High-density planting is an effective technique to optimize yields of mulched cotton. On the other hand, deficit irrigation is an emerging water-saving strategy in cotton cultivation, especially suitable for arid and water-scarce areas. However, the relationships between deficit irrigation, high-density planting, and regulation mechanisms of canopy light radiation and light use efficiency (LUE) in cotton is not yet clear. To clarify the mechanism of light interception (LI) and the LUE of cotton canopies, three irrigation treatments [315 (50% Fc), 405 (75% Fc, farmers’ irrigation practice), and 495 mm (100% Fc), where Fc was the field capacity] with three plant densities [13.5, 18.0 (farmers’ planting practice), and 22.5 plants m²] were applied. The findings of this research revealed that, under deficit irrigation, the above-ground dry matter (ADM) was reduced by 5.05% compared to the farmers’ irrigation practice. Over both years and across all plant densities, LI and LUE under deficit irrigation decreased by 8.36% and 4.79%, respectively, relative to the farmers’ irrigation practices. In contrast, LI and LUE for the highest irrigation level increased by 10.59% and 5.23%, respectively. In the case of the interaction (plant density and irrigation level), the ADM under deficit irrigation and high-density combination increased by 7.69% compared to the control (farmers’ irrigation × sowing practices interaction effects). The LI and LUE also exhibited an increase in 1.63% and 6.34%, respectively. Notably, the LI effect of the middle and upper cotton canopy under film drip irrigation reached 70%. A lower irrigation level resulted in a higher percentage of LI in the lower canopy region. The leaf area index, light interception rate, and extinction coefficient escalated with the increase in plant density. Under deficit irrigation treatment, the LI of the 0–30 cm canopy in high plant density settings increased by 8.6% compared to the control (farmers’ irrigation × sowing practices interaction effects). In conclusion, deficit irrigation and increased plant density improved the interception of LI and LUE of cotton canopy. These findings may help the farmers to optimize their agricultural management strategies in water-deficient areas.

The optimization of plant density plays a crucial role in cotton production, and deficit irrigation, as a water-saving measure, has been widely adopted in arid regions. However, regulatory mechanisms governing nitrogen absorption, transportation, and nitrogen use efficiency (NUE) in cotton under deficit irrigation and high plant density remain unclear. To clarify the mechanisms of N uptake and NUE of cotton, the main plots were subjected to three irrigation amounts based on field capacity (Fc): (315 [W1, 0.5 Fc], 405 [W2, 0.75 Fc, farmers’ irrigation practice], and 495 mm [W3, 1.0 Fc]). Subplots were planted and applied at three densities: (13.5 [M1], 18.0 [M2, farmers’ planting practice], and 22.5 [M3] plants m⁻²). The results revealed that under low-irrigation conditions, the cotton yield was 5.1% lower than that under the farmer’s irrigation practice. In all plant densities and years, the nitrogen uptake of cotton increased significantly with the increase in irrigation. However, excessive irrigation resulted in nitrogen accumulation and migration, mainly concentrated in the vegetative organs of cotton, which reduced the NUE by 9.2% compared with that under farmers’ irrigation practice. Concerning the interaction between irrigation and plant density, under low irrigation, the nitrogen uptake of high-density planting was higher, and the yield of seed cotton was only 2.9% lower than that of the control (the interaction effect of farmers’ irrigation × plant density), but the NUE was increased by 10.9%. Notably, with the increase in irrigation amount, the soil nitrate nitrogen at the 0–40 cm soil layer decreased, and high irrigation amounts would lead to the transfer of soil nitrate nitrogen to deep soil. With the increase in plant density, the rate of nitrogen uptake and the amount of nitrogen uptake increased, which significantly reduced the soil nitrate nitrogen content. In conclusion, deficit irrigation and high plant density can improve cotton yield and NUE. We anticipate that these findings will facilitate optimized agricultural management in areas with limited water.

Introduction: Plastic mulch is widely used in agriculture as it increases soil temperature and reduces water evaporation, thereby enhancing water use efficiency and crop yields. However, prolonged use of plastic mulch leaves residues that negatively impact soil structure, impede water and salt movement, and reduce cotton yields. While the impact of mulch contamination on water-salt distribution is recognized, the specific mechanism by which increasing the C/N ratio improves this distribution remains unclear. We hypothesize that by raising the C/N ratio, it is possible to enhance the water-salt environment, thus stabilizing lint cotton yields. Methods: A two-year field experiment (2024–2025) was conducted in Xinjiang, China. Three irrigation levels were established based on crop evapotranspiration (ETc): 0.6 ETc (I1, severe deficit), 0.8 ETc (I2, mild deficit), and 1.0 ETc (I3, full). Four C/N ratio treatments were applied: 0:1 (F0), 3:1 (F1), 6:1 (F2), and 9:1 (F3). Results: Across irrigation levels, increasing the C/N ratio of the fertilizer significantly enhanced soil water-holding capacity, reduced salt accumulation, and improved water and salt uniformity. Yield under C/N 6:1 (F2) was 7.9% higher on average than other treatments. Under different C/N ratios, mild deficit irrigation (I2) maintained higher soil moisture uniformity and desalination rate(Rs); soil salinity was 5.4% lower than under I1 but 9.5% higher than under I3; seed cotton yield was 17.0% and 0.8% higher than I1 and I3, respectively. Compared with I3F0, I2F2 increased soil moisture by 4.2%, moisture uniformity by 4.1%, reduced salinity by 1.1%, and raised yield by 9.1%. Correlation analysis showed desalination rate, moisture content, and its uniformity coefficient were significantly positively correlated with lint yield, while soil salinity was significantly negatively correlated. Discussion: A C/N ratio of 6:1 effectively improves the water-salt balance in plastic mulch-contaminated cotton fields by optimizing soil water and salt distribution, thereby increasing yield. This provides mechanistic insight and practical strategies for water-saving and salt-suppression management in arid regions.