Nitrification and occurrence of Nitrobacter by MPN-PCR in low and high nitrifying coniferous forest soils

The nitrification of three coniferous forest soils was investigated: a podzol (Fontainebleau, France) with low N deposition and no nitrate accumulation, an acid mull (Haldon, UK) with low N deposition and moderate nitrate accumulation, and a podzol (Wekerom, The Netherlands) with high N deposition and high nitrate accumulation. Twenty-one months in situ lysimeter experiments and short-term and long-term in vitro incubations were performed to respectively establish the status of NO₃^- accumulation and potential nitrification of these soils. These complementary approaches allowed to conclude that the absence of NO₃^- accumulation in Fontainebleau soil was effectively due to a lack of nitrifing activity in the whole profile while the high NO₃^- accumulation observed in the Wekerom soil essentially resulted from an active nitrifying activity in the Oh horizon of this soil. For the Haldon soil, the inadequacy between the lysimeter and the short-term in vitro experiment was discussed.Surprisingly, relatively high densities of the nitrite-oxidiser Nitrobacter (enumerated by PCR-MPN technique) were found in all the studied horizons of the three soils. Moreover, a long-term incubation experiment showed that inhibition of nitrification in the Oh Fontainebleau soil could be removed after submitting the soil samples to constant conditions during 82 days. This demonstrates that in these soils, ammonium-oxidisers (as well as nitrite-oxidisers) are present and that extrinsic factors were involved in the inhibition of nitrification.