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

Rota Wagai, Masako Kajiura, Maki Asano, Syuntaro Hiradate

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Organic matter (OM) bound to soil mineral particles (higher-density particles) tends to be more stabilized, enriched in 13 C and 15 N, 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 13 C NMR spectroscopy and δ 13 C and δ 15 N 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 δ 15 N 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 δ 15 N, together with theoretical density calculations of idealistic organo-mineral association modes, suggests that 15 N 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 13 C and 15 N 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.

Original languageEnglish
Pages (from-to)295-305
Number of pages11
JournalGeoderma
Volume241-242
DOIs
Publication statusPublished - Mar 1 2015
Externally publishedYes

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allophanic soils
fractionation
minerals
mineral
soil organic matter
organic matter
soil
Andisol
Andisols
metals
carbon nitrogen ratio
detritus
chemistry
volcanic soils
volcanic soil
soil chemistry
metal
pyrophosphates
air drying
oxalates

All Science Journal Classification (ASJC) codes

  • Soil Science

Cite this

Nature of soil organo-mineral assemblage examined by sequential density fractionation with and without sonication : Is allophanic soil different? / Wagai, Rota; Kajiura, Masako; Asano, Maki; Hiradate, Syuntaro.

In: Geoderma, Vol. 241-242, 01.03.2015, p. 295-305.

Research output: Contribution to journalArticle

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N2 - Organic matter (OM) bound to soil mineral particles (higher-density particles) tends to be more stabilized, enriched in 13 C and 15 N, 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 13 C NMR spectroscopy and δ 13 C and δ 15 N 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 δ 15 N 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 δ 15 N, together with theoretical density calculations of idealistic organo-mineral association modes, suggests that 15 N 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 13 C and 15 N 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.

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