Impact of combined application of biochar and basanite powder on soil-borne greenhouse gas emissions and nitrate leaching

Research into methods for carbon dioxide removing (CDR) is experiencing strong growth worldwide. To protect the climate, not only greenhouse gas emissions need to be reduced, but in addition, carbon sinks have to be created. At the same time, we need measures to adapt to climate change, especially in agriculture. The PyMiCCS project (Pyrolysis and Mineral Weathering for Carbon Capture and Storage) consortium under the umbrella of the CDRterra research line investigate the CDR potential and synergies of a combination of the CDR methods “biochar” and “enhanced weathering” (EW) as soil amendments. Biochar is the solid product of biomass pyrolysis and contains persistent carbonaceous compounds whereas the weathering of rock powder in agricultural soil results in the transfer of atmospheric CO₂ into dissolved bicarbonate. It is known that biochar has the potential to positively influence agronomically relevant parameters and to reduce soil-borne nitrous oxide (N₂O) emissions, while rock powder shows neutral to positive agronomical effects.

To identify synergistic effects of biochar and EW, cabbage turnip (Brassica oleracea L.) was grown in sandy soil in pseudo-lysimeters enriched with four different amendments in two application rates and compared with an untreated control variant: 1) application of wood biochar or 2) rock powder, 3) co-application of both and 4) rock-enriched biochar, produced by co-pyrolysis of wood and rock powder. Nitrate leaching and greenhouse gas emissions were measured over the cultivation period. At harvest, the yield was determined and soil samples were analyzed for enzyme kinetics from project partners.

Depending on the application rate, either no significant effects on yield were found or a significant yield increase was observed in all variants involving wood biochar, with no difference between the sole application of biochar, its co-application with rock powder, and the rock-enriched co-pyrolysis variant. Likewise, no significant differences were observed between these variants in the amount of nitrate leached, whereas the difference to the control was always clear: the biochar variants significantly reduced nitrate leaching. A similar pattern was also observed for N₂O. Our results indicate that biochar and EW can be combined without adverse interactions on the parameters studied and that rock enhancement did not negate biochar’s positive environmental effects, but synergistic effects have not yet been demonstrated.