Biochar amendments mitigate trace gas emissions in organic waste composting: a meta-analysis

Abstract

Composting is valuable for recycling resources and environmental protection, but it emits greenhouse gases, namely carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2 O), and ammonia (NH 3). Biochar can reduce greenhouse gases and NH 3 emissions, but its effects and mechanisms in composting are debated. We conducted a global meta-analysis of 123 published studies (251 paired comparisons and 1,184 observations) to identify the dominant factors and underlying mechanisms driving CO 2 , CH 4 , N 2 O, and NH 3 emissions during biochar-amended composting. Biochar amendment significantly reduced emissions of CH 4 (53.7%), N 2 O (49.8%), and NH 3 (35.9%), but had no significant effect on CO 2 emissions. The application rate of biochar was the key parameter affecting CH 4 , N 2 O, and NH 3 emissions, demonstrating a dose effect on mitigation efficacy. Biochar decreased CH 4 , N 2 O, and NH 3 emissions under most parameters, except acidic conditions. Although a high carbon:nitrogen ratio (> 30) and a low moisture content (< 55%) increased CO 2 emissions, high electric conductivity (> 4 mS/cm) impaired the mitigation effects of biochar on all target gases. To achieve optimal composting performance to mitigate trace gases, the following parameters are recommended: a carbon : nitrogen ratio of 20-30, a moisture content of 55%-65%, pH 7.5-8.5, an electric conductivity of < 4 mS/cm, and a biochar dosage of 10%-20% (w/w, dry weight). Highlights • Biochar altered CH 4 (−53.7%), N 2 O (−49.8%), NH 3 (−35.9%), but not CO 2 via meta-analysis. • The application rate of biochar exhibited a dose effect on CH 4 , N 2 O, NH 3 mitigation efficacy. • Four gas mitigation was derived of 123 studies with 1,184 observations for composting.