Ammonium sorption and ammonia inhibition of nitrite-oxidizing bacteria explain contrasting soil N(2)O production

Better understanding of process controls over nitrous oxide (N(2)O) 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, N(2)O, and genes associated with nitrification in two soils (‘L’ and ‘W’) having similar texture, pH, C, and C/N ratio. Solution-phase ammonia (slNH(3)) was also calculated accounting for non-linear ammonium (NH(4)(+)) sorption capacities (ASC). Soil W displayed greater nitrification rates and nitrate (NO(3)(−)) levels than soil L, but was more resistant to nitrite (NO(2)(−)) accumulation and produced two to ten times less N(2)O than soil L. Genes associated with NO(2)(−) oxidation (nxrA) increased substantially in soil W but remained static in soil L. Soil NO(2)(−) was strongly correlated with N(2)O production, and cumulative (c-) slNH(3) explained 87% of the variance in c-NO(2)(−). Differences between soils were explained by greater slNH(3) in soil L which inhibited NO(2)(−) oxidization leading to greater NO(2)(−) levels and N(2)O production. This is the first study to correlate the dynamics of soil slNH(3), NO(2)(−), N(2)O and nitrifier genes, and the first to show how ASC can regulate NO(2)(−) levels and N(2)O production.