Graduate Thesis Or Dissertation
 

Factors affecting nitrogen nutrition of western hemlock

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  • Preliminary fertilization trials with western hemlock have yielded responses which vary significantly from stand to stand, especially in areas near the Pacific Coast. Growth responses have varied from positive to negative in an erratic manner. Previous data did not permit validation of such hypotheses as, "the application of fertilizer to a hemlock stand results in unfavorable changes in soil chemistry," "application of fertilizer damages the feeder roots of hemlock so that uptake of water and minerals is reduced' or "nitrogen is not the only growth limiting element on many coastal hemlock sites." The main purpose of the series of experiments outlined in this thesis is to assess the effects of the application of urea fertilizer upon the rhizosphere and the consequent absorption capacity of hemlock roots. Prilled urea at the rate of 224 kg/hectare of elemental nitrogen was applied to test plots in six natural hemlock stands. Periodic soil core samples were extracted to examine the impact of urea on soil chemical properties and hemlock's primary roots. Foliage samples were collected and analyzed to determine the impact of urea on hemlock's mineral-nutrient status. Also, a litter-bag study was conducted to examine the impact of various forms of nitrogen fertilizers (urea included) on rates of litter decomposition in hemlock stands. The results of this study demonstrate that application of urea significantly alters the soil pH, increases the concentrations of extractable NH14 and NO3 nitrogen decreases the amounts of Ca, Mg and K and increases the rate of litter decomposition in the rhizosphere of western hemlock. The magnitude of these changes in soil chemical properties is maximum in the litter layer and decreases with soil depth and time. The pH and NHt1-N concentrations jfl the litter layer reach levels which are toxic to hemlock. Urea fertilization results in significant reduction in the total numbers of mycorrhizal roots and in significant changes in the relative populations of the mycorrhizal types. The nagnitude of these changes varies among the hemlock stands and is most likely a function of the vertical distribution of mycorrhizal roots and their composition. The maximum reduction in number of mycorrhizal roots occurs during the first six months and at 0-5 cm soil depth, which is also the time span and the soil depth at which the magnitude of changes in soil properties following urea application are maximum. The balance of essential nutrients in the foliage of western hemlock is also altered significantly after urea fertilization. Foliar concentration of nitrogen is increased and the concentrations of 1', Ca, Mg, Mn, Fe and B are generally reduced. Reduction in concentration of these elements following urea application is probably due to reduced uptake. The relative foliar concentrations of some essential elements (especially P) following urea treatment fall in the range considered limiting to hemlock's growth potential.
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