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Increased Electron Accepting and Decreased Electron Donating Capacities of Soil Humic Substances in Response to Increasing Temperature Wenbing Tan, Beidou Xi, Guoan Wang, Jie Jiang, Xiao-Song He, Xuhui Mao, RuTai Gao, Cai-Hong Huang, Hui Zhang, Dan Li, Yufu Jia, Ying Yuan, and Xinyu Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04131 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017
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Increased Electron Accepting and Decreased Electron Donating
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Capacities of Soil Humic Substances in Response to Increasing
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Temperature
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Wenbing Tan,†,‡ Beidou Xi,*,†,‡,§ Guoan Wang,║ Jie Jiang,┬ Xiaosong He,*,†,‡ Xuhui
6
Mao,┴ Rutai Gao,†,‡ Caihong Huang,†,‡ Hui Zhang,‡ Dan Li,‡ Yufu Jia,║ Ying Yuan,‡ and
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Xinyu Zhao‡
8 9 10 11 12 13 14 15 16 17 18 19
†
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research
Academy of Environmental Sciences, Beijing 100012, China ‡
State Environmental Protection Key Laboratory of Simulation and Control of Groundwater
Pollution, Chinese Research Academy of Environmental Sciences, Beijing 100012, China §
School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou
730070, China ║
College of Resources and Environmental Sciences, China Agricultural University, Beijing
100193, China ┬
College of Environmental Science and Engineering, Beijing Forestry University, Beijing
100083, China ┴
School of Resource and Environmental Science, Wuhan University, Wuhan 430079, China
20 21
*Corresponding Author:
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[email protected] 23
[email protected] (X. He), Phone: +86-10-84915307.
(B.
Xi),
Phone:
+86-10-84913133,
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ABSTRACT
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The electron transfer capacities (ETC) of soil humic substances (HS) are linked to the type
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and abundance of redox-active functional moieties in their structure. Natural temperature can
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affect the chemical structure of natural organic matter by regulating their oxidative
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transformation and degradation in soils. However, it is unclear if there is a direct correlation
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between ETC of soil HS and mean annual temperature. In this study, we assess the response
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of the electron accepting and electron donating capacities (EAC and EDC) of soil HS to
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temperature by analyzing HS extracted from soil sets along glacial–interglacial cycles through
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loess–palaeosol sequences and along natural temperature gradients through latitude and
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altitude transects. We show that the EAC and EDC of soil HS increase and decrease,
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respectively, with increasing temperature. Increased temperature facilitates the prevalence of
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oxidative degradation and transformation of HS in soils, thus, potentially promoting the
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preferentially oxidative degradation of phenol moieties of HS or the oxidative transformation
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of electron donating phenol moieties to electron accepting quinone moieties in the HS
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structure. Consequently, the EAC and EDC of HS in soils increase and decrease, respectively.
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The results of this study could help to understand biogeochemical processes, wherein the
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redox functionality of soil organic matter is involved, in the context of increasing temperature.
44 HSred HS
+eIncrease in EAC of soil HS
Cooler
Air temperature
Warmer
Decrease in EDC of soil HS -e-
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HS
HSox
Table of Content (TOC)
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INTRODUCTION
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Traditionally, soil humic substances (HS) are defined as a heterogeneous mixtures of natural
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organic macromolecules formed through the microbial degradation of high-plant, microbial
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and animal residues.1 In light of the emerging soil continuum model that refutes the ‘HS’
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concept, owing to the lack of evidences for the ‘humification’ process, and views the soil
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organic matter as a continuum spanning the full range from intact plant material to highly
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oxidized carbon,2 it is important to acknowledge that HS are operationally defined by the
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extraction procedure from the soils. At the same time, there is a significant body of work on
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the redox properties of HS extracted from various environmental matrices because of the great
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significance of the redox properties of HS in many environmentally relevant processes.3–5
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Under reducing conditions, dissolved and particulate HS can accept electrons directly from
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microorganisms,6,7 such as iron-reducing,6 sulfate-reducing,8 and fermenting bacteria.9
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Reduced HS can subsequently donate electrons to poorly accessible iron oxides and
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hydroxides6,10 as well as various organic and inorganic pollutants,11,12 including chlorinated
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compounds,13 nitrobenzenes,14,15 U(VI),16 and Cr(VI).17,18 Thus, the so-called electron transfer
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properties of HS can significantly affect the biogeochemical redox processes of redox-active
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pollutants.
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The electron transfer capacities (ETC) of soil HS have been primarily ascribed to their
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intrinsic physicochemical properties, due to various redox-active functional groups within
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their structures.19–23 Electron spin resonance spectroscopy provided direct evidence that
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quinone moieties act as the electron accepting functional groups during the microbial
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reduction of HS.19 Comparison of spectroscopic properties of HS with model quinones
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revealed that quinone was involved in the reduction of HS.24 Fourier transform infrared
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spectroscopy, NMR spectroscopy and pyrolysis–gas chromatography–mass spectrometry also
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pointed toward the quinones as the important redox-active functional groups in HS.25,26
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Moreover, phenolic moieties were proposed as the major electron donating functional groups
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in HS, based on the strong linear correlations of the electrons donated by a given HS mass
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with titrated phenol content,20,27 and based on the similar pH dependencies of the redox
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potential of phenol–hydroquinone mixture and the redox titration curves of HS.28
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These intrinsic physicochemical properties of natural organic matter (NOM) and HS are
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largely dependent on their oxidative transformation and degradation in soils.1,29 Natural
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temperature as a state factor is predicted to change the soil microenvironmental conditions,
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microbial community structure and activity, and plant-litter quality.30–32 Such changes may
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exert substantial influence on the oxidative transformation and degradation and, ultimately,
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the ETC of HS in soils. Thus, the fate and functionality of HS in soils are inherently linked to
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natural temperature. Uncovering the mechanisms by which the natural temperature changes
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the ETC of HS in soils is critical to better understanding the effect of natural temperature on
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environmentally relevant biogeochemical interaction processes in which the redox
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functionality of soil HS is involved.
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The well-known loess–palaeosol sequences in the Chinese Loess Plateau contain
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interbedded loess and buried palaeosol units, which are considered to have experienced
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periods of dominantly eccentricity-paced glacial and interglacial climates, respectively.33,34
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The soil set across the loess–palaeosol sequences thus provides a unique opportunity to assess
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the response of the ETC of HS to glacial–interglacial cycles. However, proxies from the
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loess–palaeosol sequences for the quantitative reconstruction of the palaeotemperature during
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geological time is lacking.35 The ‘space-for-time’ substitution approach, such as sampling
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soils across latitude or altitude gradients, has the inherent advantage of natural climatic
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gradients,36 allowing to predict the responses of the ETC of soil HS to increasing temperature.
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Soil sets across latitude gradients with equivalent mean annual precipitation can eliminate the
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disturbance of variable precipitation but give rise to heterogeneous soil parent materials.
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Conversely, soil sets across altitude gradients can partly eliminate the disturbance of different
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soil parent materials but cannot keep the mean annual precipitation constant. Each soil set
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possesses inherent advantages that can offset the disadvantages from other sets.
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In this study, we simultaneously sampled a soil set from loess–palaeosol sequences in
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Luochuan of China, a soil set across the latitude gradient along the 400 mm mean annual
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precipitation isoline from the northwest to the southwest of China, and a soil set across the
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altitude gradient in Dongling Mountain, China. This study aims (1) to determine the chemical
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structures of HS that account for the change in their electron accepting capacities (EAC) and
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electron donating capacities (EDC), (2) to explore which physicochemical properties of soil
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and plant litter are most responsible for the change in the ETC-related chemical structures of
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soil HS, (3) to investigate the association of physicochemical properties of soil and plant litter
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with temperature change and (4) to assess the link between the ETC of soil HS and
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temperature change. The results from the present study can improve our understanding of the
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important role of redox properties of HS and NOM in soil biogeochemical processes in the
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context of increasing temperature.
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MATERIALS AND METHODS
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Geological setting and sampling. Soils across loess–palaeosol sequences were collected
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from the classic Luochuan section (35.75°N, 109.42°E). Twenty-seven samples (14 palaeosols
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and 13 loess) from the loess–palaeosol section are shown in Figure S1 in the Supporting
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Information (SI). Twenty-seven and eighteen sampling sites were selected along the 400 mm
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mean annual precipitation isoline from the northwest to the southwest of China
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(29.06−53.29°N, 90.39−122.15°E; latitude gradient with mean annual air temperature ranging
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between −5.5 °C and 8.9 °C) and the Dongling Mountain (39.92−40.03°N, 115.45−115.57°E;
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altitude gradient with mean annual air temperature ranging between −0.7 °C and 10.5 °C),
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respectively. The surface soil temperatures of the sampling sites across the latitude and
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altitude gradients were unaffected by the specific geological conditions (e. g., geothermy) but
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mainly controlled by air temperature. The geographic locations and mean annual air
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temperature of the 27 sampling sites across the latitude gradient and of the 18 sampling sites
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across the altitude gradient are shown in Figures S2 and S3 and listed in Tables S1 and S2 in
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the SI. At each site across the latitude and altitude gradients, ten samples (including plant
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litter and the uppermost 10 cm of soil) were randomly chosen from soil plots (10 m × 10 m)
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and mixed. All sampling sites were far from human habitats to minimize the effect of human
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activity. All soil and plant-litter samples were maintained at −20 °C from the time of sampling
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until analysis.
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Soil HS extraction. Humic acids (HA) and fulvic acids (FA) as two of the HS fractions
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were exhaustively extracted from ground soil and quantified through a modified method
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based on the International Humic Substances Society (IHSS) protocol.37 Briefly, soil samples
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were extracted by a solution (1:1 mixture of aqueous 0.1 M NaOH and 0.1 M Na4P2O7) using
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a extractant/sample ratio of 10:1. The suspension was shaken mechanically under nitrogen gas
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in capped glass bottles for 24 h at 25 oC. The extraction procedure was repeated three times
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for each sample. The extracted HS were then separated into HA (precipitate) and FA
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(supernatant) fractions by acidifying the extract to pH 1 with 6 M HCl and subsequently
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centrifuging the extract at 5000 rpm. The HA fraction was suspended in a solution of 0.1 M
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HCl/0.3 M HF to remove mineral impurities and dialyzed until the elimination of chloride
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ions. The FA fraction was purified with adsorption resin XAD-8, and the alkaline eluate was
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made to pass through a cation exchange resin.
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Electrochemical Measurements. The electrochemical analysis was performed as
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previously described (see the SI).26 All electrochemical measurements were conducted under
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anoxic conditions (N2, atmosphere at 25 oC±1 oC). The number of electrons transferred to and
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from HA (or FA) was quantified by integration of reductive and oxidative current responses in
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mediated electrochemical reduction (MER; at Eh = –0.49 V) and mediated electrochemical
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oxidation (MEO; at Eh = +0.61 V), respectively. The integrated current responses were
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normalized to the analyzed HA (or FA) masses, to obtain the values of EAC and EDC (Eqs. 1
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and 2). I red dt EAC = F m HS
∫
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I ox
EDC =
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∫F
(1)
dt
mHS
(2)
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where Ired and Iox (integrated areas) are baseline-corrected reductive and oxidative currents in
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MER and MEO, respectively; mHS (g) is the mass of HA or FA; F (=96,485 sA/mol e-) is the
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Faraday constant.
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HS analysis. The titrated phenol content of HS was estimated from alkalimetric titration
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data through pH-based methods.38 The lignin-derived phenols of HS were extracted by classic
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alkaline copper oxidation39 and then analysed with an Agilent 7890A gas chromatograph after
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being converted to trimethylsilyl (TMS) derivatives. The lignin-derived phenols include three
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vanillyl phenols (vanillin, acetovanillone and vanillic acid), three syringyl phenols
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(syringaldehyde, acetosyringone and syringic acid) and two cinnamyl phenols (p-coumaric
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acid and ferulic acid). The carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulphur (S)
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concentrations of HS were determined with an elemental analyzer (VARIO EL cube).
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Excitation–emission matrices (EEMs) were collected on dissolved HS (adjusted to 10 mg L−1)
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in a 1 cm quartz cuvette using a Hitachi model F-7000 luminescence spectrophotometer.
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Scans were collected over increments of 5 nm for excitation (250–445 nm) and emission
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(300–550 nm) wavelengths. The EEMs were blank subtracted, corrected for inner-filter
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effects and instrument specific biases and normalized to the Raman area. PARAFAC analysis
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identified six components (Figure S4a in the SI) that co-varied independently across all soil
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sets. The excitation and emission loadings used to validate the model are shown in Figure S4b
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in the SI. Optical and fluorescence spectra were used to calculate the indices reflecting the
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physicochemical properties of organic matter, including the specific ultraviolet absorbance at
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254 nm (SUVA254),1 spectral slope (S240–400, S275–295 and SR),40 spectral area (A240–400),41 ratio ACS Paragon Plus Environment
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of ultraviolet visible absorbance at 465 nm and 665 nm (E4:E6),41 and humification index
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(HIX).42 The calculation methods for these indices are presented in detail in the SI. Solid-state
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13
C-NMR spectra of HS were acquired with a Bruker AV-300 spectrometer equipped with a 13
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direct polarization magic-angle spinning (DP-MAS) probe. The
C-NMR spectra were
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divided into four regions according to the chemical shift: alkyl C (0–50 ppm), O–alkyl C
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(50–110 ppm), aryl C (110–160 ppm) and carboxylic C (160–220 ppm). The proportion of
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each type of carbon was calculated by integrating the spectral regions.
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Soil analysis. Soil pH was measured in 1:5 soil:water (v:v) slurry. The concentrations of
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organic C, N and S were determined with an elemental analyzer (VARIO EL cube) after
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treatment with 1 M HCl to remove the inorganic carbon and freeze-dried for 24 h or more.
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The concentrations of P, K, Ca, Mg, Fe, Mn, Cu, Zn and Mo were determined by using
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inductively coupled plasma mass spectrometry (ICP-MS) after reverse aqua regia digestion.
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The soil fractions of sand (53–2000 µm), silt (2–53 µm) and clay (
