Fixation:
Nitrogen can be absorbed or converted directly from the atmosphere. The air we breath is predominately Nitrogen not oxygen.
Fixation occurs primarily from either commercial fertilizers or from organic material with high nitrogen content, such as alfalfa, or clover being tilled into the soil. If the nitrogen content of the organic material, like a plant, is high enough then the nitrogen will be absorbed from the break down of the organic material. If not then Nitrogen may be absorbed directly from the air.
In this case atmospheric nitrogen is converted to ammonia and then into organic nitrogen. Organic nitrogen must be broken down further before plants can readily use and absorb the nutrients.
Mineralization:
Mineralization is the process by which microbes decompose organic Nitrogen from waste solids, dead organic matter, and plant residues into ammonia. Rates of mineralization vary with temperature, moisture, and oxygen content of the media in which it breaks down. Mineralization most readily occurs in moist well aerated (oxygen rich) environments with a temperature between 68-95 F.
The ammonia is further broken down from NH3 to NH4 or ammonium, it is this ammonium that is converted into Nitrites and Nitrates.
this is where our solids are broken down in to a form usable by our beneficial bacteria. The breakdown of these solids uses up the nitrogen and oxygen in their immediate area. If we allow these solid to accumulate in our grow bed then we can see that at a certain level a tipping point is reached. At that point all of our beneficial bacteria suffocate and die, further adding to the solid wastes in the media. When this situation occurs Nitrification ceases and instead we have locked up nitrogen with no available oxygen for the bacteria or the plants. So we want and must have a mineralization process, but we are going to remove that process from the grow bed and let it occur in a controllable environment that we will build into our system.
Nitrification:
Nitrification is the process by which our beneficial bacteria, Nitrosomonas, convert ammonium to Nitrite. The bacteria do this to extract energy from the nitrogen contained in the ammonium. To further this process Nitrite is consumed by Nitrobacter, which in turn release Nitrate into the bio-filter medium.
Nitrification is most rapid in warm moist conditions between 67-87 F. But it is important for our purposes to note that Nitrification process cease at temperatures below 41F. As we want to feed plants and filter water for our fish, Nitrification is the critical process that we need to foster and encourage. This is done most effectively by controlling the environment so that the following 3 stages of the nitrogen cycle are done on our terms and not allowed to get out of control.
Denitrification:
Denitrification occurs when Nitrogen is lost through the conversion of nitrate to a gaseous form of nitrogen, such as nitric oxide, nitrous oxide, and dinitrogen gas. This occurs when our medium (soil, growbed, ect) is saturated and the anaerobic bacteria use nitrate as an oxygen source.
In other words when the system becomes septic all of our nitrogen begins to off gas and our oxygen is lost. Depending on the mineral content of the waste a whole host of noxious gases can be released. This is where the sewage smell comes from, gases being released in an anaerobic form of the nitrogen cycle. Ironically this is the part of the cycle that is talked about the least in aquaponics courses and books. And it’s the one area that will crash your entire system if ignored. This is very important and we will deal with it in such a way as to keep our oxygen levels high AND retrieve our locked up mineral content as well, resulting in lush mineral rich plant growth, without septic growbed conditions.
Volatilization:
Volatilization is the loss of Nitrogen through the conversion of ammonium to ammonia gas. This occurs in high pH or water temperature conditions that favor evaporation. Volatilization is higher in situations where solid waste is exposed directly to the air. After evaporation occurs almost all of the nitrogen will have been lost to the atmosphere.
This is why manures are tilled into the ground immediately. If they sit too long on the surface, the ammonium the bacteria depend on to create nitrite and nitrate will evaporate. In our case with aquaponics, we simply need to keep the solid waste from reaching the surface air. This will be done through a solids extraction technique, where they will remain at the bottom of a liquid holding area.
Lets look at the last part of the nitrogen cycle.
Immobilization:
Immobilization is the exact reverse of Mineralization. Anything living requires Nitrogen, Plants, bacteria, animals, and people. And in any given ecosystem organisms compete for available nitrogen in order to survive. In anaerobic conditions, whether in soil or any medium all of the nitrate and ammonium are taken up by the anaerobic bacteria and made unavailable to plant and animal life forms. The more solid organic material there is in the medium, the more sever immobilization will become.
Immobilization is the septic condition where the nitrogen and oxygen are made unavailable to plants.
Our goal in aquaponics should therefore be to manage the nitrogen for maximum plant uptake by removal or reduction of the Immobilization, Volitilization and Denitrification processes.
Now that we have a basic understanding of what is happening in our grow beds. We can finally talk about system design. How to set up your system, but more importantly why certain things need to be done for you to have a successful positive experience with aquaponics.
Nitrogen Cycle Summary
• Symbiosis is the key to a successful aquaponics system.
• Ammonia is formed from fish waste and decaying matter in the system
• Biofilters are where beneficial bacterias colonize and grow in order to convert toxic ammonia in to non toxic Nitrate.
• Nitrosomonas. Are bacteria that feed on ammonia and produce Nitrite.
• Nitrobacter Are bacterial that feed on nitrite and convert that nitrite into Nitrate.
• Aerobic bacteria require oxygen in order to live and flourish. Nitrosomona and Nitrobacter are aerobic bacteria
• Anaerobic bacteria do not require oxygen. These bacteria are present in solid waste or sludge zones.
• There are five stages of the Nitrogen Cycle
• Fixation is the process of atmospheric nitrogen being absorbed, converted to ammonia, and then converted to organic nitrogen
• Mineralization is the process by which microbes decompose organic Nitrogen from waste solids, dead organic matter, and plant residues into ammonia.
• Nitrification is the process by which our beneficial bacteria, Nitrosomonas, convert ammonium to Nitrite.
• Denitrification occurs when Nitrogen is lost to the atmosphere through the conversion of nitrate to a gaseous form of nitrogen. Anaerobic bacteria are a primary cause of deitrification.
• Volatilization is the loss of Nitrogen through the conversion of ammonium to ammonia gas. This occurs in high pH.