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Soil Syst. 2018, 2(2), 29; https://doi.org/10.3390/soilsystems2020029

Distinctive Roles of Two Aggregate Binding Agents in Allophanic Andisols: Young Carbon and Poorly-Crystalline Metal Phases with Old Carbon

1
National Agriculture and Food Research Organization, Institute for Agro-Environmental Sciences, 3-1-3 Kan-Nondai, Tsukuba 305-8604, Ibaraki, Japan
2
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Ibaraki, Japan
3
Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8572, Ibaraki, Japan
*
Author to whom correspondence should be addressed.
Received: 3 December 2017 / Revised: 20 April 2018 / Accepted: 27 April 2018 / Published: 7 May 2018
(This article belongs to the Special Issue Soil Organic Matter Dynamics)
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Abstract

Interaction of organic matter (OM) with soil mineral components plays a critical role in biophysical organization (aggregate structure) as well as in biogeochemical cycling of major elements. Of the mineral components, poorly-crystalline phases rich in iron (Fe) and aluminum (Al) are highly reactive and thus contribute to both OM stabilization and aggregation. However, the functional relationship among the reactive metal phases, C stability, and aggregation remains elusive. We hypothesized that relatively young C acts as a binding agent to form the aggregates of weak physical stability, whereas the reactive metal phases and older C bound to them contribute to stronger aggregation. Using four surface horizons of Andisols having a gradient of soil C concentration due to decadal OM management, we conducted sequential density fractionation to isolate six fractions (from <1.6 to >2.5 g cm−3) with mechanical shaking, followed by selective dissolution and radiocarbon analysis. After 28 years of no-till with litter compost addition, not only C and N but inorganic materials including the reactive metal phases (pyrophosphate-, oxalate-, and dithionite-extractable metals) showed clear shifts in their concentrations towards lower-density fractions (especially <2.0 g cm−3) on a ground-area basis. This result was explained by the binding of compost-derived OM with soil particles. Major portions of the reactive metal phases in bulk samples were distributed in mid-density fractions (2.0–2.5 g cm−3) largely as sonication-resistant aggregates. Theoretical density calculations, together with depletion in radiocarbon (Δ14C: −82 to −170‰) and lower C:N ratio, implied that the sorptive capacity of the reactive metal phases in these fractions were roughly saturated with pre-existing OM. However, the influx of the compost-derived, modern C into the mid-density fractions detected by the paired-plot comparison suggests decadal C sink in association with the reactive metal phase. Our results supported the concept of aggregate hierarchy and further provided the following new insights. At the high hierarchy level where shaking-resistant aggregates form, soil organo-mineral particles appeared to be under a dynamic equilibrium and the changes in OM input regime controlled (dis)aggregation behavior due to the binding effect of relatively young C. At a lower hierarchy level, the reactive metal phases were bound to N-rich, 14C-depleted OM and together functioned as persistent binding agent. Our study suggests that the recognition of binding agents and aggregate hierarchy level would help to untangle the complex organo-mineral interactions and to better understand soil C stability. View Full-Text
Keywords: organo-mineral interaction; density fractionation; Selective dissolution; short-range-order minerals; radiocarbon; conservation agriculture; long-term field experiment; andosol; volcanic soil; aggregate hierarchy organo-mineral interaction; density fractionation; Selective dissolution; short-range-order minerals; radiocarbon; conservation agriculture; long-term field experiment; andosol; volcanic soil; aggregate hierarchy
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Wagai, R.; Kajiura, M.; Uchida, M.; Asano, M. Distinctive Roles of Two Aggregate Binding Agents in Allophanic Andisols: Young Carbon and Poorly-Crystalline Metal Phases with Old Carbon. Soil Syst. 2018, 2, 29.

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