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 nitrogen mineralisation in southern Sweden. Uptake of a mixture of amino acids (alanine, glutamine, glycine), that varied between 1.6 and 6.3 µmol 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 correlated with measures related to soil acidity (Ellenberg's R-value, soil nitrate and total carbon) but not, as hypothesised, with the total amount of mineralised nitrogen. The total demand on nitrogen by all components of the ecosystem would probably have described the extent to which competition among and between plants and microbes induced nitrogen limitation. In a methodological study two grasses were exposed to pH 3.8, 4.5 and 6.0 and to 50, 100 and 250 µmol l(-1) of three amino acids. Uptake was also compared between intact plants and excised roots. The treatment response varied considerably between the species which stresses the importance of studying intact plants at field-relevant pH and concentrations.