Amending soil with biochar (pyrolized biomass) is suggested as a globally applicable approach to address climate change and soil degradation by carbon sequestration, reducing soil-borne greenhouse-gas emissions and increasing soil nutrient retention. Biochar was shown to promote plant growth, especially when combined with nutrient-rich organic matter, e.g., co-composted biochar. Plant growth promotion was explained by slow release of nutrients, although a mechanistic understanding of nutrient storage in biochar is missing. Here we identify a complex, nutrient-rich organic coating on co-composted biochar that covers the outer and inner (pore) surfaces of biochar particles using high-resolution spectro(micro)scopy and mass spectrometry. Fast field cycling nuclear magnetic resonance, electrochemical analysis and gas adsorption demonstrated that this coating adds hydrophilicity, redox-active moieties, and additional mesoporosity, which strengthens biochar-water interactions and thus enhances nutrient retention. This implies that the functioning of biochar in soil is determined by the formation of an organic coating, rather than biochar surface oxidation, as previously suggested.
Bibliographical noteFunding Information:
We thank E. Struve, K. Mayer and P. Riede for laboratory support, G. Müller for support at Canadian Light Source, P. Ingio and F. Zeitvogel for support during STXM data evaluation, H. Chen for support at NHFML, W. Gerber for photographs, D. Mitchell for input on EELS and A. Flicker and M. Nowak for FTIR access and support. N.H. was financially supported by a BMBF PhD scholarship provided by the Rosa Luxemburg Foundation, Berlin, Germany. T.B. and R.B.Y. were supported by the Agriculture and Food Research Initiative Competitive (grant no. 2013-67019-21359) from the USDA National Institute of Food and Agriculture and a National Science Foundation SusChEM Award (grant no. EAR1451494). The co-composted biochar was obtained from a compost that was produced during a ‘Short Term Scientific Mission’ (STSM) granted to NH by the EU COST Action TD1107 ‘Biochar as option for sustainable resource management’. Access to the ASTEM Titan3 60–300 at TU Graz was enabled by European Union through the EC Grant ESTEEM2 (20141212‐Hagemann and 20150703-Hagemann). Research described in this work was performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada and the University of Saskatchewan. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Division of Materials Research through DMR 11-54790, the State of Florida and Florida State University. We thank E. Graber and K. Spokas for valuable discussions.