Technical note: Using static chambers with large footprints to reduce variability in measurements of greenhouse gases from uneven surfaces

Abstract

A large-volume static chamber was developed and field-tested as an extension for a semi-automated flux chamber to measure greenhouse gas emissions (GHGs) from uneven compost surfaces. Conventional chambers often have a small footprint (<0.1 m²), limiting accuracy on heterogeneous piles. The chamber extension has a volume of 0.156 m³ and a footprint of 0.385 m² compared with 0.004 m³ and 0.032 m² (volume and area of the commercial chamber). It includes four adjustable rods to measure headspace height, four lateral flanges to help press the chamber extension into the compost surface, a mixing fan, and a T-fitting on the sample line for manual gas sampling. Its performance was evaluated against the original semi-automated flux chamber using a paired t-test on CO₂ and CH₄ fluxes from municipal windrows and stockpiles of uncomposted material. The CH₄ and CO₂ fluxes measured by the two systems were statistically similar (P > 0.05) on the windrow and stockpile. However, the modified chamber exhibited lower variability, with overall coefficients of variation (CV) for CO₂ flux at the peak of the windrow of 41.9% compared to 45.8% for the commercial chamber, while for CH₄ flux the CV was 84.1% versus 132.7%, respectively. For stockpiles, the CV for CO₂ flux was 64.4% for the modified chamber and 80.6% for the commercial chamber, while for CH₄ flux, the CV was 126.4% versus 162.6%, respectively. These results demonstrate that a chamber extension with a larger footprint facilitates accurate and more stable measurements of GHGs from uneven surfaces such as compost windrows.