Management of pasture soils: biochar stability, carbon storage potential and its effect on production and quality

The use of biochar has been proposed as a stable carbon (C) amendment with long-term carbon (C) storage potential in agricultural soils while improving primary productivity. However, this concept has not been widely tested in contrasting soils under temperate pasture systems. To address this knowledge gap, a ¹³C-labelled biochar, produced from Eucalyptus saligna biomass by slow pyrolysis (450° C; δ¹³C -36.7‰) was surface (0−10 cm) applied in C₃ dominated, annual temperate pasture systems across Arenosol, Cambisol and Ferralsol. The results show that only 2% of the applied biochar-C was mineralised in a relatively clay- and C-poor Arenosol, 4.6% in a clay- and C-rich Cambisol, and 7% in a clay- and C-rich and earthworm-abundant Ferralsol over 12 months. Biochar application increased soil C stock, while the mean residence time of biochar-C, an indicator of its stability in soil, decreased with increasing native C content and/or pasture productivity across the soils i.e. Arenosol (71 years) < Cambisol (39 years) < Ferralsol (29 years). Biochar application increased pasture growth rate only on two occasions over 12 months in the Ferralsol but not in the other pasture-soil systems. The biochar-C recovery to 12−30 cm depth varied as 1.2% (Arenosol), 2.7% (Cambisol) and 15.7% (Ferralsol) after 12 months. Cumulative CO₂-C emission from native soil-plant sources was lower (p <0.10) in the biochar-amended vs.via mineralisation in the Arenosol and Ferralsol but in the Cambisol. This migration of biochar to deeper soil layers could enhance C sequestration potential in soil systems.