Organic nitrogen is omnipresent in nature. The variations of dissolved organic nitrogen (DON) concentration are usually a result of microbial activity, anthropogenic influence, and/or other physical-chemical processes (e.g. photo-mineralization). Therefor Organic nitrogen is omnipresent in nature. The variations of dissolved organic nitrogen (DON) concentration are usually a result of microbial activity, anthropogenic influence, and / or other physical-chemical processes (eg photo-mineralization).