Fate of 15N labelled nitrate and ammonium in a fertilized forest soil

Abstract

The possibility that long-term atmospheric nitrogen pollution and fertilization of forest soil may serve as a basis for adaptation for enhanced transformation rates of NO 3 − and NH 4 + in soil bacteria was elucidated in a laboratory bioassay. Bacteria extracted from soils that had been fertilized at various rates for the last 30 yr were characterised with respect to their capability to reduce or oxidise different nitrogen sources. The same soils were used under oxic or anoxic conditions to quantify denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and nitrification. 15NO 3 − and 15NH 4 + were added as tracers to the soils, which were incubated in bottles for 3 to 5 d. Concentrations of 15N 2O in headspace and 15NO 3 − and 15NH 4 + in soil extracts were determined by gas chromatography–mass spectrometry. Total numbers of bacteria were similar in all soils and ranged from 3 to 4×10 8 cells g −1 d.wt. of soil. Between 50 and 70% of the isolated strains were capable of reducing nitrate and the majority of them reduced nitrate to ammonium. About 0.01‰ of all isolates were classified as nitrifiers. Both nitrate reducers and nitrifiers were more common in fertilized soils than in the unfertilized control soil. The foremost fate of added 15NO 3 − and 15NH 4 + in all soils was immobilisation. More than 85% was immobilised in anoxic soils and between 64 and 97% in the oxic soils, with the lowest quantities in fertilized soils. As regards the remaining, non-immobilised N, DNRA dominated over denitrification, as could be expected from the higher frequency of ammonifying bacteria compared with denitrifiers. There was no obvious relationship between NH 4 + produced and the amount of fertilizer applied, whereas denitrification was negatively correlated with the amount of fertilizer applied. Nitrifying activity was low in all soils with no obvious relationship between NO 3 − produced and fertilizer applied. Hence, no correlation was found between the relative abundance of N transforming bacteria and the transformation activity. The N flux followed essentially the same pattern as that seen for product formation. The DNRA flux was higher than that of both denitrification and nitrification. DNRA and denitrification fluxes were highest in the control soil, whereas the nitrification flux was low in all soils. The absence of evidence for adaptation to enhanced rates of transformation of NO 3 − and NH 4 + in soil bacteria exposed to long-term N fertilization is reflected by the low concentration of extractable inorganic N in the fertilized soils. As a consequence of the quantitative importance of immobilization of added N, differences in physiological capacity evolved in soil bacteria to immobilize and mobilise N may determine the rates by which inorganic N is available for plant growth or lost to groundwater and air.