Field-scale performance of biochar-amended soil covers for landfill methane oxidation

A field validation of three biochar-amended soil covers (2%, 10%, and 100% biochar-amended soils) along with a soil control cover was conducted within the intermediate cover of an active municipal solid waste (MSW) landfill in conjunction with laboratory studies evaluating the effect of biochar in enhancing methane (CH4) oxidation in cover soils. Baseline CH4 emissions and pre-existing site conditions were characterized prior to installation of test plots simulating three cover designs evaluated in related laboratory studies. Static chamber measurements of surface CH4 fluxes and sampling of soil pore gas at different depths were conducted across the 8-month monitoring period to assess cover performance. Surface fluxes from the test plots exhibited wide spatial variability, with one location emitting fluxes > 1100 g CH4 m−2 day−1 in one survey. Potential rates of CH4 oxidation were determined in batch assays of exhumed soil core subsamples following termination of the field trial and ranged from ~1 to 350 μg CH4 g−1 day−1. The heterogeneity of the waste led to nonuniform CH4 loads in the test plots. The soil control test plot was exposed to higher CH4 loads and biochar-amended test plots were exposed to significantly lower CH4 loads. As a result, the soil control test plot showed higher CH4 oxidation rates (257–289 μg CH4 g−1 day−1) than the biochar-amended test plots. Similarly, the soil control plot also showed higher relative abundance of methanotrophs which was positively correlated with the CH4 oxidation rates. The test plot with 10% biochar-amended soil experienced CH4 loads nearly 25% of that in soil control and still showed CH4 oxidation rates (260 μg CH4 g−1 day−1) comparable to that of soil control which showed the efficacy of biochar amendment in enhancing CH4 oxidation rates. The environmental and CH4 exposure conditions affected the microbial community composition in the test plots and showed the dominance of type I methanotrophic genus such as Methylomonas and Crenothrix spp. Overall, the waste heterogeneity led to nonuniform CH4 exposure conditions at each test plot making it hard to distinctly quantify the effect of biochar amendment on CH4 oxidation rates.