LAUSR

Key message During the first summer, wood biochar amendments increased soil temperature, pH, and soil CO 2 effluxes in a xeric boreal Scots pine forest. The increase of soil CO 2 efflux could be largely explained by increases in by soil temperature. Higher biochar application rates (1.0 vs 0.5 kg m −2 ) led to higher soil CO 2 efflux while the pyrolysis temperature of biochar (500 or 650 °C) had no effect on soil CO 2 efflux. Context Using biochar as a soil amendment has been proposed to increase the carbon sequestration in soils. However, a more rapid soil organic matter turnover after biochar application might reduce the effectiveness of biochar applications for carbon sequestration. By raising the pyrolysis temperature, biochar with lower contents of labile carbohydrates can be produced. Aims To better understand the effects of biochar on boreal forest soil, we applied two spruce biochar with different pyrolysis temperatures (500 °C and 650 °C) at amounts of 1.0 and 0.5 kg m −2 in a young xeric Scots pine forest in southern Finland. Methods Soil CO 2 , microbial biomass, and physiochemical properties were measured to track changes after biochar application during the first summer. Results Soil CO 2 increased 14.3% in 1.0 kg m −2 treatments and 4.6% in 0.5 kg m −2 . Soil temperature and pH were obviously higher in the 1.0 kg m −2 treatments. Differences in soil CO 2 among treatments disappear after correcting by soil temperature and soil moisture. Conclusion Biochar increased soil CO 2 mainly by raising soil temperature in the short term. Higher biochar application rates led to higher soil CO 2 effluxes. The increase in soil CO 2 efflux may be transient. More studies are needed to get the optimum biochar amount for carbon sequestration in boreal forest.