Straw-derived biochar optimizes water consumption, shoot and root characteristics to improve water productivity of maize under reduced nitrogen

Optimizing water and nitrogen (N) utilization to enhance crop yields under resource constraints is crucial. Straw and its biochar, combined with N fertilizer, are commonly used to improve soil carbon storage and crop growth. However, the effects of straw and N fertilizer management on water consumption, root and shoot characteristics, N uptake, and maize productivity remain unclear. To address this knowledge gap, a three-year (2019–2021) field experiment was conducted in Northwest China. We compared two straw incorporation methods [straw (SI) and straw-derived biochar (BI)] with straw removal (NI) at four N application rates [0 (N0), 225 (N225), 300 (N300), and 375 kg ha–1 (N375)]. Results indicated that compared with NI, both SI and BI significantly increased grain yield (GY), N uptake, and water productivity (WP) (SI < BI; P < 0.05). The maximum GYs were achieved with SIN300 and BIN225, respectively. Notably, compared with SIN300, BIN225 significantly enhanced GY by 10.8% and 5.8% and improved WP by 19.2% and 9.9% (P < 0.05). This improvement was mainly attributed to the increased water consumption after tasseling and crop transpiration (T) in evapotranspiration (ET). Furthermore, N application resulted in increased root distribution in shallow soil layers (0–0.3 m). Under BIN225, roots exhibited a longer, thinner and deeper profile, minimizing root redundancy and enhancing root efficiency in water and N absorption during the reproductive stage of maize. In contrast, SIN300 resulted in shorter, thicker, and shallower roots, leading to a reduced shoot-root ratio of 12.2% (P < 0.05). Based on the normalization and fitting curves, BI combined with reduced N (240 kg ha–1) improved WP by 24.5%, achieving 98.7% of the maximum GY for drip-irrigated maize (16.98 Mg kg–1). Overall, these findings provide a novel straw strategy for sustainable field management in arid irrigation agriculture and similar ecosystems.