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

Abstract

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, flux