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UBC Theses and Dissertations

Evaluating the effects of nitrogen urea fertilization on western redcedar growth, soil carbon stocks, and greenhouse gas emissions on Haida Gwaii, B.C. Boyer, Christophe Richard


Western redcedar (Thuja plicata) is a major component of native forests on Haida Gwaii, British Columbia, Canada, and a culturally indispensable species to the Haida people. However, its natural regeneration is unachievable due to intensive grazing by the island’s overabundant population of Sitka black-tailed deer. Forest fertilization with nitrogen (N)-urea has been shown to increase the growth rate of western redcedar in the Pacific Northwest, thus increasing aboveground carbon (C) sequestration. N-urea fertilization can also increase soil C storage by increasing litter inputs and decreasing the rate of soil organic matter decomposition. Using a paired-tree design, we tested the effects of N-urea fertilization (200 kg N/ha⁻¹) on western redcedar growth, soil C, N, and microbial biomass C in the forest floor and mineral soil one year following fertilization at six replicate plots in 14–19-year-old stands on Graham Island, Haida Gwaii. We also measured soil greenhouse gas fluxes (CO₂, CH₄, N₂O) for a period of 56 days and one year after fertilization to evaluate the net benefit of fertilization on forest C storage. N-urea pellets were rapidly mineralized, releasing a flush of CO₂ in the first 4 days after application. A sustained increase in CO₂ emissions was observed for 2 months after application, but emissions reverted to levels of controls after 12 months. Fertilization had no effect on CH₄ or N₂O emissions. N-urea had no effect on the change in soil C or N in any soil fraction from 2021-2022 but did increase microbial biomass C in the mineral soil in 2 of the 6 plots. Fertilized trees showed a significant increase in biomass, and thus C sequestration, compared to controls in 2 of the 6 plots, which was greater than the amount of C lost via increased CO₂ emissions. Increases in the growth rate of tree height and diameter after one year suggest that western redcedar is responding to fertilization, which may lead to an increase in aboveground C storage. However, further studies will be crucial in determining the long-term response of western redcedar growth to N-urea fertilization and evaluating changes in soil C storage.

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