The importance of ammonia volatilization in estimating the efficacy of nitrification inhibitors to reduce N2O emissions: A global meta-analysis.

ABSTRACT

Nitrification inhibitors (NIs) have been shown to be an effective tool to mitigate direct N2O emissions from soils. However, emerging findings suggest that NIs may increase soil ammonia (NH3) volatilization and, subsequently, indirect N2O emission. A quantitative synthesis is lacking to evaluate how NIs may affect NH3 volatilization and the overall N2O emissions under different environmental conditions. In this meta-analysis, we quantified the responses of NH3 volatilization to NI application with 234 observations from 89 individual studies and analysed the role of experimental method, soil properties, fertilizer/NI type, fertilizer application rate and land use type as explanatory factors. Furthermore, using data sets where soil NH3 emission and N2O emission were measured simultaneously, we re-evaluated the effect of NI on overall N2O emissions including indirect N2O emission from NH3 volatilization. We found that, on average, NIs increased NH3 volatilization by 35.7% (95% CI 25.7-46.7%) and increased indirect N2O emission from NH3 emission (and subsequent N deposition) by 2.9%-15.2%. Responses of NH3 volatilization mainly varied with experimental method, soil pH, NI type and fertilizer type. The increase of NH3 volatilization following NI application showed a positive correlation with soil pH (R2 = 0.04, n = 234, P < 0.05) and N fertilizer rate (R2 = 0.04, n = 187, P < 0.05). When the indirect N2O emission was considered, NI's N2O mitigation effect decreased from 48.0% to 39.7% (EF = 1%), or 28.2% (EF = 5%). The results indicate that using DMPP with ammonium-based fertilizer in low pH, high SOC soils would have a lower risk for increasing NH3 volatilization than using DCD and nitrapyrin with urea in high pH, lower SOC soil. Furthermore, reducing N application rate may help to improve NIs' overall N2O emission mitigation efficiency and minimize their impact on NH3 volatilization.