Synthesis of air pollution patterns and nutrient composition during organic fertilizer production: a meta-analytical study

Composting is vital for managing organic waste and producing fertilizer; however, it also releases potent greenhouse gases (GHGs: CH4, CO2, and N2O), and odorous volatiles (NH3, H2S, and volatile organic compounds [VOCs]). Uncontrolled processes can also lead to significant nutrient loss. While various control measures have been implemented to mitigate these adverse effects, a comprehensive quantitative analysis of their impact on gaseous emissions and fertilizer quality has been lacking. This meta-synthesis, based on 1,683 observations from 135 studies identified through systematic database searches assessed four categories of control measures: biological (microbial inoculants), chemical (biochar, gypsum), physical (aeration, bulking agents), and mechanical (turnover frequency, electric fields). The main findings show that these measures generally improved composting outcomes. Specifically, they led to increased temperatures (average increase of ~48%), enhanced nutrient retention (e.g., average nitrogen content increased by ~89%, humic acid by ~29%), and reduced phytotoxicity (e.g., average germination index improved by ~73%). They also contributed to carbon stabilization (average C/N ratio decreased by ~38%) and significantly lowered emissions of GHGs and VOCs. For instance, average CH4, N2O, NH3, and CO2 emissions were reduced by approximately 69%, 83%, 78%, and 78%, while H2S and VOCs emissions saw reductions of around 41% and 42%, respectively. Notably, feedstock type and initial C/N ratio were identified as key factors influencing emission profiles, often exceeding the effects of control measures. This study offers evidence-based guidance for selecting tailored strategies to reduce GHG and VOC emissions from composting while improving fertilizer quality.