Composting is a valuable strategy for recycling resources and protecting the environment in organic waste management. During this process, organic waste is converted into sanitized, stable and nutrient-rich humus through microbial metabolism. However, composting also has its downside, which is greenhouse gas (GHG) emissions, especially carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), as well as ammonia (NH3).

Various techniques are used to mitigate these emissions, including the addition of exogenous materials such as biocharBiochar is considered a promising avenue due to its exceptional physicochemical properties such as high specific surface area, porous structure and numerous surface functional groupsBiochar is believed to mitigate emissions by regulating the microbial community and adsorbing gases.

Recent global meta-analysis, which included 123 published studies with 1,184 observations, confirmed the effectiveness of biochar. Overall, The addition of biochar significantly reduced CH4 emissions by 53.7 %, N2O by 49.8 % and NH3 by 35.9 %However, it did not have a statistically significant impact on CO2 emissions.

Application rate as a key factor

The analysis showed that The application rate (AR) of biochar was a key parameter affecting CH4, N2O and NH3 emissions. Mitigation effectiveness demonstrated a nonlinear dose-dependent effect. For example, the largest reductions in CH4 (−86.1 %) and NH3 (−60.2 %) were observed at AR doses above 10.0 % (w/w). While low AR doses (< 5.0 % w/w) even significantly increased N2O emissions (51.4 %), high AR doses (> 10.0 % w/w) strongly suppressed them (−93.7 %). These results suggest that biochar at low doses can promote the nitrification process, which increases N2O emissions. Regression analysis suggests that the optimal application rate range to achieve joint mitigation of CH4, N2O and NH3 is 10 %–20 % (w/w, dry weight).

Optimizing composting conditions

The mitigating effects of biochar may be compromised by certain composting conditions. Biochar reduced CH4, N2O and NH3 emissions under most conditions, except in acidic environments. High electrical conductivity (EC > 4 mS/cm) weakened the mitigating effects of biochar on all monitored gases. In addition, high carbon to nitrogen ratio (C/N > 30) and low moisture content (MC < 55 %) significantly increased CO2 emissions.

The following optimal parameters are recommended for stable and effective mitigation of gas emissions:

• C/N ratio: 20 – 30
• Moisture content (MC): 55 % – 65 %
• pH: 7.5 – 8.5
• Electrical conductivity (EC): < 4 mS/cm
• Biochar dosage: 10 % – 20 % (w/w, dry weight).

Contribution to nitrogen and carbon

In addition to reducing gas emissions, biochar has demonstrated other benefits. Biochar significantly reduced total nitrogen loss (TN loss) by 41.3 %It also reduced the NH4+ content (by 17.4 %) and increased the NO3− content (by 17.2 %) in the compost, indicating that biochar stimulates nitrification and optimizes nitrogen conservation. Although biochar did not significantly change total carbon losses (TC loss and TOC loss), its refractory structure may contribute to carbon stocks and stabilization in the composting system.

The conclusion of the meta-analysis highlights that biochar offers a triple benefit: emission mitigation (CH4, N2O, NH3), nitrogen nutrient conservation and carbon stock stabilization. Adherence to the proposed optimal composting conditions and biochar application is crucial to achieve synergistic gas reduction in organic waste composting. JRi

The study is published in the journal Nitrogen Cycling .