Biochar increases tree biomass in a managed boreal forest, but does not alter N2O, CH4, and CO2 emissions

Biochar soil amendment may provide the forestry sector with a formidable
tool to simultaneously sequester carbon (C) in the soil and aboveground by
enhancing plant productivity, yet several key uncertainties remain.
Crucially, empirical evidence of long-term effects of biochar management
on vegetation and on greenhouse gas emissions in forest ecosystems is
scarce. Using a large field experiment in a young managed boreal forest in
northern Sweden, we investigated the effects of biochar (applied either on
the soil surface or mixed in the soil 8–9 years prior to this study) on
supply rates of soil nutrients, on survival and growth of planted Pinus
sylvestris, on community composition of the understory vegetation, and on
forest floor fluxes of N2O, CH4 and CO2. We found that biochar promoted P.
sylvestris survival only when biochar was applied on the soil surface.
Conversely, biochar enhanced P. sylvestris growth overall, resulting in a
19 % increase in C stored in biomass. Biochar also altered the composition
of the understory vegetation, especially when mixed into the soil, and
promoted a more resource-conservative community (i.e., with more
ericaceous shrubs and less graminoids and forbs). Meanwhile, supply rates
of the main soil nutrients were largely unaffected by biochar. Finally, we
found that biochar did not alter overall N2O and CO2 emissions and CH4
uptake from the forest floor. Our findings show that biochar amendment
increased the net C input to the system, since, besides directly
increasing soil C stocks, biochar enhanced biomass growth without
increasing soil C losses. Therefore, our study suggests that biochar could
potentially be used for emissions abatement in intensively managed boreal
forests.