Estimation of charcoal (char) in soils

A method for the semiquantitative estimation of charcoal (char) in soils is reported. The technique, performed on the <53 mu m fraction of soils, utilizes the highly aromatic nature of char and its relative stability to two hours of high energy ultraviolet photo-oxidation compared to other soil carbon fractions. The proportion of char in the <53 mu m fraction is estimated as the aromatic carbon (C) content measured by solid-state nuclear magnetic resonance spectroscopy using cross polarization and magic angle spinning. Corrections are made for the underestimate of aryl C by the cross polarization method in highly condensed aromatic char structures and for the small lignin content that occurs in some, samples. Evidence is also provided for the occurrence of >90% of soil char in the <53 mu m fraction. Because of the assumptions used, estimates of char by this approach must be considered to be semiquantitative, but are conservative and provide estimates of the minimum char content of a soil.