Nitrite intensity explains N management effects on N2O emissions in maize

Abstract

It is typically assumed that the dependence of nitrous oxide (N2O) emissions on soil nitrogen (N) availability is best quantified in terms of ammonium (NH4+) and/or nitrate (NO3−) concentrations. In contrast, nitrite (NO2−) is seldom measured separately from NO3− despite its role as a central substrate in N2O production. We examined the effects of three N fertilizer sources and two placement methods on N2O and N dynamics in maize over two growing seasons. Cumulative N2O emissions were well-correlated with NO2− intensity (NO2I) but not with NO3− (NO3I) or NH4+ (NH4I) intensity. By itself, NO2I explained more than 44% of the overall variance in N2O. Treatment effects on N2O and NO2I were similar. When conventional urea (U) was applied using mid-row banding (MRB), both N2O and NO2I increased by a factor of about 2 compared to broadcast/incorporated (BI). When polymer-coated urea (PCU) was the N source, MRB placement increased both N2O and NO2I compared to BI only in the wetter of the two years. When urea with microbial inhibitors (IU) was the N source, N2O and NO2I were lowest across both years and were less affected by placement than U or PCU. A 50/50 mix of IU and U reduced N2O and NO2I compared to U alone, suggesting that a mixed N source may provide an economical N2O mitigation strategy. Our results show that practices which reduce NO2− accumulation have the potential to also reduce N2O emissions, and that separate consideration of NO3− and NO2− dynamics can provide more insight than their combined dynamics as typically quantified.