Nitrogen-15 and bromide tracers of nitrogen fertilizer movement in irrigated wheat production

In irrigated agricultural systems, NO₃ leaching is believed to result from high fertilizer rates combined with the need to periodically leach salts from surface soil horizons. The purpose of this research is to estimate N fertilizer movement in the soil of commercial fields of flood-irrigated wheat (Triticum spp.) while documenting best management practices (BMPs). Potassium bromide and ¹⁵N labeled ammonium sulfate were applied as tracers of N fertilizer movement to 1- x 1-m microplots replicated 9 or 10 times in three commercial fields of flood-irrigated wheat. The soil was sampled at harvest to a depth of 2.4 to 4.0 m. More fertilizer was applied at two out of thee sites and more irrigation water was applied at all sites than recommended by BMPs. Bromide recovery in the soil and plant tissue at harvest was 29, 68, and 61% of that applied at the three sites. Most of the ¹⁵N measured in the soil profile was contained in the 0- to 0.3-m increment of soil. Recovery on ¹⁵N in the soil and plant tissue at harvest was 54, 54, and 69% of that applied. The Br recovery data suggests that 32 ± 28% (standard deviation) of the ¹⁵N applied may have leached. The highest leaching potential was measured at the site that adhered to BMPs for N fertilizer management but had the most permeable soil. Nitrate leaching in flood-irrigated wheat production seemms inevitable even if BMPs for N fertilizer management are followed.