N retention after N addition in four experimental stands of Norway spruce in southern Sweden

Human activities, especially during the last century, have altered the pools and fluxes of nitrogen (N). For example, the rate of N input into the terrestrial global N cycle has approximately doubled (Vitousek et al., 1998). The main sources of additional N are fossil fuel combustion and various agricultural measures (such as the production and use of N fertilizer and use of N2-fixing crops).