Nitrogen losses by denitrification can be large from waterlogged Vertosols growing sorghum

Abstract

Key findings • Denitrification can lead to a significant loss of fertiliser nitrogen if soils become waterlogged after heavy rain. Waterlogging depletes the soil of oxygen, so microorganisms use nitrate instead of O2 as an electron‑acceptor, converting nitrate to the gases nitric oxide, nitrous oxide and di‑nitrogen. • We used 15N isotope‑labelled urea to trace the fate of urea applied at sowing to field‑grown sorghum. Trials were located on black Vertosol soils at two sites – near Tamworth and Quirindi – in the 2012–13 summer season. • In‑crop rainfall was above average for part of the season at the Tamworth site, but well above average at the Quirindi site. • Between 12 and 45% of the N applied as 15N urea in 2012–13 was not recovered in either soil or plant samples, so was presumed lost through denitrification.