Ammonium sorption and ammonia inhibition of nitrite-oxidizing bacteria explain contrasting soil N2O production

Better understanding of process controls over nitrous oxide (N₂O) production in urine-impacted 'hot spots' and fertilizer bands is needed to improve mitigation strategies and emission models. Following amendment with bovine (Bos taurus) urine (Bu) or urea (Ur), we measured inorganic N, pH, N₂O, and genes associated with nitrification in two soils ('L' and 'W') having similar texture, pH, C, and C/N ratio. Solution-phase ammonia (slNH₃) was also calculated accounting for non-linear ammonium (NH₄ ⁺) sorption capacities (ASC). Soil W displayed greater nitrification rates and nitrate (NO₃ ^-) levels than soil L, but was more resistant to nitrite (NO₂ ^-) accumulation and produced two to ten times less N₂O than soil L. Genes associated with NO₂ ^-oxidation (nxrA) increased substantially in soil W but remained static in soil L. Soil NO₂ ^-was strongly correlated with N₂O production, and cumulative (c-) slNH₃explained 87% of the variance in c-NO₂ ^-. Differences between soils were explained by greater slNH₃in soil L which inhibited NO₂ ^-oxidization leading to greater NO₂ ^-levels and N₂O production. This is the first study to correlate the dynamics of soil slNH₃, NO₂ ^-, N₂O and nitrifier genes, and the first to show how ASC can regulate NO₂ ^-levels and N₂O production.