Characterisation of biochar amended soil by hydropyrolysis

Biochar, a solid product of biomass pyrolysis, attracts research interest due to its potential value for long-term carbon sequestration with additional agronomic benefits. The environmental potential and limitation of biochar in soil applications requires a full understanding of stability and the fate of carbon fractions and trace contaminants, such as PAHs. It is well known that a variable component of biochar is labile (degradable on annual/decadal timescales) and hence, only a proportion of total carbon in biochar provides long-term carbon sequestration. Among the various analytical techniques applied to the determination of the labile fraction and black carbon (BC) in soil, hydropyrolysis (HyPy) combined with GC-MS is an emerging approach not yet applied to biochar amended soils in long term studies. In HyPy, pyrolysis is assisted by high hydrogen pressure to facilitate the reductive removal of labile organic matter (defined as non-BCHyPy), so isolating a highly stable portion of the BC (BCHyPy) that is predominantly composed of >7 ring aromatic domains [1]. The aim of this study was to evaluate the fate of organic carbon of biochar amended soils in a four year field study. A large set of soils were amended with biochar from slow pyrolysis of pruning orchard in a vineyard at a rate of 22 t ha-1. Soils were sampled in three different years after treatment following a randomized block layout with 5 replicates. We used HyPy to remove the labile organic carbon from total organic carbon in biochar and amended soils in order to enable reliable quantification and characterisation of the BCHyPy and non-BCHyPy fractions. The distribution of PAHs generated by HyPy was compared to that of solvent extractable PAHs determined by a validated GC-MS procedure [2] to gather information on the leaching of PAHs from the biochar into the soil. [1] W. Meredith, P.L. Ascough, M.I. Bird, D.J. Large, C.E. Snape, Y. Sun, E.L. Tilston, Geochim. Cosmochim. Acta, 2012, 97, 131-147. [2] D. Fabbri, A.G. Rombolà, C. Torri, K.A. Spokas, J. Anal. Applied Pyrol., 2013, 103, 60-67