Adverse Effects of Ammonia on Nitrification Process: the Case of Chinese Shallow Freshwater Lakes

摘要

Nitrification is a process in which ammonia is oxidized to nitrite (NO_2~-) that is further oxidized to nitrate (NO_3~-). The relations between these two steps and ambient ammonia concentrations were studied in surface water of Chinese shallow lakes with different trophic status. For the oxidations of both ammonia and NO_2~-, more eutrophic lakes generally showed significantly higher potential and actual rates, which was linked with excessive ammonia concentrations. Additionally, both potential and actual rates for ammonia oxidation were higher than those for NO_2~-oxidation in the more eutrophic lakes, while in the lakes with lower trophic status, both potential and actual rates for ammonia oxidation were almost equivalent to those for NO_2~- oxidation. This can be explained by the excessive unionized ammonia (NH_3) concentration that inhibits nitrite-oxidizing bacteria in the more eutrophic lakes. The laboratory experiment with different ammonia concentrations, using the surface water in a eutrophic lake, showed that ammonia oxidation rates were proportional to the ammonia concentrations, but NO_2~- oxidation rates did not increase in parallel. Furthermore, NO_2~- oxidation was less associated with particles in natural water of the studied lakes. Without effective protection, it would be selectively inhibited by the excessive ammonia in hypereutrophic lakes, resulting in NO_2~- accumulation. Shortly, the increased concentrations of ammonia cause a misbalance between the NO_2~--producing and the NO_2~--consuming processes, thereby exacerbating the lake eutrophication.