Effects of Adding Different Corn Residue Components on Soil and Aggregate Organic Carbon

Abstract

Soil organic carbon (SOC) plays a vital role in maintaining soil fertility and ecosystem sustainability, with crop residues serving as a key carbon input. However, how different maize residue components influence SOC stabilization across aggregate sizes and fertility levels remains poorly understood. This study investigated the effects of maize roots, stems, and leaves on SOC dynamics and aggregate-associated carbon under low- and high-fertility Brown Earth soils through a 360-day laboratory incubation. Results revealed that residue incorporation induced an initial increase in SOC, followed by a gradual decline due to microbial mineralization, yet maintained net carbon retention. In low-fertility soil, leaf residues led to the highest SOC content (12.08 g kg⁻¹), whereas root residues were most effective under high-fertility conditions (18.93 g kg⁻¹). Residue addition enhanced macroaggregate (>0.25 mm) formation while reducing microaggregate fractions, with differential patterns of SOC distribution across aggregate sizes. SOC initially accumulated in 0.25–2 mm aggregates but gradually shifted to >2 mm and <0.053 mm fractions over time. Root residues favored stabilization in high-fertility soils via mineral association, while stem and leaf residues promoted aggregate-level carbon protection in low-fertility soils. These findings highlight the interactive roles of residue type and soil fertility in regulating SOC sequestration pathways.

1. Introduction

Intensive agricultural practices and excessive fertilization have contributed to widespread soil degradation, reducing soil fertility and productivity [1,2]. In response, incorporating plant residues into the soil has emerged as a promising strategy to enhance soil organic carbon (SOC) storage while mitigating the environmental impact of straw burning [3]. SOC, a key component of soil organic matter, plays a critical role in maintaining soil structure, fertility, and overall ecosystem stability [4]. However, the effects of plant residue incorporation on SOC dynamics vary depending on plant species, residue composition, and environmental conditions [5].

Northeast China’s Mollisol region, supporting over 40% of national maize production, confronts severe SOC depletion (0.8–1.2% annual decline) under intensive monoculture systems and extreme climatic stresses, particularly 15–25 annual freeze-thaw cycles that disrupt aggregate integrity [6]. The stabilization efficiency of maize residue components (roots, stems, leaves) across aggregate classes and fertility gradients remains unquantified, despite suboptimal fertility prevailing in 68% of regional croplands [7]. Different residue types (roots, stems, and leaves) exhibit distinct biochemical profiles, driving variations in decomposition rates and their differential contributions to SOC stabilization [8]. Roots typically have higher lignin content and decompose more slowly, whereas stems and leaves contain greater proportions of easily degradable compounds such as cellulose and hemicellulose, leading to faster decomposition [9,10]. These differences affect SOC turnover and stabilization in soil aggregates, which are the fundamental structural units of soil and primary reservoirs of SOC [11]. Understanding how different plant residues contribute to SOC sequestration within soil aggregates is essential for optimizing soil management practices.

Soil aggregates are critical mediators of SOC storage, with stabilization mechanisms operating across physical, chemical, and biochemical pathways [12]. Macroaggregates (>250 μm) physically shield particulate organic matter from microbial access, while microaggregates (<250 μm) and mineral-associated fractions chemically stabilize SOC through organo-mineral interactions and biochemical recalcitrance [11,12]. Long-term practices like residue incorporation enhance aggregate formation by binding particles via microbial-derived glomalin and root exudates, whereas excessive chemical fertilization disrupts aggregate hierarchy, accelerating SOC mineralization [13,14]. Straw amendments particularly stimulate macroaggregate development, preferentially sequestering labile carbon in occluded fractions, while lignin-rich residues (e.g., root residues) may promote chemical stabilization in silt–clay complexes [15]. However, the interplay between residue types (e.g., stems/leaves vs. roots) and soil fertility in directing these mechanisms remains unresolved [16].

Manure and biochar studies highlight physicochemical controls on SOC retention, maize residue types differentially influence aggregate formation: soluble compounds (e.g., stems and leaves) stimulate microbial activity and transient macro aggregation, while structural polymers (e.g., roots) sustain microaggregate stability [17,18]. Crucially, soil fertility regulates these processes by modulating microbial decomposition efficiency and mineral saturation states [19,20]. This knowledge gap impedes targeted strategies for SOC accrual across heterogeneous soil systems. Our study elucidates how maize residue types interact with fertility gradients to govern SOC stabilization pathways in distinct aggregate classes, providing mechanistic insights for sustainable carbon management.

To address these knowledge gaps, this study investigates the effects of different maize residue types (roots, stems, and leaves) on SOC content and its distribution within various soil aggregate fractions. A controlled laboratory incubation experiment was conducted under constant temperature and humidity conditions, using brown soil with two fertility levels. We hypothesized that root residues would promote greater SOC stabilization in microaggregates under high fertility, while leaves and stems would contribute more to macroaggregate-associated SOC under low fertility. Understanding these interactions will provide valuable insights into sustainable residue management practices that enhance soil carbon sequestration and improve agricultural productivity.

2. Materials and Methods

2.1. Experimental Site

The soil used in this study was collected from the long-term experimental station at Shenyang Agricultural University (41°49′ N, 123°34′ E). This long-term experiment began in the spring of 1987, with each plot covering an area of 69 m², arranged in a randomized block design with three replicates. The site has been continuously cultivated with maize, following seasonal planting schedules typical of northern China, with sowing occurring in mid-to-late April and harvesting in mid-to-late September. The soil type is Brown Earth according to China Soil Classification System (according to the US Soil Taxonomy). The basic physic-chemical properties of soil from 0–20 cm depth in 1987 were total SOC 9.05 g kg⁻¹, total nitrogen (N) 1.00 g kg⁻¹, total phosphorus 0.5 g kg⁻¹, alkali-hydrolysable N 67.4 mg kg⁻¹, available phosphorus 8.4 mg kg⁻¹, pH (H₂O) 6.39, sand 16.7%, silt 58.4%, clay 24.9% and bulk density 1.05 g cm⁻³ [21].

2.2. Soil Sampling Maize Residues Samples

In this study, all soil samples were collected from the 0–20 cm layer of brown soils. The soils from two different fertilization treatments were selected: one from a long-term unfertilized control treatment (low-fertility soil, LF) and the other from a long-term organic fertilizer treatment (high-fertility soil, HF), which receives an annual application of 270 kg N hm⁻² in the form of pig manure. The organic manure used contained 150 g kg⁻¹ organic matter and 10 g kg⁻¹ total N. In November 2016, soil samples were collected from the 0–20 cm plow layer. After collection, plant roots were removed, and the soil was air-dried, sieved through a 2 mm mesh, and stored for further incubation experiments.

At this site, continuous maize (Zea mays L.) was sown in early May and harvested in early October 2016; maize residues were processed by drying at 105 °C for 30 min to halt enzymatic activity, followed by drying at 60 °C for 8 h. The maize roots, stems, and leaves were then cut into 2 cm segments, ground, and sieved through a 40-mesh screen. The processed plant residues were stored in airtight containers in a dry environment until use. The physicochemical properties of the soil and plant materials used in the experiment are presented in

Table 1. Basic characteristics of bulk soil and the maize plant samples.

Item	Soil Organic Carbon (g kg−1)	Total Nitrogen (g kg−1)	Lignin (g kg−1)	C/N
LF	10.10	1.10	-	9.18
HF	17.80	2.20	-	8.09
Root	440.76	6.55	135.32	67.18
Stem	440.08	6.45	79.02	68.22
Leaf	400.80	5.70	45.78	70.32

LF—Low-fertility soil; HF—High-fertility soil.

.

2.3. Incubation Experiment

In June 2018, approximately 120 g of air-dried soil was weighed and adjusted to 12% of its field capacity, then pre-incubated for seven days. To minimize moisture loss, samples were covered with perforated parafilm. The pre-incubated soil was thoroughly mixed with ground maize roots, stems, or leaves, each added at 1% of soil weight [6,22]. Eight treatments were established: (1) LF soil without maize residue, (2) LF soil with maize roots, (3) LF soil with maize stems, (4) LF soil with maize leaves, (5) HF soil without maize residue, (6) HF soil with maize roots, (7) HF soil with maize stems, and (8) HF soil with maize leaves (Figure 1). The study employed a completely randomized design with 144 incubation vessels (eight treatments × three biological replicates × six sampling intervals). At each sampling interval (Days 1, 7, 30, 60, 120, 180, and 360 in June 2019), 24 vessels (1 replicate per treatment) were randomly selected using a stratified randomization algorithm. All the incubation experiments were carried out at 25 °C and at 60% of WHC. Then, a portion of the sampled soil was used for aggregate fractionation, while the remaining soil was air-dried, ground, and sieved through a 100-mesh screen for SOC analysis immediately.

2.4. Soil Aggregate Fractionation and SOC Analysis

After the incubation experiments were completed (in June 2019), aggregates were separated by using the wet-sieving method with a Soil Aggregate Analyzer (Model SAA 8052, Shanghai, China) [23,24]. For each replicate (n = 3 per treatment), a 50 g air-dried soil sample was pre-wetted for 20 min on a 2 mm sieve at room temperature, and then gently shaken in distilled water in a series of sieves. Soil samples were automatically moved up and down 3 cm at a speed of 30 repetitions per min for 30 min. The aggregate fractions remained on the 2 mm sieve (>2 mm aggregates), 0.25 mm sieve (0.25–2 mm aggregates), and 0.053 mm sieve (0.053–0.25 mm aggregates), and those that passed through the 0.053 mm sieve (<0.053 mm aggregates) were collected separately. Each aggregate fraction was quantitatively transferred to a large polyethylene container and allowed to settle at room temperature for 24 h. The sedimentation was collected and then dried at 60 °C, weighed, and analyzed for SOC using an elemental analyzer (Vario EL II, Elementar Analysensysteme, Hanau, Germany).

The QA/QC procedures for SOC measurements using the elemental analyzer were conducted as follows: (1) The analyzer was calibrated using certified reference materials (e.g., acetanilide or soil standards with known SOC content). Calibration curves were validated before sample analysis, and blank runs were performed to correct for background interference. (2) Each soil sample was analyzed in triplicate to ensure reproducibility. Replicate variability was monitored, and samples with a coefficient of variation (CV) > 5% were reanalyzed. (3) Internal standards (e.g., control soils with predetermined SOC) were run every 10 samples to check instrument drift.

2.5. Data Analysis

A mixed model with repeated measures was performed for all data. Fertility levels (low and high), and maize residues (root, stem, and leaf) were fixed effects, and time series was regarded as repeated measures; the replicates were random effects. All data satisfied the assumption of a linear mixed-effects model. To allow for correlation between repeated measures on the same treatment, a first-order antedependence correlation model was assumed for the residuals within a plot [25]. To test the effects of residues (root, stem, and leaf), fertility levels (high and low), and their interactions on the dependent variables at each sampling day, we used two-way analysis of variance (ANOVA) followed by Duncan’s post-hoc test. All statistical analyses were performed with the IBM SPSS 19.0 (IBM, Armonk, NY, USA) software package with significant differences at the p < 0.05 level. Graphs were drawn using Origin 8 (Origin Lab, Northampton, MA, USA).

3. Results

3.1. Effects of Maize Residue Addition on Soil Aggregate Composition

In the low-fertility soil without residue addition, the percentage of >2 mm aggregates ranged from 0.58% to 1.92% throughout the incubation period, 0.25–2 mm aggregates accounted for 35.9% to 54.44%, 0.053–0.25 mm aggregates constituted 24.93% to 57.73%, and <0.053 mm aggregates ranged from 6.32% to 17.15% (Figure 2a). In the high-fertility soil without residue addition, the proportion of >2 mm aggregates ranged from 0.86% to 5.38%, 0.25–2 mm aggregates constituted 66.75% to 68.05%, 0.053–0.25 mm aggregates accounted for 21.97% to 22.35%, and <0.053 mm aggregates ranged from 3.99% to 18.12% (Figure 2b).

Maize residue addition consistently enhanced macroaggregate (>0.25 mm) formation while reducing microaggregate (<0.25 mm) proportions in both soil types (Figure 3). In low-fertility soil, 0.25–2 mm aggregates dominated (60–70% of total), with >2 mm aggregates increasing fivefold by day 360. The 0.25–2 mm fraction peaked at day 180 before declining, whereas <0.053 mm microaggregates decreased to 1.96% ± 0.85% at termination. High-fertility soil exhibited analogous trends but with distinct treatment effects: root amendments caused the most pronounced reduction in 0.25–2 mm aggregates (p < 0.05), while stem treatments preferentially depleted <0.053 mm fractions. Notably, leaf amendments uniquely increased 0.053–0.25 mm aggregates by day 360. Temporal patterns revealed phase-dependent reorganization—initial increases in 0.25–2 mm aggregates (days 1–180 in low-fertility; days 1–56 in high-fertility) preceded structural maturation toward larger (>2 mm) or smaller (<0.25 mm) fractions.

Compared to low-fertility soil, the percentage of >2 mm aggregates in root and stem treatments was slightly higher in high-fertility soil. The percentage of microaggregates in root and leaf treatments was higher in high-fertility soil than in low-fertility soil, though the difference was not significant (p > 0.05). Soil aggregate composition was significantly influenced by residue type (p < 0.001), while the effects of time and fertility level were not significant. The interactions between time and residue type (p < 0.05) and fertility level and residue type (p < 0.01) were significant. The three- and four-factor interactions were also significant (p < 0.001), with the three-factor interaction of time + fertility level + residue type being significant at p < 0.01 (

Table 2. ANOVA analysis of the effects of maize residue, fertility level, and incubation time on soil organic carbon and organic carbon in soil aggregates of Brown Earth (humus horizon, 0–20 cm).

Factor	d.f.	SOC (g kg−1 soil)	Percentage (%)	TOC Aggregate (g kg−1 soil)
Time (T)	5	**	*	*
Fertility (F)	1	***	*	***
Maize (M)	2	*	ns	*
Class (C)	3	ns	***	***
T × F	5	**	*	*
T × M	10	***	*	*
T × C	15	*	***	***
F × M	2	***	**	*
F × C	3	*	***	***
M × C	6	*	**	***
T × F × M	10	***	**	*
T × F × C	15	ns	**	***
T × M × C	30	ns	***	***
F × M × C	6	ns	***	***
T × F × M × C	30	ns	**	***

Note: *, **, ***, and ns indicate significant differences at the 5% level, 1% level, 0.1% level, and no significance, respectively.

).

3.2. SOC Content in Soil

Maize residue addition elevated SOC by 17–20% relative to controls (low-fertility: 10.10 ± 0.27 g kg⁻¹; high-fertility: 17.80 ± 0.45 g kg⁻¹), with significant three-way interactions (time × fertility × residue type, p < 0.001). High-fertility soils consistently retained 1.5× higher SOC than low-fertility soils. In low-fertility soil, stem amendments initially maintained higher SOC than roots/leaves (p < 0.05), but leaf treatments surpassed others by day 360 (Figure 4a) (p < 0.05). Conversely, high-fertility soil prioritized root-derived carbon: root amendments sustained the highest SOC after day 56 (p < 0.05), while leaf treatments caused persistent depletion (Figure 4b).

3.3. Effects of Maize Residue Addition on SOC Content in Aggregate

The SOC content in various aggregates of high-fertility soil was higher than that in low-fertility soil (except for the 0.25–2 mm fraction) (p < 0.05) (Figure 5). The SOC content of >2 mm aggregates in low-fertility soil without residue addition ranged from 0.12 to 0.37 g kg⁻¹, while that of 0.25–2 mm, 0.053–0.25 mm, and <0.053 mm aggregates ranged from 4.47 to 6.78 g kg⁻¹, 2.57 to 5.22 g kg⁻¹, and 0.24 to 1.87 g kg⁻¹, respectively (Figure 5a). In the high-fertility soil, organic carbon content of > 2 mm aggregates without residue addition ranged from 0.18 to 1.24 g kg⁻¹, while that of 0.25–2 mm, 0.053–0.25 mm, and <0.053 mm aggregates ranged from 9.44 to 12.03 g kg⁻¹, 2.79 to 5.89 g kg⁻¹, and 0.58 to 2.65 g kg⁻¹, respectively (Figure 5b).

In these two types of soil, the SOC content in >2 mm and <0.053 mm aggregates gradually increased with prolonged incubation time. However, it decreased over time in 0.25–2 mm aggregates, while the SOC content in 0.053–0.25 mm aggregates showed an initial decline followed by an upward trend (Figure 6). After the incubation, in the low-fertility soil, the SOC content in >2 mm aggregates under leaf treatment was lower than that under root and stem treatments, whereas the SOC content in <0.053 mm aggregates under root treatment was lower than that under stem and leaf treatments. No significant differences (p > 0.05) were observed in the organic carbon content of 0.25–2 mm and 0.053–0.25 mm aggregates among the three maize residue treatments.

In the high-fertility soil, the temporal trends of SOC content in aggregates of all size fractions after straw addition were consistent with those in the low-fertility soil (Figure 6). The SOC content in each aggregate fraction was higher in the high-fertility soil than in the low-fertility soil. At the end of the incubation period, the trends of SOC content changes across all aggregate fractions under the three straw treatments also mirrored those observed in the low-organic-matter soil. However, in <0.053 mm aggregates, the SOC content under root treatment was higher than that under stem and leaf treatments.

4. Discussion

4.1. Effects of Corn Residue Addition on Soil Aggregate Composition

The addition of maize residues from different plant parts increased the proportion of macroaggregates (>0.25 mm) while reducing microaggregates (<0.25 mm), indicating enhanced aggregation of microaggregates into macroaggregates. This aligns with findings from previous studies that reported significant increases in macroaggregate formation following crop residue incorporation [23]. High-fertility soil exhibited greater macroaggregate formation than low-fertility soil, suggesting that nutrient-rich conditions enhance soil structural stability [11].

The increase in macroaggregate formation can be attributed to the decomposition of organic components and microbial activity, which facilitate aggregation. Crop residue addition introduces labile organic matter, which is rapidly decomposed by soil microbes, leading to the production of microbial byproducts that act as binding agents for soil particles, promoting macroaggregate formation [26,27]. In our 360-day incubation, >2 mm aggregates increased in both soil types, while 0.25–2 mm aggregates showed no clear trend, possibly due to opposing shifts in 1–2 mm (decrease) and 0.25–1 mm (increase) fractions. The <0.25 mm fraction declined significantly over time (p < 0.05), with the <0.053 mm subfraction decreasing most markedly, demonstrating that residue addition promotes microaggregate coalescence into macroaggregates.

Our findings demonstrate that maize residue incorporation drives hierarchical aggregate formation by enhancing microaggregate coalescence into macroaggregates (>0.25 mm), particularly in nutrient-rich soils where favorable biochemical conditions amplify microbial processing of organic substrates. This aligns with the “aggregate hierarchy model” [28], wherein labile residue-derived carbon stimulates microbial activity, generating transient binding agents (e.g., polysaccharides, fungal hyphae) that promote particle aggregation [26,29]. The preferential stabilization of organic carbon in >2 mm macroaggregates and <0.053 mm silt–clay fractions—coupled with the significant decline in free microaggregates (<0.25 mm)—suggests dual carbon sequestration pathways: physical occlusion in macroaggregates and organo-mineral complexation in fine fractions. These mechanisms are critical for developing sustainable residue management strategies to enhance soil carbon stocks while improving structural stability.

4.2. Effects of Corn Residue Addition on SOC Content

The incorporation of maize residues from various plant parts led to a gradual decline in SOC content over time in both long-term unfertilized and organically fertilized soils. This observation aligns with previous studies indicating that crop residue addition influences SOC dynamics [30]. In this study, we observed that the initial SOC decline was followed by a stabilization phase, suggesting a shift from labile to more recalcitrant carbon pools over time [31].

The differing decomposition rates among maize residue components can be attributed to their varying biochemical compositions. Stems and leaves contain higher proportions of readily decomposable soluble organic matter (e.g., cellulose and hemicellulose) than roots, leading to a more rapid initial decline in SOC when these residues are added to the soil [18,32]. In our study, during the early incubation phase (1–7 days), SOC in both low- and high-fertility soils amended with stems and leaves decreased rapidly. However, by the late incubation phase (180–360 days), SOC content in low-fertility soil with leaf addition exceeded that with root addition, likely due to the faster decomposition of soluble components in leaves, facilitating microbial stabilization [33,34]. In contrast, high-fertility soil amended with stems showed no significant SOC changes during this period.

The quantity of added stover also plays a crucial role in SOC dynamics. Higher stover inputs have been associated with increased SOC accumulation, as they provide more substantial carbon sources for soil microbes, enhancing microbial activity and SOC stabilization [15]. In our study, despite the relatively low stover input, soluble fractions decomposed over time, leaving residual stover in the soil [35,36]. Although stems and leaves decomposed more readily than roots, microbial breakdown of roots by day 180 resulted in comparable SOC levels across all residue treatments in high-fertility soil. By day 360, low-fertility soil amended with leaves showed significantly higher SOC content than root- and stem-amended treatments (p < 0.05), while roots showed a higher SOC content than stem- and leaf-amended treatments in high-fertility soil after day 180 (p > 0.05).

The differential SOC sequestration mechanisms are primarily mediated by soil fertility through its modulation of microbial resource availability: high-fertility soils enhance microbial metabolic efficiency to preferentially stabilize recalcitrant root-derived carbon via mineral association. In addition, co-metabolism of root-derived polyphenols and mineral surfaces promotes persistent organo-mineral complexation, with XRD analyses revealing increased Fe-oxalate phases indicative of ligand-promoted mineral weathering [37]. On the other hand, low-fertility soils favor the physical protection of labile leaf residues within aggregates due to constrained microbial decomposition [38]. Concurrently, residue-type effects are driven by their biochemical composition—lignin-rich roots in high-fertility soils promote organo-mineral complexation, while cellulose-dominated leaves in low-fertility soils amplify microbial necromass accumulation through enhanced priming effects.

4.3. Effects of Corn Residue Addition on Soil Aggregate Organic Carbon Content

Unaggregated clay particles exhibit higher organic carbon content than aggregates but contribute minimally to total SOC storage due to their low abundance [30]. In this study, maize residue addition significantly increased aggregate-associated organic carbon, particularly in 0.25–2 mm aggregates. While the percentage of this fraction declined over time, the organic carbon content in >2 mm aggregates gradually increased. The 0.053–0.25 mm fraction exhibited an initial decrease followed by recovery, while the <0.053 mm fraction demonstrated persistent enrichment of organic carbon.

The increase in organic carbon content within large macroaggregates (>2 mm) suggests that residue addition promotes organic carbon stabilization in larger aggregates. Macroaggregates serve as protective environments for organic matter, reducing its accessibility to decomposers and thereby enhancing carbon sequestration [25,39]. In our study, pre-incubation sieving (2 mm) established 0.25–2 mm aggregates as the dominant fraction. Macroaggregate organic carbon content increased with aggregate size, while microaggregates (<0.25 mm) displayed an initial decrease followed by an increase, consistent with previous findings [35,36]. These discrepancies may arise from differences in incubation duration and soil type (e.g., long-term experimental soil vs. field soil).

The continuous input of maize residues likely enhanced macroaggregate stabilization through two interconnected pathways: (1) direct physical entanglement by root-derived structures and fungal hyphae [31], and (2) biochemical binding via microbial exudates and transient cementing agents (e.g., polysaccharides) derived from residue decomposition [33,40]. While our study did not quantify mechanical aggregate stability metrics such as mean weight diameter (MWD), the observed temporal increases in organic carbon content within >2 mm and <0.053 mm aggregates (Figure 5 and Figure 6) suggest a dynamic reorganization of aggregate architecture, potentially reflecting shifts in binding efficiency or microbial processing of residue-derived substrates. Future investigations incorporating MWD measurements alongside isotopic tracing of residue-derived carbon will be essential to disentangle the relative contributions of physical and biochemical stabilization mechanisms.

These findings collectively suggest that maize residue amendment effectively enhances macroaggregate formation (>2 mm) and promotes organic carbon stabilization within fine particles (<0.053 mm), thereby improving overall aggregate stability through dual mechanisms of physical protection and chemical binding [41]. In summary, the addition of maize residues influences SOC dynamics, soil aggregate composition, and aggregate-associated organic carbon content. These effects are modulated by residue type and soil fertility levels, underscoring the importance of tailored residue management practices to enhance soil health and carbon sequestration.

While our controlled incubation elucidated temporal carbon dynamics across aggregate classes, three key limitations warrant consideration: (1) The absence of mechanical stability metrics (e.g., mean weight diameter) precludes direct linkage between aggregate-associated carbon and structural resilience; (2) Laboratory conditions (constant moisture/temperature) may overestimate field-scale aggregation processes influenced by seasonal fluctuations; (3) The lack of microbial functional data (e.g., biomass, community composition, enzyme activities) limits mechanistic interpretation of how residue quality regulates microbial processing and subsequent SOC stabilization within aggregates; (4) Without isotopic tracing, we cannot differentiate residue-derived carbon from native SOC stabilization pathways. Future field studies integrating ¹³C-labeled residues and stability assays are needed to validate these mechanisms under natural pedoclimatic regimes.

5. Conclusions

Soil organic carbon (SOC) dynamics play a pivotal role in regulating organic matter turnover and sustaining soil fertility. Our findings demonstrate that maize stover incorporation serves as a sustainable practice to enhance SOC retention while reducing reliance on residue burning, thereby mitigating air pollution. This study systematically evaluated the distinct effects of maize residue components (roots, stems, and leaves) on SOC dynamics, aggregate composition, and aggregate-associated carbon stabilization. While the addition of residue transiently elevated SOC levels during initial decomposition, gradual microbial mineralization led to subsequent depletion, with sustained net retention by day 360. Notably, residue type and soil fertility jointly governed SOC trajectories: in low-fertility soil, leaf amendments retained significantly higher SOC (12.08 g kg⁻¹) than root or stem treatments, whereas root amendments maximized SOC retention (18.93 g kg⁻¹) in high-fertility soil.

Maize residues preferentially enhanced macroaggregate (>0.25 mm) formation while reducing microaggregate (<0.25 mm) proportions, with root residues exerting the strongest influence on aggregate dynamics. Soil fertility modulated carbon allocation pathways through residue-aggregate interactions, though residue biochemical composition dominated over temporal and fertility-driven effects in shaping aggregation patterns. Enhanced stability arose from dual mechanisms: (1) physical encapsulation of labile carbon within macroaggregates and (2) chemical stabilization of recalcitrant carbon in fine fractions (< 0.053 mm). Root-driven multistage aggregation emerged as the central pathway for structural optimization, highlighting the critical role of residue quality in SOC sequestration.

This study provides mechanistic insights into residue-specific SOC stabilization under controlled conditions. Future research should integrate isotopic tracing (e.g., ¹³C-labeled residues) to partition residue-derived versus native SOC contributions and validate these dynamics under field conditions. Such efforts will refine residue management strategies to maximize carbon sequestration while aligning with site-specific soil fertility gradients.