Variability of nitrification potentials in patches of undergrowth vegetation in primary Scots pine stands

We investigated whether the distribution of undergrowth species could explain small-scale spatial variability of nitrate production in primary Scots pine stands. This was done by measuring the nitrification potential, i.e. the accumulation of NO3--N in homogenized samples of the ectorganic layer, in vegetation patches that consisted of single species. These were the grass Deschampsia flexuosa and the dwarf shrubs Empetrum nigrum, Vaccinium myrtillus and Vaccinium vitis-idaea. The nitrification potential varied extremely among the vegetation patches (CV = 142). This variability was observed for all undergrowth species. Hence, we found no indications that the chemical composition of the plant species, e.g. the relatively high content of polyphenolics and terpenoids of the dwarf shrubs, was important as a regulator of the nitrification process. The nitrification potential was not significantly correlated with NH4+-N, pH or moisture and was only poorly predicted by the N mineralization potential (R(2) = 0.157). Thus, differences in the nitrification potential between the patches could neither be explained by the composition of the undergrowth vegetation nor by the availability of NH4+-N.