Nitrogen acquisition in four co-existing species from an upland acidic grassland

The concentration of both nitrate and ammonium nitrogen was measured in soil taken from an upland acidic (pH 4.5) grassland habitat, containing four co-existing species, Deschampsia flexuosa (L.) Trin., Festuca ovina L., Juncus squarrosus L. and Nardus stricta L. Both nitrate and ammonium nitrogen were found to be present in the soil, in similarly small quantities. The effect of both sources of nitrogen on relative growth rate was studied, and an attempt was made to determine whether nitrate or ammonium nitrogen is the immediate source of nitrogen for these plants using assays of nitrate reductase (EC 1.6.6.2) and ammonium uptake. All four species showed larger growth rates on the same concentration of ammonium nitrogen compared to nitrate nitrogen. All species showed low activities of leaf nitrate reductase, even in plants grown on 18 mol nitrate m⁻³. Ammonium uptake activity appeared to be higher in species which showed the lowest nitrate reductase activity and least response to increasing nitrate concentration in the growth medium.