Irrigation and nitrogen fertilization are key practices for stabilizing crop production in semi-arid croplands. However, how water-nitrogen coupling alters soil-microbial stoichiometry and thereby coordinates microbial carbon metabolism and adaptive root growth remains poorly understood. Based on a two-year in-situ field experiment in the Guanzhong Plain, we measured soil and microbial C:N:P stoichiometry, extracellular enzyme activities, and root traits, and used path analysis to identify pathways regulating carbon use efficiency (CUE) and root morphology. Water and nitrogen inputs significantly increased soil nutrients and microbial biomass. Under the W2N3 treatment (70-85% FC, 180 kg hm-2), microbial C:P and N:P ratios reached their maxima, increasing by 35% and 58%, respectively. Water and nitrogen inputs alleviated microbial nutrient limitation. Under W2N3, vector length (VL) decreased by 11%, vector angle (VA) increased by 15%, and CUE increased by 12%. Water and nitrogen inputs promoted root development, and the W2 treatment markedly optimized root distribution in the 0-20 cm soil layer. Path analysis further showed that variation in CUE was closely correlated with nutrient limitation-related indices, while changes in root architecture were more strongly influenced by the indirect effect of microbial stoichiometry ((3 = 0.71) than by direct effects of water and nitrogen (both (3 = 0.50). Overall, W2N3 optimized soil-microbial stoichiometry and coordinated microbial metabolic efficiency and root growth, providing a process-based framework for understanding belowground ecological regulation in semi-arid areas.