Elsevier

Geoderma

Volume 375, 1 October 2020, 114483
Geoderma

Organo-mineral interactions and soil carbon mineralizability with variable saturation cycle frequency

https://doi.org/10.1016/j.geoderma.2020.114483Get rights and content

Highlights

  • Organo-mineral associations were Al-dominated at high saturation frequency.

  • Carboxylic and aromatic C chemically interacted with Fe(III) at the molecular scale.

  • C mineralizability (per unit SOC) increased (2X) at high saturation frequency.

  • Post-anaerobic DOC mineralizability increased (10X) at high saturation frequency.

Abstract

The response of mineral-stabilized soil organic carbon (SOC) to environmental change is a source of uncertainty in the understanding of SOC cycling. Fluctuating wet-dry cycles and associated redox changes in otherwise well-drained soils may drive mineral dissolution, organic carbon (OC) mobilization, and subsequent OC mineralization. However, the extent to which rapid fluctuations between water-saturated and unsaturated conditions (i.e., flashy conditions) result in long-term changes in mineral composition and organo-mineral interactions is not well understood. In this study, the effect of variable saturation frequency on soil mineral composition, mineral-associated OC, and OC mineralizability was tested using selective dissolution, bulk spectroscopy, microscale imaging, and aerobic-anaerobic incubation experiments. Previous water table fluctuation measurements and diagnostic profile characteristics at Hubbard Brook Experimental Forest (NH) were used to identify soils with high, medium, and low saturation frequency regimes (defined by historical water table cycling frequency; i.e., water table presence and recession in the upper B horizon). We found the amount of OC released during extractions targeting non-crystalline minerals was of similar magnitude as extracted iron (Fe) in lower saturation frequency soils. However, the magnitude of extracted OC was 2.5 times greater than Fe but more similar to extractable aluminum (Al) in higher saturation frequency soils. Bulk soil Fe was spatially more strongly correlated to soil organic matter (SOM) in lower saturation frequency soils (Spearman Rank rs = 0.62, p < 0.005), whereas strong correlations between Al and SOM were observed in higher saturation frequency soils (rs = 0.88, p < 0.005) using nanoscale secondary ion mass spectrometry (NanoSIMS) imaging. Characterization of bulk soil Fe with X-ray absorption spectroscopy showed 1.2-fold greater Fe(II) and 1-fold lower contribution of Fe-organic bonding in soils with high saturation frequency. Fe(III) interactions with carboxylic and aromatic C were identified with 13C nuclear magnetic resonance (NMR) spectroscopy Fe(III) interference experiments. Additionally, carboxylic acid enrichment in high saturation frequency soils quantified by C K-edge X-ray absorption spectroscopy point towards the role of carboxylic functional groups in Al-organic in addition to Fe-organic interactions. In our incubation experiments, a doubling in short-term CO2 evolution (per unit total soil C) was detected for high relative to low saturation frequency soils. Further, an order of magnitude increase in CO2 evolution (per unit water-extractable OC) following anaerobic incubation was only detected in high saturation frequency soils. The observed shift towards Al-dominated SOC interactions and higher OC mineralizability highlights the need to describe C stabilization in soils with flashy wet-dry cycling separately from soils with low saturation frequency or persistent saturation.

Introduction

The accumulation and persistence of soil organic carbon (SOC) is considered to be predominantly governed by environmental variables (Schmidt et al., 2011, Lehmann and Kleber, 2015). Water saturation and associated anoxic conditions generally decrease SOC mineralization rates, leading to SOC accumulation under persistent saturation (e.g., in wetlands or flood plains) (Kayranli et al., 2010, Sutfin et al., 2016, Mayer et al., 2018). However, redox conditions in otherwise aerobic, well-drained soils can vary widely due to both spatial heterogeneity (e.g., anaerobic microsites) and flashy (i.e., rapidly fluctuating) saturation-drying cycles (Silver et al., 1999, Keiluweit et al., 2017). Reducing conditions resulting from short-term saturation events in otherwise aerobic soils may induce mineral dissolution and release SOC that was previously associated with the mineral solid phase (Buettner et al., 2014, Herndon et al., 2017, Coward et al., 2018). This relationship between water content and carbon bioavailability with frequent cycles of rapid saturation and draining is a point of uncertainty in SOC cycling, particularly in light of predicted shifts in moisture regimes with global climate change (Christensen et al., 2013).

The mechanism of mineral-SOC interaction is expected to influence the degree of susceptibility to dissolution and subsequent transformations (Winkler et al., 2018). Overall, SOC association with redox-active elements (e.g. iron, Fe, and manganese, Mn) may be more sensitive to fluctuating redox than other elements (e.g., aluminum, Al). Non- or semi-crystalline Fe phases that rapidly dissolve and re-precipitate under fluctuating redox conditions (including short-ranged order (SRO) phases, Fe-organic complexes, and Fe co-precipitated with SOC) may have increased susceptibility to dissolution under increased saturation frequency, with associated mobilization of Fe(III)-associated SOC (Buettner et al., 2014, Ginn et al., 2017, Barcellos et al., 2018, Chen et al., 2018, Chen and Thompson, 2018). However, Fe(II) may also form stable Fe(II)-organic complexes in the presence of high OC input (Bhattacharyya et al., 2018). Co-precipitation of SOC with reduced or re-oxidized Fe may also provide surface-area independent stabilization reactions (Kleber et al., 2015). For semi-crystalline minerals, Fe and Al solubilities are likely to co-vary, and Fe oxidation–reduction reactions may indirectly affect the solubility of other colloidal materials via changes in soil solution chemistry (e.g., pH) (Thompson et al., 2006a). This potential for wide variation in Fe-Al-SOC dynamics with variable saturation frequency complicates prediction of the relative role of Fe vs. Al.

With increased saturation frequency, long-term shifts in soil mineral composition and elemental ratios are likely, resulting from leaching of mobile elements, changes to mineral weathering and/or crystallization, or indirect effects on soil solution chemistry (Thompson et al., 2006a, Thompson et al., 2011, Das et al., 2019). In particular, lower-pH soils with high Fe and Al contents may experience shifts in Fe crystallinity and elemental composition (e.g., increased Al to Fe ratio) in the long-term due to the mobilization of soluble Fe(II) with increased saturation frequency (Thompson et al., 2006b, Thompson et al., 2011, Inagaki et al., 2020). Experimental exposure to rapid redox oscillations has also been linked to increased Fe reduction rates in Fe-rich soils (Ginn et al., 2017, Barcellos et al., 2018, Winkler et al., 2018). While Fe reduction occurs in generally aerobic soils (Yang and Liptzin, 2015, Hall et al., 2016), the effect of short-duration saturation and rapid draining events on soil mineral composition, organo-mineral interactions, and associated availability of SOC is not fully understood in upland, otherwise well-drained soils.

To test the hypothesis that higher frequency of saturated–unsaturated cycles increases the bioavailability of mineral-stabilized SOC, we used upland forest soils that span a naturally occurring gradient in mineral soil saturation cycle frequency at Hubbard Brook Experimental Forest (Woodstock, NH) (Fig. 1). Differences in saturation cycle frequency of the studied soil profiles are a result of bedrock-induced limitation on drainage (Bailey et al., 2014, Gillin et al., 2015, Gannon et al., 2017). We applied bulk soil characterization, selective extractions, and bulk spectroscopy to identify changes in reactive metal and SOC properties and organo-mineral interactions as a function of soil saturation frequency. We also related observations of soil properties to SOC mineralization potential under fluctuating anaerobic–aerobic conditions in laboratory incubations.

Section snippets

Soil profile categorization by saturated–unsaturated cycle frequency

Soil sampling was conducted over a slope transect in Hubbard Brook Experimental Forest (Woodstock, NH) Watershed 3 (W3), a hydrological reference watershed for which a hydropedological unit (HPU) classification system was initially developed and verified (Bailey et al., 2014) (Supplementary Fig. A1). Soils in W3 are Wisconsinan glacial deposits (basal and ablation till) over Silurian Rangeley Formation bedrock (sillimanite-grade pelitic schist and calc-silicate granulite) (Gannon et al., 2017).

Basic soil characterization

Overall, soil pH increased in deeper horizons for low and high saturation frequency category soils, but did not change appreciably with depth for medium saturation frequency soils (Supplementary Table A5). For transitional horizons and C horizons, the lack of observations at medium and high saturation frequency precluded comparisons across saturation frequency soils. For eluvial and spodic horizons, pH increased with increasing soil saturation frequency (Supplementary Table A5). Total C and N

Contrasting mineral-organic interactions across variable saturation frequency soils

In this study, increases in the ratio of bulk soil Al to Fe (Fig. 2), OC release of similar magnitude to Al with selective dissolution (Fig. 3), and Al-SOM spatial associations in the fine fraction (Fig. 4) point towards a shift from Fe- to Al-dominated mineral-SOC interactions in higher saturation frequency soils. In the same study system, increasing Al to Fe ratios in higher saturation frequency soils have been identified in other areas of the watershed (e.g., in Bhs podzol spodic horizons) (

Conclusions

The extent of saturation frequency and hydrology-driven pedogenic processes in this system resulted in notable shifts in Fe redox state and in the composition of Fe phases, underscoring the influence of redox fluctuations even in short-duration, flashy saturation events. The observed transition from Fe(III)-organic interactions in lower saturation frequency soils towards Al-dominated interactions in higher saturation frequency soils indicates relevant changes in potential SOC stabilization

Data availability

Data associated with this manuscript are published in a Cornell University Ecommons repository entitled: “Carbon and metal characterization and incubation studies conducted on soil samples collected in June 2016 from Hubbard Brook Experimental Forest”, available at: https://doi.org/10.7298/412j-t911

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding for this study was provided by the NSF IGERT in Cross-Scale Biogeochemistry and Climate at Cornell University (NSF Award #1069193) and the Institute for Advanced Study (IAS) from the Technical University of Munich (TUM) through the Hans-Fisher Senior Fellowship. Additional research funds were provided by the Andrew W. Mellon Foundation and the Cornell College of Agriculture and Life Sciences Alumni Foundation, USA. Hubbard Brook Experimental Forest is operated and maintained by the US

References (78)

  • E. Herndon et al.

    Influence of iron redox cycling on organo-mineral associations in Arctic tundra soil

    Geochim. Cosmochim. Acta

    (2017)
  • W. Huang et al.

    Optimized high-throughput methods for quantifying iron biogeochemical dynamics in soil

    Geoderma

    (2017)
  • T.M. Inagaki et al.

    Subsoil organo-mineral associations under contrasting climate conditions

    Geochim. Cosmochim. Acta

    (2020)
  • M. Jankowski

    The evidence of lateral podzolization in sandy soils of Northern Poland

    Catena

    (2014)
  • M. Kleber et al.

    Mineral-organic associations: formation, properties, and relevance in soil environments

    Adv. Agron.

    (2015)
  • H. Knicker et al.

    N-15 and C-13 CPMAS and solution NMR studies of N-15 enriched plant material during 600 days of microbial degradation

    Org. Geochem.

    (1995)
  • E.J. Lundquist et al.

    Wet-dry cycles affect dissolved organic carbon in two California agricultural soils

    Soil Biol. Biochem.

    (1999)
  • S. Mayer et al.

    Drivers of organic carbon allocation in a temperate slope-floodplain catena under agricultural use

    Geoderma

    (2018)
  • C.M. Preston et al.

    Effects of variations in contact times and copper contents in a 13C CPMAS NMR study of samples of four organic soils

    Geoderma

    (1984)
  • I. Schöning et al.

    Intimate association between O, N-alkyl carbon and iron oxides in clay fractions of forest soils

    Org. Geochem.

    (2005)
  • A. Thompson et al.

    Iron-oxide crystallinity increases during soil redox oscillations

    Geochim. Cosmochim. Acta

    (2006)
  • A. Thompson et al.

    Iron solid-phase differentiation along a redox gradient in basaltic soils

    Geochim. Cosmochim. Acta

    (2011)
  • M.-L. Vermeire et al.

    Is microbial reduction of Fe (III) in podzolic soils influencing C release?

    Geoderma

    (2019)
  • E. Viollier et al.

    The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters

    Appl. Geochem.

    (2000)
  • P. Winkler et al.

    Contrasting evolution of iron phase composition in soils exposed to redox fluctuations

    Geochim. Cosmochim. Acta

    (2018)
  • D. Barcellos et al.

    Faster redox fluctuations can lead to higher iron reduction rates in humid forest soils

    Biogeochemistry

    (2018)
  • A.J. Berry et al.

    XANES calibrations for the oxidation state of iron in silicate glass

    Am. Mineral.

    (2003)
  • C. Black et al.

    Methods of Soil Analysis Part 1: Physical and Mineralogical Properties, Including Statistics of Measurement and Sampling

    (1965)
  • R.R. Bourgault et al.

    Chemical and morphological distinctions between vertical and lateral podzolization at Hubbard Brook

    Soil Sci. Soc. Am. J.

    (2015)
  • L. Breiman

    Random forests

    Mach. Learn.

    (2001)
  • C. Chen et al.

    Properties of Fe-organic matter associations via coprecipitation versus adsorption

    Environ. Sci. Technol.

    (2014)
  • K. Chen et al.

    Stabilization of natural organic matter by short-range-order iron hydroxides

    Environ. Sci. Technol.

    (2016)
  • C. Chen et al.

    Influence of pO2 on iron redox cycling and anaerobic organic carbon mineralization in a humid tropical forest soil

    Environ. Sci. Technol.

    (2018)
  • C. Chen et al.

    Ferrous iron oxidation under varying pO2 levels: the effect of Fe(III)/Al(III) oxide minerals and organic matter

    Environ. Sci. Technol.

    (2018)
  • Christensen, J.H., Krishna Kumar, K., Aldrian, E., An, S.-I, Cavalcanti, I.F.A., de Castro, M., Dong, W., Goswami, P.,...
  • DeCiucies, S., 2018. When less is more: priming of soil organic matter by pyrogenic carbon (M.S. thesis). Cornell...
  • B.P. Degens et al.

    Repeated wet-dry cycles do not accelerate the mineralization of organic C involved in the macro-aggregation of a sandy loam soil

    Plant Soil

    (1995)
  • J. Gannon et al.

    Lateral water flux in the unsaturated zone: a mechanism for the formation of spatial soil heterogeneity in a headwater catchment

    Hydrol. Process.

    (2017)
  • C.P. Gillin et al.

    Mapping of hydropedologic spatial patterns in a steep headwater catchment

    Soil Sci. Soc. Am. J.

    (2015)
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    Present address: Virginia Tech, 220 Ag-Quad Ln, Blacksburg, VA, 24061, USA.

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