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Biochar application to agricultural soils is gaining acceptance as a method to increase the soil organic C reserves, thereby mitigating climate change. Biochar-amended soils have lower nitrous oxide (N2O) emissions in many cases, but the mechanisms are not well established. Denitrification, the microbially-mediated reduction of NO3 to N2, is the dominant source of N2O emitted from agricultural soils. The aim of our study was to determine the influence of biochar on denitrification and elucidate possible mechanisms. The experimental unit was a flask containing 20 g of soil mixed with biochar, with the following treatments: 2 soil types (surface and subsurface soil), 4 biochar rates (0, 0.2, 0.4 and 0.6 g), 2 nitrogen fertilizer rates (0 and 100 mg NO3-N) and 2 acetylene levels (0 and 10% headspace), arranged as a full factorial with 5 replicates. Soil moisture content was adjusted to 84% water-filled pore space, headspace was flushed with argon and acetylene added (10% treatment only) and flasks were incubated at 25oC for 24 hours. Headspace gases were sampled from each flask at 0, 2, 4 and 6 hours after the incubation ended and the N2O concentration was measured with a gas chromatograph. There was a significant reduction in N2O emission with increasing rate of biochar in the surface soil but not in the subsurface soil. The difference suggests a smaller denitrifying community or less available substrates in the subsoil. These results suggest biochar can affect denitrifying activities and thus reduce the N2O emission from soil.
biochar, denitrification, mechanism, nitrous oxide, anaerobic
Soil and water