The effect of Chinese Milk Vetch (CMV) residues as a cover crop on the performance of rice plants and nitrogen mobilization and mineralization in paddy soil was evaluated in a pot-culture experiment. Three treatments were included in the trial, i.e. without exogenous-N (Control or CK), urea fertilizer as the sole N-source (N) and urea plus CMV (NM). The results revealed higher amounts of total-N, inorganic-N, acid hydrolysable-N and non-acid hydrolysable-N in the soil under NM, followed by N and CK treatments at tillering, booting and maturity stages of rice. A similar trend was observed for various soil ammonia oxidizing bacteria, aerobic and anaerobic bacteria. Correlation coefficients exhibited a negative correlation of residual exogenous-¹⁵N with only amino acid nitrogen (P < 0.01). Moreover, low abundance of the key functional gene amoA in rice was found in CK treatment. From the results it can be concluded that the nitrogen from organic source can improve the availability of total-N, inorganic-N and NH₄in the soil, especially at the later stages of rice growth.

This study investigates the effects of various treatments on soil pH, SOC, and crop yield in red soil with a pH of 4.25 through a two-year field experiment, using a rapeseed and sesame cropping system. The treatments included the control (CK); lime (CaO) (L); fully fermented pig manure (M); a calcium–magnesium–phosphate fertilizer (P); lime and fully fermented pig manure (LM); lime and the calcium–magnesium–phosphate fertilizer (LP); fully fermented pig manure and the calcium–magnesium–phosphate fertilizer (MP); and lime, fully fermented pig manure, and the calcium–magnesium–phosphate fertilizer (LMP). Then, the changes in yield, soil pH, and SOC were analyzed. The results showed that, among all treatments, the LMP treatment produced the highest yields for both rapeseed (93.62%) and sesame (45.10%); they increased by 93.62% and 45.10% compared with that for CK. Furthermore, these treatments with lime application increased the soil pH. During the rapeseed season, compared with CK, the soil pH values increased by 0.87, 0.75, 0.90, 1.03, 1.24, 1.18, and 1.45 units in the L, M, P, LM, LP, MP, and LMP treatments, respectively. Moreover, they increased by 0.66, 0.34, 0.51, 0.95, 0.82, 0.72, and 1.03 units, respectively, in the sesame season. Similarly, in terms of yield, the highest pH was observed in the LMP treatment for both the rapeseed and sesame seasons. In contrast to soil pH, the effects of these treatments on SOC were less pronounced. Furthermore, the relationship between soil pH and crop yields was significant (R², p < 0.05). In addition, fitted equations indicated a higher yield response (5.17%) in rapeseed compared with that in sesame (2.32%), while soil pH increased by 0.1 unit. Therefore, the combined application of lime, composted pig manure, and calcium–magnesium–phosphate is an effective strategy to reduce soil acidification and improve crop yield in highly acidified red soils, with the increase in soil pH having a more substantial impact on crop yield than the increase in SOC.

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.

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.