Objectives This study aimed to investigate the effects of localized application of controlled-release urea on the three-dimensional spatial distribution of maize root systems and nitrogen uptake and utilization. Specifically, it sought to clarify the advantages of slow-release urea over traditional uniform fertilization and localized quick-release fertilization in maintaining a sustained nutrient supply and matching with root growth, thereby providing a theoretical basis for the efficient utilization of slow-release nitrogen fertilizers.

Methods A soil column experiment was conducted using maize and nitrogen fertilizer as the experimental subjects. Four treatments were established: no nitrogen application (CK), uniform urea application (UU), localized urea application (LU), and localized slow-release urea application (SU). X-ray computed tomography (X-ray CT) was employed for in situ scanning and three-dimensional reconstruction of root systems. Measurements of plant biomass, nitrogen uptake, and soil nitrogen content were integrated to analyze the relationships among root morphological traits, spatial distribution pattern, and nutrient uptake under different fertilization regimes.

Results Compared with UU and LU, SU increased above-ground nitrogen uptake by 43.8% and 36.9%, respectively, thereby significantly enhancing total maize nitrogen uptake. The SU treatment also substantially increased soil nitrate nitrogen content. Within the localized fertilization zone (2−6 cm soil layer), nitrate nitrogen concentrations in SU treatment were significantly higher than those in the CK, UU, and LU treatments by 200%, 80%, and 103%, respectively. The three-dimensional reconstruction based on X-ray CT revealed that the SU treatment significantly promoted root growth. Particularly in the localized fertilization zone (2−6 cm soil layer), root length density reached 7.3 cm/cm³, representing increases of 182% and 101% relative to the UU and LU treatments, respectively. Moreover, within the fertilization microzone (a 50 cm³ volume surrounding the fertilizer granules), root length under the SU treatment was 3.3 times of that under the LU treatment. Furthermore, correlation analysis indicated that root morphological parameters (root length, root surface area, and root length density) were significantly and positively correlated with soil nitrate nitrogen content and above-ground nitrogen uptake, whereas mean distance exhibited a significant negative correlation with nitrogen uptake.

Conclusions Localized application of slow-release urea promotes sustained root proliferation in fertilized zones by providing a continuous supply of nitrogen, enhances the spatial coupling between roots and nutrients, and significantly improves maize nitrogen nutrition. X-ray CT technology effectively reveals the spatial characteristics of root-fertilizer interactions and provides crucial technical support for studying root-fertilizer interactions.