Application of biochars to soils is gaining popularity because of their ability for sequestering atmospheric carbon as well as bringing beneficial agricultural and environmental effects. Biochars have potential to improve soil fertility and enhance crop productivity as well as reduce greenhouse gas (GHG) emissions and nutrient leaching. However, one of the important knowledge gaps in biochar research is whether these beneficial effects exist after several years of field application.
To know this, we measured properties of plant and soil, fluxes of greenhouse gases, and leaching of nitrogen from four already established field experiments on various types of Finnish soils where biochars were applied 2-8 years prior to this study. This includes two of AgriChar long-term biochar experiments from Viikki, sampling was done in 2018.
The major effects observed were:
1) No effects of biochar on crop yield were observed on two field experiments but the spruce biochar increased barley grain yield by 65% after 7 years of field application in a coarse-textured Umbrisol in Viikki. As a result, CH4 + N2O emitted per ton of grain yield is reduced by 43%. Similarly, in a clayey Cambisol, where spruce and willow biochars were applied 2 years ago, only the spruce biochar significantly increased oat grain yield by 75%.
2) In the clayey Cambisol, the increased crop yield by the spruce biochar was related to its ability to hold nitrate ions, which was available for plant uptake. However, such effect was not observed for the willow biochar. We hypothesize that the ability of retaining soil nitrate is related to the surface area of biochar since the surface area of the spruce biochar (328 m2 g-1) was much higher than the willow biochar (1.3 m2 g-1).
3) In the same clayey Cambisol field, where mineral nitrogen leaching test was also measured, we found that biochar has potential to reduce N2O flux and nitrate leaching especially during the peak emission or leaching period. However, no clear effects on fluxes of N2O or CH4 were observed on other field experiments.
Based on this, it can be concluded that the positive effects of biochars such as increase in crop yield and plant N availability, decrease N2O emissions and N leaching could be observed even after several years of biochar application, however, it depends on soil and biochar types as well as the environmental conditions.
Please read the full study here: Subin Kalu et al. 2022. Potential of Biochar to Reduce Greenhouse Gas Emissions and Increase Nitrogen Use Efficiency in Boreal Arable Soils in the Long-Term