The current farming system is heavily reliant on chemical fertilizers, which negatively affect soil health, the environment, and crop productivity. Improving crop production on a sustainable basis is a challenging issue in the present agricultural system. To address this issue, we assumed that the combined use of organic manure and inorganic nitrogen (N) fertilizers can improve rice grain yield and soil properties without the expense of the environment. This study explores the combined effects of cattle manure (CM), poultry manure (PM), and chemical fertilizer (CF) on soil properties, rice growth, physiology, and grain yield and quality. Six treatments in the following combinations were included: T₁—no N fertilizer; T₂—100% CF; T₃—60% CM + 40% CF; T₄—30% CM + 70% CF; T₅—60% PM + 40% CF; and T₆—30% PM + 70% CF. Results showed that across the seasons, treatment T₆ increased the net photosynthesis rate, total biomass, grain yield, and amylose content by 23%, 90%, 95%, and 10%, respectively, compared with control. This increment in net photosynthetic rate and growth was the result of 24%, 14%, 19%, and 20% higher total root length, root surface area, root volume, and root diameter, respectively. Improvements in these attributes further enhanced the grain yield and nitrogen use efficiency of rice. No significant difference between T₄ and T₆ was observed. The correlation analysis also confirmed that root morphological traits were positively correlated with grain yield, N uptake, and biomass accumulation. Similarly, improvement in grain yield and NUE was also associated with improved soil properties, i.e., bulk density, soil porosity, soil organic carbon, and total N under combined organic and inorganic N fertilizers treatment. Conclusively, the integration of 30% N from PM or CM with 70% N from CF (urea) is a promising option not only for higher grain yield and quality of rice but also for improved soil health. This study provides a sustainable nutrient management strategy to improve crop yield with high nutrient use efficiency.

Leaf nitrogen (N) concentration plays an important role in biochemical and physiological functions, and N availability directly influences rice yield. However, excessive N fertilization is considered to be a root cause of environmental issues and low nitrogen use efficiency. Therefore, the selection of appropriate nutrient management practices and organic amendments is key to maximizing nitrogen uptake and maintaining high and sustainable rice production. Here, we evaluated the effects of different 15N-labelled nitrogen sources (urea, ammonium nitrate, and ammonium sulfate at 315 kg ha−1) with or without biochar (30 t ha−1) on paddy soil properties, root growth, leaf gas exchange, N metabolism enzymes, and N uptake in the early and late seasons of 2019. We found significant differences among N fertilizer sources applied with or without biochar (P < 0.05). Across the seasons, the combination of biochar with N fertilizers significantly increased soil organic carbon by 51.21% and nitrogen availability by 27.51% compared with N fertilizers alone. Correlation analysis showed that rice root morphological traits were strongly related to soil chemical properties, and higher root growth was measured in the biochar treatments. Similarly, net leaf photosynthetic rate averaged 9.34% higher, chlorophyll (Chl) a concentration 12.91% higher, and Chl b concentration 10.05% higher in the biochar treatments than in the biochar-free treatments across the seasons. Notably, leaf 15N concentration was 23.19% higher in the biochar treatments in both seasons. These results illustrated higher activities of N metabolism enzymes such as NR, GS, and GOGAT by an average 23.44%, 11.26% and 18.16% in the biochar treatments across the seasons, respectively. The addition of biochar with synthetic N fertilizers is an ecological nutrient management strategy that can increase N uptake and assimilation by ameliorating soil properties and improving the morpho-physiological factors of rice.

Biochar is an important soil amendment that can enhance the biological properties of soil, as well as nitrogen (N) uptake and utilization in N-fertilized crops. However, few studies have characterized the effects of urea and biochar application on soil biochemical traits and its effect on paddy rice. Therefore, a field trial was conducted in the early and late seasons of 2020 in a randomized complete block design with two N levels (135 and 180 kg ha⁻¹) and four levels of biochar (0, 10, 20, and 30 t ha⁻¹). The treatment combinations were as follows: 135 kg N ha⁻¹ + 0 t B ha⁻¹ (T1), 135 kg N ha⁻¹ + 10 t B ha⁻¹ (T2), 135 kg N ha⁻¹ + 20 t B ha⁻¹ (T3), 135 kg N ha⁻¹ + 30 t B ha⁻¹ (T4), 180 kg N ha⁻¹ + 0 t B ha⁻¹ (T5), 180 kg N ha⁻¹ + 10 t B ha⁻¹ (T6), 180 kg N ha⁻¹ + 20 t B ha⁻¹ (T7) and 180 kg N ha⁻¹ + 30 t B ha⁻¹ (T8). The results showed that soil amended with biochar had higher soil pH, soil organic carbon content, total nitrogen content, and mineral nitrogen (NH₄⁺-N and NO₃⁻-N) than soil that had not been amended with biochar. In both seasons, the 20 t ha⁻¹ and 30 t ha⁻¹ biochar treatments had the highest an average concentrations of NO₃^–-N (10.54 mg kg⁻¹ and 10.25 mg kg⁻¹, respectively). In comparison to soil that had not been treated with biochar, the average activity of the enzymes urease, polyphenol oxidase, dehydrogenase, and chitinase was, respectively, 25.28%, 14.13%, 67.76%, and 22.26% greater; however, the activity of the enzyme catalase was 15.06% lower in both seasons. Application of biochar considerably increased the abundance of ammonia-oxidizing bacteria (AOB), which was 48% greater on average in biochar-amended soil than in unamended soil. However, there were no significant variations in the abundances of ammonia-oxidizing archaea (AOA) or nitrite-oxidizing bacteria (NOB) across treatments. In comparison to soil that had not been treated with biochar, the average N content was 24.46%, 20.47%, and 19.08% higher in the stem, leaves, and panicles, respectively. In general, adding biochar at a rate of 20 to 30 t ha⁻¹ with low-dose urea (135 kg N ha⁻¹) is a beneficial technique for improving the nutrient balance and biological processes of soil, as well as the N uptake and grain yield of rice plants.

Improving rice production in modern agriculture relies heavily on the overuse of chemical fertilizer, which adversely affects grain quality. Biochar (BC) application is well known for enhancing rice yield under reduced nitrogen (N) application. Therefore, we conducted a two-year field experiment in 2019 and 2020 to evaluate RVA profile characteristics, grain milling, and appearance qualities under four BC rates (0, 10, 20, 30 t ha⁻¹) in combination with two N levels (135 and 180 kg ha⁻¹). The results showed that BC at 30 t ha⁻¹ along with 135 kg N ha⁻¹ improved rapid visco-analyzer (RVA) profile attributes, including peak viscosity (4081.3), trough viscosity (3168.0), break down (913.3), final viscosity (5135.7), and set back (1967.7). Grain yield, grain rain length, milled rice rate, percent grains with chalkiness, amylose, and starch content were improved by 27%, 23%, 37%, 24%, 14%, and 8%, respectively, in the plots treated with the combination of 30 t BC ha⁻¹ and 180 kg N ha⁻¹. A positive coefficient of correlation was observed in RVA profile, milling, and apparent quality of rice with soil properties. These results suggested that BC at 20 to 30 t ha⁻¹ in combination with 135 kg N ha⁻¹ is a promising option for enhancing grain yield, RVA profile, appearance, and milling quality.

Grain filling can directly influence rice yield. However, there is limited information on the growth relationship among grains at different positions on the entire panicle during grain filling. In this study, field experiments were conducted in 2014–2015 to compare the growth dynamics of grains at various positions for two rice cultivars (Nongle 1 and Guifeng 2). The results showed that a high similarity and a slow–fast–slow trend of dry-matter accumulation occurred in all primary branches. However, the maximum grain growth rates of the top primary branches were 86% and 44% higher than basal primary branches of Nongle 1 and Guifeng 2, respectively. Similarly, the maximum final grain weights were 32% and 18% greater in the top primary branches than in the basal primary branches of Nongle 1 and Guifeng 2, respectively. In contrast, the active grain filling duration was 1.5 and 1.3 times longer in the basal primary branches than the top primary branches of Nongle 1 and Guifeng 2, respectively. The time to reach the maximum rate of grain growth of the basal primary branches for Nongle 1 and Guifeng 2 was 2.2 and 2.5 times longer than those of the top primary branches, respectively. Based on cluster analysis of growth characteristics of all primary branches, Group I (superior primary-branches) was considered to be the fastest for grain filling and greatest for dry matter weight, followed by Group II (medium primary-branches). The poorest growth occurred in Group III (inferior primary-branches). Therefore, the yield of poor-filling grains at the basal panicle could be achieved primarily by improving the growth of Group III.

Current farming systems are highly reliant on chemical fertilizers (CF), which negatively affect soil health, the environment, and crop productivity. Substituting organic fertilizer for chemical fertilizer (CF) is an important agricultural practice that improves soil health and crop productivity and is eco-friendly. To explore the effects of organic fertilizer in the form of cattle manure (CM) or poultry manure (PM) combined with CF on soil properties, leaf physiological traits, and grain physiochemical characteristics of rice, a 2-year field experiment was conducted in a dual cropping system with six treatments: Neg-CF (no N fertilizer control), Pos-CF (100% CF), High-CM (60% CM + 40% CF), Low-CM (30% CM + 70% CF), High-PM (60% PM + 40% CF), and Low-PM (30% PM + 70% CF). The results exhibited that the adding of organic manure significantly improved soil chemical traits such as soil organic C (SOC), total N (TN), and available N (AN). Similarly, applying the combined fertilizer led to significant increases in the leaf net photosynthetic rate (Pn), SPAD values, and rice grain nutritional and cooking characteristics according to Rapid Visco-Analyzer (RVA) profile. Average increases in SOC, TN, Pn, starch content (SC), and amylose content (AC) were 16%, 12%, 9%, 7%, 9%, and 12%, respectively, across the year in the Low-PM compared to the Pos-CF treatment. Moreover, the manure amendments significantly altered the RVA profile attributes, including peak viscosity, tough viscosity, final viscosity, and the setback and pasting temperatures compared to Pos-CF. Linear regression analysis revealed that SOC and TN were positively associated with leaf Pn and SPAD values. Further, leaf Pn and SPAD were positively correlated with rice grain SC and AC. The correlation heat map analyses revealed a positive correlation in the RVA profile between the soil properties and leaf physiological traits. These correlation analyses showed that the increase in soil chemical traits and leaf physiological activities played a significant role in higher rice grain nutritional and cooking quality. Overall, the findings of this study show that the integrated use of organic fertilizers and CF in rice fields enhances soil functionality and the quality of rice on a sustainable basis.