Biochar chemistry defined by 13 C-CPMAS NMR explains opposite effects on soilborne microbes and crop plants

Numerous recent studies have demonstrated that biochar may significantly reduce the incidence of plant diseases caused by airborne and soilborne pathogens, although contrasting results have also been reported. In this work, we investigated how biochar affects crop plant and soilborne microbe growth. Aims of this study were: i) to analyze the chemical changes occurring in four organic feedstocks (e.g. wood chips, organic urban waste, Zea mays residues, and Medicago sativa hay) when pyrolyzed at 300 °C and 550 °C by using 13 C NMR spectroscopy and SEM (Scanning Electron Microscopy); ii) to assess how biochar affects growth of five bacteria, nine fungi, and three crop plants; and iii) clarify the relationships between biochar chemistry and its effect on target species. As pyrolyzation temperature increased, organic matter chemistry of all products changed significantly, with a progressive loss of O-alkyl C, di-O-alkyl C, and methoxyl and N-alkyl C, coupled with an enrichment in aromatic C types. Untreated urban waste and Medicago hay severely inhibited Lepidium, Lactuca and Solanum root growth, whereas no inhibitory effects were found for the other feedstocks. However, these phytotoxic effects largely decreased after pyrolyzation. In contrast to the crop plants, fungi and bacteria thrive on most of the unprocessed organic materials but showed reduced growth and development or complete growth inhibition on biochars obtained at 300 °C and 550 °C. Soilborne microbes demonstrated remarkably similar correlation patterns between their growth to the organic feedstock and biochar chemical components. This work demonstrates that defining organic matter quality by 13 C NMR extends our understanding of the impact of biochar on crop plants and key components of the soil food-web.