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Greenhouse gases (GHGs) are produced during the composting process, but few studies have measured emissions from a full-scale windrow of composting green-waste. This is important for evaluating composting as a waste management option and for understanding how changes to current composting management practices could help reduce emissions. This study uses micrometeorological mass balance (MMB) and open flow-through chamber techniques to measure emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from a windrow of composting green-waste in Northern California. The MMB technique yielded mean upwind–downwind concentration differences over the study period that showed sourcing of all three GHGs. CO2 showed a stronger signal than CH4 and N2O. A strong diel pattern was found in the concentration differences at lower levels and fluxes of CO2, with substantial noise likely obscuring any possible daily patterns for CH4 and N2O. Fluxes normalized by the time since the previous turn event revealed an initial rapid rise in CO2 concentration differences (at lower levels) and fluxes, peaking close to 13 h after the turn event followed by a gradual decline. The same pattern was not as clear for the other two gases but overall declines in concentration differences and fluxes were apparent with increasing time since the previous turn event. Substantial differences between MMB and chamber calculated fluxes were found, due to both differences in the techniques as well as sampling frequency. [ABSTRACT FROM AUTHOR]

The process of composting is a source of greenhouse gases (GHG) that contribute to climate change. We monitored three field-scale green waste compost windrows over a one-year period to measure the seasonal variance of the GHG fluxes. The compost pile that experienced the wettest and coolest weather had the highest average CH4 emission of 254 ± 76 g C day-1 dry weight (DW) Mg-1 and lowest average N2O emission of 152 ± 21 mg N day-1 DW Mg-1compared to the other seasonal piles. The highest N2O emissions (342 ± 41 mg N day-1 DW Mg-1) came from the pile that underwent the driest and hottest weather. The compost windrow oxygen (O2) concentration and moisture content were the most consistent factors predicting N2O and CH4 emissions from all seasonal compost piles. Compared to N2O, CH4 was a higher contributor to the overall global warming potential (GWP) expressed as CO2 equivalents (CO2 eq.). Therefore, CH4 mitigation practices, such as increasing O2 concentration in the compost windrows through moisture control, feedstock changes to increase porosity, and windrow turning, may reduce the overall GWP of composting. Based on the results of the present study, statewide total GHG emissions of green waste composting were estimated at 789,000 Mg of CO2 eq., representing 2.1% of total annual GHG emissions of the California agricultural sector and 0.18% of the total state emissions. [ABSTRACT FROM AUTHOR]