N-15 ABUNDANCE OF SURFACE SOILS, ROOTS AND MYCORRHIZAS IN PROFILES OFEUROPEAN FOREST SOILS

N-15 natural abundances of soil total N, roots and mycorrhizas were st udied in surface soil profiles in coniferous and broadleaved forests a long a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in delta(15)N of soil tot al N of up to 9 parts per thousand from the uppermost horizon of the o rganic mor layer down to the upper 0-5 cm of the mineral soil. The del ta(15)N 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 parts p er thousand depleted under such conditions. In experimentally N-enrich ed 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 delta(15)N with depth in soil total N was smaller. An increase in delta(15)N of the surface soil was even observed on experimentally N-enriched plots, although o ther data suggest that the N fertilizer added was depleted in N-15. In such cases roots could be enriched in N-15 relative to soil total N, suggesting that labelling of the surf ace soil is via the pathway: - a vailable pools of N plant N - litter N. Under N-limiting conditions ro ots of different species sampled from the same soil horizon showed sim ilar delta(15)N. By contrast, in experimentally N-enriched forest delt a(15)N of roots increased in the sequence: ericaceous dwarf shrubs<Sco ts pine<grass, suggesting increasing use of inorganic N along the sequ ence. Complementary studies at the major transect sites had shown that 90-99% of fine tree roots had ectomycorrhizas (ECMs). ECMs were 2 par ts per thousand more enriched than corresponding non-mycorrhizal fine roots. Fungal stripped off ECMs were 2.4-6.4 parts per thousand enrich ed relative to the remaining root core. It is suggested that a flux of N through ECMs to aboveground pasts in N-limited forests would leave N-15 enriched compounds in fungal material, which could contribute to explain the observed delta(15)N profiles if fungal material is enriche d, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isot opically lighter soil surface in forests: plant uptake of N-15-deplete d N and its redeposition onto the soil surface by litter-fall.