Soil Nitrogen Transformations And Retention During A Deciduous To Coniferous Successional Transition

Abstract

The mineralization, retention and movement of soil nitrogen (N) was investigated in forest types which encompass one of the most dramatic plant successional transitions in the boreal forest---the shift from deciduous, mid-succession, stands of balsam poplar (Poputus balsamifera) to coniferous, late-succession, stands of white spruce (Picea glauca). Nitrogen is an essential nutrient that often limits plant productivity in the boreal forest. Nitrogen uptake by plants is constrained by the activity of soil microbes and their associated exoenzymes which depolymerize (break down) organic molecules and release forms of N that are useable by plants (e.g., amino acids, ammonium and nitrate). The availability of labile carbon (C) is generally thought to limit soil microbes; however, it has been hypothesized that soil microbes in floodplain stands of balsam poplar are actually N limited. Balsam poplar trees also have large N requirements; thus, the overall demand for N is considerable in these stands and biological N retention should be high. In contrast, lower primary productivity and more recalcitrant soil organic matter in white spruce stands should result in comparatively less immobilization and less retention of N in this stand type. Experimental N additions resulted in the acceleration of net N mineralization and nitrate leaching in both stand types, probably because biological N demand was rapidly satiated. In balsam poplar soil, net nitrification was greatly stimulated by N additions; while in white spruce soil only net ammonification was stimulated; indicating that different mechanisms control ammonium oxidation or nitrate immobilization in these stands. Nitrogen additions did not affect soil microbial biomass in either stand. Results from a laboratory soil incubation study indicate that, compared to mid-succession soil, soil organic matter in late succession stands was more labile and the mineralization of C and N were significantly more temperature sensitive. Thus, climatic warming may result in the release of a larger proportion of soil C and N from late succession stands. A separate study examining soil solution N concentrations and movement showed that the Tanana River is a source of active layer nitrate during the growing season in both mid and late succession stands.