Nitrogen response of herbs and graminoids in experiments with simulated acid soil solution

Sand-solution experiments involving 15 herbs and 13 graminoids were performed to study how biomass was effected by N concentrations of 50, 250 and 1250 μM, respectively, in a simulated acid forest soil solution. The concentrations of the various components corresponded with those found in soil solutions in deciduous forests of southern Sweden. The experiments were undertaken to study the extent to which species are affected differently by increases in the concentration of N as compared with that of the other nutrients. The underlying hypothesis was that species with a high demand for N and a lesser demand for other nutrients are particularly competitive in areas with acidic soil and a high degree of N deposition. The graminoids were found to be favoured more than the herbs by the addition of N, 46% displaying a significantly greater biomass at 250 than at 50 μM N as compared with only 7% for the herbs. Some species attained their highest biomass at 1250 μM N and others at 50 μM N. It is concluded that at the highest N concentration, growth was limited for most species by the supply of other nutrients and that at the intermediate N concentration the graminoids were more efficient than the herbs in utilising N. It was found that the Ellenberg N indicator values could not be used to predict biomass production under the conditions studied since they failed to adequately explain the response to treatment by N.