Sustained Post-Anthesis Source Supply Underpins the Biochar-Induced Shift in Grain Yield Response to Nitrogen in Irrigated Winter Wheat

Whether biochar alters the nitrogen (N) response of irrigated winter wheat by coordinating the grain-sink establishment with post-anthesis source supply remains unclear. Two independently randomized field trials compared 0 and 20 t ha−1 biochar (B0 and B20) across eight N rates (0–350 kg N ha−1 in 50 kg increments). Biochar interacted significantly with the N rate. B20 significantly increased the yield at 200–350 kg N ha−1 but not at 0–150 kg N ha−1. It raised the fitted yield-maximizing N rate by 55–57 (95%CI 34.9–80.0) kg N ha−1 and the predicted maximum yield by 1.0 (95%CI 0.70–1.29) t ha−1. The grain number (GN) increased with N but was unaffected by biochar. In contrast, B20 maintained a greater grain weight at 150–350 kg N ha−1 and increased post-anthesis dry matter production (PostDM) by 11.0–43.5% at 200–350 kg N ha−1, without affecting the apparent pre-anthesis dry matter translocation. Under a high N, a greater PostDM coincided with enhanced post-anthesis N uptake, canopy persistence, and water-use efficiency. Integrated analyses highlighted associations of yield with the estimated GN and PostDM. Treatment means showed concurrent increases in GN and PostDM through 200 kg N ha−1 under B0 and 250 kg N ha−1 under B20, followed by increasing GN and declining PostDM at higher tested rates. Biochar raised the yield ceiling by sustaining the post-anthesis source supply for an N-expanded grain sink rather than by increasing the sink capacity at a given N rate.