Cao, Y., Wang, X., Bai, Z., Chadwick, D., Misselbrook, T., Sommer, S.G., Qin, W., Ma, L. (2019)

Composting of solid waste can be associated with a loss of the agronomic value (nutrient loss), as well as a source of environmental impact through the emission of the greenhouse gases (GHG) nitrous oxide (N2O) and methane (CH4) and volatilization of ammonia (NH3). Additives have been considered as a useful option to mitigate these environmental emissions, but the wider effects of using different additives on multiple gas emissions is still uncertain. Here, a global meta-analysis was conducted using 105 studies with 303 paired comparisons, to quantify the impact of different additives on NH3 and GHG emissions, considering different composting feedstock properties. On average, additives reduced the total nitrogen (TN) loss (46.4%), NH3 (44.5%), N2O (44.6%) and CH4 (68.5%) emissions, and total GHG emissions expressed as global warming potential (GWP) (54.2%) during composting. Chemical, physical and microbial additives all significantly reduced TN loss and NH3 emission; however, the strongest effect was observed for chemical additives under the condition of low moisture content (≤65%), low C/N ratios (≤20) or alkalinity (pH > 7.5). Specifically, PO43− and Mg2+ salt had the greatest mean mitigation for TN loss (68.0%) and NH3 emission (62.0%). In contrast, physical additives (e.g. biochar and zeolite) were more effective at reducing the total GHG emissions (67.2%) compared with chemical additives because of the greater mitigation of N2O emission. Low moisture content (≤65%) or low C/N ratios (≤20) enhanced the effectiveness of chemical additives in reducing TN loss and NH3 emission. Physical additives were suggested to be more effective in reducing N2O emission at low moisture content (≤65%) or high C/N ratios (>20 to ≤ 30). The magnitude of the mitigation of TN loss and NH3 emission increased as the dosage rate of PO43− and Mg2+ salt/superphosphate increased between 0 and 20%. This meta-analysis suggests that an optimized combination of chemical and physical additives tailored to feedstock characteristics and application dosage may be a promising approach for the synergistic mitigation of NH3 and GHG emissions.

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