Use of strontium isotopes and foliar K content to estimate weathering of biotite induced by pine seedlings colonised by ectomycorrhizal fungi from two different soils

Pinus sylvestris seedlings, colonised by ectomycorrhizal (EM) fungi from ei ther of two different soils (untreated forest soil and a limed soil from a clear cut area), were grown with or without biotite as a source of K. The b iotite was naturally enriched in Sr-87 and the ratio of Sr-87/Sr-86 in the plant biomass was estimated and used as a marker for biotite weathering and compared to estimates of weathering based on foliar content of K. Differen t nutrient regimes were used to expose the seedlings to deficiencies of K w ith and without an application of nitrogen (NH4NO3) in excess of seedling d emand. The seedlings were grown for 220 days and the elemental composition of the shoots were analysed at harvest. The EM colonisation was followed by analysing the concentration of ergosterol in the roots and the soils. Bact erial activity of the soil was estimated by the thymidine incorporation tec hnique. The concentration of organic acids in the soil solution was measure d in the soil in which seedlings colonised by EM fungi from the untreated f orest soil were grown. It was found that seedlings colonised by EM fungi fr om untreated forest soil had taken up more K in treatments with biotite add ition compared to seedlings colonised by EM fungi from the limed forest soi l (p < 0.05). Seedlings from untreated forest soil had larger shoots and co ntained more K when grown with biotite compared to KCl as K source, indicat ing that biotite had a stimulatory effect on the growth of these seedlings which was not related to K uptake. Seedlings from the limed soil, on the ot her hand, had similar foliar K content when grown with either biotite or KC l as K source. The larger uptake of K in seedlings from untreated forest so il was not an effect of a more developed EM colonisation of the roots since seedlings from the limed soil had a higher ergosterol concentration both i n the soil and in the roots. Nutrient regimes had no significant influence on the total uptake of K but the Sr-87/ Sr-86 isotope ratio in the plant bi omass indicated that seedlings grown with excess nitrogen supply had taken up proportionally less Sr from the biotite (1.8% of total Sr content) compa red to seedlings grown with a moderate nitrogen supply (5.0%). Furthermore, seedlings grown with excess nitrogen supply had a reduced fungal colonisat ion of roots and soil and bacterial activity was lower in these soils. The Sr-87/Sr-86 ratio in the plant biomass was positively correlated with funga l colonisation of the roots (r(2)=0.98), which may indicate that the fungus was involved in releasing Sr from the biotite. Uptake of K from biotite wa s not related to the amount of organic acids in the soil solution. Oxalic a cid was positively related to the amount of ergosterol in the root, suggest ing that oxalic acid in the soil solution originates from the EM symbionts. The accuracy of the estimations of biotite weathering based on K uptake by the seedlings in comparison with the Sr-87/Sr-86 isotope ratio measured in the shoots is discussed.