Effects of Irrigation Amount and Salinity Levels on Maize (Zea mays L.) Growth, Water Productivity and Carbon Emissions in Arid Region of Northwest China

To address the growing scarcity of freshwater resources, the use of saline water for agricultural irrigation is gaining increasing attention. This study presents findings from a two-year field experiment conducted during the 2023 and 2024 maize-growing seasons in northwestern China. The objective of the experiment was to evaluate the main and interactive effects of saline irrigation water on soil water–salt dynamics, maize growth, photosynthetic characteristics, water productivity, and carbon emissions. The experiment involved nine treatments with three irrigation amounts: 4500 m3 hm−2 (W1), 5625 m3 hm−2 (W2), and 6750 m3 hm−2 (W3), combined with three water salinity levels: 0.85 g L−1 (S1, freshwater), 3 g L−1 (S2), and 5 g L−1 (S3). Results indicated that both irrigation amount and salinity level significantly affected water–salt dynamics, with more soil accumulating in the 0–100 cm soil layer under saline irrigation water; however, this effect diminished with higher irrigation amounts. The maximum leaf area index and plant height were affected by both the irrigation amount and salinity level, as well as their interaction. Photosynthetic capacity declined with increasing salinity of the irrigation water, ultimately reducing grain yield and irrigation water use efficiency. Compared to freshwater (S1), the average maize grain yield under S2 and S3 treatments decreased by 6.28% and 15.43% in 2023 and by 7.82% and 17.48% in 2024, respectively. Additionally, for the same irrigation amount, higher salinity of the irrigation water (S2, S3) significantly reduced total soil CO2 emissions, with reductions of 10.08% and 27.53% in 2023, and 11.97% and 28.01% in 2024, respectively. In summary, to optimize the utilization of saline water, enhance maize yield, and improve soil carbon sequestration, we recommend maintaining the salinity of irrigation water below 3 g L−1, and using an irrigation amount of 6750 m3 hm−2 (W3S2) for optimal outcomes in the study area.