Nitrogen-Utilization by Plant-Species from Acid Heathland Soils .2. Growth and Shoot/Root Partitioning of No3- Assimilation at Constant Low Ph and Varying No3-/Nh4+ Ratio

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(-3) N, varying the NO3-/NH4+ ratio up to 40% nitrate. In addition, measurements of NRA, plant chemical composition, and biomass allocation were carried out on a complete NO3-/NH4+ replacement series up to 100% nitrate. With the exception of M. caerulea, the partial replacement of NH4+ by NO3- tended to enhance the plant's growth rate when 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+, grasses reduced nitrate in the shoot only; roots did not become involved in the reduction of nitrate until zero ambient NH4+. The dwarf shrubs, being species that assimilate N exclusively in their roots, 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 a rather irregular pattern. Free nitrate accumulated in the tissues of grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO4- liproportion of absorbed ammonium highly preferentially (relative uptake ratio for NO3- <0.20), D. flexuosa showed a strong preference for NO3- at low external nitrate (the relative uptake ratio for NO3- reaching a value of 2.0 at 10% NO3-). The ecological significance of this prominent high preference for NO3- at low NO3-/NH4+ ratio by D. flexuosa and its consequences for soil acidification are briefly discussed. [KEYWORDS: Ammonium; heathland plants; no3-/nh4+ ratio; nitrate; nitrate reductase activity; soil acidification; specific absorption rate Nitrate reductase-activity; root temperature; lolium-perenne;ionic-balance; ammonium; acidification; nutrition]