Water and Nitrogen Management Drive Soil Nutrient Dynamics and Microbial–Enzyme Activity in Silage Maize Systems in Northwest China

Efficient water and nitrogen management is essential for maintaining soil fertility and achieving sustainable agricultural production, especially in arid oasis regions where soil degradation and nutrient loss are common challenges. However, the interactions between irrigation regimes, nitrogen application, and soil biological processes in such environments remain insufficiently understood. This study investigated the effects of water and nitrogen management on the sustainability of sandy soil nutrients within the context of the sustainable development goals during silage maize cultivation in the oasis irrigation area of the Hexi Corridor, Northwest China. Four irrigation regimes and five nitrogen management regimes were tested. The results indicate that ammonium nitrogen (NH4+-N) varied significantly during the jointing stage (W4 treatment), ranging from 3.52 to 16.38 mg/kg (p < 0.05). Nitrate nitrogen (NO3−-N) exhibited significant differences during the tasseling stage (W1 treatment), with a range of 6.16–21.58 mg/kg (p < 0.05). Soil total phosphorus (STP) gradually declined from early to late growth stages, ranging from 0.20 to 0.97 g/kg. Regarding enzyme activity, alkaline phosphatase (ALP) increased progressively throughout the growth period, with a range of 0.02–0.14 mg/g/d, while urease (URE) showed a declining trend, ranging from 0.25 to 0.66 mg/g/d. Water management exerted a significant negative effect on soil enzyme activity (p < 0.05), while nitrogen fertilization had a minimal impact on soil microbial communities (p > 0.05). Growth stage and irrigation regime are key regulators of the soil–microbe–enzyme activity system. The crop’s nutrient demand cycles and microbially mediated nutrient transformations exhibited strong dependence on growth stage. Enzyme activity is notably and positively affected by nitrogen inputs and plant developmental stages, while microbial biomass is mainly regulated by soil C, N, and P contents and enzyme activities. These findings provide a scientific basis for implementing water-saving irrigation and high-efficiency fertilization strategies in oasis agricultural systems.