Excessive fertilization is often applied to produce rice. To reduce nitrogen loss and improve nitrogen use efficiency (NUE), we studied the effects of application depth (surface application, 5 and 10 cm) and shape of nitrogen fertilizers (row application and deep application of large granular fertilizer) on rice growth, soil N distribution and ammonia volatilization. The results showed that grain yield, shoot biomass and total dry biomass of the treatment with N in large granular fertilizer applied at 10 cm depth were significantly higher than those of all other treatments. Moreover, compared with the surface application, the N recovery efficiency and the N agronomic efficiency of deep application treatments were enhanced by 18.1–52.3% and 35.6–95.6%, respectively. Deep application significantly increased NH₄⁺-N concentration at their fertilization points. During the growth season, N in large granular fertilizer treatments (mixed with clay to form an unusually large pellet of 1.0–1.5 cm in diameter) distributed closer to the roots, while N in other treatments, including row application treatments, was more widely distributed. Compared with the surface application, deep application significantly reduced NH3 volatilization and NH₄⁺-N concentration in surface water by 58.7–64.8% and 26.0–72.5%, respectively. Furthermore, the NH₃ volatilization from large granular treatment was 7.6–11.0% lower than that in the row application. In conclusion, applying N in large granular fertilizer at 10 cm depth reduces ammonia volatilization, and improves rice growth and grain yield, indicating improved NUE and lowered environmental risks.

While the application of manure to improve soil quality has attracted attention, the effect of pig manure application rates on soil acidity remains poorly understood. In this study, we analyzed the changes and correlations between soil acidity, pH buffer capacity (pHBC), soil chemical properties, and crop yields after 8 years of pig manure application at different rates (i.e., 0, 7.5, 15, 30, and 45 Mg ha⁻¹) in a red upland soil (Ultisol). With an increase in the application rates, the crop yields were 0.77–8.85 times higher; the pH was enhanced by 0.4–0.8 units; and the soil organic matter (SOM), pHBC, iron activation (Feo), exchangeable calcium (ExCa), and exchangeable magnesium (ExMg) contents increased by up to 74.1%, 92.7%, 36.7%, 62.2%, and 48.7%, respectively, whereas that of total exchangeable acid (ExAcid) decreased by 17.2–52.9%. The crop yields were positively related to the soil pH but negatively correlated with ExAcid. Redundancy analysis revealed ExAcid and pHBC were more sensitive than pH was to the other chemical indicators. ExAcid was negatively correlated with SOM and ExCa; pHBC was positively correlated with ExMg, TN, SOM, and Feo. In conclusion, the crop yield could be improved by adjusting the soil acidity characteristics, and the application of pig manure reduced the soil acidity, with an optimal application rate of 15 Mg·ha⁻¹.

Soil acidification is a significant threat to agricultural sustainability, particularly in paddy fields, where acidic conditions can limit crop productivity and soil health. This study aimed to explore the combined effects of alkaline amendments—lime, magnesia, and silicon fertilizer—on the acid-neutralizing capacity (ANC) of paddy soils and the rice yield, with the objective of identifying effective strategies to mitigate soil acidification and enhance agricultural productivity. From 2018 to 2021, a four-year field trial in Hunan tested lime, magnesia, and silicon fertilizers. Soil samples (0–20 cm depth) were collected once post-harvest in 2021 to evaluate the cumulative treatment effects. After four years, the control soil pH was 6.12. Lime and light magnesia treatment increased it to 6.70 and 6.99, respectively. Silicon fertilizer showed no significant difference (pH 6.05). ANC analysis revealed the following anti-acidification capacity ranking: light magnesia > lime > control > silicon fertilizer. Light magnesia boosted the rice yield by 13.02% over the control. Statistical analyses indicated a significant positive correlation between the soil acid-neutralizing capacity and pH (ANC4.0 = 7.53 × pH − 30.00, R²adj = 0.70; ANC5.0 = 6.96 × pH − 37.49, R²adj = 0.58). The rice yield was correlated with exchangeable magnesium (yield = 0.42 × Ex-Mg + 24.54, R²adj = 0.44). The continuous application of lime and light magnesia enhanced the nutrient availability and soil anti-acidification, with light magnesia also improving the rice yield. These findings provide insights to aid in enhancing soil quality and agricultural productivity in acid-affected regions.

Fertilization regimes impact the carbon cycle processes in paddy soils. However, the effects of shifting fertilization regimes on the structure of microbial communities and functional genes involved in soil carbon (C)-cycling remain unclear. A long-term field experiment was established with three paired fertilization shift treatments: chemical fertilizer (CF) and CF to normal-rate organic fertilizer (CF-NOM); normal-rate organic fertilizer (NOM) and NOM to CF (NOM-CF); high-rate organic fertilizer (HOM) and HOM to CF (HOM-CF). Metagenomic sequencing and bioinformatics analysis were employed to investigate the effects of fertilization shifts on soil C-cycling microbial community structure, functional genes, and environmental factors. The results showed that compared to CF treatment, CF-NOM significantly increased soil organic carbon (SOC), mineral-associated organic carbon (MAOC), particulate organic carbon (POC), microbial biomass carbon (MBC), dissolved organic carbon (DOC), and the emissions of CO₂ and CH₄ (p < 0.05). The NOM-CF led to significant reductions in MAOC, MBC, DOC, and CO₂ and CH₄ emissions. The HOM-CF shift caused significant decreases in SOC, MAOC, POC, MBC, DOC, and CO₂ and CH₄ emissions. Fertilization shifts had no significant effect on the α-diversity of C-cycling microbial communities (p > 0.05), but β-diversity showed a significant restructuring of community composition. Network analysis indicated that fertilization shifts increased positive microbial correlations while reducing network modularity. C-cycling functional genes responded sensitively to fertilization disturbances, especially key genes in the carbon fixation pathway (cdhDE, cooS). Redundancy analysis indicated that soil bulk density (BD) and POC are key environmental factors regulating functional differences in carbon metabolism, which collectively influenced microbial community structure and functional gene abundance along with other factors. We concluded that the C-cycling process in paddy soil was greatly altered by shifts in fertilization regimes, influenced by microbial diversity, functional genes, and network structure linked to soil characteristics.