[M-IS03] Exploring the role of soil in earth science: ecological/biogeochemical linkage and beyond

[MIS03-P11] Soil organic matter and aggregate hierarchy revisied: a case study using an Andisol

*Rota WAGAI¹, Maki ASANO², Masako KAJIURA¹, Yudzuru INOUE³, Chie HAYAKAWA⁴, Syuntaro HIRADATE¹ (1.National Institute of Agro-Environmental Science, 2.University of Tsukuba, Graduate School of Life & Environmental Sciences, 3.Saitama University, Graduate School of Science and Engineering, 4.The University of Tokyo, Graduate School of Agricultural and Life Sciences)

Soil aggregate structure is likely to exert fundamental control on soil biology (e.g., microbial activity and community composition) and soil biogeochemistry including the stabilization and mobilization of soil organic matter (SOM). Aggregate hierarchy concept (Tidall and Oades, 1982) has been verified for most temperate and tropical soil types except for Andisols developed from tephra and volcanic rocks are known for strong aggregate stability and high SOM storage capacity. We recently showed the first evidence of hierarchical structure in Andisol (Asano and Wagai, 2013). After maximum dispersion (achieved only after 5 kJ/mL sonication following sodium saturation), most of macro- and micro-aggregates were broken down to much smaller particles (<0.2 and 0.2-2 um) that accounted for dominant proportion of total C, N, and extractable phases of Al, Si, and Fe. These <2um particle-size fractions appeared to act as persistent binding agent, contributing to the high physical stability of the aggregates. When the same soil was fractionated by density, the aggregates resistant to mechanical shaking (i.e. higher level in hierarchical structure) showed progressive changes in chemistry along density gradient (Wagai et al. 2014). Among the intermediate density fractions (2.0-2.5 g/cc) where >80% of total C and N and the extractable metals are present, we found general decrease in C:N ratio and delta 14C and concurrent enrichment of 13C and 15N in accord with previous reports.

The next key question is how the particle-size fractions isolatable after the maximum dispersion are assembled together to form the shaking-resistant aggregates of different density. We attempted to answer this question by examining the organic and inorganic chemistry (C, N, 13C, 15N, d14C, 13C-NMR, extractable metals) and the surface characteristics (SSA by N2-BET and XPS) of the size and density fractions. We will also discuss the nature of organo-mineral interactions inferred from our results and compare it with the previously-suggested concepts such as aggregate hierarchy (Tidall and Oades, 1982), and "zonal layering" model (Kleber et al. 2007).

References:
Asano, M and Wagai, R. (2014) Evidence of aggregate hierarchy at micro- to submicron scales in an allophanic Andisol. Geoderma. 216, 62-74.
Kleber, M. et al. (2007) A conceptual model of organo-mineral interactions in soils: self-assembly or organic molecular fragments into zonal structures on mineral surfaces. Biogeochem. 85: 9-24.
Tisdall, J.M. and Oades, J.M. (1982) Organic-matter and water-stable aggregates in soils. J. Soill Sci. 33: 141-163.
Wagai, R. et al. (2015) Nature of soil organo-mineral assemblage examined by sequential density fractionation with and without sonication: Is allophanic soil different? Geoderma 241-242: 295-305.