Reduced environmental impact from using controlled-release nitrogen fertilizer in two public bare-root tree nurseries

Background/Question/Methods

A critical issue in soil management involves balancing the tradeoffs between plant production and environmental impact. Specifically, nursery managers seek to reduce nitrogen (N) input while enhancing N-fertilizer use efficiency. This goal appears simple, yet the application of conservation management in bare-root tree nurseries has been a significant challenge primarily because the nurseries are typically located on sandy soils. The rapid drainage of sandy soils require heavy irrigation during the growing season, especially in the variable climate of Wisconsin, which often leads to a significant amount of nitrate leaching. Studies highlight the use of controlled-release fertilizers (CRF) as a tool for enhancing N-fertilizer use efficiency. However, there is little information to guide nursery managers on the appropriate N-fertilizer rate to maintain traditional levels of seedling productivity and reduce environmental impacts. The objectives of this study were to determine the appropriate amount of CRF additions, measure their contributions to nitrate leaching and production of northern red oak (Quercus rubra) in two Wisconsin bare-root nurseries, and transfer this information to nursery growers. To meet these objectives, we conducted a field study over two years evaluating rates of CRF and conventional fertilizer (CON): CRF1 (272 kgN ha^-1), CRF2 (161 kgN ha^-1), and CON (259 kgN ha^-1).

Results/Conclusions

Consistently over the two growing seasons, total biomass, stem diameter, and specific leaf area under CRF1 and CRF2 additions showed no significant differences compared with the conventional fertilizer. However, root:shoot ratio in CRF2 was larger than CRF1 and CON at the time of transplant. Stem height and number of first order lateral roots were smaller in CRF2 than CON and CRF1, yet all treatments at the time of transplant exceeded seedling quality standards. Regarding environmental impact, nitrate concentrations of soil water at 60cm depth showed no significant difference between CRF1 and CON, but both were more than CRF2. Therefore we now recommend to nursery managers in Wisconsin that shifting soil N management from CON to CRF additions would not only reduce nitrate leaching, but would maintain seedling quality. This nutrient management tool will be discussed relative to its ecological impact and educational outreach in Wisconsin tree nurseries.