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 N-2 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 NH4+ and NO3- as collected on ion exchange membranes. We also estimated N-2 fixation activity of the N-2-fixing plants by the natural abundance of N-15 of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N-2-fixing plants was almost totally derived from N-2 fixation. Plots containing N-2-fixing species generally had significantly higher soil N and N availability than a control plot without N-2-fixing plants. Taken together, all measurements indicated that N-2-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden.