Organic Fertilizer Type and Rate Regulate Rice Canopy Function, Nitrogen Use Efficiency, and Yield Formation

Balancing rice yield and nitrogen use efficiency (NUE) under reduced nitrogen (N) input remains challenging. This study aimed to evaluate effects of fertilizer type and N rate on canopy function, yield formation, and NUE; examine source–sink relationships; and identify strategies balancing yield and NUE. A two-year field experiment (2024–2025) compared chicken manure (CM), pig manure (PM), and rapeseed cake fertilizer (RCF) at four N rates (135, 180, 225, and 270 kg N ha−1), with conventional chemical fertilization at 270 kg N ha−1 as the control. Across both years, RCF increased grain yield by 8–9% at 135 and 180 kg N ha−1 versus CM and PM, while maintaining higher leaf area index, SPAD, and canopy source capacity from heading to mid-grain filling. Grain yield was strongly and positively correlated with population spikelet number (r = 0.881, p < 0.001). Partial least squares structural equation modeling (PLS-SEM; 2025) indicated that RCF positively affected canopy source capacity (1.807 ***), which in turn positively influenced nitrogen accumulation and translocation (0.441 ***), strengthening source–sink coordination (0.512 ***). The analytic hierarchy process (AHP; 2025) showed superior integrated performance of RCF treatments for yield and NUE. These results suggest that C2 (RCF supplied at 180 kg N ha−1) may help improve rice yield and NUE in the Huaibei region.