Inoculating microbial inoculants to speed up the decomposition of returning straw is currently a hot topic. Meanwhile, the soil moisture content (SMC) could change the diversity, abundance, and metabolism of the soil microbial community structure, which affects the straw degradation rate under the straw returning condition. In this research, rumen microorganisms with strong decomposing abilities in natural systems were used as inoculants to promote straw decomposing and returning to the field. The effects of the SMC on straw decomposition under rumen fluid (RF)-induced returning were investigated. Experiments were conducted for 30 days with typical paddy soil in the south of China under conditions of 30%, 70%, and 100% SMC. With an increase in the SMC within a certain range (30~100%), the decomposition rate of straw showed a trend of first rising and then falling. Treatments of 70% SMC with RF addition generally achieved the maximum rate of straw degradation. The peak value was 49.96%, which was 2.67-fold higher than the treatments of 30% SMC with RF addition (18.74%) and 24.1% higher than those of the control with 70% SMC (40.3%) (p < 0.05). Moreover, a straw structural analysis proved that at 70% SMC, microorganisms from RF favored the destruction of functional groups on the straw surface and the degradation of cellulose. Meanwhile, it was shown that RF could promote the decay of straw, leading to increments in enzyme activities and soil nutrients. The higher the soil moisture content, the higher the key soil enzyme activities. This indicates that the diversity and abundance of cellulose-degrading bacteria and fungi in soil microorganisms and rumen microorganisms were changed with different soil moisture contents. The experimental findings suggest an innovative way to further utilize rumen microorganisms.

As natural selenium (Se)-rich soil in China is generally characterized by a high geological background of cadmium (Cd), the safe utilization of such seleniferous soil remains a challenge. The accumulating evidence shows that the threshold value of the Se:Cd ratio is a determinant of regulating Cd accumulation in plants. However, the factors modulating the soil’s Se:Cd ratio in selenium-enriched regions are not well understood. Here, a comprehensive study aimed at quantitatively analyzing the effects of land-use types, parent-material types, and soil properties on the distribution and influencing factors of Se, Cd, and the Se:Cd ratios. According to land use and parent-material types, 77 soil samples were collected in Yuanzhou District, a typical naturally seleniferous area in the subtropical hilly area. The results suggested that, compared with quaternary red clays (qrc), the Se content of soils derived from river and lake sediments (rls) and weathered acidic crystalline rocks (wacr) decreased by 5.81%–19.75%, while the weathered quartzite (wq)-derived soils was increased significantly. The soil Cd content in an orchard was significantly reduced compared with that in a paddy field. A redundancy analysis (RDA) revealed that SOM, Total K, and Total P significantly affected the changes in Se and Cd contents. In addition, the land-use type had the most significant effect on the Se:Cd ratio, with a regression coefficient of −0.6999 analyzed by the binary logistic regression model (p < 0.05). Furthermore, pH and Total K were the critical soil properties in controlling the Se:Cd ratio. The study indicated that the Se:Cd ratio in natural selenium-rich soil was mainly regulated by land-use types. Therefore, it is a feasible measure to regulate the Se:Cd ratio by using agronomic practices, mainly regulating soil pH, for the safe utilization of selenium-rich soil with a high Cd background.

To address cadmium pollution in China’s cultivated land, chitosan, inorganic and organic selenium were used to modify rice husk charcoal for cadmium inhibition. Basic physicochemical properties of rice husk carbons were characterized (BET, FTIR, XRD, Zeta potential). Kinetic and isothermal adsorption experiments studied the adsorption of Cd²⁺ by modified biochar under different pH and dosages. A350 and C350 had pore changes, and B350 had a smoother surface. The polarity and Zeta potential of A350, B350, and C350 differed. B350 and C350’s kinetic adsorption fit the pseudo second order model, A350’s fit both the pseudo first and second order. Their isothermal adsorption fit Langmuir (B350, C350) and Freundlich (A350). Intraparticle diffusion was three-stage with single-layer chemical adsorption. The pH increase raised removal and adsorption of CK350, A350, B350, and C350. The dosage increase hiked removal but cut unit adsorption. A350 had the highest max adsorption (57.845 mg/g). All modifications enhanced Cd²⁺ adsorption, and the effect could be altered by adjusting pH and dosage.

Microbial necromass carbon (MNC) is the dominant contributor to soil organic carbon (SOC). However, the contribution of MNC in different soil compartments to SOC sequestration has not been comprehensively studied, especially under the organic fertilizers input. To address this gap, we conducted a rice root box experiment by adding organic fertilizer (straw and straw biochar) and chemical fertilizer alone to red loamy paddy soil, respectively. We found that although SOC accumulation was stimulated by both biochar and straw in the rhizosphere, more substantial SOC was sequestered in the rhizosphere due to biochar addition (increased by 25.82% compared to straw addition). Additionally, the input of organic fertilizers resulted in varying degrees of MNC retention in the different soil compartments. Compared with that in bulk soil, fungal necromass carbon (FNC) content was reduced by 1.37% and 7.06%, and bacterial necromass carbon (BNC) content was reduced by 5.53% and 9.49% in the rhizosphere and hyphosphere, respectively, following straw addition. Conversely, the addition of biochar leads to a significant increase of FNC (increased by 2.92%) and BNC (increased by 2.00%) in the rhizosphere compared with bulk soil. However, straw addition also significantly enhanced SOC thermal stability within the rhizosphere and hyphosphere soils. Based on partial least squares path modeling, we found that SOC thermal stability was significantly and positively influenced by FNC, which was strongly associated with carbon degradation gene abundance. These results emphasize the critical role of soil compartments in SOC sequestration under organic fertilizer application and underscore the importance of FNC in enhancing SOC stability in the rhizosphere.

The prevalent issue of cadmium (Cd) in naturally selenium (Se)-enriched soils has significantly impacted the safe utilization of Se-rich soils. Although Se antagonizes Cd and is affected by Se:Cd stoichiometry, the mechanism behind this interaction remains unclear. To reveal the relation between the soil bioavailable Se:Cd molar ratio (AMR-Se:Cd) and the transformation of Se and Cd fractions, as well as to identify the principal controlling factors, we conducted a study in Shanggao County, Yichun City, a naturally Se-rich area in Jiangxi Province, and quantitatively analyzed the distribution features of Se, Cd, and AMR-Se:Cd across different soil types and land use types. The results demonstrated that soil AMR-Se:Cd was statistically positively correlated with the bioavailable Se content (r = 0.331, p < 0.01) and had a negative correlation with the bioavailable Cd content (r = −0.402, p < 0.001). Cd was transformed from highly bioavailable fractions to less bioavailable fractions as the AMR-Se:Cd increased. A suitable AMR-Se:Cd was conducive to achieving a higher mobility of Se (mobility factor of 12.31%) and a lower mobility of Cd (mobility factor of 23.49%) simultaneously. Spearman correlation analysis and partial least squares path modeling revealed that soil type and land use type modulated the morphological transformation of soil Se and Cd by influencing changes in free Fe-Al oxides and soil organic matter content, which in turn altered the AMR-Se:Cd. Therefore, the findings of this study can offer guidance for regulating the appropriate AMR-Se:Cd in Se-rich soils through management practices to enhance the bioavailability and mobility of soil Se while diminishing the bioavailability and mobility of Cd.

Rice grain is a primary dietary source of cadmium (Cd), a heavy metal toxic to humans. Reducing Cd accumulation in rice through selecting and breeding low-Cd-accumulating cultivars is very important. However, field-based screening for low-Cd rice cultivars remains labor-intensive and time-consuming. In this study, we identified molecular marker genotypes that can distinguish high- and low-Cd-accumulating rice cultivars. We developed corresponding genotypes for marker-assisted selection of low-Cd cultivars in both early and late rice varieties. Fifty-nine locally adapted, high-yielding early rice cultivars and thirty-seven locally adapted, high-yielding late rice cultivars were grown in two fields with different soil Cd levels and genotyped using molecular markers associated with grain Cd accumulation. We identified five early rice cultivars that consistently showed low Cd accumulation, with grain Cd concentrations below the food safety threshold of 0.2 mg kg⁻¹ across two paddy fields. For early rice, we developed two low-Cd combined molecular marker genotypes (Multi-LCL1 and Multi-LCL2) that had significantly lower grain Cd content compared to Multi-LCL3 and Multi-LCL4. For late rice, the low-Cd combined molecular marker genotype Multi-CL1 showed substantially reduced grain Cd levels relative to Multi-CL2-CL5. These findings suggest that the combined molecular marker genotypes Multi-LCL1/LCL2 for early rice and Multi-CL1 for late rice are practical tools for quickly identifying cultivars with low Cd accumulation potential.