Biodegradable Plastic Film Residues Impede Soil Organic Carbon Sequestration and Macroaggregate-Associated Carbon Storage in Agricultural Soil

Abstract

The progressive replacement of conventional plastic films with biodegradable alternatives in agricultural systems has led to the accumulation of diverse plastic residues in soils, exerting documented impacts on microbial-mediated ecological processes. However, systematic investigations into how these residues influence organic carbon (C) turnover and inter-aggregate C flows remain critically lacking. This study investigated the effects of diverse plastic film residues on organic C decomposition dynamics and aggregate-associated C sequestration through a 60-day soil incubation experiment. Two representative plastic film types—conventional polyethylene (PE) and biodegradable polylactic acid + polybutylene adipate-co-terephthalate (PAT)—were incorporated into agricultural soil under contrasting organic matter input regimes: with maize straw addition (St) and without any straw addition. The results demonstrated that, in the absence of maize straw, both PE and PAT residues enhanced native soil organic C (SOC) mineralization. Notably, PAT elevated the cumulative CO₂ emission by 7.4% (P < 0.05) relative to the control. PE slightly reduced the final SOC content but increased the proportion of soil gates (Mi) and silt plus clay (S + C) toward Ma. Conversely, PAT exerted a negligible effect on final SOC content but reduced Ma by 40.9% (P < 0.05) and increased Mi by 33.4% (P < 0.05), driving C redistribution from Ma to Mi. In contrast, with the addition of maize straw, both St + PE and St + PAT treatments reduced organic C mineralization and diminished the increases in SOC content. Specifically, St + PAT decreased the cumulative CO₂ emission by 1.9% (P < 0.05) and lowered the SOC content by 7.1% (P < 0.05) compared to straw addition alone (St). Both St + PE and St + PAT also lowered Ma formation; notably, St + PAT significantly reduced Ma by 33.6% and diminished C flow from Mi and S + C into Ma. In conclusion, biodegradable film residues may impede SOC sequestration and macroaggregate-associated C storage by stimulating the mineralization of native SOC and suppressing organic matter decomposition after crop residue input in soil. These findings provide novel insights into the mechanisms governing SOC turnover and C stabilization via soil aggregation in the context of accumulating plastic wastes.

1. Introduction

Soil organic carbon (SOC) plays a vital role in the global carbon cycle; its sequestration has paramount implications for climate change mitigation, soil fertility enhancement, and ecosystem function maintenance [1,2]. Exogenous organic inputs fundamentally determine SOC sequestration efficiency, with net accumulation occurring when microbial mineralization of native SOC is outpaced by new SOC formation [3,4]. Crop residue return is widely adopted in agroecosystems to promote SOC sequestration [5,6], as both plant-derived structural compounds and microbial metabolic products contribute to SOC accrual [7,8]. However, microbially mediated decomposition and utilization of crop residues are strongly regulated by physicochemical and biological processes, where soil microenvironmental dynamics serve as pivotal determinants [9,10]. Elucidating organic matter decomposition responses to soil microenvironment disturbances is therefore essential for advancing our understanding of SOC accumulation mechanisms in agricultural soils.

SOC accumulation is mechanistically tied to soil aggregate formation, where SOC primarily functions as a transient binding agent for aggregates. Crop residues supply diverse organic agents via microbial decomposition, thereby enhancing soil’s aggregation and structural stability [3,4]. Aggregates, in turn, critically protect SOC by restricting microbial access and enzymatic activity [11,12]. Aggregate size classes reflect distinct SOC stabilization mechanisms [13]: for instance, different-sized aggregates physically encapsulate free organic matter to varying degrees. Furthermore, mineral-associated organic C (MAOC)—derived predominantly from microbial necromass and byproducts—readily adheres to soil minerals, forming relatively stable SOC fractions distributed across aggregate components [14,15]. Thus, aggregates and their encapsulated SOC limit organic C’s accessibility to decomposers through multiple pathways. Investigating soil aggregation dynamics and SOC fractionation is therefore essential to elucidate the mechanisms underlying SOC’s sequestration and stability.

The application of plastic mulch effectively suppresses weed growth while enhancing soil’s thermal and hydraulic properties, leading to its global adoption across diverse crops—particularly in arid and semi-arid agroecosystems [16,17]. Nevertheless, intensive deployment coupled with suboptimal recovery practices has induced persistent accumulation of plastic residues (e.g., microplastics < 5 mm) in agricultural soils [18,19]. These exogenous inputs significantly alter soil’s physicochemical architecture, including pore connectivity, bulk density, and water retention capacity [20]. Furthermore, microplastic-derived effects, such as high specific surface area and plasticizer leaching, compromise soil’s microbial functionality [21,22]. Ecotoxicological responses emerge when plastic residues surpass threshold concentrations, with recent agricultural soil assessments confirming microplastic contamination across intensive farming regions in China [22].

The persistent nature and microplastic accumulation of conventional polyethylene (PE) films drive global demand for biodegradable plastic alternatives [19,23]. Nevertheless, despite evidence showing accelerated fragmentation of biodegradable films into microplastics relative to PE films [24], the transient impacts of plastic residues on biota-driven processes through soil microclimate modification have not been well studied. Crucially, biodegradable plastics’ effects on SOC turnover dynamics and aggregate-associated C fluxes have received limited attention [23].

Biodegradable plastics can function as exogenous C inputs to soil systems, directly modifying SOC pools while indirectly mediating aggregate-associated C dynamics through microbial activity [25]. These microplastics stimulate specific microbial functions and enhance exoenzymatic activities, thereby accelerating SOC mineralization [25,26]. Paradoxically, field studies report divergent outcomes: biodegradable film mulching increased SOC relative to polyethylene (PE) films in some trials [27] yet concurrently reduced both SOC stocks and proportions of relatively large aggregates in others [28]. The contradictory evidence indicates that the mechanisms through which plastic residues regulate SOC sequestration and soil aggregation still remain unclear, particularly regarding dynamic monitoring of organic C decomposition and aggregate-associated C transformation. We hypothesize that biodegradable film residues may impede SOC accumulation and the formation of relatively large aggregates, as emerging studies demonstrate their capacity to induce substantial positive priming effects by serving as exogenous C sources [10,26].

Therefore, we designed a 60-day soil incubation experiment to elucidate the mechanisms governing organic C mineralization and soil aggregation—specifically, C partitioning among aggregate fractions—under the influence of diverse plastic film residues following crop residue incorporation. Throughout the incubation, dynamic monitoring tracked organic C mineralization in soils amended with conventional polyethylene (PE) and biodegradable polylactic acid + polybutylene adipate-co-terephthalate (PAT) residues, while SOC variation and fractionation within aggregate classes were quantified across treatments with/without crop residue input.

2. Materials and Methods

2.1. Soil, Straw, and Plastic Film Preparations

The soil used for incubation was collected from a field at the experimental station (34°17′44″ N, 108°04′10″ E; 524.7 m elevation) at Northwest A&F University, Yangling, Shaanxi Province. Located on the Guanzhong Plain in China’s southern Loess Plateau, the site experiences a temperate continental monsoon climate characterized by a mean annual temperature of 13 °C and mean annual precipitation of 600 mm. The soil is classified as a calcareous clay loam, a Eum-Orthic Anthrosol (Udic Haplustalf in the USDA system) [3]. The soil has borne a monoculture with winter wheat since 2002. Long-term integrated fertilization management includes annual applications of 240 kg N ha⁻¹ yr⁻¹ (urea, 46% N), 100 kg P₂O₅ ha⁻¹ yr⁻¹ (calcium superphosphate, 12% P₂O₅), and full straw return. Surface soil (0–20 cm depth) sampled in October 2021 was sieved (<2 mm) to remove coarse roots, visible crop residues, leaves, and stones, followed by air-drying at ambient temperature. Key soil properties were pH (1:1 soil/water): 8.05, SOC: 12.60 g·kg⁻¹, and total nitrogen (TN): 1.15 g·kg⁻¹.

The maize straw used in the incubation experiment was sourced from an adjacent field following maize harvesting in late October 2021. Immediately after collection, the straw was chopped, oven-dried at 75 °C for 72 h, and processed into 2 mm segments. Chemical characterization revealed organic C content: 444 g·kg⁻¹, TN: 8.33 g·kg⁻¹, and total phosphorus: 0.88 g·kg⁻¹.

Conventional polyethylene (PE) and biodegradable (PLA+PBAT, simply designated as PAT) films were manufactured by Tongyuan Plastic Products Co., Ltd. (Guangzhou, China). The PE film consisted exclusively of polyethylene, whereas the PAT film was mainly composed of polylactic acid + polybutylene adipate-co-terephthalate. Both film types were fragmented into approximately 5 × 5 mm pieces using a cryogenic mill. Prior to incubation, all fragments were subjected to a UV sterilization lamp for 10 min to minimize pre-existing microbial contamination.

2.2. Experimental Design and Soil Incubation

The incubation experiment was initiated at the end of 2021 using a 2 × 3 factorial design with two maize straw addition doses (12 g kg⁻¹ soil (St) and 0 g kg⁻¹ soil (control)) and the following three plastic film addition regimes: no film residue addition as the control (Ct), conventional polyethylene film residue addition (PE), and biodegradable PLA+PBAT film residue addition (PAT). The experiment combined two factors, i.e., straw addition doses (S) and added film types (F); each factorial combination (n = 6 treatments) included three replicates. The film residues were applied at a dose of 0.1% (w/w), according to the previous study [29].

Following amendment with mineral nutrients dissolved in distilled water (88 mg N·kg⁻¹ as NH₄NO₃ and 113.04 mg P·kg⁻¹ as KH₂PO₄), soils were pre-incubated for 7 days at 25 °C under 70% water-holding capacity (WHC). Aliquots of 200 g of dry-weight equivalent soil were transferred to 500 mL sterilized plastic jars, followed by thoroughly mixing with the chopped maize straw (2 mm) and/or plastic film residues (5 × 5 mm). The soil moisture was readjusted to 70% WHC before sealing the jars with gas-tight lids. Each jar contained a 20 mL vial of 1 M NaOH for CO₂ absorption. Three blank jars (soil-free) containing NaOH traps served to correct for atmospheric CO₂. All jars were incubated at 25 °C in darkness for 60 days. Moisture was maintained through gravimetric monitoring with distilled water replenishment.

2.3. CO₂ Efflux Measurement and Soil Sampling

Soil CO₂ efflux was quantified at incubation days 1, 2, 3, 4, 5, 7, 10, 15, 20, 30, 40, and 60. The NaOH-containing vials were replaced with fresh traps at each sampling point. Trapped CO₂ was precipitated as BaCO₃ by adding 1.0 mol·L⁻¹ BaCl₂, followed by titration against standardized 0.1 M HCl using phenolphthalein as the indicator.

Terminal soil sampling was conducted on day 60 of incubation. Following the manual removal of visible film residues and straw debris, each sample was subdivided into three portions. The first portion of fresh soil sample was immediately stored at 4 °C for analyses of dissolved organic carbon/nitrogen (DOC/DON), as well as determinations of microbial biomass carbon/nitrogen (MBC/MBN) using the chloroform fumigation–extraction method [30]. The second portion was air-dried and sieved sequentially through 2 mm and 0.15 mm meshes for physicochemical characterization. The third portion was preserved as undisturbed cores, fragmented along natural planes, sieved through an 8 mm mesh, and then air-dried for aggregate fractionation.

2.4. Aggregate Separation and Organic C Determination

Soil aggregates were separated using standardized wet-sieving [31]. Following pre-wetting with distilled water (25 °C) for 5 min, undisturbed air-dried samples underwent sequential sieving through 2 mm, 0.25 mm, and 0.053 mm mesh sieves. The samples were mechanically agitated in deionized water at a speed of 30 times per minute for 10 min (3 cm vertical amplitude). The soils retained on each sieve and leached in the solution were collected and dried at a temperature of 50 °C, and then weighed to measure the mass of aggregates. Given pre-incubation and 2 mm sieving, >2 mm macroaggregates were negligible. The final fractions comprised 2–0.25 mm (macroaggregates, Ma), 0.25–0.053 mm (microaggregates, Mi), and <0.053 mm (silt and clay, S + C).

The particle organic C (POC) and mineral-associated organic C (MAOC) within both Ma and Mi were isolated via density-based separation using sodium hexametaphosphate dispersion [32,33]. First, 30 mL of 5 g·L⁻¹ sodium hexametaphosphate solution was introduced into 10 g of aggregates (Ma/Mi), and the mixture was oscillated for 18 h. After complete dispersion, each mixture was sieved through a 0.053 mm aperture, yielding a POC fraction (>0.053 mm) and an MAOC fraction (<0.053 mm).

The soils before and after incubation, as well as the separated aggregate fractions, were all measured for their organic C content using the K₂Cr₂O₇–H₂SO₄ wet oxidation method [34]. POC and MAOC separated from both Ma and Mi were dried and measured for organic C content using an elemental analyzer (Vario MACRO cube, Elementar, Frankfurt, Germany).

2.5. Data Calculations

The mean weight diameter (MWD) calculation involves collecting three aggregate components, determining their mass proportions, and computing the MWD of the aggregates, which was calculated using the following equation [33]:

$$\mathrm{M}\mathrm{W}\mathrm{D}={\sum }_1^3{X}_i\times P_i$$

(1)

where X_i represents the average particle diameter (mm) of each aggregate category, while P_i denotes the mass proportion of each aggregate size.

Contents of organic C, POC, and MAOC (g C kg⁻¹ soil) indicate their net distributions in each soil fraction, and they were calculated using the following equation [33]:

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}=C_i\times P_i$$

(2)

where C_i represents the organic C content of a certain fraction in each aggregate (g C kg⁻¹ aggregate), while P_i denotes the mass proportion of the corresponding aggregate category (%).

2.6. Statistical Analysis

Prior to parametric analyses, dataset compliance with normality and homoscedasticity assumptions was verified using distribution diagnostics and variance homogeneity tests. All statistical procedures were executed in SPSS Statistics v.19.0 (IBM Corp., Armonk, NY, USA). Soil CO₂ efflux dynamics and cumulative emissions were subjected to a two-way (S and F) repeated-measures ANOVA with incubation time (T) as the within-subject factor, incorporating Greenhouse–Geisser sphericity corrections when necessary. Terminal soil properties—including SOC, DOC, DON, MBC, and MBN contents, POC and MAOC contents within aggregate classes, and their net contents in soil—were comparatively analyzed via two-way (S and F) factorial ANOVA. For all ANOVA models, differences in the experimental means were all compared using Fisher’s least significant difference (LSD) test at the α = 0.05 significance level. Inter-variable relationships across measurement indices were quantified using Pearson correlation coefficients (r). Graphical representations were generated in OriginPro 2022 (OriginLab, Northampton, MA, USA).

3. Results

3.1. Soil CO₂ Release

The incubation experiment showed significant effects of straw addition dose (S), plastic residue addition (F), and their interaction (S × F) on both temporal effluxes and cumulative emissions of soil CO₂ (Figure 1, P < 0.05). Incubation time (T) exerted substantial influence on CO₂ effluxes and cumulative emissions. Peak CO₂ efflux rates occurred predominantly during the initial 14-day phase across all treatments, followed by progressive declines in temporal effluxes and asymptotic stabilization of cumulative emissions (Figure 1). The addition of straw exerted the greatest increases in CO₂ effluxes and emissions: treatments with straw (St) consistently yielded higher effluxes and cumulative emissions compared to straw-absent controls, even masking film-residue-induced differences. Significant film effects were only detectable in treatments with straw (St) during the early incubation stages (Figure 1A). Specifically, St + PAT exhibited a lower CO₂ efflux than both St and St + PE (straw + polyethylene film) in the first several days, establishing diverse trajectories in cumulative CO₂ emission until the termination of the experiment (Figure 1B).

Three timepoints (15, 30, and 60 d) were selected to represent the early, middle, and late phases of the incubation period, respectively. Apart from St causing great increases in CO₂ emission relative to controls, F also exerted distinct effects on CO₂ emissions throughout specific incubation phases (Figure 2; P < 0.05). Moreover, F significantly interacted with S in modulating CO₂ emission, demonstrating diverse film-residue-induced effects between the straw-amended treatments and straw-absent treatments. Specifically, in straw-absent treatments, PAT showed negligible influence on CO₂ emission during the early and mid-incubation phases relative to Ct. However, PAT triggered a 7.4% elevation in cumulative CO₂ emission during the late incubation phase (60 d), compared to Ct (P < 0.05). In contrast, relative to straw addition alone (St), St + PAT significantly reduced the CO₂ emission from the early phase of incubation, ultimately reducing the cumulative emission of CO₂ by 1.9% (P < 0.05).

3.2. SOC Content and Related Indicators

Throughout the 60-day incubation, SOC contents were significantly governed by S and S × F interaction, wherein straw addition dominated SOC accumulation and enhanced the SOC contents by 7.3% on average, relative to straw-absent controls (Figure 3; P < 0.05). Crucially, F effects exhibited divergence mediated by straw addition status: in straw-absent treatments, PAT maintained the SOC content at a level almost equivalent to Ct, while PE induced marginal depletion; conversely, in straw-amended treatments, both St + PE and St + PAT reduced the increases in SOC, with St + PAT causing a pronounced 7.1% depletion relative to St (P < 0.05).

Throughout the 60-day incubation, DOC, DON, MBC, and MBN contents exhibited specific differences among the various treatments (Figure 4). Crucially, DOC and MBC demonstrated much higher sensitivity to S, F, and their interaction (S × F) compared to DON and MBN. Notably, DON and MBN were dominantly governed by S rather than F: straw addition elevated DON and MBN by 13.1% and 119%, respectively, relative to straw-absent controls (Figure 4B,D; P < 0.05).

DOC content exhibited significant responses to S and S×F, with straw addition elevating DOC by 23.7% on average, relative to straw-absent controls (Figure 4A; P < 0.05). Crucially, F exhibited straw-mediated divergence in affecting DOC: in straw-absent treatments, PE and PAT both depressed DOC relative to Ct, exhibiting the order Ct > PE > PAT, where PAT induced a significant 17.6% reduction in DOC (P < 0.05); however, in straw-amended treatments, both St + PE and St + PAT enhanced the DOC content, with St + PE specifically increasing DOC by 11.4% relative to St (P < 0.05).

MBC content was significantly regulated by S, F, and S × F (P < 0.05). Straw addition elevated MBC by 86.1% on average, relative to straw-absent controls (Figure 4C; P < 0.05). Crucially, F exhibited straw-mediated divergence in affecting MBC: in straw-absent treatments, both PE and PAT significantly enhanced MBC by 21.3% and 32.9%, respectively, relative to Ct (P < 0.05); however, in straw-amended treatments, St + PE and St + PAT reduced MBC by 9.7% and 4.9%, respectively, relative to St (P < 0.05).

3.3. Soil Aggregation and SOC Distribution

Throughout the 60-day incubation, the mass proportions of macroaggregates (Ma), microaggregates (Mi), and silt + clay (S + C) were significantly influenced by S, F, and their interaction (S×F) (Figure 5A; P < 0.05), wherein straw addition increased Ma by 106% but reduced Mi and S + C by 24.6% and 38.9%, respectively, relative to straw-absent controls (P < 0.05). F exhibited straw-mediated divergence in affecting the mass proportions of aggregates: In straw-absent treatments, PE elevated Ma by 32.7% while reducing both Mi and S + C (S + C decreased 16.2%), whereas PAT reduced Ma by 40.9% but increased Mi by 33.4%, relative to Ct (P < 0.05). Conversely, in straw-amended treatments, relative to St, both St + PE and St + PAT decreased Ma but increased Mi and S + C; notably, St + PAT significantly lowered Ma by 33.6% while elevating Mi and S + C by 17.8% and 78.8%, respectively (P < 0.05).

S, F, and their interaction (S × F) significantly influenced the MWD of soil aggregates, with straw addition elevating MWD by 77.3% relative to straw-absent controls (Figure 5B; P < 0.05). Specifically, in straw-absent treatments, PE increased MWD by 25.1%, whereas PAT reduced it by 27.4%, relative to Ct (P < 0.05); conversely, in straw-amended treatments, both St + PE and St + PAT decreased MWD, with St + PAT notably exhibiting a 29.6% reduction relative to St (P < 0.05).

Throughout the incubation, neither S, F, nor their interaction (S × F) significantly altered organic C content across all aggregate fractions, except for F exerting an effect exclusively on Mi (Figure 6A). The net distribution of organic C across aggregate fractions—Ma, Mi, and S + C—was significantly governed by S, F, and S × F (Figure 6B; P < 0.05), with straw addition elevating Ma-associated organic C by 101% while reducing organic C in Mi and S + C by 26.9% and 36.6%, respectively, relative to straw-absent controls (P < 0.05). Crucially, in straw-absent treatments, PE increased Ma-associated organic C by 22.9% but suppressed organic C in Mi and S + C by 11.0% and 15.2%, respectively, relative to Ct; conversely, PAT reduced Ma-associated organic C by 41.8% yet enhanced Mi-associated organic C by 48.3% (P < 0.05). However, in straw-amended treatments, both St + PE and St + PAT decreased Ma-associated organic C relative to St (by 10.8% and 32.7%, respectively) while elevating organic C in Mi and S + C, most prominently in St + PAT, with increases of 16.0% and 70.9% in Mi and S + C, respectively (P < 0.05).

Throughout the incubation, both POC and MAOC contents across the Ma and Mi of soil demonstrated relatively low sensitivity to S, F, and their interaction (S × F) (Figure 7A,B). Therein, straw addition decreased the POC contents by 17.5% and 15.5% in Ma and Mi, respectively, relative to straw-absent controls (P < 0.05), indicating the dilution effect caused by variation in aggregate proportions.

The net distribution of POC and MAOC across aggregate fractions was significantly governed by S, F, and their interaction (S×F) (Figure 7C,D; P < 0.05), wherein straw addition elevated Ma-associated POC and MAOC by 71.5% and 116%, respectively, relative to straw-absent controls, while suppressing Mi-associated POC and MAOC by 36.9% and 24.5%, respectively (P < 0.05). Critically, in straw-absent treatments, PE increased MAOC in Ma but reduced it in Mi, with negligible POC effects relative to Ct, whereas PAT decreased both POC and MAOC in Ma yet enhanced them in Mi (P < 0.05); conversely, in straw-amended treatments, POC and MAOC in Ma both exhibited the order St > St + PE > St + PAT, while MAOC in Mi followed the order St > St + PE > St + PAT, with minimal POC changes in Mi (P < 0.05).

3.4. Correlation Analysis

Distinct straw-mediated regulation governed the correlations among soil indicators, necessitating separate analyses for straw-absent and straw-amended treatments (Figure 8). In straw-absent treatments, cumulative CO₂ emissions exhibited significant correlations (P < 0.05) with multiple indicators, except for SOC content: specifically, CO₂ emission was positively correlated with the mass proportion of Mi and Mi-associated organic C (including POC and MAOC) but negatively correlated with DOC content and net organic C in Ma-associated organic C, especially the POC in Ma (Figure 8A; P < 0.05). In contrast, in straw-amended treatments, both the final SOC content and CO₂ emissions were positively correlated with Ma proportion, MWD, and Ma-associated organic C (including POC and MAOC) while negatively correlating with Mi and S + C proportions, as well as their inner organic C (Figure 8B; P < 0.05).

4. Discussion

4.1. Organic C Mineralization

Our results demonstrate that maize straw amendment primarily governs soil respiration, inducing significantly elevated CO₂ emissions in straw-amended treatments relative to straw-absent controls (Figure 1 and Figure 2). This phenomenon can be mechanistically attributed to the straw-triggered expansion of the soil labile C pool [35], wherein microbial communities utilize readily degradable compounds as preferential substrates to accelerate growth rates and metabolic turnover [6,36]. The rapid depletion of these labile fractions compels microorganisms to enhance extracellular hydrolytic enzyme production, facilitating accelerated SOC mineralization [37]. This process was corroborated by concurrent increases in MBC and MBN in the straw-amended treatments in our study (Figure 4). Consequently, straw incorporation reconfigures soil C cycling by amplifying labile C availability and stimulating CO₂ release.

Plastic film residue incorporations at 0.1% (w/w) exerted straw-dependent effects on CO₂ emissions. In straw-absent soils, both conventional and biodegradable film residues tended to enhance CO₂ release, consistent with prior research [10,38,39], which can be primarily attributed to improved soil porosity and aeration promoting microbial mineralization [39]. Notably, biodegradable plastic residues further induced a positive priming effect, mainly attributable to the provision of ecological niches and C sources for microorganisms [10,26,38], consequently yielding higher CO₂ emissions than conventional film residues (Figure 2).

In contrast, when integrated with crop straw incorporation, both conventional and biodegradable plastic films consistently reduced CO₂ emissions, with biodegradable films exhibiting greater suppression than conventional films (Figure 2). These results were mechanistically linked to the following: (1) film fragments disrupting water/nutrient transport beyond straw-induced permeability, reducing microbial metabolic activity [40,41]; (2) biodegradable film components (e.g., di-butylphthalate) inhibiting microbial diversity and function [42,43], particularly delaying the decomposition of straw structural compounds—such as cellulose, chitin, pectin, and lignin—via polymer/plasticizer leaching [44,45]; and (3) biodegradable film residues possibly inducing selective enrichment of K-strategist microbes with low C-use efficiency over r-strategists under straw–film co-incorporation [46]. These conjectures were also evidenced by St + PAT yielding lower CO₂ emission than St without reducing MBC (Figure 2 and Figure 4). Although St + PAT reduced the CO₂ emission to a relatively small ratio (1.9%) compared to St, this might have been due to the short duration of the incubation experiment and the sufficient inputs of fresh straw C sources and nutrients. The inhibitory effect of biodegradable plastic film residues on soil respiration may be more obvious, especially in long-term field practice. Collectively, we speculated that microbial population dynamics and metabolic pathway shifts jointly drive significant S×F interactions’ effects on CO₂ release (Figure 1 and Figure 2).

4.2. SOC Sequestration

The formation of new SOC from exogenous organic materials constitutes a critical replenishment for mineralization-induced SOC loss [6,36]. Under conditions of zero straw input, while both plastic film types increased native SOC mineralization, biodegradable film residues elevated the SOC stock relative to conventional film residues (Figure 2 and Figure 3). This was mainly due to supplemental organic C derived from film biodegradation, although biodegradable microplastic in soil may represent transitional C [22]. Our study could not clarify the process of plastic film deterioration and biodegradation; thus, dynamic quantification of particle-size-fractionated plastic residues and tracking the partitioning of biodegradable plastics into CO₂ and microbial biomass are needed in the future.

Nevertheless, exogenous straw amendment replenishes SOC through the incomplete decomposition of structural tissues and microbial byproducts [6]. Our study demonstrated that both plastic film residues, particularly the biodegradable film residues, reduced the increase in SOC when combined with straw incorporation (Figure 3), likely attributable to reduced necromass production from suppressing the dominance of r-strategists, which have relatively faster growth and turnover rates [45,46], thereby decreasing the production of incompletely decomposed straw debris such as POC via diminished catabolic activity. This was evidenced by reduced CO₂ emission and macroaggregate-associated POC—yet stable MBC—in St + PAT (Figure 2, Figure 4 and Figure 7). Collectively, these findings indicate that film-specific modulation of SOC sequestration efficiency depends on exogenous C inputs. Future research should employ ¹³C isotope labeling and amino sugar biomarkers to quantitatively partition crop residue versus microbial-derived SOC pools and elucidate plastic film controls on plant C transformation mechanisms.

DOC, DON, MBC, and MBN are labile soil C and N components primarily generated from microbial metabolic processing of organic substrates, serving as proxies for microbial metabolic activity and substrate availability [22]. In the present study, maize straw incorporation markedly accelerated microbial growth, driving concurrent increases in MBC and its correlated fractions (DOC, DON, and MBN) (Figure 4). As a directly utilizable microbial C source [47,48], DOC exhibits an inverse correlation with MBC due to consumption during biomass synthesis, explaining their consistent negative relationship regardless of straw input (Figure 8). The muted responsiveness of DON and MBN to film residue additions likely reflects non-limiting soil N conditions during short-term incubation (Figure 4). Crucially, although film additions induced variations in DOC and MBC within straw-absent soils, high temporal variability of these transient pools is inherent. High-resolution temporal monitoring beyond single-timepoint measurements is needed to clarify the impact mechanisms of plastic waste in the future.

4.3. Soil Aggregation and SOC Fractions

Aggregate assembly follows a hierarchical model wherein fresh organic matter decomposes into intermediates and microbial byproducts that complex with clay minerals to form primary particles; these progressively coalesce into microaggregates and, ultimately, develop into macroaggregates via binding agent mediation [49]. Macroaggregates constitute the dominant SOC-accumulating fraction due to their numerical abundance in soils [50]. Although susceptible to physical disruption, macroaggregate reformation is facilitated by exogenous organic inputs whose decomposition-derived intermediates directly participate in re-aggregation [51], explaining our observed straw-induced macroaggregate proliferation and consequent MWD elevation in straw-amended treatments (Figure 5). Concurrently, macroaggregate expansion enhanced intra-aggregate SOC storage and redirected C flow from finer fractions toward macroaggregates (Figure 6).

However, film residue additions differentially modulated soil macro-aggregation and inter-aggregate C flow. Without exogenous straw input, PE and PAT film residues exerted divergent modulation on aggregation (Figure 5), reflecting microbial activity’s dual role in promoting aggregation via binding agents while destabilizing aggregates through labile C consumption. PE film residues enhanced soil porosity and aeration, stimulating microbial metabolism, as evidenced by elevated CO₂ and MBC (Figure 2 and Figure 4). The resultant microbial necromass generally contained highly labile C (such as carbohydrates, amino acids, and oligopeptides) that would be easily stabilized via association with soil minerals [36]. Consequently, PE residues promoted macroaggregate formation and C redirection from finer aggregates to macroaggregates, wherein MAOC dominated the net SOC accrual in macroaggregates (Figure 5, Figure 6 and Figure 7). Conversely, PAT residues induced aggregate disintegration due to the higher stimulating priming effect [10,38]. In addition, it has been shown that biodegradable microplastics may participate in the formation of microaggregates [23,26]. Therefore, PAT reduced macroaggregates but increased microaggregates (Figure 5 and Figure 6), validating prior findings [52,53]. Concurrently, decreased POC and MAOC in macroaggregates indicated C flow toward microaggregates, due to the relatively easier accessibility in macroaggregates for decomposers than in microaggregates (Figure 7). Collectively, straw-absent soils exhibited SOC mineralization and redistribution dominated by aggregate dynamics, explaining the CO₂ emission’s correlation with microaggregate proportion, intra-microaggregate SOC, and microaggregate-associated POC and MAOC (Figure 8).

However, in straw-amended systems, both PE and PAT film residues—particularly PAT film residues—suppressed microbial metabolic activity due to the physical impedance of water/nutrient fluxes [41], consequently inhibiting macro-aggregation and MWD development while attenuating C flow from finer fractions to macroaggregates (Figure 5 and Figure 6). Critically, biodegradable films may further compromise microbial functionality via compositional complexity and di-butylphthalate leachates [42,43], with depressed catabolic rates and r-strategist dominance depletion reducing straw decomposition efficiency and microbial-derived SOC production [45,46], ultimately yielding lower POC and MAOC in macroaggregates under St + PAT versus St + PE treatments, while elevating MAOC retention in microaggregates and silt + clay fractions (Figure 7). Collectively, microbial-driven SOC accrual and aggregation dominated the straw-amended incubations, as evidenced by positive correlations among SOC content, macroaggregate proportion, MWD, and macroaggregate-associated organic C (Figure 8).

Comprehensively, PE and PAT film residues differentially modulated C allocation among soil aggregate fractions, primarily attributable to the absence of plant residues, wherein native SOC turnover and PAT microplastics constituted the principal drivers of aggregate reformation. Conversely, when co-incorporated with maize straw, both film residues induced similar directional shifts—albeit with magnitude variations—in inter-aggregate C flow, reflecting sufficient straw-decomposition-derived organic C as the core aggregation driver, with film residues merely exerting limited modulation on aggregation efficiency through microbial metabolic alteration. Our results provide important implications for the mechanisms governing SOC sequestration under plastic waste accumulation. Only by considering these findings can we improve our ability to further improve soil fertility and eco-environmental quality under the current scenario by better managing organic solid waste in the field.

5. Conclusions

We demonstrated that conventional PE and biodegradable PAT film residues divergently governed short-term (60 d) organic C turnover and inter-aggregate C flow in response to crop residue inputs. Under native SOC-dominated conditions (no exogenous residue), PAT residues—compared to PE—provoked stronger mineralization of native SOC while depressing macroaggregate reformation and elevating microaggregates, redirecting C flow from macro- to microaggregates. Conversely, with maize straw as the dominant C source, PAT film residues reduced organic C mineralization and SOC accrual compared to PE, concurrently diminishing the increase in macroaggregate proportion and attenuating C flow from finer aggregates. Critically, our research revealed that PAT film residues compromised SOC sequestration by accelerating native SOC mineralization and impeding the organic substrate decomposition after crop residue input, providing novel knowledge on the mechanisms governing SOC sequestration and aggregate-associated C storage under plastic waste accumulation.