Nitrification rate limitation

Nitrification is limited in most soils by the supply rate of NH4+ (40, 41). Competition exists between nitrifiers and vegetation, which may both be limited by the availability of NH4 +. This microbial demand for NH4 +, coupled with the high cation-exchange capacity of most temperate forest soils, leads to surface-water NH4+ concentrations that are usually undetectable. Nitrification rates may also be limited by inadequate microbial populations, lack of water, allelopathic effects (toxic effects produced by inhibitors manufactured by vegetation), or by low soil pH. 

Nitrate flux from aerobic zones to anaerobic sites For nitrate reduction to occur in wetlands, nitrate must be present in anaerobic zones. Thus, nitrate reduction rates are regulated by transport of nitrate either by diffusion or by mass flow from aerobic zones to anaerobic portions of the soil. Similarly, the rate of nitrification and the oxygen availability in the soil regulate nitrate concentrations in aerobic zones of the soil. In wetlands with limited inputs of nitrate from external sources, nitrification and atmospheric deposition are the primary sources of nitrate. In these systems, denitrification rates are tightly coupled to nitrification rates. 

Both potential nitrification and denitrification rates decreased with distance from the inflow along the entrophic gradient in WCA-2A and the rates were positively correlated to total phosphorus (White and Reddy, 2003). Higher nitrification rates in soils from entrophic areas were due to high ammonium levels, whereas higher denitrification rates were due to greater availability of bioavail-able carbon. Both nitrifiers and denitrifiers were limited by phosphorus in the oligotrophic areas of WCA-2A (White and Reddy, 1993,2003). 

The kinetics of nitrification are a fnnction of several factors the most important of which inclnde pH, temperature, and the concentrations of ammoifia and dissolved oxygen. Experience has shown that the optimum pH of nitrification lies between 7.2 and 8.8. Outside this range, the rate becomes limited. As shown in the nitrification reaction, acidity is produced. If this acidity is not buffered by addition of sufficient alkalinity, the pH could control the process and the kinetics become pH limited. We have not addressed the mathematics of this issue. 

Assimilation of NOs" assimilation of NOs requires the synthesis of NOs and N02 reductases, associated active transport systems, and the turnover of cellular ATP and NADPH (Chapter 32 by Berges and Mulholland, this volume McCarthy, 1981 Syrett, 1981). Further, supply of NOs is limited by rates of nitrification and vertical... 

In the acetylene inhibition technique, acetylene is added to a water sample, which inhibits the reduction of N2O to N2 (Sorensen, 1978). The accumulation of N2O is then measured using gas chromatography and an electron capture detector and the denitrification rate is taken to be equal to the total N2O flux. One potential problem is incomplete inhibition of N2O reduction to N2, particularly in the presence of hydrogen sulfide, a compound commonly found under anaerobic conditions. Another potential problem with the technique is that acetylene also inhibits nitrification, a process that often supplies the NOs and N02 substrates for denitrification. To inhibit nitrification is to inhibit denitrification if it is at aU substrate limited (Hynes and Knowles, 1978). 

Analysis of N20 during nitrification and denitrification rate studies has been limited. Punshon and Moore (2004) further developed the method of Barnes and Owens (1998) to extract N20 produced from either NH4+ or by... 

Often, the rates of fertilization in intensively managed agriculture are intended to satiate the needs of crop plants for these chemicals, so their productivity will not be limited by nutrient availability. However, excessive rates of fertilization have important environmental costs. These include the contamination of ground water with nitrate eutrophication of surface waters caused by nutrient inputs (especially phosphate) acidification of soil because of the nitrification of ammonium to nitrate large emissions of nitrous oxide and other nitrogen gases to the atmosphere, with implications for acid rain and Earth s greenhouse effect and the need to use herbicides to control the weeds that flourish under artificially nutrient-rich conditions. 

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