Interactive Effects of Irrigation Amount, Irrigation Salinity, and Nitrogen Rate on Soil Enzyme Activities and Maize...

Brackish water and fertilizer must be used more efficiently to sustain maize production in arid regions affected by freshwater scarcity and soil salinization. Yet, the coupled effects of irrigation amount, irrigation salinity, and nitrogen application on soil biochemical processes and maize productivity under mulched drip irrigation remain unclear. We conducted a two-year field experiment (2023-2024) in a sandy loam field in arid northwestern China to evaluate irrigation amount (450 mm, W1; 675 mm, W2), irrigation salinity (1.3 dS m^-1, S1; 3.5 dS m^-1, S2; 5.7 dS m^-1, S3), and nitrogen rate (225 kg ha^-1, N1; 330 kg ha^-1, N2; 435 kg ha^-1, N3). The experiment measured four soil enzyme activities, soil physicochemical properties, and maize yield. Higher irrigation increased soil water storage by about 27%, but it also increased salt accumulation by about 9% across the two seasons. Increasing irrigation salinity from S1 to S3 reduced urease, alkaline phosphatase, protease, and sucrase activities by 8.52%, 6.72%, 13.75%, and 3.65%, respectively, suggesting weaker nutrient turnover and uptake under salinity stress. By contrast, the higher irrigation amount (W2) increased these activities by 10.27%, 9.73%, 11.08%, and 9.47%, respectively. Structural equation modeling indicated that soil salt accumulation constrained yield mainly by reducing grain number rather than 1000-grain weight. Although higher irrigation increased mean yield, the integrated evaluation identified W1S2N2 as the best combination. This regime produced the highest grain yield (18,355.76 kg ha^-1) and irrigation water productivity (40.79 kg mm^-1), which were 82.17% and 155.26% higher than the corresponding lowest values, while maintaining relatively high partial factor productivity of nitrogen (55.62 kg kg^-1) in 2024. These findings suggest that controlled deficit irrigation (450 mm) and moderate nitrogen input (330 kg ha^-1) are recommended for long-term sustainability in arid regions, but longer-term salinity monitoring is required before a broad recommendation can be made when using brackish water irrigation.