Biochar buffers yield penalties under nitrogen reduction in arid irrigated cotton by improving soil functional status and nitrogen-use efficiency

Reducing nitrogen (N) inputs without sacrificing yield is a major challenge for sustainable cotton production in arid irrigated regions, where salinity risk and soil functional decline frequently constrain crop performance. Although biochar is widely regarded as a promising soil amendment, its agronomic performance in high-input cotton systems remains insufficiently resolved, particularly with respect to whether it primarily increases yield per se or helps maintain yield when N input is reduced. Here, we conducted two-year field experiment in northern Xinjiang, China, using a split-plot design with biochar (0 and 10 t ha -1 ; applied once before sowing in 2023) as the main-plot factor and three N rates (0, 240, and 360 kg N ha -1 ) as subplots under a mulched drip-irrigated cotton system. We quantified soil physicochemical and biological properties, enzyme activities, soil quality index (SQI), seed-cotton yield, partial factor productivity of applied nitrogen (PFPN), and economic return. Biochar consistently reduced soil electrical conductivity by 17% and increased gravimetric soil water content by 10%, and these changes were accompanied by improved soil functional status. Without biochar, reducing N from 360 to 240 kg ha -1 decreased seed-cotton yield by 13.7–29.5%. With biochar, however, the reduced-N treatment-maintained yields statistically comparable to the conventional N rate. In 2023, biochar combined with reduced N produced the highest SQI (0.54), 22.1% higher than that of the corresponding no-biochar treatment. Economic benefits showed a delayed response, with biochar increasing net return by 25.4%–33.9% in the second year under reduced- and zero-N treatments despite lower returns in the application year due to the initial amendment cost. Random forest and structural equation modeling further indicated that biochar-related improvements in soil physical and enzymatic domains were closely associated with higher SQI, improved PFPN, and yield stability. Overall, biochar primarily served to buffer yield penalties under moderate N reduction rather than to enhance yield additively under high N input. These findings provide field-based evidence that improving soil functional status can support input-saving cotton production in arid irrigated systems.