Bridging rice yield gaps through integrated nutrient management in the Northeastern Hill region of India: Field study evidence and 33-year APSIM evaluation

Context and objective The Northeastern Hill (NEH) region of India, a national priority zone for sustainable agricultural intensification, faces persistently low rice productivity driven by suboptimal nutrient management, highly acidic soils with spatially heterogeneous soil organic carbon (SOC), and increasing climate variability. In these regions yield gaps and their interaction with SOC status remain unquantified, and long-term impacts of integrated nutrient management (INM) on productivity, SOC dynamics, and drought resilience across soil carbon gradients are limited in the literature. This study aimed to quantify rice yield gaps for contrasting cultivars across SOC gradients, evaluate INM strategies that simultaneously sustain yields and build SOC, and assess drought resilience under supplemental irrigation and INM. Methods Field experiments conducted at ICAR-NEH, Umiam, Meghalaya, using two contrasting rice cultivars, the high-yielding Shahsarang and the traditional landrace Mendri, were used to calibrate and validate APSIM-Oryza. Model parameters were calibrated using automated PEST optimisation. Seven nutrient management scenarios, ranging from baseline farmer practice to integrated organic-mineral nutrient management, were simulated over a 33-year period under three hypothetical soil organic carbon (SOC) classes representing contrasting SOC levels, as well as at the experimental field site for each cultivar. Integrated nutrient management scenarios included 50–75% of the recommended nitrogen (N) dose combined with 5–10 t/ha farmyard manure. Long-term meteorological trends were assessed using the Mann-Kendall test and Sen's slope estimator, while drought years were identified using the Standardized Precipitation Index (SPI). Results and conclusions Meteorological analysis revealed intensifying rainfall (+67.9 mm/yr), and asymmetric nighttime warming (+0.023°C/yr). Simulated nitrogen-limited yield gaps ranged from 1.6 to 3.1 t/ha (40–57% of Y w /Y p ) in the experimental field to 3.7 t/ha (68%) under low-SOC (C1) conditions, with SOC-driven nitrogen mineralization emerging as the primary modelled determinant of actual farmer yields under no-input conditions. INM combining 75% recommended N with 5 t/ha farmyard manure maintained grain yields comparable to 100% RDN while increasing SOC by 2.7–6.2% relative to 100% RDN after 33 years of simulation. Supplemental irrigation increased drought-year yields by 6–17%, while INM reduced drought-year yield losses by 5.4–14% in low-SOC soils. Yield responses to additional inputs diminished progressively in high-carbon soils, consistent with SOC saturation dynamics. Significance This study provides the simulation-based, multi-scenario assessment of rice yield gaps and long-term SOC dynamics across soil carbon gradients in the NEH region. The finding that INM simultaneously closes yield gaps, reduces SOC depletion, and strengthens drought resilience within a practically feasible intervention, and that optimal management differs systematically by soil carbon class, offers a quantitative basis for spatially differentiated extension recommendations across the heterogeneous smallholder rice landscapes of similar agroecosystems.