Nature of soil organo-mineral assemblage examined by sequential density fractionation with and without sonication: Is allophanic soil different?

Organic matter (OM) bound to soil mineral particles (higher-density particles) tends to be more stabilized, enriched in ¹³C and ¹⁵N, and has a lower C:N ratio. Yet how these variations in OM chemistry are linked to the nature of organo-mineral assemblage remains poorly understood, especially in allophanic soils where high amounts of OM are stabilized by interactions with reactive inorganic phases such as short-range-order (SRO) minerals. We thus assessed the extent to which the degree of aggregation and its disruption during fractionation control the distribution and chemistry of the soil organo-mineral particles across six density fractions using a volcanic soil (allophanic Andisol) based on selective dissolution, microscopy (SEM), solid-state ¹³C NMR spectroscopy and δ¹³C and δ¹⁵N analyses. Intermediate-density fractions (2.0-2.5gcm^-3) accounted for 63-86% of organic C and N, 73-93% of pyrophosphate-extractable iron and aluminum (Fep, Alp), and 78-95% of oxalate-extractable metals (Feo, Alo) in the bulk soil sample. While air-drying pretreatment had little effect, sonication during fractionation led to (i) fragmentation of both plant detritus and some of the aggregates of 30-100mm sizes, (ii) release of occluded low-density fraction (<1.6gcm^-3) which largely originated from the aggregates of 1.6-2.0gcm^-3 density range, and (iii) redistribution of organo-mineral particles (15-16% of total OM and 7-19% of the extractable metals) within the intermediate density fractions. Positive correlation of Alp with C:N ratio and negative correlation of Alp with δ¹⁵N among the fractions suggest preferential binding of Alp phase (e.g., organo-Al complexes) to decaying plant detritus. Positive correlation of Alo and Feo with δ¹⁵N, together with theoretical density calculations of idealistic organo-mineral association modes, suggests that ¹⁵N enrichment may be coupled with OM binding to SRO minerals and with the formation of physically-stable aggregates of micron/submicron sizes in accord with our conceptual model (Asano and Wagai, 2014). The general pattern of ¹³C and ¹⁵N enrichment and C:N decline with increasing particle density remained largely unchanged despite the sonication effects detected, indicating that sonication-resistant organo-mineral assemblages largely control the observed patterns. The similarity in the density-dependent changes of OM chemistry between the studied Andisol and the soils with crystalline clay and metal oxide mineralogies in previous studies strongly suggests a common biogeochemical control which deserves further investigation.