Nitrogen dynamics in primary successional soils on the Tanana River of interior Alaska

As succession determines the patterns of ecosystem development, it also provides a temporal framework in which to investigate the controls of nitrogen-cycling. The object of this study was to examine patterns of nitrogen-cycling within primary successional soils. Estimates of N mineralization, nitrogen fixation, and denitrification were made within open shrub, alder, and white spruce stands, representing early, mid-, and late successional stages, respectively. Net NH{dollar}sb4sp+{dollar} mineralization and net nitrification, measured using the polyethylene bag technique, were significantly different between sites and among forest floors and mineral soils. The alder forest floor had the highest observed mineralization rates, {dollar}<{dollar}1-21 ug N {dollar}cdot{dollar} g dry soil{dollar}sp{lcub}-1{rcub}{dollar} {dollar}cdot{dollar} d{dollar}sp{lcub}-1{rcub}{dollar}, with the white spruce forest floor exhibiting rates of {dollar}<{dollar}1-2 and mineral soils {dollar}<{dollar}1. Seasonal patterns of N mineralization were more pronounced in the alder forest floor: high net NH{dollar}sb4sp+{dollar} mineralization in early summer and high net nitrification in late summer. Immobilization of nitrogen was observed at all sites. Laboratory studies indicate temperature as a limiting factor of N mineralization in early and mid-successional stages, while both temperature and moisture were limiting in later succession. Laboratory studies suggest heterotrophic nitrification may be important in the alder forest floor. Denitrification activity was low to undetectable at all sites, the highest observed rate was in alder forest floor samples, 220 g N {dollar}cdot{dollar} ha{dollar}sp{lcub}-1{rcub}{dollar} {dollar}cdot{dollar} d{dollar}sp{lcub}-1{rcub}{dollar}. Potential denitrification was low, the alder forest floor exhibited the highest rate, 3.4 Kg N {dollar}cdot{dollar} ha{dollar}sp{lcub}-1{rcub}{dollar} {dollar}cdot{dollar} d{dollar}sp{lcub}-1{rcub}{dollar}. Laboratory studies indicated denitrification was more limited by carbon and NO{dollar}sb3sp-{dollar} than temperature or moisture. Nonsymbiotic nitrogen fixation was low to undetectable and highly variable. The highest observed nitrogenase activity was associated with alder root nodules, 163 Kg N {dollar}cdot{dollar} ha{dollar}sp{lcub}-1{rcub}{dollar} {dollar}cdot{dollar} yr{dollar}sp{lcub}-1{rcub}{dollar}.