Free living nitrogen-fixation in ponderosa pine/Douglas-fir forests in western Montana
Nitrogen (N) is a primary limiting nutrient in all ecosystems. Therefore, a thorough understanding of N cycling processes in forest ecosystems is required to minimize N losses to fire, harvesting, and other forms of land management. The influence of fire, fire exclusion and forest restoration treatments on non-symbiotic N-fixation in the forest ecosystem has been poorly studied. Over the past 100 years, fire has been greatly excluded from low elevation, fire maintained forests as a result of active fire suppression as well as land management activities that create discontinuities in landscape fire patterns. Previous studies have shown this activity to inhibit recolonization of sites by symbiotic N-fixing plant species. The lack of these important N fixing species may make non-symbiotic, free-living N fixing bacteria a more important source for N recovery in these forest ecosystems following disturbance. Recent studies also suggest that free-living N-fixing bacteria colonizing decomposing woody roots have the capacity to fix large amounts of N. The purpose of these studies was to investigate the effect of fire, fire exclusion, and forest restoration on the N contribution of non-symbiotic N-fixing bacteria (colonizing soil, woody roots, and soil crusts) to the forest ecosystem and how their contribution compares to symbiotic N-fixers in Western Montana. Studies were conducted in the laboratory and at numerous field sites throughout western Montana. In order to determine the N-fixation activity of organisms in these systems, we used the acetylene reduction technique. Neither time since fire, nor restoration treatment had any direct influence on free living N-fixation in soil or woody roots. Moisture and N availability were the potent drivers of free living N-fixation in western Montana. Nitrogen-fixation rates were low in decomposing woody roots in these ecosystems and woody roots do not contribute a significant amount of N to low elevation ponderosa pine/Douglas-fir forests in western Montana. Free-living N-fixing bacteria in soils were found to make a significant, yet modest N contribution to the forest ecosystem. Nitrogen demand by trees and shrubs are being maintained by residual soil organic N, symbiotic N fixation, and wet and dry N deposition in the Inland Northwest
free living bacteria, nitrogen fixation, ponderosa pine, roots
Networked Digital Library of Theses and Dissertations