Comparing methane fluxes from stored liquid manure using micrometeorological mass balance and floating chamber methods

Storage of animal manure is a significant source of methane (CH4), a greenhouse gas. Comparison of gas fluxes from stored manure measured with chamber and micrometeorological methods has yielded contrasting results. The objective of this study was to compare CH4 fluxes from stored liquid swine manure measured using a micrometeorological mass balance (MMB) with 1) those measured with a floating chamber method, and 2) modeled CH4 fluxes based on manure temperature and volatile solids (VS). For the MMB method, air samplers were installed on three air sampling towers (4 intakes per tower) positioned around a concrete manure storage tank (diameter 18.7 m; depth 3.5 m). Air samplers were also positioned at the inlet and outlet of the non-steady state chamber. Mean half-hourly CH4 concentrations in air samples were measured using a tunable diode laser trace gas analyzer during June 2006. The floating chamber was positioned at four locations, near the manure inlet (L1 and L2) and far from the manure inlet (L3 and L4). Mean CH4 flux measured with MMB (1.65 mg m−2 s−1) was 80% of that measured using the floating chamber method (2.37 mg m−2 s−1). The largest difference in mean flux occurred when the chamber was at L2, where high temporal variability in half-hourly fluxes due to enhanced CH4 ebullition was observed. Fluxes measured using the MMB method were averaged over a larger area (29–134 m2 vs. 0.37 m2 for chamber), so that areas outside this ‘hot-spot’ were also considered. No significant differences in the mean fluxes obtained by both methods were observed in L3 and L4. Hence, flux overestimation by the floating chamber was related to chamber placement in relation to manure inlet were VS accumulated. Mean CH4 flux obtained with the chamber was much higher than predicted. Agreement between CH4 flux means obtained with the MMB and chamber method during the latter part of the study, and the fact that fluxes obtained with the MMB method were constrained within the predicted flux range increases our confidence in the MMB approach used.