Landscape controls on nitrogen availability in a western forest watershed

Abstract:

Global increases in anthropogenic nitrogen (N) deposition and declines in winter snowpack are expected to alter the productivity of montane forests in the Western U.S. In this study, we focused on the topographic controls on seasonal N patterns in a mountainous landscape. The experiment was conducted in the North Fork of the Elk Creek (NFEC) watershed within the Lubrecht Experimental Forest, located in western Montana. The NFEC is a managed watershed and was logged approximately 80 years ago; thus, this watershed offers a unique opportunity to examine topographic and biological controls on N dynamics within a relatively even aged forest. In 2014 and 2015, we examined changes in soil N dynamics in four sites that varied in aspect (north and south) and elevation (high and low). Within each site, we also examined micro-topographic controls by sampling soil from both the hollow and the side slope. From pre-snowmelt to the end of the growing season (March – October), we collected snow cores and soil cores within each site to characterize the heterogeneity of soil N. We also installed resin strips to measure the exchangeable N within the upper soils. Across the entire watershed, we found that seasonal NH₄⁺ and NO₃^- concentrations were highest in April, as well as at the higher elevation sites. This pattern corresponded with higher snowpack and snow water equivalent values and was likely due to higher mineralization rates during the winter. We also found that across all four sites, hollow soils had higher NH₄⁺ and NO₃^- concentrations relative to soils from the hillslope, and this was largely due to increased organic matter and high soil moisture content found in the hollows. We also found a significant interaction between time of year, site, and micro-topography, suggesting that during the early growing season, NH₄⁺ and NO₃^- concentrations were largely modulated by snowpack depth, which in turn was controlled by topography. Soil NH₄⁺ and NO₃^- concentrations for samples collected for the remainder of the growing season is currently being analyzed, but preliminary results suggest that topography plays a large role in regulated soil N dynamics via controls on soil water content and organic matter accumulation.