Nitrogen utilization by plant species from acid heathland soils

Seven heathland species, four herbaceous plants and three dwarf shrubs, were tested for their capacity to utilize NH4+ or NO3− . When cultured in solution at pH 4.0 with 2mol m−3 N, all species showed similar growth responses with respect to N source. Nitrate was assimilated almost equally well as ammonium, with relative growth rate generally averaging 5–8% lower for NO3− grown plants, albeit not always significantly. However, N source was significantly and consistently correlated with biomass partitioning, as NH4+-fed plants allocated more dry matter to shoots and less to roots when compared to NO3− -fed plants. The strong difference in biomass partitioning may relate to the relative surplus of carbon per unit plant N (or, alternatively, the relatively suboptimal rate of N assimilation per unit plantC) in NO3−-fed plants Inherently slow-growing dwarf shrubs accumulated virtually no free nitrate in their tissues and reduction of nitrate was strictly root-based. Faster-growing herbaceous plants, however, partitioned the assimilation of nitrate over both shoots and roots, thereby accumulating relatively high tissue NO3− levels. Ion uptake rates depended clearly on the ‘relative shoot demand’. At similar shoot demands, especially in the herbaceous species, specific uptake rates for N and total inorganic (non-N) anions were higher in NH4+ -fed plants, whereas the uptake rate for total (non-N) cations was higher in NO3−-fed plants. Rate of P uptake was enhanced with increasing plant demand, but was independent of the N source. Net H+ extrusions ranged from 1.00 to 1.34 H+ per NH4+, and from −0.48 to −0.77 H+ per NO3− taken up.