Short-term effects of biochar on soil CO2 efflux in boreal Scots pine forests

The use of biochar as a soil amendment has been proposed to increase the carbon (C) sequestration in soils. However, a more rapid soil organic matter turnover after biochar application might reduce the effectiveness of biochar applications for C sequestration. Data on the effects of biochar on soil C turnover is particularly important in boreal forests where large quantities of forest harvest residues would be available as feedstock for biochar production. To better understand the effects of biochar on boreal forest soil, we established a split-plot experiment where two spruce biochar produced with different temperatures (500°C and 650°C) were applied at a rate of 1.0 kg m^-2 and 0.5 kg m^-2 in a young xeric Scots pine forest in southern Finland. Measurement of soil CO₂ effluxes and microbial biomass were used to investigate changes in soil C dynamics. Biochar application increased the rate of soil CO₂ efflux by 10.6% across all biochar treatments and significantly (P<0.05) in 1.0 kg m^-2 treatments. Soil microbial biomass remained unchanged. Soil temperature was 0.1 to 0.5°C higher in the biochar-amended treatments. Further analysis revealed that when soil CO₂ efflux was corrected for the changes in soil temperature and soil moisture, there were no significant differences between treatments. We conclude that increase in soil CO₂ efflux was attributed to warmer soils at the initial stage after biochar application to the soil surface; changes in soil chemical properties did not have any detectable effect on soil respiration.