Understanding the legacy impact of biochar on soil function and carbon stocks – evidence from a 13-year field experiment

Incorporation of biochar, a carbon (C) dense and stable (over centuries or millennia) product of pyrolysis of organic material, into soil (particularly agricultural soil) has been proposed as a potential method of atmospheric carbon dioxide removal (CDR). However, assessing the impact of biochar application on agricultural soils, particularly over time, will be key to understanding the wider impact on ecosystem function. Here, using one of the longest running biochar field experiments in the UK, we evaluate the soil biological, physical and chemical impact of biochar 13 years after the initial application, at a plot (bulk soil from 50 t ha-1 biochar application vs control) and ‘charosphere’ scale (soil brushed from the biochar surface, and the biochar surface itself), as well as the impact of field exposure on biochar C stability and textural properties.The organic C density of the biochar plots (4.89 kg C m-3) was higher than the control (3.32 kg m-3) plots, confirming the persistence of both biochar and soil derived organic C. Stable polycyclic aromatic carbon (SPAC) content, a measure of the long-term chemical stability of biochar C, of the original (non-field aged char) and field aged biochar was determined by hydropyrolysis (HyPy). The original biochar had a higher SPAC content compared to the field aged biochar, driven by one outlier, suggesting the initial biochar may have been heterogeneous in its quality and stability. Gas chromatography-mass spectrometry analysis of the HyPy-released labile fraction showed no compositional changes among samples with similar SPAC contents, indicating negligible degradation.16S and ITS rRNA sequencing revealed divergent trends in the beta diversity of bacterial and fungal communities. The 16S bacterial community associated with the biochar surface differed from the bulk control and biochar soils and soil brushed from the biochar surface. Conversely, the ITS fungal community was different in the bulk control soil compared to biochar associated soils (bulk biochar, soil brushed from the biochar surface and the biochar surface itself). Soil pH and nitrogen (N) availability seemed to be the drivers of differences in soil properties, with pH significantly higher in the soil brushed from the biochar surface (pH 6.67) and biochar (pH 6.79) itself than the bulk soil (pH 5.22 and pH 5.30 for the control and biochar bulk soil, respectively), while extractable and available N was highest in the soil brushed from the biochar surface.Overall, we show that, after 13 years, biochar application had a positive influence on soil C stocks. The chemical stability of the biochar had diminished by very little, with even the low quality (low SPAC content) char persisting. Soil function at a bulk soil level was relatively unchanged between control and biochar plots. However, at the charosphere (biochar surface) level, changes in the composition of the fungal and bacterial community may drive some changes in function, likely driven by soil pH and the biochar’s ability to retain nutrients, specifically N. The results presented here reinforce the durability of biochar application to soil as a CDR method, in the medium term.