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