Different Impacts of Early and Late Rice Straw Incorporation on Cadmium Bioavailability and Accumulation in Double-Cropping Rice

Abstract

Straw return is widely adopted to promote agricultural sustainability, but it can also increase cadmium (Cd) bioavailability in contaminated paddy soils, potentially leading to higher Cd accumulation in rice grains. Although numerous studies have investigated straw incorporation, the specific differences between early- and late-season straw return regarding Cd dynamics within double-cropping rice systems remain inadequately characterized. To address this knowledge gap, we conducted a two-year field experiment comparing early-rice (ER) and late-rice (LR) straw return, complemented by controlled pot experiments simulating ER (ER-S, ER-CK; July–September 2023) and LR (LR-S, LR-CK; December 2022–March 2023) straw incorporation. The results revealed that the Total-Cd exhibited an upward trend following both ER and LR straw incorporation. The ER treatment caused a rapid yet short-lived increase in CaCl₂-extractable Cd (CaCl₂-Cd) concentration, peaking around 60 days following straw return and exhibiting a 28.83% increase compared to the LR treatment. In contrast, the LR treatment induced a slower but more prolonged Cd release, with CaCl₂-Cd concentration peaking around 210 days and exhibiting a 34.89% increase relative to the ER treatment. Additionally, at the late-rice stage, grain Cd concentration in the ER treatment increased by 23.64% relative to the LR treatment. In the subsequent year, grain Cd concentrations in the LR treatment increased significantly by 32.12% to 45.08% compared to the ER treatment for both early- and late-rice crops. These differences were attributed to variations in straw decomposition rates, soil pH, and redox potential between warm, aerobic summer–autumn conditions and cooler, anaerobic winter–spring conditions. This suggests that returning late-rice straw constitutes an elevated hazard to soil health and rice safety compared to early-rice straw return.

1. Introduction

In 2020, China was among the world’s leading producers of straw, generating nearly 856 million tons of straw [1,2]. For the sustainability of agriculture, the Chinese government has banned the open burning of crop residues and promoted straw return as a sustainable alternative [3]. This practice has been shown to improve soil fertility and porosity, as well as the potential of soil to retain nutrients and its organic carbon content [4,5]. Nevertheless, incorporating straw into cadmium (Cd)-contaminated fields may increase the likelihood of soil and subsequent crops being exposed to elevated Cd levels [6]. Cd is highly toxic [7] and can accumulate in crops and enter the human food chain [8]. Long-term Cd exposure is strongly associated with various human health issues, including osteoporosis and cancers [9].

The return of Cd-containing straw has been documented to significantly increase Cd concentrations and accelerate its migration through the soil, thereby promoting Cd enrichment in rice [6,10]. Long-term straw incorporation (>4 years) has been shown to sustain an increase in soil Cd bioavailability by 38.64–53.95%, whereas straw removal significantly reduces total soil Cd by 16.93–27.30% and available Cd by 8.23–21.05% [11]. This effect arises because crop straw not only serves as a major source of Cd input to farmland [12,13] but also alters key soil properties, such as pH, redox potential (Eh), and organic matter (OM), that regulate Cd bioavailability [14]. During straw decomposition, large quantities of dissolved organic carbon (DOC) are released, which can form complexes with Cd and facilitate its desorption from soil particles [15]. The associated decrease in soil pH and Eh further enhances Cd mobility and bioavailability [16,17,18,19]. Moreover, straw addition can transform Cd from stable to more labile fractions and alter microbial community composition, indirectly affecting Cd solubility [20]. However, in the double-cropping rice regions of southern China, no studies have yet reported whether the incorporation of early- and late-rice straw exerts differential effects on Cd accumulation in soil and in subsequent rice crops.

Significant differences have been shown to exist in soil temperature, moisture, and aeration between the early-rice and late-rice straw incorporation stages [21,22]. Consequently, the straw decay process, organic carbon content and fraction in soil, soil aggregate distribution and stability, and greenhouse gas emissions all vary significantly [21,22,23,24]. Straw decomposition rates are strongly influenced by environmental conditions, including meteorological factors, soil properties, and agronomic practices, which in turn affect the release of straw-derived Cd and consequently its bioavailability [14,25,26]. Previous studies have shown that when straw was added to early-rice fields, the aerated aerobic environment led to faster decomposition, while straw added to late-rice fields experienced slower decomposition due to the flooded anaerobic environment. This variation results in an increased influx of carbon, which significantly elevates the concentrations of DOC in the soil during early-rice cultivation [21], thereby enhancing Cd bioavailability. In summary, we hypothesize that a significant difference exists between the effects of early- versus late-rice straw return on the chemical interactions of Cd in soil. However, available data remain limited. Determining whether early or late straw return more effectively mitigates Cd accumulation in soil and rice under a double-cropping system is critical for managing Cd-contaminated farmland while maintaining the soil fertility benefits of straw incorporation. Therefore, targeted experiments are required to elucidate the differential impacts of these two straw-return practices on soil Cd bioavailability.

The field and pot experiments of this study were conducted within this context. Our objective was to: (i) examine the differences in how early- and late-rice straw incorporation affected soil physicochemical properties and Cd bioavailability; (ii) investigate the accumulation of Cd in rice tissues under different straw return patterns; and (iii) elucidate the underlying mechanisms. The findings aim to provide theoretical support for mitigating Cd accumulation in both rice and soil in systems where straw is returned.

2. Materials and Methods

2.1. Experimental Site and Material Description

Field experiments were conducted in Yongan Town, Liuyang City, Hunan Province, China (28°14′58″ N, 113°20′15″ E). The site is under a subtropical monsoon climate, characterized by an average annual temperature of 16.7 to 17.6 °C, 1568.8 mm of evenly distributed rainfall, 1490 to 1850 h of sunshine, and a frost-free period of 235 to 293 days. The Changsha Meteorological Bureau provided the meteorological information.

The pot experiment was conducted in the Agricultural Environmental Engineering Training Center of Hunan Agricultural University. The texture of the soil used in the field and pot tests was 15.40% sand, 50.23% silt, and 34.37% clay, which was classified as silty loam according to the USDA texture triangle. The paddy field described above served as the source of both soil and straw samples. For the experiments, the soil was first air-dried and then passed through a 2 mm nylon sieve. Rice straw returned to the field was cut into strips 5–10 cm long before application. The straw Cd content was 2.18 mg kg⁻¹.

Table 1. Basic soil properties of the experiments.

Properties	pH	CEC (cmol kg−1)	OM (g kg−1)	CaCl2-Cd (mg kg−1)	Total-Cd (mg kg−1)
Field experiment	5.61	24.01	42.79	0.16	0.65
Pot experiment	5.25	23.18	36.76	0.34	1.03

CEC: cation exchange capacity; OM: organic matter; CaCl₂-Cd: CaCl₂-extractable Cd in soil; Total-Cd: total Cd in soil.

lists the basic indicators of the original soil.

2.2. Experimental Design

2.2.1. Field Experiment

The study was conducted over two consecutive years, from 2022 to 2023, with monthly continuous monitoring conducted from July 2022 to July 2023. The experimental treatments included early-rice straw return (ER) and late-rice straw return (LR). In the ER treatment, early-rice straw was cut into 5–10 cm lengths and incorporated into the top 0–20 cm of soil, while the late-rice straw was removed after harvesting. In the LR treatment, early-rice straw was removed, and the late-rice straw was cut into 5–10 cm lengths and placed on the field surface until it was incorporated into the 0–20 cm tillage layer of soil just before planting early rice in the 2nd year. The early-rice harvest and straw incorporation into the field occurred in mid-July each year, while the late-rice harvest and straw return took place in early November. The amount of straw incorporated in the ER treatment was 3.28 ± 0.08 t/ha annually, while in the LR treatment it was 8.32 ± 0.85 t/ha. The experiment consisted of six 6 m × 10 m plots, each separated by 0.5 m. Three independent plots per treatment were arranged as replicates, following a randomized complete block design. Two common regional rice varieties, Xiangzaoxian 45 (early rice) and Tianyouhuazhan (late rice), were selected for the field experiment. Pesticide control and fertilization were combined and carried out in accordance with regional planting practices.

2.2.2. Pot Experiment

This study consisted of twelve pots: six for simulated early-rice straw incorporation treatments (ER-S and ER-CK, conducted from July to September 2023) and six for simulated late-rice straw incorporation treatments (LR-S and LR-CK, December 2022 to March 2023). Each pot measured 0.3 m in height, 0.3 m in diameter and was filled with 10 kg of sieved soil. Subsequently, 10 g of dried straw was packed into a nylon bag, and ten bags were mixed with the soil (a total equivalent to 100 g dried straw). Furthermore, the straw was incorporated into the soil in summer by burying, while in winter, it was shallowly mixed with the surface soil. The ER-S treatment was left in a flooded condition (flooded layer: 4 cm), while the LR-S treatment was kept under moist conditions. All other management practices were applied consistently in the two sets of simulated pot experiments.

2.3. Sample Preparation

2.3.1. Soils

Experimental soils were collected monthly from the top 20 cm of the field, spanning from the sowing of late rice in 2022 to the maturity of early rice in 2023. In each plot, samples were taken both before sowing and at maturity in 2022 and 2023. Soil was sampled by collecting equal amounts of soil from the four corners (avoiding field boundaries) and the center of each plot, then thoroughly mixed to form a composite sample. For the pot experiment, soil samples were collected on days 15, 30, 45, 60, 75, and 90 from five locations within each pot. All soil samples were air-dried and passed through either a 10-mesh or 100-mesh sieve before physicochemical analysis.

2.3.2. Plants and Straws

Samples of rice were taken during the tillering, heading, grain filling, and maturity stages of the field experiments. For each treatment plot, five whole rice plants were randomly selected and rinsed with deionized water. Each form of rice tissue was separated. All plant samples were first dried at 105 °C for 1 h, then gradually dried at 65 °C, pulverized by grinding to <0.5 mm particles, and stored in polyethylene bags. In the pot experiment, a nylon mesh bag was withdrawn from the incubator on days 15, 30, 45, 60, 75, and 90. Straw samples were triple-rinsed with tap water and ultrapure water, oven-dried at 105 °C until constant mass, pulverized by grinding to <0.5 mm particles, and stored in polyethylene bags.

2.4. Sample Analysis

Soil pH and Eh were determined under a soil–water ratio of 1:2.5 using a portable meter (PHS-3C, Leici, Shanghai, China). CEC was determined using the sulfuric acid and barium chloride forced reduction methods [27]. The colorimetric method using potassium dichromate oxidation was employed to determine the OM content [28]. DOC was extracted with K₂SO₄ (0.5 mol L⁻¹) and analyzed with a TOC automatic analyzer (Vario TOC, Elementar, Langenselbold, Germany). The soil was treated with a 0.01 mol L⁻¹ CaCl₂ solution to extract extractable Cd (CaCl₂-Cd) [29]. Total Cd (Total-Cd) concentration was measured after acid digestion was performed using a mixed acid solution of HCl–HNO₃–HClO₄ (4.5:1.5:5, v/v/v). The chemical fractions of Cd in the soil were determined through a continuous fractionation protocol based on the Community Reference Bureau (BCR) method [30,31]. A 1:2 soil-to-water ratio with oscillation was used to extract the soil solution, and the extract was analyzed using ICP-MS (NexION 350, PerkinElmer, Shelton, CT, USA).

Dried rice samples were digested with 10 mL of HNO₃. Cd concentrations in the digested samples were determined using ICP-OES (Optima 8300, PerkinElmer, Shelton, CT, USA). To ensure analytical accuracy, GBW07405-GSS-5 soil and GBW07603-GSV-2 plant materials were obtained from the China National Reference Materials Center (Beijing, China) and used for calibration and quality control.

2.5. Statistical Analysis

The ratio of Cd content in rice tissues to Cd content in soil (bioconcentration factor, BCF) was determined.

$$\mathrm{BCF}=\mathrm{C}{\mathrm{d}}_{R\mathrm{i}\mathrm{c}\mathrm{e}\mathrm{tissues}}/\mathrm{C}{\mathrm{d}}_{S\mathrm{o}\mathrm{i}\mathrm{l}}$$

(1)

The ratio of Cd content in aboveground organs to Cd content in roots (transfer factor, TF) was determined in rice.

$$\mathrm{TF}=\mathrm{C}{\mathrm{d}}_{\mathrm{Stem},\mathrm{Leaf},\mathrm{Rice}}/\mathrm{C}{\mathrm{d}}_{\mathrm{Root}}$$

(2)

The rate of straw decomposition was determined by comparing the original dry weight of the straw in the nylon mesh bag to its weight at subsequent time intervals.

$${\omega }_{\mathrm{t}}=(10-{\mathrm{m}}_{\mathrm{t}})/10$$

(3)

Here, ω_t represents the straw decomposition rate on day t after the start of the pot experiment, while m_t denotes the remaining straw mass (g) on day t.

Statistical analyses were conducted using IBM SPSS 26.0 (IBM Corp., Armonk, NY, USA). One-way ANOVA (Duncan’s multiple range test) and an unpaired t-test were applied to analyze the significance of differences between treatments. Statistical significance was indicated using different letters and asterisks. Significant differences between treatments are indicated by different letters (p < 0.05), whereas *, **, and *** denote significance at p < 0.05, 0.01, and 0.001, respectively. Graphs—including heat maps and correlation matrices—were generated using Origin 2024 and Canoco 5. Spearman’s correlation analysis was employed, and redundancy analysis was conducted using multiple linear regression (MLR) to fit relationships among soil indicators, plant Cd content, and environmental factors.

3. Results

3.1. Variations in Soil Properties

3.1.1. Soil pH and Eh

In field experiments (Figure 1A), soil pH in the ER treatment was lower than in the LR treatment during the first 7 months following early-rice straw incorporation. The LR treatment exhibited a lower pH than the ER treatment in the subsequent 5 months when the late-rice straw was plowed into the soil. In the pot experiments (Figure 1B,C), compared with their respective controls (ER-CK and LR-CK), the ER-S and LR-S treatments reduced soil pH by 0.07–0.16 and 0.04–0.13 units, respectively. Furthermore, the soil pH from August to December 2022 was higher than from January to July 2023, which was contrary to the monthly rainfall pattern. The change in Eh throughout the year was opposite to that of the pH values. After the early-rice straw was returned, the ER treatment caused a decrease in soil Eh compared to the LR treatment (August–November 2022), and then the two treatments displayed the opposite pattern (January–June 2023). In the pot experiments, incorporating rice straw also lowered soil Eh at each time point, with significant differences observed between the ER-CK and ER-S treatments (p < 0.05).

3.1.2. Soil CEC

CEC exhibited two primary characteristics: higher values during the late-rice season and lower values in winter and the early-rice season (Figure 2A). Additionally, straw incorporation increased soil CEC (p > 0.05). In the summer pot experiment, CEC content was higher than that observed in winter, and the straw return treatment resulted in higher CEC than the control (Figure 2C). For example, ER-S and LR-S treatments increased soil CEC by 1.92–11.28% and 2.18–7.09% after 15 days of straw incorporation, compared to the ER-CK and LR-CK treatments (p > 0.05), respectively.

3.1.3. Soil OM and DOC

From August to November (Figure 2B), the ER treatment significantly increased the OM content by 1.09–24.57% (p < 0.05), primarily due to the incorporation of early-rice straw. The OM content in the LR treatment increased by 6.17–14.86% from January to May following the addition of late-rice straw, compared to the ER treatment. In July of the following year, the OM content in the ER treatment began to rise again. Overall, the increase in OM resulting from early-rice straw return was more rapid and substantial; however, the increase from late-rice straw return was slower and more gradual. This pattern was also evident in the pot experiments (Figure 2D,E). The OM content increased with straw incorporation during summer, particularly within the first 45 days, compared to ER-CK. DOC in the ER-S treatment rose dramatically by 9.22–28.87% after 30 days relative to ER-CK (p < 0.05). For the LR-S treatment, OM and DOC contents also showed increases (p > 0.05).

3.2. Variations in Soil Cd Concentrations

3.2.1. Total Cd

In both the field and pot experiments (Figure 3A, right and B), the Total-Cd exhibited an upward trend following straw incorporation, with most increases being statistically significant (p < 0.05). Compared with the LR treatment, Total-Cd levels increased by 3.85% to 18.86% under the ER treatment between August and February. In contrast, during the following March to July, Total-Cd in the LR treatment was 5.50% to 13.14% higher than that observed under the ER treatment. For the ER treatments, the increase in Total-Cd concentration was rapid and pronounced, whereas that in the LR treatments was slow and gradual. In the field experiment, higher Total-Cd concentrations in the ER covered the period from August 2022 to February 2023, which covered late-rice growth and the slack winter season. Following the late-rice harvest in November 2022, when the straw was left on the field surface, the Total-Cd content remained relatively stable for the first three months and began to rise in March 2023 when the straw was incorporated into the soil. The pot experiment also confirmed these results. Significant differences between ER-CK and ER-S were observed on days 30 and 45, and differences between LR-CK and LR-S were observed only after day 90. Furthermore, the concentrations of Total-Cd in the ER-S treatment increased by 1.91–8.81% on days 30–75 in contrast to the ER-CK treatment. Assuming that the incorporation of late-rice straw similarly causes an increase in soil Total-Cd over a seven-month period (March to September), this span would overlap with the early- and late-rice growing seasons of the following year. This means that the addition of late-rice straw into the field affects the next two rice crops, while early-rice straw return influences only the subsequent late-rice crop.

3.2.2. Extractable Cd

In the field experiment, the trend in CaCl₂-Cd concentrations mirrored that of the Total-Cd (Figure 3A, left). Soil CaCl₂-Cd concentrations increased significantly due to straw incorporation—from September to February in the ER treatment and from March to July in the LR treatment. CaCl₂-Cd concentrations of the ER treatment increased by 14.32–28.83% from September to February compared with the LR treatment, whereas the CaCl₂-Cd in the LR treatment increased by 5.75–34.89% between March and July compared to the ER treatment.

The chemical fractions of Cd in the soil also varied significantly, likely influenced by straw incorporation (Figure 3D). Both early- and late-rice acid-soluble Cd (Aci-Cd) fractions showed an increase due to the straw incorporation, consistent with the observed trends in CaCl₂-Cd concentrations. Furthermore, straw incorporation increased the oxidizable Cd (Oxi-Cd) fraction and reduced the reducible Cd (Red-Cd) fraction. In this study, the effect persisted for four months following early-rice straw incorporation and for seven months following late-rice straw incorporation. In contrast, the residual Cd (Res-Cd) fraction was minimally affected. Straw incorporation not only results in a new input of exogenous Cd but also facilitates the transformation of Cd from the Fe/Mn-bonded form to the exchangeable form in the soil, thereby improving its bioavailability [32,33,34,35]. Organic matter released from straw contributes to the formation of Oxi-Cd.

In the pot experiments (Figure 3C), the CaCl₂-Cd concentrations in the ER-S and LR-S treatments were lower than those in the ER-CK and LR-CK treatments, respectively, in the first 30 days. An opposite trend emerged over the subsequent 60 days, and after straw return, the CaCl₂-Cd concentrations gradually increased in both treatments. Moreover, the CaCl₂-Cd concentration was lower in the ER treatment than in the LR treatment, likely due to differences in pH and Eh. Compared with ER-S treatment, the CaCl₂-Cd in LR-S treatment increased by 2.02–55.44%. The decomposition characteristics and Cd content of the straw are provided in the Supporting Information (Figure S1).

3.2.3. Cd and Fe Concentration in Soil Solution

Ferric (hydr)oxides serve as key adsorption surfaces for Cd in soil, and Cd bioavailability is influenced by the transfer of Fe at the solid–liquid interface. In the field experiments (Figure 4A,B), between August and February, soil solution Cd in the ER treatment was significantly higher (p < 0.01) than that of the LR treatment, except in January. Especially in August and November, the soil solution Cd concentrations of the ER treatments were enhanced by 94.07% and 134.73% compared to the LR, respectively. From April, the Cd concentrations in the LR treatments began to increase, peaking in May, which was 102.8% higher relative to the ER treatment. Straw return also increased the soil solution Fe concentrations. During the first four months, the soil solution Fe concentrations increased significantly in the ER treatment. Subsequently, the soil solution Fe concentrations rose rapidly in December after LR treatment, reaching levels 96.17% higher than those of ER, and this trend was maintained for eight months. During the period of early- and late-rice straw return, Cd concentration in the soil solution gradually decreased after August, reached its minimum in winter slack, and then increased from April to June. This dynamic change was consistent with the fluctuation in soil pH but inversely related to Fe concentrations.

In the pot experiments (Figure 4C,D), Cd concentrations in the LR treatment were significantly higher than those in the ER treatment, whereas the Fe concentrations showed the opposite trend. In contrast to the ER-S treatment, Cd concentration in the LR-S treatment was 18–121 times higher, whereas Fe concentration in the ER-S treatment was 367–4761 times higher than that in the LR-S treatment. These differences were primarily attributed to distinct redox conditions and soil pH between the simulated early- and late-rice straw incorporations. Incorporating straw generally enhanced the concentrations of both soil solution Cd and Fe after 15 d; however, the change in Cd concentration was not pronounced in the ER treatments. These results suggested that the release of Cd from straw, along with the reductive dissolution and secondary mineralization of iron oxide, may explain the observed increase in soil solution Cd following straw incorporation.

3.3. Cd Concentration in Rice Organs

Regardless of whether it was early or late rice, the addition of rice straw enhanced Cd concentration in subsequent rice organs (Figure 5). For late rice in the first year (Figure 5A), at the maturity stage, compared to the LR treatment, the Cd concentrations in the leaves, stems, and roots under the ER treatment were 12.50%, 13.06%, and 29.01% higher, respectively. Relative to LR treatment, root Cd increased significantly at the heading and grain-filling stages due to straw return (p < 0.05). Similarly, Cd concentration in stems increased at the grain-filling stage, but did not increase significantly in other rice tissues at other stages. For early rice in the second year (Figure 5B), Cd contents increased by 36.78% to 67.02% in leaves, 25.59% to 104.43% in stems, and 18.40% to 61.68% in roots compared to the ER treatment. Moreover, significant differences between the two treatments were observed throughout the rice growth period. Incorporating previous rice straw increased the Cd concentrations in grains by 23.64% in late rice (first year) and 45.08% in early rice (second year), respectively. Notably, the increase in Cd content in late-rice tissues, particularly in brown rice, was generally lower than that in early rice tissues following straw incorporation.

At maturity, only rice roots were present, and the BCF for roots was 16.60% higher in the ER treatment than in the LR treatment (Figure 6A). Furthermore, there were minimal differences in the TF values of late rice between the two treatments. This indicates that incorporating early-rice straw had no significant impact on Cd enrichment in late-rice tissues. Nevertheless, in early rice, the BCF values of rice tissues were generally higher under the LR treatment (Figure 6B). This suggests that incorporating late-rice straw promotes the accumulation of Cd in the early-rice crop in the following year. For example, at the maturity stage, compared to the ER treatment, the BCF values of grains, leaves, stems, and roots in the LR treatment were significantly increased by 37.52%, 35.82%, 19.04%, and 22.91%, respectively. Combining the results of Figure 6C,D, the return of rice straw slightly increases the translocation of Cd to rice stems, leaves, and grains. During the filling stage, the TF value for stems exhibited a significant increase in the LR treatment compared to that in the ER treatment.

3.4. Consecutive Two-Year Field Experiment

To further evaluate the differences in Cd concentrations and bioavailability within soil–rice systems under early-rice and late-rice straw return patterns, Total-Cd, CaCl₂-Cd, and Cd concentrations in late rice grains were determined at the maturity stage of late rice in 2023. Data on soil Cd and grain Cd content at maturity for each rice crop over a two-year period were integrated as follows:

3.4.1. Soil Total Cd and Extractable Cd

A general increase in Total-Cd was observed in both ER and LR treatments following straw incorporation (Figure 7A). Compared to the LR treatment, significantly higher Total-Cd concentrations were found in the ER treatment only during the late-rice season following early-rice straw incorporation. However, the LR treatment exhibited higher soil Total-Cd concentration after late-rice straw incorporation, and this effect persisted until the next late-rice season. As shown in Figure 7B, the soil CaCl₂-Cd concentration exhibited a continuous upward trend, except during the period when early rice was not cultivated in 2023. Consistent with the trends shown for Total-Cd, the incorporation of early-rice straw significantly increased the CaCl₂-Cd concentration in the late-rice soil. In contrast, the return of late-rice straw had a prolonged influence on Cd bioavailability, affecting the cultivation of both early and late rice in the subsequent year, indicating a more persistent effect on soil Cd dynamics.

3.4.2. Cd Accumulation in Grain

Grain Cd concentrations showed an upward trend, regardless of whether early- or late-rice straw was returned (Figure 7C). In 2022 and 2023, the incorporation of early-rice straw increased Cd accumulation in late-rice grains in the ER treatment; however, this effect was limited to the same growing season. The late-rice straw return in 2022 resulted in significantly higher Cd concentrations in both early and late rice grains in 2023. Specifically, in late rice of 2022, grain Cd concentration under the ER treatment was 23.64% higher compared with that of the LR treatment. The Cd concentration in grains (both early and late rice) of the LR treatment in 2023 was significantly higher by 45.08% and 32.12%, respectively, compared to the ER treatment. This difference may be attributed to the varying durations during which soil Cd concentrations remained elevated. These results indicate that in Cd-contaminated paddy fields, the return of late-rice straw may pose a greater long-term risk to soil health and rice safety.

The data reveal a critical trade-off: late-rice straw incorporation enhanced soil OM and CEC more sustainably than early-rice straw incorporation (Figure 2A,B), aligning with its role in long-term soil fertility. Paradoxically, this ‘benefit’ coincided with prolonged Cd bioavailability (Figure 3B) and significantly higher grain Cd (Figure 7C). This underscores a fundamental conflict in sustainable agriculture—practices that improve soil structure (e.g., late-rice straw return) may inadvertently threaten food safety in Cd-contaminated regions. Policymakers should therefore prioritize region-specific guidelines: in Cd-polluted areas, late-rice straw removal may be essential despite its value for carbon sequestration.

3.5. Correlation Analysis of Environmental Factors with Cd Availability

Meteorological data from July 2022 to July 2023 are provided in the Supplementary Materials (Figures S2 and S3). Both straw return and meteorological conditions significantly influenced the bioavailability of soil cadmium. As shown in Figure S4, soil CaCl₂-Cd content was significantly negatively correlated with soil pH (r = −0.34, p < 0.01) and Fe concentration in soil solution (Fe-SS, r = −0.55, p < 0.01). These correlations suggest that straw return alters Cd bioavailability by modifying soil pH and Fe-SS concentration. Additionally, soil CaCl₂-Cd content showed significant positive correlations with accumulated temperature (AT, r = 0.41, p < 0.01) and total illumination time (TIT, r = 0.56, p < 0.01), while Cd concentration in soil solution (Cd-SS) correlated positively with AT (r = 0.62, p < 0.01) and TIT (r = 0.63, p < 0.01). These results demonstrate that meteorological factors also influence soil Cd bioavailability, resulting in seasonal differences in the overall effects of straw return.

As shown in Figure 8A,B, the RDA1 + RDA2 values are 90.64% and 85.09%, respectively—both exceeding 50%, indicating that the RDA analysis can effectively reflect the correlations among various indicators. Specifically, as can be seen from Figure 8A,B, soil pH is negatively correlated with soil CaCl₂-Cd content, suggesting that the decrease in soil pH after early- or late-rice straw return is a primary reason for the increase in soil CaCl₂-Cd content. From Figure 8A, it can be observed that soil OM content is positively correlated with soil CaCl₂-Cd content and polished rice Cd content. However, in Figure 8B, soil OM content is negatively correlated with soil CaCl₂-Cdd content and grain Cd content. Additionally, Figure 8A reveals that Fe-SS content is negatively correlated with rice root, stem, leaf, and polished rice Cd content, while Figure 8B shows that Fe-SS content is positively correlated with rice root, stem, leaf, and grain Cd content. Therefore, the above results indicate that late-rice straw return may increase the risk of elevated soil Cd bioavailability and grain Cd accumulation.

4. Discussion

4.1. Impact of Straw Return on Cd Bioavailability

In this study, both early- and late-rice straw return led to increased CaCl₂-Cd content, which aligns with existing findings [36]. First, a sizable quantity of active Cd is directly released when Cd-contaminated straw is added to the paddy soil. Additionally, during the early and intermediate phases of straw decomposition, significant amounts of DOC are formed [37]. Specific functional groups on DOC molecules have strong complexation ability with Cd, thereby increasing Cd solubility [38]. The correlation analysis of environmental factors with Cd availability is provided in the Supplementary Materials (Figure S4 and Figure 8). Results indicated that pH and Eh were significantly correlated with CaCl₂-Cd concentrations. Furthermore, the impact of pH and Eh on the concentration of CaCl₂-Cd differed between early and late rice (Figure 8). The fixation and adsorption of Cd by soil may be weakened by a pH drop [39]. Furthermore, straw incorporation is a critical factor influencing changes in soil Cd speciation, with the most mobile form of soil Cd being Aci-Cd [32]. Red-Cd, Oxi-Cd, and Res-Cd are the three generally stable forms of Cd that require specific conditions to change into a form usable by crops. In this research, straw incorporation increased the proportion of Aci-Cd, which indicates that straw incorporation resulted in the partial conversion of weakly active Cd into a highly active fraction in the paddy soil. Soil samples that had been returned to straw showed lower amounts of Red-Cd. Our results contrast with those of Chen et al.’s pot experiment [40], which reported an increase in soil pH after straw incorporation. This discrepancy may stem from differences in straw and soil properties [41]. In our study, the proportion of Aci-Cd increased because straw incorporation into paddy fields reduced soil Eh, intensifying the reduction and dissolution of iron oxides. This subsequently decreased the residual fraction of Cd [42].

4.2. Cd Bioavailability Differences Between the Field with Early-Rice and Late-Rice Straw Return

Several factors influence the rate at which straw decomposes and releases nutrients into the soil, including its chemical composition, soil properties, and climatic conditions [43,44]. There is a general consensus that soil microbial biomass, activity, and structure play crucial roles in straw decomposition and are affected by climate variables such as temperature and precipitation [17,45,46,47]. Soil warming enhances microbial activity and respiration, while soil moisture significantly impacts microbial and enzyme activity [48,49,50]. Straw decomposition in soil begins with the breakdown of cellulose by Firmicutes, followed by the degradation of hemicellulose by Bacteroidetes [51]. This leads to a significantly increased decomposition rate during summer. Furthermore, compared with the mulching and returning of late-rice straw in winter–spring, the incorporation of early-rice straw in summer and autumn accelerates straw decomposition. Research shows that straw decomposition in summer–autumn was 2–3 times faster than in winter–spring, with a higher carbon release rate [52]. This results in different soil OM and DOC contents. Consequently, soil OM contents in the early-rice straw treatments increased more rapidly and reached higher peaks than those observed in late-rice straw return treatments.

Critically, the temporal dynamics of Total-Cd, CaCl₂-Cd, and OM in this study exhibited similar trends, which were closely related to the straw decomposition process. The AT and TIT were significantly correlated with soil CaCl₂-Cd content. Elevated temperatures promote the decomposition of rice straw, thereby facilitating the release of Cd into the soil [53,54,55]. In addition, solar radiation may induce the photodecomposition of CdS in soil, altering Cd speciation and enhancing its mobility and bioavailability [56]. Compared to October, the higher temperature and illumination intensity in June—corresponding to the grain-filling stages of late and early rice, respectively—allow for greater Cd uptake and accumulation in early rice than in late rice.

Cd concentration in the soil solution is an essential determinant of its bioavailability in soil [57]. Prior experiments have demonstrated that straw incorporation can increase Cd concentration in the soil solution [58]. Paradoxically, however, ER treatments (summer–autumn) showed lower Cd concentrations in the soil solution than LR treatments (winter–spring) despite faster decomposition. This phenomenon is likely due to the formation of metal sulfides (e.g., FeS) in flooded anaerobic environments [58]. The reduction in Eh induces the reductive dissolution of Fe(III) oxides, releasing Fe²⁺ into the soil solution [59,60,61]. Simultaneously, sulfide ions (S²⁻) produced through sulfate reduction act as substrates for FeS formation [62]. As a consequence, the generated FeS can co-precipitate with Cd, decreasing Cd solubility in the soil solution [63,64]. The activity of sulfate-reducing bacteria is strongly influenced by temperature, with activity levels reported to be up to six times higher at room temperature compared to lower temperatures [65]. Therefore, the formation of FeS is likely more pronounced during the summer–autumn period, which helps to explain the reduced soil Cd concentration observed following early-rice straw incorporation.

While this study offers a robust quantification of the temporal dynamics of Cd bioavailability under early- and late-rice straw incorporation, several limitations should be considered when extrapolating the results to broader field applications. The experiments were conducted at a single site, and multi-location data encompassing diverse soil types, Cd contamination levels, climatic conditions, and rice cultivars are currently lacking; consequently, definitive thresholds or recommendations for straw return in paddy fields with different Cd contamination levels cannot yet be established. Future research should validate these findings under a wider range of field conditions and develop refined straw application thresholds that optimize soil fertility benefits while minimizing the risk of Cd accumulation. Moreover, the relatively short experimental duration limits the ability to predict equilibrium Cd loading or to determine thresholds for potential irreversible contamination. Long-term (>5 years) monitoring is necessary to model Cd flux under continuous straw management and to identify safe application frequencies. Additionally, the aging of Cd species—specifically the gradual transformation from exchangeable to residual fractions over extended periods—remains insufficiently investigated, yet it may play a critical role in mitigating Cd risks in chronically contaminated soils.

4.3. Differences in the Uptake of Cd in the Later Rice Crop

Incorporating straw into paddy fields has been demonstrated to enhance Cd bioavailability, thereby facilitating its uptake and accumulation in crop tissues [35,66]. Based on the findings of this study, early rice in the second year of the LR treatment exhibited a greater increase in Cd accumulation in plant tissues compared to late rice in the first year of the ER treatment. RDAs revealed that Cd concentrations in rice tissues were positively correlated with Eh and negatively correlated with OM content (Figure 8). Both a more pronounced decrease in Eh and an increase in OM were observed following early-rice straw incorporation relative to late-rice straw incorporation. The grain-filling stage is a critical period for Cd uptake in rice [67,68]. During this stage, soil CaCl₂-Cd concentrations in early rice under the LR treatment in the second year were significantly higher than those in late rice under the ER treatment in the first year. This is consistent with the earlier observation that higher temperatures and longer light exposure during the grain-filling period of early rice led to increased Cd bioavailability, thereby promoting Cd accumulation in rice tissues. Moreover, the abundant thermal and light resources enhance root growth and expand the surface area of fine roots. Ultimately, increased Cd uptake by early-rice plants was driven by elevated root respiration and rhizosphere microbial activity, along with a reduction in iron plaque formation on root surfaces [54,69,70].

Straw incorporation has been reported to increase the BCF of Cd in subsequent rice crops [71]. Although the return of both early- and late-rice straw elevated Cd content in rice tissues, paradoxically, the BCF of Cd in all rice organs under the ER treatment did not show a significant increase following early-rice straw incorporation. This phenomenon is attributed to Cd–Fe competition, whereby the ER treatment reduced the soil Cd:Fe ratio during the grain-filling stage, thereby limiting Cd accumulation in rice tissues [72]. Notably, the concentration of Fe in the soil solution following early-rice straw return was significantly higher than that after late-rice straw incorporation (Figure 4D), which promoted the formation of iron plaque and subsequently inhibited the uptake of Cd in the late-rice crop under the ER treatment [73].

Remarkably, the incorporation of early-rice straw predominantly facilitated Cd uptake in late rice within the same year. In contrast, the incorporation of late-rice straw in subsequent years resulted in an increased uptake of Cd in both early and late rice. The above phenomenon can be attributed to the prolonged effects of straw incorporation on soil Cd dynamics, as both early and late rice experienced a 7-month period of elevated Cd levels. Therefore, higher Cd concentrations after early-rice straw return only affected the late-rice planting period and the winter slack. In comparison, the effect of late-rice straw incorporation persisted until the maturity stage of late rice in the second year. This trend is consistent with the Cd concentrations observed in rice tissues, which were strongly correlated with both total Cd and soil CaCl₂-Cd [74,75].

The potential health risks associated with the observed variations in Cd accumulation were assessed by comparing grain Cd concentrations to China’s maximum permissible limit (0.20 mg/kg, National Food Safety Standard GB 2762-2022) [76]. The results show that Cd levels in rice grains under both ER and LR straw incorporation treatments consistently exceeded this safety threshold. Specifically, Cd concentrations in ER-treated grains surpassed the limit by 77.75–81.86%, while those in LR-treated grains exceeded it by 43.76–163.85%. This significant and persistent exceedance highlights a clear potential health risk associated with rice produced under these straw management practices. Notably, the LR treatment exhibited a substantially greater degree of exceedance than the ER treatment due to its higher BCF of Cd in rice tissues, indicating a more serious health risk. In conclusion, cultivating double-cropped rice on Cd-contaminated farmland may be addressed through a targeted straw return strategy: returning early-rice straw at 5–10 cm length while removing late-rice straw. However, considering the practical difficulty of removing early-rice straw in actual agricultural production, in areas with mild Cd contamination (0.3 mg/kg ≤ total soil Cd concentration ≤0.6 mg/kg), a management strategy of incorporating early-rice straw while removing late-rice straw can be adopted. This study establishes a theoretical framework for comparing seasonal Cd behavior in soil under straw return within double-cropping systems, offering new approaches for the safe utilization of Cd-contaminated straw and remediation of polluted farmland, contributing to sustainable agriculture.

5. Conclusions

This research demonstrates the significant impact of straw incorporation on Cd bioavailability and accumulation in rice within a double-cropping system. These findings illustrate that, relative to the incorporation of late-rice straw, the incorporation of early-rice straw had a significantly lower impact on soil Cd bioavailability and the subsequent accumulation of Cd in the later rice crop. The findings emphasize the critical influence of timing in straw incorporation on soil Cd dynamics and rice safety. Early-rice straw incorporation leads to a rapid but short-lived increase in soil Cd bioavailability, while late-rice straw incorporation results in prolonged Cd release and uptake. These insights suggest a strategic approach to straw management, particularly in Cd-contaminated areas. Prioritizing the removal of late-rice straw can help mitigate long-term Cd accumulation in soil and rice, enhance soil health and rice safety, and contribute to sustainable agriculture.