Global nitrous oxide emission factors from agricultural soils after addition of organic amendments: A meta-analysis

Agricultural soils receiving synthetic fertilizers and organic amendments containing nitrogen contribute a large part to anthropogenic nitrous oxide (N2O) emissions. As a source of nitrate that undergoes reduction to N2O, organic amendments also change soil C availability and redox potential, which influences the N2O emission factor (EF) of organically-amended soils. The objective of this study was to conduct a meta-analysis of N2O EF from agricultural soils receiving organic amendments. A global survey of peer-reviewed literature resulted in the selection of 38 studies including 422 observations at 43 sites in 12 countries. The analysis yielded a global EF for all organic sources, EForg, equal to 0.57 ± 0.30%, which is lower than the IPCC default EF of 1 for synthetic fertilizers. Three groups of organic amendments with similar EFs were identified: the high-risk group including animal slurries, waste waters and biosolids (1.21 ± 0.14%); the medium-risk group including solid manure, composts + fertilizers, and crop residues + fertilizers (0.35 ± 0.13%); and the low-risk group including composts, crop residues, paper mill sludge and pellets (0.02 ± 0.13%). The EF was higher when soils received organic amendments in combination with synthetic fertilizers, such as liquid manures + fertilizers (2.14 ± 0.53%), composts + fertilizers (0.37 ± 0.24%), and crop residues + fertilizers (0.59 ± 0.27%). The EF was modulated by amendment (C/N ratio), soil (texture, drainage, organic C and N) and climatic (precipitation) factors. For example, EFs were on average 2.8 times greater in fine-textured than coarse-textured soils. We recommend site-specific EFs that consider organic amendment chemistry, soil characteristics, climate conditions and whether the organic amendment is applied alone or in combination with synthetic fertilizers.