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-) slNH3 explained 87% of the variance in c-NO2−. Differences between soils were explained by greater slNH3 in 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.