| Summary: | Direct emissions from commercial-scale composting are uncertain. We used micrometeorological methods to continuously measure greenhouse gas (CO _2 , CH _4 , N _2 O) emissions from full composting of green waste and manure. We measured oxygen (O _2 ), moisture, and temperature continuously inside the composting pile, and analyzed chemical and physical characteristics of the feedstock weekly as potential drivers of emissions. Temperature, moisture, and O _2 all varied significantly by week. Feedstock porosity, C:N, and potential N mineralization all declined significantly over time. Potential net nitrification remained near zero throughout. CH _4 and CO _2 fluxes, indicators of feedstock lability, were variable, and most emissions (75% and 50% respectively) occurred during the first three weeks of composting. Total CH _4 emitted was 1.7 ± 0.32 g CH _4 kg ^−1 feedstock, near the median literature value using different approaches (1.4 g CH _4 kg ^−1 ). N _2 O concentrations remained below the instrument detection. Oxygen, moisture and temperature exhibited threshold effects on CH _4 emissions. Net lifecycle emissions were negative (−690 g CO _2 -e kg ^−1 ), however, after considering avoided emissions and sinks. Managing composting piles to minimize methanogenesis—by maintaining sufficient O _2 concentrations, and focusing on the first three weeks—could reduce emissions, contributing to the climate change mitigation benefit of composting.
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