The effect of soil temperature on soil nitrogen form availability and nitrogen uptake by conifers of British Columbia




Boczulak, Stacy Avni

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With climate change, forest soils of British Columbia (B.C.) will likely undergo significant increases in temperature. Changes in temperature may differentially alter steps of N cycling, altering the amount of N in various pools of the cycle. Furthermore, plant species may show a preference for certain N forms available in soils, such as ammonium, nitrate or organic N. Changes in soil N forms and plant N preferences can shift competitive interactions among conifer species in B.C. forests. Using a greenhouse incubation of forest soils from two elevations, I aimed to determine how temperature affects N cycling in soils that differ in temperature adaptations. With a conifer growth experiment where ammonium, nitrate and a mix of amino acids were applied to trees, I studied N form preferences and uptake rates of three conifer species from contrasting environments (Pseudotsuga menziesii, Picea sitchensis, and Picea engelmannii), and how N uptake in these species reacted to increases in soil temperature. Results show that the abundance of all N forms increased with temperature, but the response to warming was stronger in soils from a low elevation. Furthermore, ammonium and soluble organic N in soils increased faster with warming than nitrate. Nitrification potential was higher in the low elevation soil. This indicates that rates of soil processes, producing plant available N may increase with warming and the balance of different N forms may change. Differences in the abundance, composition, or activity of soil biota at these two locations likely caused dissimilar reactions to warming in two chemically and physically similar soils. Conifers exhibited preferences towards N forms, and these preferences are likely due to adaptation to the N form most available in native soils. On average, Douglas-fir showed preference for nitrate (a N form commonly found in warmer areas), Sitka spruce preferred ammonium (a N form high in cooler areas), and Engelmann spruce showed equal preference for organic N and ammonium (organic N is usually abundant in very cold areas). Preference as indicated by plant growth changed when species were grown at different temperatures, showing ability for acclimation in these conifers. Understanding that a soil’s history greatly affects its response to perturbation is important if we are to make predictions on how N cycling in soils may change with changing climate. Knowing how conifers utilize available soil nutrients at different temperatures will help to predict species’ future performance, composition and abundance in B.C. forests as soils warm and tree lines move north or to higher elevations.



Conifer, Nitrogen, Plant Physioogy, Nutrient Preference, Soil