The natural abundance of 15N in litter and soil profiles under six temperate tree species: N cycling depends on tree species traits and site fertility

Aims We investigated the influence of tree species on the natural N-15 abundance in forest stands under elevated ambient N deposition. Methods We analysed delta N-15 in litter, the forest floor and three mineral soil horizons along with ecosystem N status variables at six sites planted three decades ago with five European broadleaved tree species and Norway spruce. Results Litter delta N-15 and N-15 enrichment factor (delta N-15(litter)-delta N-15(soil)) were positively correlated with N status based on soil and litter N pools, nitrification, subsoil nitrate concentration and forest growth. Tree species differences were also significant for these N variables and for the litter delta N-15 and enrichment factor. Litter from ash and sycamore maple with high N status and low fungal mycelia activity was enriched in N-15 (+0.9 delta units) relative to other tree species (European beech, pedunculate oak, lime and Norway spruce) even though the latter species leached more nitrate. Conclusions The delta N-15 pattern reflected tree species related traits affecting the N cycling as well as site fertility and former land use, and possibly differences in N leaching. The tree species delta N-15 patterns reflected fractionation caused by uptake of N through mycorrhiza rather than due to nitrate leaching or other N transformation processes.