Soil carbon increased by twice the amount of biochar carbon applied after 6 years: Field evidence of negative priming

Applying biochar to agricultural soils has been proposed as a means of sequestering carbon (C) while simultaneously enhancing soil health and agricultural sustainability. However, our understanding of the long-term effects of biochar and annual versus perennial cropping systems and their interactions on soil properties under field conditions is limited. We quantified changes in soil C concentration and stocks, and other soil properties 6 years after biochar applications to corn (Zea mays L.) and dedicated bioenergy crops on a Midwestern US soil. Treatments were as follows: no-till continuous corn, Liberty switchgrass (Panicum virgatum L.), and low-diversity prairie grasses, 45% big bluestem (Andropogon gerardii), 45% Indiangrass (Sorghastrum nutans), and 10% sideoats grama (Bouteloua curtipendula), as main plots, and wood biochar (9.3 Mg/ha with 63% total C) and no biochar applications as subplots. Biochar-amended plots accumulated more C (14.07 Mg soil C/ha vs. 2.25 Mg soil C/ha) than non-biochar-amended plots in the 0–30 cm soil depth but other soil properties were not significantly affected by the biochar amendments. The total increase in C stocks in the biochar-amended plots was nearly twice (14.07 Mg soil C/ha) the amount of C added with biochar 6 years earlier (7.25 Mg biochar C/ha), suggesting a negative priming effect of biochar on formation and/or mineralization of native soil organic C. Dedicated bioenergy crops increased soil C concentration by 79% and improved both aggregation and plant available water in the 0–5 cm soil depth. Biochar did not interact with the cropping systems. Overall, biochar has the potential to increase soil C stocks both directly and through negative priming, but, in this study, it had limited effects on other soil properties after 6 years.