Hydraulic activation of stomata (HAS), driven by hygroscopic salt films on leaf surfaces, can create an additional pathway for water loss beyond conventional stomatal vapour diffusion, yet this process remains largely overlooked in studies of plant–atmosphere interactions. We conducted the first field-based assessment of HAS-related responses following foliar interception of hygroscopic urea ammonium nitrate (UAN) fertilizer, using eddy covariance (EC) data from five cropland sites (2005–2021). During the 10-day period following UAN application, vegetation conductance (G_veg) increased significantly across barley, winter wheat and maize, while significant increases in the marginal water cost of carbon gain (λ) and stomatal sensitivity (G₁) were observed in barley and winter wheat. This pattern could not be attributed to changes in canopy microclimatic conditions or short-term crop growth, and was not observed with alternative liquid mineral fertilizers. Gross primary productivity (GPP) and inferred transpiration (T) also increased following UAN application, with generally greater responses in T than in GPP in most crop–site combinations. The results suggest that UAN-induced HAS alters crop carbon–water exchange at the ecosystem scale. Neglecting HAS in current models may bias predictions of ecosystem water use in fertilized cereals, with broader implications under widespread hygroscopic aerosol deposition.