Meta-analysis of the differences of soil nitrous oxide emission factors between whole years and growing seasons

Nitrous oxide (N2O) from croplands is a major source of anthropogenic N2O emissions. An N2O emission factor (EF) is proposed by the Intergovernmental Panel on Climate Change (IPCC) for greenhouse gas (GHG) inventories as the response of direct emissions from fertilizer nitrogen applied. The N2O EF varies with management-related factors and other environmental factors, and the IPCC emission factor database requires the experimental duration of these EFs to cover at least a year, including growing seasons and fallow periods, reflecting the impacts of weather conditions during the whole year. However, a large number of N2O field measurements have been conducted during the growing season only, instead of the whole year, and thus cannot be used for deriving N2O EFs. The aim of this study it to identify the differences between EF for whole years (EFwy) and EF for growing seasons (EFgs) and their controlling factors at regional and plot scales. At the regional scale, 364 and 761 EFs were calculated for whole years and growing seasons as the percentage of fertilizer N input converted to N2O emissions from 199 published studies. Although there was no generally significant differences between them (0.81% vs 0.87%, P>0.05), the differences in all climatic zones are significant (Warm-moist: 1.19% vs 0.38%; Warm-dry: 0.70% vs 0.51%; Cool-moist: 0.71% vs 1.15%; Cool-dry: 1.01% vs 0.30%; P<0.05). For crop types, the differences between EFwy and EFgs were found non-significant (P>0.05) except for wheat. Therefore, different crop compositions for EFwy and EFgs in each climatic zone led to the distinctions between EFs during whole years and growing seasons at regional scale. At plot scale, from 15 field studies, 98 pairs of EFwy and EFgs were collected. In general EFwy was slightly lower than EFgs and 93% of the variance can be explained by the regression (EFwy=0.88EFgs, R2=0.93). Additionally, we found that freeze-thaw cycles, climatic zones, crop types, soil bulk density and clay content have important impacts on the ratio of EFwy to EFgs. Our results suggest that the difference between EFwy and EFgs at regional scale can be attributed to crop compositions and at plot scale EFwy is generally lower than EFgs and can be adjusted by surrounding environmental factors. Use of this knowledge in GHG inventories could improve data availability and efficiency of EF in the global N2O budget, refine the accuracy of fertilizer-induced N2O emissions, and help to bridge the knowledge gap of the differences in EFs between whole years and growing seasons.