Synopses & Reviews
Improving nitrogen (N) management in bare-root nurseries hinges on achieving a balance between the production of quality tree seedlings and the impact of this management on the environment. This goal is especially challenging in the production of Northern red oak (Quercus rubra) (NRO) seedlings, the most numerous, but relatively understudied hardwood species grown in the coarse sandy soils of Wisconsin State Nurseries. In this dissertation, I investigated four key areas to improve our understanding of N cycling related to NRO seedling production and N fertilization in bare-root nurseries. In chapter one, I summarize the current state of science regarding N management of NRO in bare-root tree nurseries. Chapter two focuses on the development and testing of a model to improve estimates of N release from controlled-release fertilizer (CRF) broadcast to soil surfaces that vary in moisture and temperature, typical of most bare-root operations. Chapter three describes a study that found a range of N fertilization rates (259--517 mg N seedling-1 over two years) in Wisconsin State nurseries did not yield significant changes in the biomass and N content of NRO seedlings, however marked reductions in N fertilizer uptake efficiency (NFUE) with increased rates of N fertilization. Using stable isotopes of N, the study described in chapter four tested a relatively new practice of late-season fertilization in Wisconsin state nurseries. While this regime had no effect on morphology, biomass, and NFUE of NRO seedlings, estimates of NFUE that did not account for N contributions from mineralization and reabsorption within the plants significantly overestimated the contribution of fertilizer N. Biomass and N allocation following fertilization are also assessed in this study. Finally, chapter five demonstrates that a reduced CRF application (60% N of conventional rate) in Wisconsin state nurseries yielded similar morphology, biomass, and N uptake compared to conventional fertilization while reducing N loss potential and net N mineralization. Overall, this research demonstrates that improved understanding of N cycling in bare-root nurseries of Wisconsin will improve the production of NRO seedlings, reduce the environmental impact associated with N fertilization, and minimize the potential errors between methods to calculate NFUE.