The role of biochar in the mitigation of CO2 emissions has been extensively studied in agricultural soils but is not well understood in Japanese forest soils, especially in relation to CO2 emissions from applied biochar and native soil C (i.e., the priming effect; PE). We hypothesized that the type of biochar and/or the application method (mixed or sprinkled) affect the direction and magnitude of PE in forest soil, and in particular, negative PE can be achieved relatively easily if biochar produced under higher temperature conditions were sprinkled on the soil surface. To test our hypothesis, we measured CO2 emissions from biochar-amended brown forest soil in Japan and examined its PE by conducting a medium-term (~4 months) incubation study. As substrates, we used plain straw from the C4 grass Miscanthus sinensis (SU) and two qualities of biochar produced from it at either 300°C (BC300) or 800°C (BC800) and compared two application methods: mixed into or sprinkled onto the soil. BC800 had a greater C content and C:N ratio as well a lower volatile matter content and higher nonvolatile matter content than BC300. SU had the lowest C content and C:N ratio of all the substrates. We found that biochar quality (volatile and nonvolatile matter content) was clearly related to the decomposition rate when mixed into the soil, but we could not find this relationship when biochar was sprinkled onto the soil. The addition of biochar to the soil induced a positive PE in the early stages (except for BC800 sprinkled) but suppressed CO2 emissions from native soil organic matter (negative PE) in the later stages, whereas the PE with SU application was always positive regardless of application method. Our results suggest that when biochar is sprinkled onto soil it is more likely to suppress soil-derived CO2 emission than when mixed in soil; however, the trend based on biochar quality was unclear.
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