Nitrous oxide emissions from vermicompost preparation and application phases: Emission factors based on a meta-analysis

Vermicomposting is a biotechnology similar to composting but is more effective at degrading organic waste. Earthworms engulf the waste and digest it through the gut to form manure fertilizer. However, the nitrogen loss caused by nitrous oxide (N2O) can reduce the fertilization value of vermicompost and contribute to global warming. In the last decade, an increasing number of studies have focused on the effect of vermicomposting on N2O emissions. However, the results are contradictory, and a quantitative summary of these studies is still missing. In this study, a meta-analysis was conducted to evaluate the effects of the two phases (vermicompost preparation and vermicompost application) of vermicomposting on N2O emissions and their relationship with substrate and soil properties (e.g., substrate type, C/N ratio, soil pH, and moisture). Our results showed that the vermicompost preparation insignificantly increased N2O emissions compared with traditional composting (without earthworms), and the amounts of N2O generated during the phase of preparation were 3-143 times higher than those of the application phase. Interestingly, it is the vermicompost application phase, rather than preparation, that showed a positive effect size on N2O emissions (87 %), and the effects were highly dependent on the soil and substrate C/N ratio. A vermicomposting duration of <= 90 days promoted N2O emissions, and the vermicomposting effect size of the cumulative N2O emission was positively correlated with the temperature, soil C/N ratio and soil organic carbon (SOC) content but negatively correlated with the water-filled pore space (WFPS). Our findings indicated that, in the two phases of vermicomposting, organic matter was decomposed with the simultaneous consumption of oxygen, which accelerated N mineralization in the environment and in the guts of earthworms. As a result, nitrification and microsite denitrification were promoted, which enhanced the production of N2O. Our meta-analysis highlighted the necessity of connecting the effect of vermicomposting with soil and substrate C/N ratio to mitigate soil N2O emissions.