Schuytema, Gerald S., and Nebeker, Alan V. 1999. Effects of ammonium nitrate, sodium nitrate and urea on red-legged frogs, Pacific treefrogs and African clawed frogs. Bulletin of Environmental Contamination and Toxicology 63:357-364.
The effects of ammonium nitrate and sodium nitrate on red-legged frog (Rana aurora) embryos/larvae and effects of urea on Pacific treefrog (Pseudacris regilla) and (Xenopus laevis) tadpoles were determined in static-renewal tests. The 16-day LC50s for R. aurora were 71.9 mg/L NH4-N for ammonium nitrate and 636.3 mg/L NO3-N for sodium nitrate. The 10-day LC50s for urea were 8396 mg/L N for P. regilla and 9180 mg/L N. for X. laevis. LOAEL values ranged from 13.2 mg/L NH4-N and <29.1 mg//L NO3 -N for R. aurora embryos to 6000 mg//L N for P.regilla and X. laevis tadpoles exposed to urea. The ammonium ion appears to be the primary cause of mortality in the ammonium nitrate exposures. The levels of NO3-N present in solutions containing sufficient NH4-N to cause adverse effects were insufficient to have an effect on survival or growth. Potential harm to amphibians could occur if sensitive life stages were impacted by NH4-N or NO3-N in agricultural runoff or drainage for a sufficiently long period. While urea appears to be relatively non-toxic to amphibians at normally applied concentrations, its effects when combined with ammonium compounds remains to be tested.