Straw return as an adaptive strategy to offset soil C and N depletion and increase maize yield in long-term film mulching systems

Film mulching systems increase crop yield across extensive drylands, but long-term application often tends to progressively deplete soil carbon (C) and nitrogen (N), threatening yield potential and sustainability. Whether—and how quickly—straw return can offset this depletion under varying mulching intensities remains unclear. A 9-year field experiment on China’s Loess Plateau thus evaluated straw return effects on soil hydrothermal dynamics, soil organic carbon (SOC) and total nitrogen (TN) characteristics, and maize yield under three mulching intensities (quantified by mulch ratio): no mulching, half-film mulching, and full-film mulching. In years 8–9, film mulching alone (without straw return) increased grain yield by 57.1%–59.3% relative to no mulching, but caused declines in SOC, TN, and the C/N ratio by 27.1%–43.2%, 21.2%–34.5%, and 6.4%–12.5%, respectively, with depletion trend intensified with increasing mulching intensity. Continuous straw return, initiated three years before the experiment ended, substantially improved soil quality and constrained SOC and TN declines to 19.0%–29.9% and TN decline to 15.9%–27.3%, respectively. These offsetting effects also enhanced with mulching intensity as soil hydrothermal regulation was progressively strengthened, fully offsetting the C/N ratio decline under full-film mulching. Moreover, straw return further enhanced maize yield under film-mulching, raising the yield gain over the control (no mulching without straw return) from that under mulching alone to 67.8%–79.2%. Overall, 3-year straw return demonstrated the potential to offset soil C and N depletion while further increasing maize yield in 6-year film mulching systems, though full offsetting was predicted to require at least 2–3 additional years. These findings support adaptive sustainable soil management in extensively film mulching systems.