# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 2 The Biological Nitrogen Cycle  
The nitrogen cycle is the biogeochemical loop that supplies nitrogen to living organisms and keeps it recirculating in the biosphere (Figure 1). Nitrogen undergoes a variety of chemical reactions (oxidations and reductions) (Table 1) that produce N-compounds with oxidation states ranging from +5 (NO\({}_{3}^{-}\)) to \(-3\) (NH\({}_{4}^{+}\)) [24].  
The main and most abundant source of nitrogen is the atmosphere, where it is found in molecular form (N\({}_{2}\)). In this form, it is generally not available to plants, which limits primary productivity in most ecosystems [25]. Only a small group of highly specialized prokaryotes are capable of using nitrogen from the atmosphere, taking N\({}_{2}\) from the air and  
\begin{table}
\begin{tabular}{c l c} \hline  & **Process** & **Chemical Reaction** \\ \hline \(\blacksquare\) & Nitrogen fixation & N\({}_{2}\) + 8 H\({}^{+}\) + 8 e\({}^{-}\) \(\rightarrow\) 2 NH\({}_{3}\) + H\({}_{2}\) \\ \(\blacksquare\) & Ammonia equilibrium & NH\({}_{3}\) + H\({}_{2}\)O \(\rightarrow\) NH\({}_{4}\)\({}^{+}\) + OH\({}^{-}\) \\ \(\blacksquare\) & Ammonium oxidation & NH\({}_{4}\)\({}^{+}\) + 3/2 O\({}_{2}\) \(\rightarrow\) NO\({}_{2}\)\({}^{-}\) + H\({}_{2}\)O + 2 H\({}^{+}\) \\ \(\blacksquare\) & Nitrite oxidation & NO\({}_{2}\)\({}^{-}\) + 1/2 O\({}_{2}\)\(\rightarrow\) NO\({}_{3}\)\({}^{-}\) \\ \(\blacksquare\) & Nitrification & NH\({}_{4}\)\({}^{+}\) + 2 O\({}_{2}\)\(\rightarrow\) NO\({}_{3}\)\({}^{-}\) + H\({}_{2}\)O + 2 H\({}^{+}\) \\ \(\blacksquare\) & Denitrification & 2 NO\({}_{3}\)\({}^{-}\) + 10 e\({}^{-}\) + 12 H\({}^{+}\)\(\rightarrow\) N\({}_{2}\) + 6 H\({}_{2}\)O \\ \(\blacksquare\) & Anammox & NO\({}_{2}\)\({}^{-}\) + NH\({}_{4}\)\({}^{+}\)\(\rightarrow\) N\({}_{2}\) + 2 H\({}_{2}\)O \\ \hline \end{tabular}
\end{table}
Table 1: Chemical reactions in the nitrogen cycle.  
Figure 1: The biological nitrogen cycle.  
reducing it to ammonium (NH4+), a nitrogenous form that can be uptaken by plants. This process is known as biological nitrogen fixation. While certain nitrogen-fixing organisms are free-living, others need to form symbiotic associations with a host to complete the process. [26]. Among the latter, the rhizobial and actinorhizal symbiotic systems stand out for their high efficiency [27]. All nitrogen-fixing organisms are characterised by the nitrogenase enzyme complex, which is responsible for the reduction reaction of N2 to NH3/NH4+. In this way, the soil is enriched in nitrogenous compounds that can be assimilated by the plants that serve as food for herbivores, and through them for carnivores, thus incorporating nitrogen into their organism. Later, through excretion and the decomposition of plant and animal residues, nitrogen returns to the soil, this time in the form of ammonium (NH4+); this process is named mineralization. This NH4+ remains in the soil to be absorbed by plants that do not have the nitrogenase system [28].  
During nitrification process, a series of aerobic nitrifying bacteria transform the NH3/NH4+ produced during mineralization initially into nitrite (NO2-), and finally, it is oxidized to nitrate (NO3-), the nitrogenous form primarily assimilated by the radicular system of plants. The nitrification process is particularly important as it produces an extra supply of nitrogen available to plants. Finally, NO3- is transformed by denitfying bacteria into gaseous nitrogen (N2), which returns to the atmosphere, completing the cycle [29].  
The anammox (anaerobic ammonium oxidation) process, also makes a significant contribution to the nitrogen cycle, turning over NO2- and NH4+ to N2 gas. This conversion is performed by bacteria of the phylum Planctomycetes, which are present in anoxic freshwater and marine environments [30].  
Nitrogen and its cycle are the key for different forms of life as its multiple oxidation states confer to nitrogen a significant biological relevance in the context of microbial metabolism [31].