The effect of varying active layer and soil climate on net nitrogen mineralization and foliar nitrogen in a boreal watershed

Thesis (M.S.) University of Alaska Fairbanks, 2004In the Alaskan boreal forest, black spruce (Picea mariana) is the forest type most severely limited by nitrogen mineralization. Nitrogen cycling in upland black spruce forests of the Interior is affected by the interactions between permafrost, soil climate, and litter quality. Permafrost and soil climate also play a significant role in nitrogen acquisition of shrubs, which are important for biomass turnover and element cycling in the understory. This study took place in a boreal watershed and addressed the question of how variation in soil climate and active layer between north and south-facing aspects affected 1) net nitrogen mineralization rate and 2) foliar nitrogen concentrations in understory shrubs. I hypothesized that south-facing aspects, with warmer, drier soils and deeper active layers would have higher mineralization rates and support deciduous and evergreen shrubs with higher nitrogen status. Contrary to my predictions, net mineralization rate was not explained by active layer or soil climate. In support of my hypothesis, I found shrubs generally had higher foliar N on south-facing aspects. My study concludes that 1) the typical controls of net mineralization do not operate over the small scale of this study and 2) although more favorable soil conditions supported shrubs with higher foliar N, seasonal and spatial differences in foliar N cannot be categorized by growth strategy.General introduction -- The effects of active layer depth and soil climate on net nitrogen mineralization rate in a boreal watershed -- Seasonal foliar nitrogen concentrations of three dominant understory shrubs in relation to permafrost in a boreal forest watershed -- General conclusions -- Future research