Nitrification in adjacent cultivated and pasture landscapes – a comparison
van Kessel, C.
Variability in nitrification and autotrophic nitrifying communities were assessed at shoulder and footslope positions of a cultivated and a pasture landscape. The sampling design was a systematic grid design and all analysis were carried out for soils collected from 10 grid points at the shoulder and the footslope position of each landscape. Gross nitrification was measured in intact soil cores using 15N isotope pool dilution technique. NH4+ oxidizers were enumerated using the most probable number technique and Michaelis-Menten kinetics for NH,+ oxidation was determined in soil slurries. Gross nitrification did not vary between or within studied sites and remained less than 20% of the Vmax. Population densities of NH4+ and NO2-, oxidizing bacteria at the cultivated site were about 10-fold higher than those at the pasture site. The Vmax for NH4+ oxidation was 28 and 15 ug g-1 soil d-1 at the cultivated and the pasture site respectively and varied significantly between sites. The apparent Km for NH4+ oxidation varied from 5 to 8 uM at the cultivated site and they were about 10 to 100-fold lower than those observed for the pasture site. Although in situ nitrification did not differ between sites, significantly higher V, indicated that cultivation practices enhance nitrification by sustaining a larger nitrifying community having a higher substrate affinity. NH4+ oxidizing bacterial communities were different between sites with respect to the substrate affinity and the competitive ability for the substrate.
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