Chamber measurement methods and aeration effect on greenhouse gas fluxes during composting

Composting has the potential to mitigate methane (CH4) and nitrous oxide (N2O) emissions from manure.  The heterogeneous nature of emitting surfaces makes it difficult to quantify these emissions.  CH4 and N2O fluxes measured using eight small chambers (0.72 m2) and a mega chamber (90 m2) were compared, and the effect of aeration on the fluxes during composting was studied.  Two batches of compost were placed in three channels and 2-3 small flux chambers were deployed on each channel.  The channels were enclosed by a building serving as a mega chamber.  Chamber location significantly affected gas fluxes, pointing to strong spatial heterogeneity.  Mean CH4 fluxes from the small chambers were similar or 1.4 times higher compared to the mega chamber.  Mean N2O fluxes from the small chambers were 50%-55% lower compared to the mega chamber.  Channel edges, not captured by the small chambers, were potentially significant ‘hot spots’ for N2O production.  When only small chambers are used for flux measurements, a large number should be strategically positioned to cover different areas of the emitting surface so as to capture a representative flux.  On the other hand, if a few small chambers are used, they should be moved frequently to different locations on the emitting surface.  Temporal variations in CH4 and N2O fluxes were similar for all the chambers, including periods with active aeration.  Correlation of total aeration time with CH4 fluxes was insignificant (r = -0.097), but was positive with N2O (r = 0.556).  The flushing of stored CH4 at the onset of aeration, likely promoted fluxes, as opposed to the expected flux decrease with higher aeration time.  The purging of stored N2O enhanced the expected stimulation of N2O production at high aeration times, resulting in the positive trend observed for N2O fluxes.  Our results suggest that a mega chamber that covers a larger emitting surface area can avoid biases in flux estimates due to spatial variability of the source.   Keywords: chamber measurements, compost, greenhouse gases, aeration, flu