Long-term effects of softwood biochars on boreal soils: results from two experiments through 13 years on soils, nutrient cycling and crops

Biochars have notable potential in sequestering atmospheric carbon over long terms and offer various agricultural and environmental co-benefits. However, there is limited information about the long-term effects of added biochars, as there are only handful of continuously monitored biochar field experiments expanding over a decade, and especially poorly have been studied the long-term effects from boreal regions. The soils in northern colder boreal regions typically have higher carbon content and undergo continuous freeze-thaw cycles. Therefore, effects of biochars in these regions may differ from those observed in warmer climates.

We studied the effects of a single application of softwood biochars on two contrasting boreal agricultural soils (nutrient-poor, coarse textured Umbrisol and fertile, fine-textured Stagnosol), both with high initial soil organic carbon contents, over 13 years following the application in 2010 or 2011. We focused on plant yield formation as well as nutrient uptake dynamics of all major Finnish field crops, as well as on soil physical properties and greenhouse gas emissions.

We found that the ability of biochar to enhance the supply of nutrients to plants and hence to improve the crop biomass yield exists in boreal conditions, although these effects were minimal and not consistent over the years. Biochar notably increased plant K content, and reduced the plant content and uptake of Al and Na in several years in Stagnosol. The relative plant contents of Cd and Ni in Umbrisol, and P, K, Mg, S, Al, Cu, Fe and Ni in Stagnosol increased over the years. Despite these increased plant contents, no significant improvement was observed in crop biomass yield by added biochar over the first eight years.

The enhanced plant available water and reduced bulk density previously reported during the initial years were faded in long-term, likely due to dilution of biochar concentration in topsoil. However, the potential of biochar to affect N₂O emission persisted, even seven years after the application, this contributed to reduced yield-normalized non-CO₂ GHG emissions. In the presentation, we will share also the preliminary results for the latest growing seasons 2019–2022.