Effects of nitrogen fertilizer on CH4 emission from rice fields: multi-site field observations

There is an ongoing discussion of the possible effects of nitrogen (N) application on methane (CH₄) emission from rice fields. However, the Intergovernmental Panel on Climate Change (IPCC) methodologies for estimating the national inventory of CH₄ emission from paddy rice production do not consider the effects of N addition. To assess the lack of knowledge about N addition effects on inventory estimates, we recently launched a multi-site observation campaign in major rice cultivation regions of China. The observations showed that, across various climate zones, the application of ammonium-based fertilizers at the commonly-adopted levels for fields in China (150 or 250 kg N ha^?1) generally inhibited accumulative CH₄ emission during rice season (by 28–30% on average) as compared to no N addition. An increase in application from the moderate level of 150 kg N ha^?1 to the high rate of 250 kg N ha^?1 did not significantly modify CH₄ emission. Our results suggest that disregarding the effect of N fertilization by the IPCC methodologies may not significantly bias CH₄ inventory estimates of China. In regions with much lower N addition levels, however, disregarding the effect of N fertilization may result in the underestimation of regional CH₄ emission, since these emissions were mainly derived from studies in regions with relatively high N addition rates.