Effect of Sodium Nitrate Treatment on Water and Sediment Quality in Laboratory and Pond Studies

Abstract

Sodium nitrate (NaNO3) has been widely used in shrimp aquaculture ponds as a fertilizer, a water quality enhancer and a bottom soil oxidant. This study was conducted to determine whether or not treatment with sodium nitrate can improve water quality, bottom soil condition, phytoplankton abundance and community structure and fish yield in freshwater channel catfish ponds. The study consisted of a pond study and a laboratory study. In the pond study, sodium nitrate was applied at 2 mg/L NO3--N at 2-week intervals to rectangular ponds of 400-m2 water surface area stocked with 400 channel catfish Ictalurus punctatus fingerings and 10 grass carp Ctenopharyngodon idella. Water quality, phytoplankton communities, sediment condition and fish production were compared between triplicate treatment and control ponds. The results showed that catfish production and survival rate did not differ (P>0.1) between treated and control ponds. There were higher mean concentrations of nitrate nitrogen, total nitrogen, soluble reactive phosphorus, total phosphorus, turbidity and chlorophyll a in sodium nitrate-treated ponds than in control ponds (P<0.1). Transparency was greater in control ponds (P<0.1). The pH and concentration of total alkalinity, ammonia nitrogen, and dissolved oxygen were not different between treated and control ponds (P>0.1). There were also no differences in pH and organic matter concentration of sediment (P>0.1) between control and treated ponds. However, application of sodium nitrate caused a decline in redox potential between the beginning and the end of grow-out period in sediment (P<0.1), and upon draining, sediment in treated ponds was lighter colored than that of control ponds. This suggests that nitrate treatment enhanced oxidation at the sediment surface. In the laboratory study, sodium nitrate was further investigated to determine if it would influence redox potential, denitrification rate, and the rate of organic matter decomposition when added to sediment. Results revealed no differences (P>0.05) in redox potential and organic matter concentration in sediment treated with 0 to 32 mg/kg of NO3--N. There was no increase in denitrification (P>0.05) in sediment to which nitrate was applied at 0 to 10 mg/L to the water. Dissolved oxygen declined at similar rates in water samples held in BOD bottles and treated with 0 to 8 mg/L NO3--N. Nitrate and ammonium also were compared as nitrogen source for phytoplankton. Uptake rate of ammonium by green algae was greater than that of nitrate. However, diatom and blue-green algae communities appeared to use both forms of nitrogen.