Abstract
Carbon materials (e.g., pyrogenic carbon (PyC)) are widely used in agricultural soils and can participate in various biogeochemical processes, including iron (Fe) cycling. In soils, Fe(II) species have been proposed as the main active contributor to produce reactive oxygen species (ROS), which are involved in various biogeochemical processes. However, the effects of PyC on the transformation of different Fe species in soils and the associated production of ROS are rarely investigated. This study examined the influence of PyC (pyrolyzed at 300–700 °C) on Fe(II)/Fe(III) cycling and hydroxyl radical (·OH) production during redox fluctuations of paddy soils. Results showed that the reduction of Fe(III) in soils was facilitated by PyC during anoxic incubation, which was ascribed to the increased abundance of dissimilatory Fe(III)-reducing microorganisms (biotic reduction) and the electron exchange capacity of PyC (abiotic reduction). During oxygenation, PyC and higher soil pH promoted the oxidation of active Fe(II) species (e.g., exchangeable and low-crystalline Fe(II)), which consequently induced higher yield of ·OH and further led to degradation of imidacloprid and inactivation of soil microorganisms. Our results demonstrated that PyC accelerated Fe(II)/Fe(III) cycling and ·OH production during redox fluctuations of paddy soils (especially those with low content of soil organic carbon), providing a new insight for remediation strategies in agricultural fields contaminated with organic pollutants.
Graphical Abstract
Highlights
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Pyrogenic carbon (PyC) with high EEC promoted active Fe(II) formation in anoxic paddy soils through abiotic and biotic mechanisms.
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Pyrolysis temperature affected the physiochemical properties of PyC, which regulated Fe cycling and ·OH production in soils.
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The enhanced production of ·OH led to microbial inactivation and organic pollutant degradation.
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1 Introduction
Dynamic changes of redox potential have been widely observed during underground water recharge of unconfined river aquifers, water table fluctuations of lake sediments, and flooding-drainage cycles of cultivated lands (Du et al. 2021; Honma et al. 2016; Zhang et al. 2020a). The frequent redox fluctuations in these water-soil environments lead to intense interactions between reduced substances and oxygen (O2), which consequently induce the production of reactive oxygen species (ROS) (Chen et al. 2021b; Page et al. 2013; Yuan et al. 2017). Ubiquitous ROS production in redox oscillation events has arisen increasing attention and the potential roles of ROS in multiple biogeochemical processes have been proposed (Yu and Kuzyakov 2021). As one of the most reactive ROS, hydroxyl radical (·OH, E0 = 2.8 V vs. NHE) has critical impacts on the transformation of redox-active metals (Kong et al. 2015; Liu et al. 2022b), degradation of organic pollutants (** iron: anaerobic microbial iron oxidation and reduction. Nat Rev Microbiol 4(10):752–764" href="/article/10.1007/s42773-023-00236-8#ref-CR59" id="ref-link-section-d143775191e649">2006). In paddy fields, agricultural strategies such as flooding-drainage management and application of remediation agents can greatly influence microbial community structure and soil redox conditions (Huang et al. 2021a). To maintain agricultural production, amendments such as pyrogenic carbon (PyC) have been widely applied to paddy fields with several tons per hectare per year (Liu et al. 2022; Sun et al. 2017). EPR spectrum analysis revealed that PyC500 contained abundant carbon-centered persistent free radicals (1.09 × 106 spin g−1, g-factor of 2.0027, Additional file 1: Fig. S17e), which are also involved in electron transfer processes (Xu et al. 2016).
To distinguish the contribution of PyC (abiotic) and microorganisms (biotic) to Fe reduction, soils were sterilized by 1% (m/v) HgCl2 to eliminate microorganisms. Interestingly, despite soil microorganisms being eliminated, Fe reduction was observed in both CS and YT soils with the addition of PyC300 (1.44 and 1.17 g kg−1) and PyC500 (1.16 and 0.76 g kg−1) (Additional file 1: Fig. S18). These results indicated that PyC300 and PyC500 reduced Fe minerals abiotically, which was consistent with their high EDC (Additional file 1: Fig. S17a). Abiotic reduction of Fe minerals by PyC300, PyC500, and PyC700 contributed to 22.2%, 17.1%, and 4.0% of total Fe reduction in CS soil, and 19.7%, 11.4%, and 4.6% in YT soil, respectively (Additional file 1: Table S8). Abiotic reduction by different PyC resulted in the transformation of different Fe(II) species in soils (Additional file 1: Fig. S19). Specifically, PyC300 significantly promoted the formation of 5 M HCl-Fe(II), and PyC500 promoted the formation of 0.5 M HCl-Fe(II) and 5 M HCl-Fe(II). The formation of high-crystalline Fe(II) in the presence of PyC300 and PyC500 may be due to the reversible redox reactions between quinone and hydroquinone groups, which can transform low-crystalline Fe minerals to high-crystalline minerals through a dissolution-reprecipitation mechanism (Lian et al. 2022), higher redox potential of ·OH (E0 = 2.8 V) may favor the degradation of organic pollutants in soils (Yu and Kuzyakov 2021). Degradation of IMI in the YT and CS slurries was enhanced with the addition of PyC500, which was ascribed to the higher ·OH production as aforementioned (Fig. 1). Except for exogenous anthropogenic organic pollutants, the oxidation of soil organic carbon by reactive ·OH has also been proposed (Chen et al. 2021c). Assuming that 1 mol ·OH can effectively react with humic substances to produce 0.3 mol CO2 (Goldstone et al. 2002; Page et al. 2013), the enhanced ·OH production in CS- and YT-3%PyC500 slurries would result in 5.6–9.8 μmol L–1 more CO2 production after 8-h oxygenation.
4 Conclusion
Given the importance of PyC in participating in various biogeochemical processes, an investigation into the integrated effects of PyC on Fe cycling and ROS production in soil systems is required. Under the anoxic condition, PyC (especially PyC500 with high EEC) promoted the formation of active Fe (II) species mainly through abiotically reducing Fe minerals and enhancing the relative abundance of Fe(III)-reducing microorganisms, and therefore increased ·OH production during oxygenation. With PyC amendment, accelerated Fe(II) oxidation was consistent with the higher ·OH production rate, indicating that enhanced Fe(II)/Fe(III) cycling and ·OH production occurred. The produced ·OH was capable of inducing microbial inactivation and organic pollutant degradation during oxygenation processes. Overall, this study helped to better understand the critical roles of PyC in driving Fe(II)/Fe(III) cycling and ROS production under redox conditions. Given the significance of Fe(II)/Fe(III) cycling and ROS production, these processes consequently influence the mobility of toxic metals (e.g., arsenic), degradation of organic pollutants, and nutrient cycling (e.g., C, N, and P). As a frequently used functional material, the utility of PyC to increase the content of soil organic carbon should be examined during redox fluctuations in long-time scales due to the enhanced ·OH production and probable organic carbon decomposition.
Data availability
The datasets used or analyzed during the current study are available from the corresponding author on reasonable request. Additional information, including chemicals, detailed experimental methods, and results of additional experiments in forms of figures and tables are provided in Additional file 1.
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Acknowledgements
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Funding
This research was supported by the National Natural Science Foundation of China (No. 42130707, 42107382, 42022049) and the Natural Science Foundation of Jiangsu Province (BK20200323).
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DZ and NC conceived the research idea. DH designed and performed laboratory analyses. YL, CG, XW, and DW assisted sample collections and analytical tools. CZ and GF assisted with data interpretation and mechanism discussion. DH produced the first draft of the manuscript, and all authors discussed and approved the final manuscript.
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Handling editor: Baoshan ** of the pyrogenic carbon (PyC) in (a) CS-3%PyC300, (b) CS-3%PyC500, (c) CS-3%PyC700, (d) YT-3%PyC300, (e) YT-3%PyC500, and (f) YT-3%PyC700 anoxic slurries.Fig. S10. Effects of 2,2′-bipyridine (BPY), 1 mM nitrotetrazolium blue chloride (NBT), and 4000 U L–1 catalase (CAT) on hydroxyl radical (•OH) production in Changsha (CS) slurries (a) without Pyrogenic carbon (PyC) and with (b) 3%PyC300, (c) 3%PyC500, (d) 3%PyC700 during 8-h oxygenation. Fig. S11. The effect of γ irradiation on •OH production in 40-day anoxic incubated Changsha (CS) slurries.Fig. S12. (a) Cumulative •OH generated by pyrogenic carbon (PyC) or reduced pyrogenic carbon (rPyC) under oxic condition at pH 7 (10 mM phosphate buffer). Fig. S13. Correlation analyses between cumulative •OH and total extractable Fe(II) in (a) Changsha (CS) and (b) Yingtan (YT) slurries. Fig. S14. Changes of extractable Fe(II) species in Changsha (CS) slurries without (control) and with 1% (w/w) pyrogenic carbon (PyC300, PyC500, and PyC700) during 8-h oxygenation.Fig. S15. Variation of soil suspension pH of (a) Changsha (CS) and (b) Yingtan (YT) slurries during 8-h oxygenation.Fig. S16. Changes of Fe species in Changsha (CS) and Yingtan (YT) slurries during 8-h oxygenation.Fig. S17. Physicochemical properties of pyrogenic carbon. (a) Electron donating capacity (EDC) and electron accepting capacity (EAC); (b) FTIR spectra; (c) Raman spectra; (d) electrical conductivity; (e) EPR spectra; (f) peroxidase-like activity.Fig. S18. Content of total Fe(II) in sterilized Changsha (CS) and Yingtan (YT) soils without (control) and with 3% (w/w) pyrogenic carbon (PyC300, PyC500, and PyC700) during 20-day anoxic incubation.Fig. S19. Dynamic changes of (a, e) CaCl2-Fe(II), (b, f) 0.5 M HCl-Fe(II), (c, g) 5M HCl-Fe(II), and (d, h) HF-Fe(II) of sterilized Changsha (CS) and Yingtan (YT) paddy soil without (control) and with 3% (w/w) pyrogenic carbon (PyC300, PyC500, and PyC700) during 20-day anoxic incubation.Fig. S20. Content of total Fe(II) in sterilized Changsha (CS) and Yingtan (YT) soils without (control) and with 3% (w/w) reduced pyrogenic carbon (rPyC300, rPyC500, and rPyC700) during 20-day anoxic incubation. Fig. S21. Dynamic changes of (a, e) CaCl2-Fe(II), (b, f) 0.5 M HCl-Fe(II), (c, g) 5 M HCl-Fe(II), and (d, h) HF-Fe(II) of sterilized Changsha (CS) and Yingtan (YT) paddy soil during a 20-day anoxic incubation without (control) and with 3% (w/w) reduced pyrogenic carbon (rPyC300, rPyC500, and rPyC700).Fig. S22. Pathway of abiotic formation of different Fe(II) species in the presence of pyrogenic carbon (PyC300, PyC500, and PyC700) in soil.Fig. S23. Content of total extractable Fe(II) in Chongqing (CQ), Hebei (HB), and Sichuan (SC) slurries without (control) and with 3% (w/w) PyC500 after 20-day anoxic incubation.Fig. S24. Contents of (a) CaCl2-Fe(II), (b) 0.5 M HCl-Fe(II), (c) 5 M HCl-Fe(II), and (d) HF-Fe(II)) in Chongqing (CQ), Hebei (HB), and Sichuan (SC) slurries without (control) and with 3% (w/w) PyC500 after 20-day anoxic incubation.Fig. S25. (a) Microbial compositions of Yingtan (YT) slurries.Fig. S26. Correlation analyses between initial content of 0.5 or 5 M HCl-Fe(II), consumed 0.5 or 5 M HCl-Fe(II) and cumulative •OH. Fig. S27. Instantaneous H2O2 concentration during oxygenation.Fig. S28. Peroxidase-like activities of the Changsha (CS) and Yingtan (YT) slurries. Fig. S29. Concentration of dissolved organic matter (DOM). Fig. S30. Oxygenation of 1 mM Fe2+ and cumulative •OH production with pyrogenic carbon (PyC) or reduced pyrogenic carbon (rPyC) at pH 6 (20 mM MES buffer). Fig. S31. Physicochemical properties of reduced pyrogenic carbon. Fig. S32. The XPS O1s spectra of pyrogenic carbon. Fig. S33. Content of total extractable Fe(II) in CS slurries and (b) YT slurries. Fig. S34. •OH production in Yingtan (YT) slurries without (control) and with 3% (w/w) reduced pyrogenic carbon (rPyC300, rPyC500, and rPyC700) during 8-h oxygenation. Fig. S35. Changes of extractable Fe(II) species (CaCl2-Fe(II), 0.5 M HCl-Fe(II), 5 M HCl-Fe(II), and HF-Fe(II) in (a–d) Changsha (CS) and (e–h) Yingtan (YT) slurries without (control) and with 3% (w/w) reduced pyrogenic carbon (rPyC300, rPyC500, and rPyC700) during 8-h oxygenation.Fig. S36. The correlation heatmap of physicochemical properties of pyrogenic carbons with the fitted kinetic value kobs of •OH production. The physicochemical properties include electrical conductivity (EC), electron accepting capacity (EAC), electron donating capacity (EDC), electron exchange capacity (EEC, sum of EAC and EDC), persistent free radical concentrations (PFRs), the content of oxygen, and the content of oxygen-containing functional groups (i.e., C=O, C–O–C, and C–OH) (*p ≤ 0.05).Fig. S37. Numbers of living bacteria in (a) Changsha (CS) slurries and (b) Yingtan (YT) slurries after 4-h reaction with and without 100 mM ethanol. Anoxic controls were conducted in anoxic glovebox. Fig. S38. Fluorescent microscopy images of stained cells in Changsha (CS) slurries (CS-control and CS-3%PyC500). (a, d) Before oxidation; (b, e) after 4-h oxidation; (c, f) after 4-h oxidation with addition of 100 mM ethanol to quench •OH.Fig. S39. Degradation of imidacloprid after 4-h anoxic incubation or 4-h oxygenation of Changsha (CS) and Yingtan (YT) slurries.
