Initial influence of fertilizer nitrogen types on water quality

Using different sources of nitrogen as fertilizers in nursery ponds may affect water quality and plankton responses. We evaluated water quality variables and plankton population responses when using different nitrogen sources for catfish nursery pond fertilization. We compared calcium nitrate (12% N), sodium nitrite (20% N), ammonium chloride (26% N), ammonium nitrate (34% N) and urea (45% N) in 190‐L microcosms at equimolar nitrogen application rates. Sodium nitrite‐fertilized microcosms had higher nitrite and nitrate levels during the first week; no other differences in the water quality were detected among fertilizer types (P>0.05). No differences in green algae, diatoms or cyanobacteria were detected among treatments; desirable zooplankton for catfish culture was increased in urea‐fertilized microcosms. Based on these results, any form of nitrogen used for pond fertilization should perform similarly without causing substantial water quality deterioration. Ammonium nitrate and urea contain a higher percentage of nitrogen, requiring less volume to achieve dosing levels. If both urea and ammonium nitrate are available, we recommend using the one with the least cost per unit of nitrogen. If both types of fertilizer have an equal cost per unit of nitrogen, we recommend using urea because of the potential advantage of increasing desirable zooplankton concentrations.