Contrasting residual effects of different biochar types on maize nitrogen uptake, biomass accumulation, water and nitrogen use efficiency under alternate partial root-zone drying irrigation

Although the immediate benefits of biochar in enhancing nitrogen cycling and crop productivity are well documented, its residual effects across different biochar types and irrigation regimes over successive growing seasons have not been fully elucidated. Here, we assessed the residual effects of softwood (SWB) and wheat-straw (WSB) biochar on soil–plant nitrogen (N) dynamics and maize (Zea mays L.) productivity over two growing seasons following a one-time application. Experiments were conducted in 2021 and 2022 under full (FI), deficit (DI), and alternate partial root-zone drying (APRI) irrigation. In both years, despite limited changes in water consumption and total N uptake, WSB-APRI combination improved total dry biomass (+ 13.5%), harvest index (+ 4.4%), water use efficiency (+ 26.7%), and N use efficiency (+ 10.3%). These improvements were linked to enhanced microbial activity (+ 26.8–51.2%) and soil N availability (+ 4.8–13.2%), which stimulated root growth (+ 7.4–22.7%) and N uptake (+ 7.0–17.8%) under water stress. However, under reduced irrigation in 2021, SWB markedly suppressed microbial respiration (− 42.4%) and N availability (− 29.2%), which in turn led to compromised crop performance, particularly under DI. Partial least squares path modeling revealed that microbial activity and root traits indirectly affected maize water and N use efficiency by influencing water consumption, N uptake, and biomass accumulation. Notably, excessive N uptake reduced N use efficiency, whereas biomass accumulation enhanced it. Considering the residual effects of biochar, APRI combined with WSB offers a promising approach to continuously enhance water-nitrogen coordination and maintain maize productivity under limited irrigation.