SPAD Chlorophyll Meter-Based Real-Time Nitrogen Management in Manure-Amended Lowland Rice

Precise adjustment of timing and dosage of fertilizer nitrogen (N) under real-time nitrogen management (RTNM) is crucial to maximize benefits. The study aimed to assess and recommend the best combination of threshold SPAD-502 chlorophyll meter reading (SCMR) and top dressing N rate for lowland flooded rice, amended with farmyard manure 5 t ha−1, to maximize yield, N use efficiency, and economic gains. In optimization trial, four SCMR-based N scheduling (SCMR34, SCMR36, SCMR38, SCMR40), each with three top dressing N rates (15, 20, 25 kg N ha−1) as RTNM treatments, were evaluated for crop growth and yield, nutrient use efficiency, simulated N2O emission, and economics, comparing with fixed-time N management (FTNM) (100 kg N ha−1, 4 splits). Subsequently, the best-fit SCMR treatment was validated across seven rice cultivars. Top dressing of 20 kg N ha−1 at ≤ SCMR 38 (SCMR38N20) led to a higher yield (+ 8%), N use efficiency (+ 43%), net return (+ 11%), and reduced N input (− 18%) compared to FTNM. The yield advantage with SCMR38N20 was attributed to higher internal N use efficiency and optimal source–sink balance (higher harvest index). In validation trial, the best-fit RTNM treatment (SCMR38N20) showed yield advantage (+ 7%), but significant for four out of seven cultivars. In lowland rice production, the RTNM with top dressing of 20 kg N ha−1 at ≤ SCMR38 can increase yield of medium duration cultivar (Rajendra Swata) and N use efficiency. However, cultivar-specific adjustments to the SCMR threshold and N rate are equally important for significant yield advantage