Uptake capacity of amino acids by ten grasses and forbs in relation to soil acidity and nitrogen availability

Uptake capacity of organic nitrogen was studied in solution experiments on eight grasses and two forbs growing in acid soils with relatively high nitr ogen mineralisation in southern Sweden. Uptake of a mixture of amino acids (alanine, glutamine, glycine), that varied between 1.6 and 6.3 mu mol g(-1) dw root h(-1), could not be explained by soil data from the species' field distributions (pH, total carbon and nitrogen, potential net mineralisation of ammonium and nitrate). The ratio between organic and inorganic nitrogen (methylamine) uptake was < 0.05 for the forbs, higher for the grasses with a maximum of 1.42 for Deschampsia flexuosa. The ratio was negatively corre lated with measures related to soil acidity (Ellenberg's R-value, soil nitr ate and total carbon) but not, as hypothesised, with the total amount of mi neralised nitrogen. The total demand on nitrogen by all components of the e cosystem would probably have described the extent to which competition amon g and between plants and microbes induced nitrogen limitation. In a methodo logical study two grasses were exposed to pH 3.8, 4.5 and 6.0 and to 50, 10 0 and 250 <mu>mol l(-1) of three amino acids. Uptake was also compared betw een intact plants and excised roots. The treatment response varied consider ably between the species which stresses the importance of studying intact p lants at field-relevant pH and concentrations. (C) 2000 Elsevier Science B. V. All rights reserved.