Denitrification is the bacterial process where Nitrate is reduced to gaseous Nitrogen. This is accomplished in a series or steps: Nitrate to Nitrite, Nitric oxide, Nitrous oxide and finally Nitrogen gas.
Denitrification is accomplished by primarily Heterotrophic and Autotrophic bacteria, with Heterotrophic bacteria playing the larger role. Autotrophic bacteria utilize sunlight and inorganic chemicals as an energy source, whereas Heterotrophic bacteria rely mainly on organic carbon as an energy source.
Denitrification takes place under special conditions in both terrestrial and aquatic ecosystems. It occurs where Oxygen is depleted, and bacteria utilize nitrate in the respiration process. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in environments where oxygen consumption exceeds the rate of oxygen supply, such as in some soils and groundwater, wetlands and the substrates in the bottom of a pond.
In a properly constructed eco-system pond, denitrification is a normally occurring process. A classic Koi pond, however, because of its method of construction (no rock or gravel; bare liner), does not have any areas of low or depleted Oxygen to promote the growth of these denitrifying bacteria. External methods must be used for denitrification such as trickle towers, fluidized beds and rotating biological contactors. Additionally, attention should also be given to ORP (oxidation reduction potential) levels.
In the previous post, the toxic level of Nitrate was shown to be arbitrary at best. This was focused on high end Nitrate levels. The complete absence of Nitrate (in an eco-system pond), however, is definitely cause for concern. In the absence of Nitrate, the denitrifying bacteria will utilize Sulfur as a replacement, producing Hydrogen sulfide (rotten egg gas) which is highly poisonous. It should be pointed out that, for this scenario to occur, the pond would have to be completely neglected……a total lack of maintenance or ownership responsibility or concern.
This, then, is where the Nitrogen cycle comes full circle. With very few exceptions, all living organisms rely on this continuous process for their existence.
The subject may be debated for years to come, but it has been shown that Carp (Koi) are some of the most durable and adaptable fish in existence. I think that the Australians have summed up the hardiness in Carp (Koi) by referring to them (not very affectionately) as “River Rabbits”.
We can now begin venturing into the world of aquatic life; exploring the succeeding higher levels of the Food Web and how they interact.
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