N abundance of surface soils, roots and mycorrhizas in profiles of European forest soils.

N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in δN of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0-5 cm of the mineral soil. The δN of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha year for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in δN with depth in soil total N was smaller. An increase in δN of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted inN. In such cases roots could be enriched inN relative to soil total N, suggesting that labelling of the surface soil is via the pathway: - available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar δN. By contrast, in experimentally N-enriched forest δN of roots increased in the sequence: ericaceous dwarf shrubs