Methane oxidation activity inhibition via high amount aged biochar application in paddy soil

Fresh biochar application has been recommended as an effective strategy to mitigate methane emissions from paddy soil due to its role in promoting potential methane oxidation (PMO). However, little is known about the effect of biochar on PMO after long-term aging. Here, a laboratory incubation experiment was performed to gain insight into the combined effects of biochar characteristics on soil PMO during the biochar aging process. Biochar samples aged with an HNO₃/H₂SO₄ mixture (1:3) were prepared and added at 22.5 and 2.8 t ha^-1 to paddy soil. Mixed acid with a dilution of 10- 40% indicated the aging process in paddy field properly. Smaller biochar particles, smoother surface, higher surface area and porosity were observed with the increase of mixed acid ratio. Carboxylic functional groups (COOR) on biochar surface increased accompanied with biochar aging process. Aged biochar application at 22.5 t ha^-1 inhibited paddy soil PMO, which mainly caused from the increased COOR groups induced NH₄⁺-N inhibition for PMO. Increased soil NH₄⁺-N inhibited PMO by limiting the growth of methanotrophs and nitrifiers due to decreased soil pH. In contrast, 2.8 t aged biochar ha^-1 application showed no significant difference on PMO compared with control. Our results indicated that aged biochar exerted in paddy soil have limited effect on mitigating methane emissions from paddy soil in the long term.