Nitrogen Management Strategies for Field-grown Thuja occidentalis: Comparison of Fertigation and Granular Fertilization

Efficient nitrogen (N) management is essential to optimize growth, quality, and environmental performance in field-grown Thuja occidentalis production systems. A two-year field study was conducted at two commercial sites in Quebec, Canada, to evaluate three N application strategies: conventional granular fertilization, drip fertigation, and a hybrid approach combining both methods. Treatments were arranged in a RCBD, and assessments included plant growth, aesthetic quality, foliar N concentration, soil nitrate dynamics, root distribution, and soil water status. Height growth was not significantly affected by N rate or application method. However, applying 150 kg N ha⁻¹ significantly increased trunk diameter and improved ornamental quality, including foliage density, pigmentation, and the proportion of premium-grade plants. Fertigation numerically increased foliar N concentration by up to 17% compared with granular fertilization, suggesting improved N use efficiency, although without measurable effects on height growth. Soil nitrate monitoring showed that granular fertilization generated transient nitrate peaks strongly influenced by precipitation, whereas fertigation provided more immediate nitrate availability within the root zone. Nitrate flux dynamics were closely associated with rainfall patterns. Root observations indicated that the active root zone of three-year-old Thuja was mainly restricted to the upper 30 cm of soil, where TDR measurements also identified the highest water uptake. Simulated precipitation events demonstrated that water applications exceeding 50 mm may displace nitrate below the active root zone. Overall, these findings highlight the importance of integrating climatic variability, root distribution, and soil water dynamics into N management strategies to improve productivity while minimizing environmental N losses.