For agricultural production, improving the rice harvest index (HI) through agricultural management practices is a major means to enhance water and N utilization efficiency and yield. Both irrigation regimes and nitrogen (N) rates are important aspects of agricultural management practices. However, it is unclear how the rice HI is affected by water and N. This study aimed to clarify the mechanism underlying the response of the rice HI to water and N, and to explore the most suitable water-saving and N reduction management practices to ensure yield. A two-year (2021~2022) field experiment was conducted on Mollisols in Northeast China. In this experiment, nine treatments were performed, involving three irrigation regimes (flooded irrigation, controlled irrigation, and “thin-shallow-wet-dry” irrigation) and three N rates (110, 99, and 88 kg/ha). The rice agronomic traits and transfer of photoassimilates under different water and N management practices were observed and studied; rice HI, WUE, and the NUE of rice was calculated and analyzed. The highest HI was achieved under controlled irrigation with a 99 kg/ha N rate, at values of 0.622 and 0.621 in 2021 and 2022, respectively. Controlled irrigation (CI) with an appropriate reduction in the N rate increased the proportion of productive tillers, the transfer rate of dry matter and non-structural carbohydrates (NSCs), the sugar–spikelet ratio, the grain–leaf ratio, and the leaf area index (LAI) during the heading–flowering stage. A subsequent analysis indicated that the main reason for the increase in the HI was the increase in the sugar–spikelet ratio during the heading–flowering stage. A high HI increased the rice yield by increasing the thousand-grain weight. The present study suggested that water-saving irrigation regimes and appropriate N reduction not only led to water and fertilizer resource savings but also improved agronomic characteristics during rice growth and enhanced transport capacity. Thus, these practices improved the rice HI and have enormous potential for increasing yield. Therefore, regulating the rice HI through water and N management methods should be considered an important strategy for improving rice yield.

Biochar amendment is an effective strategy for mitigating methane (CH ₄ ) emissions in paddy fields. However, the long-term efficacy of different biochar amendment strategies for mitigating CH ₄ emissions remains unclear, particularly under different water management regimes. To address this knowledge gap, a 5-year (2018–2022) field experiment was conducted to determine CH ₄ production potential, CH ₄ oxidation potential, and CH ₄ emissions in paddy fields under six treatments: flooded irrigation (F) without biochar amendment (FB0), F with once biochar amendment (12.5 t ha ⁻¹ , FB1), F with continuous biochar amendment (2.5 t ha ⁻¹ year ⁻¹ , FB5), controlled irrigation (C) without biochar amendment (CB0), C with once biochar amendment (12.5 t ha ⁻¹ , CB1), and C with continuous biochar amendment (2.5 t ha ⁻¹ year ⁻¹ , CB5). Additionally, random forest analysis and structural equation modeling (SEM) were used to elucidate interaction pathways among biochar amendment, water management, CH ₄ production and oxidation potentials, and key soil properties affecting CH ₄ emissions. In the first year, once biochar amendment demonstrated optimal CH ₄ mitigation efficacy under different water management regimes, reducing cumulative CH ₄ emissions by 18.87–36.32% compared to other treatments. However, this mitigation effect diminished progressively over 5 years under different water management regimes, with the most rapid decline occurring under C. Consequently, there was no significant difference in the 5-year cumulative CH ₄ emissions between FB1 and FB5, while CB5 achieved a significant 29.32% reduction in 5-year cumulative CH ₄ emissions compared to CB1. Random forest analysis and SEM identified soil redox potential (Eh), dissolved organic carbon (DOC), and ammonium nitrogen ( $$\text{NH}^{+}_{4}\text{-N}$$ NH 4 + -N ) as key soil properties driving CH ₄ emissions. CB5 maintained higher soil Eh and $$\text{NH}^{+}_{4}\text{-N}$$ NH 4 + -N levels with lower DOC, leading to the lowest CH ₄ production potential and highest CH ₄ oxidation potential over 5 years, ultimately achieving the greatest CH ₄ mitigation. Furthermore, CB5 achieved the lowest CH ₄ -attributed greenhouse gas intensity and net greenhouse gas emissions over 5 years. These findings highlight the necessity of long-term monitoring to accurately assess the effects of biochar amendments on CH ₄ mitigation and offer technical guidance for implementation of biochar strategies in paddy fields under different water management regimes. Highlights Water-saving irrigation rapidly diminished the CH ₄ mitigation effect of the once biochar amendment. Continuous biochar amendment combined with water-saving irrigation was the most effective strategy for CH ₄ mitigation over 5 years. Soil Eh, DOC, and $$\text{NH}^{+}_{4}\text{-N}$$ NH 4 + -N were key drivers of CH ₄ emissions over 5 years. Graphical Abstract

To improve nitrogen use efficiency (NUE) during rice cultivation, it is essential to comprehend the morphological and physiological traits of rice roots. However, in high-fertility black soil regions of Northeast China, the effects of combining biochar application with water-saving irrigation (WSI) conditions on rice root development and nitrogen utilization are still unknown. To address this knowledge gap, a combination of field experiments and ¹⁵N tracer micro-area investigations was conducted in this study. Four treatments were implemented: (i) controlled irrigation without biochar application (CB0); (ii) controlled irrigation with 2.5 t ha⁻¹ biochar application (CB1); (iii) controlled irrigation with 12.5 t ha⁻¹ biochar application (CB2); and (iv) controlled irrigation with 25 t ha^–1 biochar application (CB3). Flooded irrigation conditions without biochar treatment (FB0) were used as the control. The primary objective of this research was to identify the mechanisms by which combined WSI conditions and biochar application affect rice root development and nitrogen utilization. Biochar application enhanced rice root morphological and physiological characteristics. Optimal biochar application increased the longest root length (RL), root volume (RV), root fresh weight (RFW), root active absorption area, root bleeding intensity, and root activity (RA) of rice while also optimizing the root–shoot ratio and facilitating nitrogen absorption by roots. These changes in root morphological and physiological characteristics facilitated the absorption of fertilizer-¹⁵N and soil nitrogen by rice roots, ultimately leading to improvements in rice yields and NUEs. Notably, the rice yields, NUE, nitrogen agronomic efficiency (NAE), and nitrogen partial factor productivity (NPFP) of CB2 plants were 16.45%, 39.42%, 24.48%, and 16.45% higher than those of FB0 plants, respectively. These results highlight the effectiveness of biochar application as a strategy to ensure food security and enhance NUE under WSI conditions. Furthermore, this study suggests that the recommended optimal application amount of biochar for the black soil area of Northeast China is 12.5 t ha⁻¹.