Nitrification determination methods

THE METHOD EOR DETERMINING N REQUIREMENTS. Mineral nitrogens (N J include nitrate (NOj ), nitrite and ammonia (NH ). Nitrite is an intermediate in nitrification and denitrification and occurs in negligible amounts in the soil. The amount of ammonia is highly dependent on soil pH and increases with increasing acidity. 

Clark, D. R., Rees, A. P., and Joint, 1. (2007). A method for the determination of nitrification rates in oligotrophic marine seawater by gas chromatography/mass spectrometry. Mar. Chem. 103, 84—96. 

There have been very few comparisons of different methods for determination of nitrification rates. Enoksson (1986) compared three isotopic methods, incorporation with N-serve to inhibit nitrification, and N03 isotope dilution. She found ammonium oxidation measured with was 2-7 times greater than determined by the other methods, but pointed out that both different incubation times (from 1 to 12 days) and different substrate additions were used which would greatly alter the measured rates. Jones (1992) found a good relationship between a CO method and for measuring ammonium oxidation. Although most researchers have switched to direct assessment using there is stiU usage of the 002 method (e.g., Brion et al, 2000). 

ISO 14238 1997 Soil quality - Biological methods - Determination of nitrogen mineralization and nitrification in soils and the influence of chemicals on these processes. 

At last, the inhibition technique takes advantage of the property of acetylene to block the reduction of N O to N after it is injected into the sediment. The total amount of N O produced is then the measure for the denitrification rate as it is easy to determine by gas chromatography (Andersen et al. 1984) or by microsensors (Christensen et al. 1989). The advantage of this method is that analyses can be carried out rapidly and sensitively. Problems are (a) N O reduction is sometimes incomplete, (b) a homogenous distribution of in the pore water is difficult to maintain, (c) inhibits nitrification in the sediment meaning that the coupled system (nitrification / denitrification) might be seriously affected due to the applied method, and... 

The first two experiments yielded the following results the NH concentration began to increase from the first to the second day, it reached its maximum at the seventh, then nitrification followed. The PO " concentration increased from the second to the fifth day. The concentration of the dissolved OM in the first experiment determined by permanganate oxidisabil-ity, the albuminoid nitrogen method and BOD exceeded the concentrations observed in the control experiment without plankton by the sixtieth day (Fig. 2). 

The well-established fact that potassium interferes with the release of NH4 from clay minerals, and the effect commonly increases with concentration, has led to the use of heterotrophic bacteria as agents for the determination of availability of fixed ammonium. These organisms have a much higher requirement for potassium than do the nitrifiers. Hence, experiments have been performed (Nommik, 1965) in which carbohydrates have been added to soils containing different amounts of fixed ammonium and the amount of CO2 evolved determined. Such experiments have shown that this method of measuring the availability of fixed ammonium is fairly satisfactory provided the added energy source is readily available and free from potassium. Data on this method are still inadequate but present evidence indicates that the nitrification method is much to be preferred as a routine method. 

Briefly, any chlorine is removed from a water sample to be tested and the sample or a diluted version of it is adjusted to approximately pH 7. The pH buffer and nutrients are added along with a seed and/ or nitrification inhibitor if necessary. Two BOD bottles are filled to an extent such that insertion of the stoppers displaces all the air. The initial DO level (DOi) of the first bottle is determined using the Winkler titration method (a titrimetric determination) or using one of the modifications of this method (most commonly the azide modification). The second bottle is then incubated for 5 days, at which time the DO level (DOf) is determined ... 

Improved methods, particularly high performance liquid chromatography, are now available for the determination of phenolic acids in soils and plants but problems associated with the extraction of free and bound acids from the soil remain. The concentrations of the free acids extractable from most soils by v/ater are very low i.e. of the order of 1 pM or less. These concentrations appear to be too lov/ to have significant effects on plant grbv/th, but further studies are required of the extent to v/hich sorbed or bound phenolic acids may exert physiological effects. More information is also needed on the influence of the acids on microbial processes and, in particular, on nitrogen fixation and nitrification. ... 

---