Conservation nitrogen management in a northern Wisconsin tree nursery

Background/Question/Methods

Conservation management of nitrogen in bare-root tree nurseries has proven a considerable challenge as growers seek to maximize N uptake and minimize N leaching to groundwater. Controlled-release fertilizers (CRF) have been highlighted as an alternative to water-soluble fertilizer that may help provide this balance. In theory, the controlled N release from CRF parallels N demand of plants, effectively increasing nitrogen-use efficiency (NUE), and reducing N leaching without sacrificing seedling quality. The objective of this study was to compare the effects of conventional and controlled-release fertilization on seedlings morphology, NUE, and N leaching at a bare-root tree nursery in northern Wisconsin. In a bare-root tree nursery in Hayward, Wisconsin, red pine (Pinus resinosa) seedlings were planted in beds of loamy sand soil (Typic Udipsamments). Seedlings were fertilized over a course of two years with conventional fertilizer (223.05 kg N ha^-1 year^-1 for first year and 299.27 kg N ha^-1 year^-1 for second year), CRF1 (212.96 kg N ha^-1 year^-1 for first year and 298.15 kg N ha^-1 year^-1 for second year), and CRF2 (106.48 kg N ha^-1 year^-1 for first year and 170.37 kg N ha^-1 year^-1 for second year) using complete randomized design of five replicates per treatment. We analyzed N content in plant tissues, measured seedling morphology (stem diameter and length; biomass of leaf, stem, and roots), and estimated N leaching using mass-balance.
Results/Conclusions

We found no significant difference in seedling morphology among all three treatments in both growing seasons. The root:shoot ratios for N content and biomass also exhibited no significant difference among treatments in both growing seasons, although a trend was found with consistently lower root:shoot ratios in CRFs in first growing season. Conventional fertilizer and CRF1 showed no significant difference in N leaching in either growing season. However, CRF2, which had less amount of N input, had the greatest NUE in the first growing season and the least N leaching in both growing seasons. Based on morphological data combined with a mass-balance of each treatment, smaller N input from CRF2 reduced N leaching without sacrificing seedling morphology. Therefore, we suggest that CRFs with lower N input may reduce N leaching from red pine in this Wisconsin bare-root nursery without sacrificing seedling quality. These findings may help nursery managers improve environmental stewardship with minimal economic impacts.