| Abstract: | The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m–3N, varying the N03–/NH4+ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N03–/NH4+ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH4+ by NO3– tended to enhance the plant's growth ratewhen compared to NH4+ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO3–increasing from 0 to 100%. In the presence of NH4+, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH4+. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO3–in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO3– (proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO3– <0.20), D. flexuosashowed a strong preference for NO3– at low external nitrate(the relative uptake ratio for N03– reaching a value of2.0 at 10% NO3–). The ecological significance of thisprominent high preference for NO3– at low NO3–/NH4+ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N03–/NH4+ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate |
