Forms of organic C and P extracted from tropical soils as assessed by liquid-state C-13- and P-31-NMR spectroscopy

Transformation of soil organic phosphorus (SOP) is linked with the transfor mation of soil organic carbon (SOC). Yet, it is uncertain to which SOC stru ctures the cycling of SOP is related, especially in tropical environments. To clarify this issue, we determined the vertical distribution of extractab le C and P chemical structures in 4 soil profiles using solution C-13- and P-31- nuclear magnetic resonance (NMR) spectroscopy after extraction with 0 .1 M NaOH/0.4 M NaF (1:1). Soils were from a cabbage cultivation with annua l burning of weeds, a Pinus reforestation, a secondary forest, and a primar y forest in northern Thailand. For all profiles, signals due to O-alkyl and carbonyl C dominated the C-13- NMR spectra (up to 50 and 22% of total spec tral area, respectively). The proportions of alkyl and aryl C decreased, wh ereas carbonyl and O-alkyl C increased with soil depth. Sharp resonances at 135 and 177 ppm appeared in spectra of subsoil horizons. They indicated me llitic acid, an end- product of the oxidation of charred plant residues. Th e SOP forms comprised mainly orthophosphate diesters in the organic layer o f the forests, whereas in the mineral horizons orthophosphate monoesters do minated the chemical composition of extractable SOP. The relationships between SOC and SOP forms in the organic floor layers of the forests were clearly different from those in the mineral soil horizons, indicating changed SOM dynamics upon contact with soil minerals. In the fo rest mineral soils, significant correlations between monoester-P and O-alky l C (R = 0.84, P < 0.001) were found. Diester- P, teichoic acids, and phosp honates were positively correlated with aromatic C and negatively with O-al kyl C. At the same time, teichoic acids and phosphonates were positively co rrelated with short range-ordered Al and Fe oxide phases. These findings ca n be explained through an increasing microbial decay of aryl C and diester- P compounds that may be less effectively stabilised at lower depths.