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Freshwater aquaculture sector constitutes an important component of Indian fisheries contributing to more than 50% of the total fish production of the country. Fish being a protein-rich crop, this high level of production demands transformations of good amount of nitrogen into fish flesh. It has been grossly estimated that 0.15 million ton of nitrogen was harvested for producing 4.9 mt of fishes through freshwater aquaculture systems of India during 2013–14. This quantum of nitrogen nutrition needs to be provided to the fishes through natural food chains and/or supplementary feeds. However, in order to compensate for the low conversion efficiency of N to fish flesh under the prevailing aquatic conditions, considerably higher quantity of this input needs to be maintained in the fish pond environment. Of this, a large part remains unutilized and is liable to be lost from the culture system in various reactive forms. Various pathways, magnitudes, and dynamics of transformation of this consumed as well as unutilized nitrogen are assessed in this chapter and an estimated N budget for freshwater aquaculture is prepared. Scope of reducing the large-scale losses of reactive nitrogen from freshwater aquaculture environment has also been addressed.

High doses of nitrogenous fertilizers are often applied in fishponds but only a small part of the added nitrogen is utilized for improving primary productivity and, thereby, fish yields. A large por- tion of the nitrogen is lost from the pond environment through various mechanisms, especially nitrification. In the present investigation, we studied the effects of three nitrification inhibitors: (a) neem (Azadirachta indica) extract, (b) karanj (Pongamia glabra) extract, and (c) sodium azide (NaN3), on the transformation of applied nitrogen in simulated fishpond conditions. The study revealed that nitrification inhibitors considerably retard the rate of nitrification from the ammoni- um form of nitrogen (NH4+), resulting in a higher amount of nitrogen in the soil and water. The increased nitrogen concentration significantly increased gross primary production and, hence, improved the efficiency of the added nitrogen.