Characterization of allophanic Andisols by solid-state ¹³C, ²⁷Al, and ²⁹Si NMR and by C stable isotopic ratio, δ¹³C

Three representative allophanic Andisols in Japan were evaluated by solid-state ¹³C, ²⁷Al, and ²⁹Si nuclear magnetic resonance (NMR) and stable isotopic ratio of carbon (δ¹³C). The solid-state cross polarization magic angle spinning (CPMAS) ¹³C NMR spectra were effective in characterizing crude soil C without any chemical treatment when soil C content was > 100 g C kg^{- 1}. Aliphatic, O-alkyl, and carbonyl C were relatively abundant in the uppermost horizons, whereas aromatic C was concentrated in the subsurface horizons, showing its persistence and tolerance to degradation in subsurface horizons. Contribution ratios of C4-plant-derived C (mainly from Miscanthus sinensis) on the total CR_C4 which were evaluated from δ¹³C value, were 35% to 42%, 59% to 62%, and 50% to 53% in subsurface horizons, and 12%, 23%, and 48% in uppermost horizons, for the three soils. The decline in CR_C4 values in the uppermost horizons could be an effect of recent vegetation. Solid-state ²⁷Al and ²⁹Si NMR revealed that most part of the tetrahedral Al in volcanic glass had already weathered into octahedral Al and a large amount of allophanic constituents (allophane, imogolite, allophane-like constituents including protoimogolite) was formed in B horizon within the past 25,000 ¹⁴C y. Allophanic constituents determined by ²⁹Si NMR were compared with those dissolved by acid-oxalate, and differences between them were discussed.