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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.

Numerous coal fired plants operating in different parts of the world produce huge amount offly-ash every year. Such large scale generation of this waste material poses several problems,the most important ones being environmental pollution and occupation of large areas of landfor storage and disposal. Hence, the urgent and imperative needs to overcome these problemsthrough not only safe disposal but also gainful utilization of these materials cannot probablybe overemphasized. Among various uses of fly-ash, its application to agricultural soils hasshown encouraging results in many countries owing to the fact that the waste contents manyessential plant nutrients in varying concentrations. However, the major problem of recyclingfly-ash in agriculture is that most of the nutrient elements in this waste material remainlargely in insoluble forms and hence do not become available to plants in requiredconcentrations. Under this context, use of fly-ash along with organic materials appears to bean effective proposition. In view of high efficiency of vermicomposting biotechnology inincreasing the pace of the composting process and also in encouraging higher availability ofdifferent nutrients with the help of epigeic earthworms, a series of studies were carried out toassess the possibility of improving the usability of fly-ash in agriculture in combination withorganic wastes through adoption of vermicomposting. The results of the investigation showedvermicomposting to increase the availability of different major and micro nutrients in fly-ash.On farm trials carried out on use of vermicomposted fly-ash and organic wastes showed useof 15 ton per hectare of vennicomposted fly-ash to result in significant yield increments inpotato cultivation. This also reduced the application of 40% of mineral fertilization for thiscultivation.

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.