Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation

Abstract

Nitrous oxide (N₂ O) is a greenhouse gas that also plays the primary role in stratospheric ozone depletion. The use of nitrogen fertilizers is known as the major reason for atmospheric N₂ O increase. Empirical bottom-up models therefore estimate agricultural N₂ O inventories using N loading as the sole predictor, disregarding the regional heterogeneities in soil inherent response to external N loading. Several environmental factors have been found to influence the response in soil N₂ O emission to N fertilization, but their interdependence and relative importance have not been addressed properly. Here, we show that soil pH is the chief factor explaining regional disparities in N₂ O emission, using a global meta-analysis of 1,104 field measurements. The emission factor (EF) of N₂ O increases significantly (p < .001) with soil pH decrease. The default EF value of 1.0%, according to IPCC (Intergovernmental Panel on Climate Change) for agricultural soils, occurs at soil pH 6.76. Moreover, changes in EF with N fertilization (i.e. ΔEF) is also negatively correlated (p < .001) with soil pH. This indicates that N₂ O emission in acidic soils is more sensitive to changing N fertilization than that in alkaline soils. Incorporating our findings into bottom-up models has significant consequences for regional and global N₂ O emission inventories and reconciling them with those from top-down models. Moreover, our results allow region-specific development of tailor-made N₂ O mitigation measures in agriculture.

Keywords: emission factor; environmental factors; global meta-analysis; greenhouse gas estimate and mitigation; nitrous oxide; nonlinearity coefficient; soil pH.