Alder and lupine enhance nitrogen cycling in a degraded forest soil in Northern Sweden

Positive effects of legumes and actinorhizal plants on N-poor soils have been observed in many studies but few have been done at high latitudes, which was the location of our study. We measured N2 fixation and several indices of soil N at a site near the Arctic Circle in northern Sweden. More than 20 years ago lupine (Lupinus nootkatensis Donn) and gray alder (Alnus incana L. Moench) were planted on this degraded forest site. We measured total soil N, net N mineralization and nitrification with a buried bag technique, and fluxes of NH+ 4 and NO– 3 as collected on ion exchange membranes. We also estimated N2 fixation activity of the N2-fixing plants by the natural abundance of 15N of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N2-fixing plants was almost totally derived from N2 fixation. Plots containing N2-fixing species generally had significantly higher soil N and N availability than a control plot without N2-fixing plants. Taken together, all measurements indicated that N2-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden.