Nitrogen (N), typically supplied through fertilisers, is essential for enhancing agricultural productivity, but over-fertilisation-particularly in soils with high initial soil mineral nitrogen (NISM)-can lead to nutrient pollution of both soil and water. In regions with shallow groundwater, optimising N application is essential yet understudied, particularly in balancing environmental protection and yield maximisation. To address this gap, field experiments were conducted in China's Mu Us Sandy Land in 2019 and 2021, with a pause in 2020. The 2019 study evaluated the effects of a 200 kg N ha-1 fertilisation rate on soil water dynamics, N behavior, and spring maize growth, whereas the 2021 drip-irrigated trial tested a base rate of 73 kg N ha-1 supplemented with six additional N rates (0, 100, 150, 200, 250, and 300 kg N ha-1). Using the collected field data, a WHCNS soil-crop model was developed, calibrated with 2019 data-including soil moisture, soil N concentration, and leaf area index-from the 200 kg N ha-1 treatment, and validated across all 2021 treatments. The model, highly sensitive to crop parameters, was further optimised using PEST to improve accuracy and then used to simulate the impacts of various water and N management strategies on water use, N fate, and crop growth under shallow groundwater conditions. Simulations revealed that high NISM levels reduced the benefits of additional N for maize yield and resource use efficiency, whereas low NISM conditions responded positively to increased N applications. An optimal N application rate of 200-250 kg N ha-1, paired with a total water input of 473-516 mm, was identified as the most effective for maximising yield while minimising water and N losses.