strategy for improving soil structural quality and enhancing carbon and nitrogen retention in sloping cropland, although treatment effects became less evident with depth.

• 1
  Research developed at Guangrong Village, Hailun, China.
• Ref. 300832
• Financing statement:
  This study was supported by the National Key R&D Program of China (Grant No. 2021YFD1500802-2).

Acknowledgments:

The authors would like to express their sincere gratitude to the College of Water Conservancy and Civil Engineering, Northeast Agricultural University, the National Key R&D Program of China, and Guangrong Village, Hailun City, Heilongjiang Province for their generous support.

Data Availability Statement:

The authors declare that there are no data underlying the text.

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Edited by

• Editors:
  Toshik Iarley da Silva & Walter Esfrain Pereira

Publication Dates

• Publication in this collection
  14 Sept 2026
• Date of issue
  2026

History

• Received
  13 Sept 2025
• Accepted
  06 May 2026
• Published
  27 Aug 2026