Electron spin resonance characteristics of humic acids from a wide range of soil types

Hyperfine splitting observed in the ESR spectra of a range of Ando soil humic acids dissolved in 0.1 M KOH was similar, while the peak strength decreased with increasing degree of humification. Although continuous application of manure to upland field soils scarcely affected the free-radical concentration in their humic acids, their hyperfine structure was weakened. The solution ESR spectra of the green fraction, which contains a chromophore related to 4,9-dihydroxyperylene-3,10-quinone (DHPQ) structure, was composed of basic features with an additional hyperfine structure superimposed on them. Since the basic features were consistent with the hyperfine structure of humic acids from Ando soils and the H layer of a Podzol which do not contain the green fraction, it was considered that the additional hyperfine structure is associated with DHPQ and thus the influence of DHPQ on the ESR spectra of humic acids is minor.