In subtropical wetland ecosystems, climate change can alter both the composition of nitrogen (N) deposition and the levels of soil water, affecting nutrient acquisition. Arbuscular mycorrhizal fungi (AMF) can regulate the effects of soil water availability and the NH₄⁺: NO₃⁻ ratio on plant N acquisition. However, exactly how N acquisition is affected by soil water content and AMF symbiosis remains unclear. We transplanted the dominant wetland species, Carex thunbergii, collected in freshwater wetlands in our study area in southern China, into microcosms in a greenhouse experiment. In this context, we were able to manipulate both the amount of water supply and the NH₄⁺: NO₃⁻ ratio in the soil, combined with and without AMF symbiosis. Three months after our treatments were imposed, we examined N-uptake rate, along with plant growth and functional traits, in response to these experimental manipulations. Soil water content dominated the level of plant growth, whereas variations in plant functional traits (e.g., specific root length, SRL, and specific leaf area, SLA) could largely be ascribed to soil NH₄⁺: NO₃⁻ ratio. Collectively, plant-AMF symbiosis enhanced plant N acquisition under conditions of variable water availability, for instance promoting plant N-uptake rate more at lower level of soil water (30%) compared to higher level of soil water(70%), and also when N was applied at a 3:1 of NH₄⁺: NO₃⁻ ratio compared to other ratios. SLA was significantly negatively correlated with SRL, and thus indirectly contributed to N acquisition. However, mycorrhizal suppression decoupled the coordination between leaf and root traits, mainly affecting SRL, rather than SLA, which remained rather stable. The correlations between root and leaf traits, adjusted by AMF symbiosis, were crucial for plant N use strategies.