Vertical biochar transport in soil in a long-term field experiment in Germany

The long-term stability of biochar is valuable for carbon sequestration in soil. Biochar’s aromatic structure inhibits microbial decomposition and prolongs the mean residence time of this organic soil amendment. During residence in soil, biochar particles do not remain rigidly in place but experience several dissipation processes, including vertical and lateral transport. However, the quantitative dissipation of biochar under field conditions remains unclear due to lacking long-term field observations.

Uncertainties concerning decreasing topsoil biochar stocks were inspected in a long-term field experiment located on a loamy soil and under humid temperate conditions in Bayreuth, Southern Germany. Industrially produced biochar was applied 14 years ago. Four differing amendments were arranged in a Latin rectangle experimental design: unamended control, pristine biochar (31.5 Mg ha⁻¹), biochar mixed with compost (31.5 Mg ha⁻¹ and 70.0 Mg ha⁻¹, respectively), and co-composted biochar with addition before composting (31.5 Mg ha⁻¹ and 70.0 Mg ha⁻¹, respectively). Soil samples were retrieved in 30 cm intervals to a depth of 90 cm. Benzene polycarboxylic acids (BPCA) were analyzed as a molecular marker for biochar. Our results indicate vertical biochar transport exceeding mechanical influence by agricultural practices. While treatments differed, all amendments containing biochar showed a trend of particle transport. The highest amounts were found in soil treated with pristine biochar, while the difference between treatments decreased with increasing soil depth. Furthermore, the contribution of individual BPCAs differed between depths. Relatively, higher condensed BPCA tended to decrease vertically. This indicates a preferential vertical transport of less condensed BPCA.

This study proves vertical particle transport of biochar in soil. This valuable insight partially explains decreasing biochar stocks near the surface with increasing time. Downward movement can be beneficial for carbon sequestration in soil due to generally reduced microbial activity at lower depths. However, hereby provided and associated organic matter may alter microbial abundance and must be studied further, as must the apparent effect of different amendments and selective transport of different BPCA. This future research bears implications on biochar’s mechanisms as a carbon sequestration technology in mitigating climate change.

 

Keywords: Biochar aging, biochar transport, carbon sequestration, climate change mitigation, molecular marker, organic soil amendment, pyrogenic carbon, soil organic carbon

Funding information: EU grant no. 101059546-TwinSubDyn