Coupling nitrogen and irrigation under high-retention film mulching regulates plant growth, forage quality, and soil nutrient and carbon dynamics in maize

Introduction Film mulching, nitrogen input, and irrigation interactively regulate crop productivity and soil carbon sequestration in arid maize systems, yet their combined effects on aboveground–belowground growth, forage quality, and soil carbon fraction dynamics remain insufficiently understood. Methods A two-year field experiment was conducted in the Hexi Corridor of China using two film mulching types, three nitrogen application levels, and two irrigation regimes. We systematically evaluated maize yield, belowground biomass, plant and root traits, forage quality, soil physicochemical properties, and soil organic carbon fractions. Results After two consecutive years, high-retention film increased yield by 9.6% relative to low-retention film, but reduced belowground biomass under equivalent water and fertilizer conditions. Under deficit irrigation, reduced fertilization decreased yield but increased belowground biomass, indicating enhanced carbon allocation to roots under resource limitation. Root traits were more responsive than shoot traits to water–fertilizer coupling, with moderate fertilization promoting greater root length and root volume. Fertilization was the main driver of forage quality and soil nutrient status, whereas irrigation mainly affected acid detergent fiber (ADF), and neutral detergent fiber (NDF), and available nutrient pools. Compared with the conventional control treatment (T7), T4 increased Soil Total Nitrogen (STN) by 4.6%, Total Phosphorus (STP) by 7.6%, and Available Nitrogen (SAN) by 13.7%. High-retention film also promoted soil carbon accumulation, with Soil Organic Carbon, Particulate Organic Carbon (POC), and Mineral-Associated Organic Carbon (MAOC) under T4 increasing by 7.1%, 17.4%, and 4.5%, respectively, relative to T7. Under moderate fertilization, no significant differences were detected between T5 and T7, suggesting that moderate nitrogen reduction under high-retention film can maintain soil fertility and carbon stocks. Redundancy analysis further identified SAN, STP, and MAOC as the major factors driving variation in maize productivity and belowground biomass. Conclusions Overall, high-retention film improved maize yield, soil fertility, and soil carbon accumulation more effectively than low-retention film under comparable water and nitrogen inputs, while moderate urea reduction under high-retention film maintained productivity and soil fertility, highlighting its potential as a resource-efficient strategy for sustainable arid maize production.