Assessment of N2 fixation and N cycling by Dryas along a chronosequence within the forelands of the Athabasca Glacier, Canada

The role of both actinorhizal and leguminous N2-fixing plants during primary succession within glacial forelands has received much attention, but there are few estimates of the contribution of fixed N to neo-glacial substrates. The main objectives of our work were (i) to assess the N2-fixing ability of three Dryas taxa across a chronosequence of ca 135 yr of post neo-glacial recession, and (ii) to establish whether Dryas serves as a source of N for non-N2-fixing species. The mineral N pool was highly limited across the sere and increased from 0.3 to 1.3 pg g of soil over the chronosequence. The vascular non-N2-fixing species had negative mean delta 15 N values over the sere, ranging from -6.4 +/- 0.4 to -3.3 +/- 0.4. Dryas drummondii, present over the entire chronosequence, showed delta 15N values ranging from -6.0 +/- 0.5 to 0.32 + 0.4, and appeared not to fix N2 until the mid to late stages of the sere. Mean estimated values of percent N derived from the atmosphere fir D. drummondii ranged from 81 to 89%. The observed delta 15N mean values for D. octopetala and D. integrifolia were -3.5 +/- 0.5 and -4.9 +/- 0.3, respectively. Those values were close to the delta 15N values of the non-N2-fixing species, therefore, at this site, these species did not appear to fix N2. Four non-N2-fixing woody vascular taxa that grew adjacent to N2-fixing plants had significantly greater foliar N contents and higher (less negative) delta 15N values than non-N2-fixing woody vascular taxa grown at a distance from the N2-fixing Dryas. The delta 15N values from non-N2-fixing plants that grew adjacent to N2-fixing D. drummondii appear to contradict the widely held hypothesis that pioneer N2-fixing species facilitate the establishment of later successional species during primary succession. The absence of N2-fixing activity by D. drummondii during the early stages of the sere, the higher concentration of N, and the less negative delta 15N values for foliar N content in plants adjacent to this taxon, suggest an alternative mechanism for the spatial and temporal vegetation change during primary succession. These observations confirm and refine a hypothesis suggested earlier that N2-fixing plants stimulate, through the process of N transfer, adjacent non-N2-fixing plants that are already established.