Soil nitrogen cycling in a boreal hardwood forest in relation to the phenolic compound bearing species Ledum palustre

Ledum palustre (Labrador tea) is a late successional evergreen shrub widely distributed in boreal ecosystems that readily leaches high concentrations of soluble phenolic compounds into water. We selected this species in order to study whether leaf leachates could be responsible for the effects of plant canopy on N cycling under natural conditions. Organic matter content, soil respiration and net N mineralization were measured in organic and mineral soil horizons and gross N mineralization was also measured in mineral soils sampled underneath L. palustre in a hardwood forest dominated by Populus tremuloides and Betula neoalaskana. Soils were amended with L. palustre litter leachates and incubated in the laboratory. Because mineral soil is also influenced by the overlying organic horizon we also characterized some simple indices of the organic horizon carbon quality (lignin, cellulose and condensed tannins) to look for differences in likely decomposability.
Our objectives were i) to determine whether L. palustre presence and L. palustre leachates addition changed soil N availability, and ii) to determine the specific N cycling processes that were affected, including changes in mineralization, nitrification or immobilization. We wanted to know whether L. palustre leachates could be a mechanism through which plant presence would impact N cycling.