15N abundance of surface soils, roots and mycorrhizas in profiles of European forest soils.

15N 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 δ15N 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 δ15N 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-1 year-1 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 δ15N with depth in soil total N was smaller. An increase in δ15N of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted in15N. In such cases roots could be enriched in15N 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 δ15N. By contrast, in experimentally N-enriched forest δ15N of roots increased in the sequence: ericaceous dwarf shrubs<Scots pine<grass, suggesting increasing use of inorganic N along the sequence. Complementary studies at the major transect sites had shown that 90-99% of fine tree roots had ectomycorrhizas (ECMs). ECMs were 2% more enriched than corresponding non-mycorrhizal fine roots. Fungal sheaths stripped off ECMs were 2.4-6.4 enriched relative to the remaining root core. It is suggested that a flux of N through ECMs to aboveground parts in N-limited forests would leave 15N enriched compounds in fungal material, which could contribute to explain the observed δ15N profiles if fungal material is enriched, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isotopically lighter soil surface in forests: plant uptake of 15N-depleted N and its redeposition onto the soil surface by litter-fall.