Measurement of NO emissions over the whole year is necessary for estimating reliable emission factors.

Nitrous oxide emission factors (NO-EF, percentage of NO-N emissions arising from applied fertilizer N) for cropland emission inventories can vary with agricultural management, soil properties and climate conditions. Establishing a regionally-specific EF usually requires the measurement of a whole year of NO emissions, whereas most studies measure NO emissions only during the crop growing season, neglecting emissions during non-growing periods. However, the difference in NO-EF (ΔEF) estimated using measurements over a whole year (EF) and those based on measurement only during the crop-growing season (EF) has received little attention. Here, we selected 21 studies including both the whole-year and growing-season NO emissions under control and fertilizer treatments, to obtain 123 ΔEFs from various agroecosystems globally. Using these data, we conducted a meta-analysis of the ΔEFs by bootstrapping resampling to assess the magnitude of differences in response to management-related and environmental factors. The results revealed that, as expected, the EF was significantly greater than the EF for most crop types. Vegetables showed the largest ΔEF (0.19%) among all crops (0.07%), followed by paddy rice (0.11%). A higher ΔEF was also identified in areas with rainfall ≥600 mm yr, soil with organic carbon ≥1.3% and acidic soils. Moreover, fertilizer type, residue management, irrigation regime and duration of the non-growing season were other crucial factors controlling the magnitude of the ΔEFs. We also found that neglecting emissions from the non-growing season may underestimate the NO-EF by 30% for paddy fields, almost three times that for non-vegetable upland crops. This study highlights the importance of the inclusion of the non-growing season in the measurements of NO fluxes, the compilation of national inventories and the design of mitigation strategies.