Millennium timescale carbon stability in an Andisol: How persistent are organo-metal complexes?

Abstract

Significant linear correlation between organic carbon (OC) and reactive aluminum and iron (metal) are often observed across a wide range of acidic soils including Andisols, and often explained by the formation of stable organo-metal complexes. However, the chemical variation and the long-term stability of the complexes remain unclear. We thus compared two buried A horizons (the burial period of 0.5 and 4.6 kyr, based on tephra chronology) to examine the changes in OC chemistry as well as its association with the metal phases extractable by sodium pyrophosphate (PP) and acid oxalate (OX). Both horizons were subdivided into six layers, and then the layer with the highest OC in each horizon was subjected to particle-size fractionation after maximum dispersion. 14C ages of the bulk and the size fractions were consistent with the tephra-based burial ages. Younger 14C age was shown for the organo-mineral fractions of finer sizes in both horizons, in accord with our previous study using a surface Andisol. PP-extractable metal concentration (metalPP) showed a highly significant linear correlation with OC across all the layers and the fractions from both horizons, whereas OX-extractable metal showed the correlation only within the younger buried horizon. Solid-state 13C NMR spectroscopy showed a stronger metalPP correlation with alkyl C than with aromatic C especially among the older horizon layers and the 14C age variation among the size fractions in this horizon was not related to aromaticity. Significant decline in the proportion of O-alkyl C and concurrent increase in that of aromatic C from the younger to older horizon may suggest preferential degradation of O-alkyl C under the buried condition. Over the burial time, however, the correlation between metalPP and aromatic C weakened whereas metalPP-alkyl C correlation increased. These results imply a shift in the dominant organic ligand in the organo-metal complexes. Furthermore, the shift in the distribution of total OC and metalPP from 0.2–2 μm fraction to <0.2 μm fraction suggested a physical weakening of microaggregates where organo-metal complexes likely acted as glue. The observed changes imply reduced OC stability, which may lead to the eventual disappearance of buried A horizons. The current study demonstrated that, while organo-metal complexes appeared persistent based on the OC-metalPP correlation, the changes in the nature of OC-metal association including the partial breakdown of microaggregates took place, in the millennium timescale.