Response of needle-litter decomposition and soil nitrogen mineralization to logging-debris manipulation and competing vegetation control in western Oregon and Washington Public Deposited

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  • Growing societal demand for forest products is pressuring managers to increase productivity from a finite land area, and it is expected that increased supply will come mostly from expansion of intensively managed stands. The USDA Forest Service and numerous collaborators created the Long-Term Soil Productivity (LTSP) network of research sites across North America to investigate the implications of intensive management. The purpose of the LTSP research program is to examine effects of management disturbances on soil productivity, evaluate standards for soil quality monitoring, understand fundamental relationships between soil, forest management practices, and long-term productivity, and to examine ways to mitigate adverse disturbance effects. Research in this thesis was conducted at the Matlock, WA and Molalla, OR LTSP affiliate sites, which were specifically designed to examine effects of contemporary management practices on growth and productivity of Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) as well as the driving soil factors associated with productivity. Three levels of logging-debris manipulations (bole-only, whole-tree, bole-only with debris piling) and two herbicide treatments (initial and sustained control) were replicated using a randomized complete block design at each study site. A stratified sampling scheme was used to characterize needle-litter decomposition (using a litterbag study) and associated nutrient dynamics as well as net nitrogen (N) mineralization (using in situ sequential coring) in the mineral soil. Rates of needle-litter decomposition were highest in conditions characterized by thick debris coverage. Needles acted as an N sink, immobilizing 14 to 40 kg N ha-1 y-1 initially. Needles were a source of potassium (K) and calcium (Ca), and a minimal source of phosphorus (P) and magnesium. Soil net N mineralization results were highly variable, likely reflecting extensive soil variability, although some patterns were observed. Nitrate-N was the dominant inorganic N form in soils, and accumulations between 25-45 mg N kg-1 soil yr-1 were found at sites. A faster Douglas-fir needle-litter decomposition rate was observed in bole-only logging-debris treatments at Matlock (lower productivity site), which also retained higher N, P, K, and Ca percentages than other treatment types. No treatment differences in needle-litter decomposition and nutrient-release dynamics were observed at Molalla (higher productivity site), possibly resulting from greater resource availability. At both sites, a combination of soil net N mineralization in the range of 25-75 mg N kg-1 soil yr-1 and high rates of initial N immobilization in decomposing needle materials suggests that N is being retained in response to the three logging-debris and two vegetation control treatments. Early dynamics of these two soil processes suggest that soil N pools are initially conserved.
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