Nitrification at the community level across a cultivated and a pasture landscape
Rajapaksha, R. M. Chandani P.
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Nitrification is a key N transformation process which provides one of the major mineral N sources for biota in terrestrial ecosystems and influences atmospheric and hydrospheric chemistry via secondary reactions of its products. This study was conducted to assess the relative contribution of autotrophs and heterotrophs to nitrification activity across a cultivated and a pasture landscape. Shoulder and footslope complexes were chosen as experimental units and assays were carried out in soil obtained from randomly selected grid points (10) at each landscape position. Gross autotrophic and heterotrophic nitrification rates were determined in situ and in the laboratory using the ¹⁵N isotope pool dilution technique in conjunction with the biochemical block C₂H₂. Michaelis-Menten kinetics for NH₄⁺ oxidation were quantified in soil slurries. Population densities of autotrophic nitrifiers were obtained by the MPN technique. Heterotrophic nitrifiers were isolated using two selective media and identified based on fatty acid profiles and taxonomic morphology. Total nitrification ranged from 3.8 to 4.9 μg N g⁻¹ soil d⁻¹ at both sites but at low moisture conditions declined to 2.9 and 2.2 μg N g⁻¹ soil d⁻¹ at the cultivated and the pasture landscape, respectively. Heterotrophic nitrification contributed 17-22% and 14-31% to the total nitrification at the cultivated and the pasture landscape, respectively. Among heterotrophic nitrifiers isolated and identified, several strains oxidized both NH₄⁺ and peptone to NO₂⁻ and/or NO₃⁻ and two strains produced N₂O. Athrobacter spp. were the predominant heterotrophic nitrifier among the soil isolates. The effects of cultivation and topography on the nitrification activity of both groups of nitrifiers were not statistically significant. The average Vmax of NH₄⁺ oxidation approached a maximum of 35 μg N g⁻¹ soil d⁻¹ at the cultivated landscape, whereas at the pasture landscape it remained below 20 μg N g⁻¹ soil d⁻¹. Conversely, apparent Kₘ values at the pasture landscape were approximately 2 to 20-fold higher than those of the cultivated landscape and varied from 16 to 165 μM. Ammonium-oxidizers at the cultivated landscape were characterized by a lower apparent Kₘ for NH₄⁺ oxidation at about 10-fold greater population densities (3.4 x 10⁶ g⁻ soil) compared with those of the pasture landscape. Ammonium-oxidizers at both landscapes operate adaptive mechanisms toward the substrate fluxes and survive under oligotrophic conditions. At the cultivated landscape, autotrophic nitrification was controlled by soil moisture and pH, whereas heterotrophic nitrification was limited by organic C. Substrate availability was identified as the main driving variable of autotrophic nitrifiers at the pasture landscape.