Measurements of gross N transformation rates are important to properly understand N cycling processes in agricultural soils where both productive and consumptive processes occur. The objective of the study was to determine the effect of repeated application of dairy‐waste compost (DC), liquid dairy‐waste (LW), and ammonium sulfate (AS) on gross N mineralization and nitrification rates and N supplying potential of an agricultural soil. Our goal was to examine both production and consumption of inorganic N for their effects on the balance between N supply from treated dairy‐wastes and plant N demand. Treatments were applied at rates approximately equivalent to 100 and 200 kg available N ha −1 for 6 yr annually. Field‐based N 15 pool dilution techniques and laboratory incubation experiments were employed to measure gross rates and mineralization potential of the soil. Both levels of DC raised the labile organic N pool significantly but only the high level DC significantly increased the decomposition rate constant ( k ). The mean gross N mineralization rates for 1999 to 2002 for the high levels of DC, LW, and AS were 5.72, 2.89, and 1.27 mg N kg −1 d −1 , whereas gross nitrification rates were 10.24, 1.57, and 0.74 mg N kg −1 d −1 , respectively. Net mineralization rates were <35% of gross rates while nitrate consumption was not significant under any treatment. Variability in gross rates was high in the soils receiving DC, which could be due to presence of hotspots of labile organic matter. Elevated gross N transformation rates in plots receiving DC indicate the dynamic nature of this agricultural soil after repeated applications of dairy‐waste.
Local associations between anammox bacteria and obligate aerobic bacteria in the genus Nitrosococcus appear to be significant for ammonia oxidation in oxygen minimum zones. The literature on the genus Nitrosococcus in the Chromatiaceae family of purple sulfur bacteria (Gammaproteobacteria, Chromatiales) contains reports on four described species, Nitrosococcus nitrosus, Nitrosococcus oceani, 'Nitrosococcus halophilus' and 'Nitrosomonas mobilis', of which only N. nitrosus and N. oceani are validly published names and only N. oceani is omnipresent in the world's oceans. The species 'N. halophilus' with Nc4T as the type strain was proposed in 1990, but the species is not validly published. Phylogenetic analyses of signature genes, growth-physiological studies and an average nucleotide identity analysis between N. oceani ATCC19707T (C-107, Nc9), 'N. halophilus' strain Nc4T and Nitrosococcus sp. strain C-113 revealed that a proposal for a new species is warranted. Therefore, the provisional taxonomic assignment Nitrosococcus watsonii is proposed for Nitrosococcus sp. strain C-113T. Sequence analysis of Nitrosococcus haoAB signature genes detected in cultures enriched from Jiaozhou Bay sediments (China) identified only N. oceani-type sequences, suggesting that different patterns of distribution in the environment correlate with speciation in the genus Nitrosococcus.
An agricultural soil was treated with dairy-waste compost, ammonium-sulfate fertilizer or no added nitrogen (control) and planted to silage corn for 6 years. The kinetics of nitrification were detemined in laboratory-shaken slurry assays with a range of substrate concentrations (0–20 mM NH4+) over a 24-h period for soils from the three treatments. Determined concentrations of substrate and product were fit to Michaelis–Menten and Haldane models. For all the treatments, the Haldane model was a better fit, suggesting that significant nitrification inhibition may occur in soils under high ammonium conditions similar to those found immediately after fertilization or waste applications. The maximum rate of nitrification (Vmax) was significantly higher for the fertilized and compost-treated soils (1.74 and 1.50 mmol N kg−1 soil day−1) vs. control soil (0.98 mmol kg−1 soil day−1). The Km and Ki values were not significantly different, with average values of 0.02 and 27 mM NH4+, respectively. Our results suggest that both N sources increased nitrifier community size, but did not shift the nitrifier community structure in ways that influenced enzyme affinity or sensitivity to ammonium. The Km values are comparable to those determined directly in other soils, but are substantially lower than those from most pure cultures of ammonia-oxidizing bacteria.
