NITROGEN MINERALIZATION IN UPLAND PRECAMBRIAN SHIELD CATCHMENTS - CONTRASTING THE ROLE OF LICHEN-COVERED BEDROCK AND FORESTED AREAS

The upland boreal forest at the Experimental Lakes Area (northwestern Ontario, Canada) is characterized by treed soil islands interspersed w ithin lichen and moss-covered bedrock outcrops. N mineralization was 2 .5-fold and net nitrification was 13-fold higher on an areal basis ove r bedrock surfaces because of high mineralization rates under lichen a nd moss patches. The higher average soil temperature in lichen and mos s patches could not account for the difference in mineralization rates . Lichens did not provide a significant additional source of N because they did not fix atmospheric N. A refractory conifer litter with a hi gh C:N probably favours the immobilization of N in forest islands. Bur ied bag and in situ core incubations yielded similar net N mineralizat ion rates but core incubations underestimated net nitrification rates. Both methods did not adequately measure dissolved organic N (DON) pro duction rates because soil disturbance caused high initial DON concent rations. The higher export of mineral N from bedrock surfaces is proba bly a combination of the lower retention of N in precipitation and lea ching of mineralized N from lichen and moss patches.