Nutrient losses and nitrogen mineralization on forested watersheds in Oregon's Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w6634601j

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  • Nutrient losses of the biologically responsive anions, nitrate and bicarbonate together with the major cations, were monitored on 14 small watersheds in Oregon's Coast Range and evaluated in relation to management-induced disturbance. Mixed forests of Douglas-fir and red alder had dominated these high-nitrogen sites prior to treatment. A paired-watershed experimental design with duplicated treatments and untreated controls was employed at three study sites to examine effects of deforestation treatments on nutrient retention. The treatments represent combinations of conventional techniques for converting mixed hardwood-shrub forests to conifer stands: 1) herbicide suppression of competing alder and shrub layer; 2) pre-harvest herbicides and clearcut Logging; and 3) clearcut logging, herbicides and slash burning. During the approximately two years of streamwater monitoring following treatments, no significant increases in dissolved solids were observed. No consistent differences appeared between treatments or between cut and uncut watersheds. Nitrate concentrations (2.8 ppm N maximum) never exceeded U. S. Public Health Service (1962) standards for drinking water. Clearfelling and herbicide treatment on one south-facing watershed did result in abnormally high concentrations of the bicarbonate anion (82 ppm maximum) and the sodium cation (21 ppm maximum) during summer base flows. Total bicarbonate loss was small because streamflow was low, preventing excessive loss of accompanying cations. Moderately high nitrate levels were characteristic of all watersheds, regardless of treatment, due to the presence of red alder. Average streamwater concentrations of nitrate ranged from 0.4 to 1.9 ppm N, for all samples collected from the individual watersheds. Soil solution samplers were used to monitor the nitrate and bicarbonate anions in the soil profile under a residual red alder stand and an adjacent alder site recently cleared. Samples of soil solution were extracted weekly during the spring from three soil depths, 10, 65, and 130 cm. Higher concentrations of nitrate were found in soil solution than were observed in the streamwater. Concentration decreased with soil depth in both the residual stand and devegetated area. Nitrate uptake and immobilization by tree roots does not appear essential for nitrogen conservation on the gently sloping reliefs examined. Nitrogen mineralization was further scrutinized using a soil incubation experiment with soils from under Douglas-fir and under red alder. Sixteen combinations of four soil moistures and four temperatures were utilized for study over three incubation periods. Nitrification rates are substantially greater in alder soils; while ammonification rates appear similar for the two soils within the normal operating regimes of temperature and moisture. In very wet alder soils the mobile nitrate anion is reduced to the less mobile ammonium cation and ammonification rates are minimal. This behavior appears to be important in nutrient retention during winter flushings when high soil moisture is prevalent in the lower soil profile and in wet source areas (slowly draining areas) that characterize parts of these watersheds. The combinations of temperature and moisture exhibiting the highest rates of nitrogen mineralization (suggesting unstable states) were not encountered in the field under stands or in cleared situations. Soil temperatures above 21°C combined with moist but unsaturated conditions results in the highest rates of nitrate production. This, and the observed behavior of the nitrogen-rich watersheds, suggest that nitrate losses in streamwater following forest disturbance are only likely in climatic regions of summer-surplus precipitation.
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