Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage

Journal Pre-proofs Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage Xiaolei Huang, Wenjing Kang, Junjie Guo, Lei Wang, Haiyan Tang, Tingliang Li, Guanghui Yu, Wei Ran, Jianping Hong, Qirong Shen PII: DOI: Reference:

S0048-9697(19)34719-9 https://doi.org/10.1016/j.scitotenv.2019.134728 STOTEN 134728

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Science of the Total Environment

Received Date: Revised Date: Accepted Date:

27 June 2019 24 September 2019 28 September 2019

Please cite this article as: X. Huang, W. Kang, J. Guo, L. Wang, H. Tang, T. Li, G. Yu, W. Ran, J. Hong, Q. Shen, Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.134728

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Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage

Xiaolei Huanga,b,c, Wenjing Kangc, Junjie Guoc, Lei Wangd,e, Haiyan Tangc, Tingliang Lia,b, Guanghui Yuf, Wei Ranc*, Jianping Honga,b and Qirong Shenc

a

College of Resources and Environment, Shanxi Agricultural University, Taigu, Shanxi,

080301, China b

National Experimental Teaching Demonstration Center for Agricultural Resources

and Environment, Shanxi Agricultural University, Taigu, Shanxi, 080301, China c

Jiangsu Provincial Key Lab for Organic Solid Waste Utilization, National Engineering

Research Center for Organic-based Fertilizers, Jiangsu Collaborative Innovation Center for Solid Organic Waster Resource Utilization, Nanjing Agricultural University, Nanjing, 210095, China d

Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural

Sciences, Nanjing, 210014, China e

Scientific Observation and Experimental Station of Arable Land Conservation of

Jiangsu Province, Ministry of Agriculture and Rural Affairs, Nanjing, 210014, China f

Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, PR

China * Corresponding author. Tel: +86 25 84399188; Fax: +86 25 84396975. E-mail address: [email protected] (W, Ran). 1

Highlights 

Organic fertilization promoted the formation of short-range-ordered minerals in paddy soils.



Organic fertilization enhanced the mineral availability for carbon binding in waterdispersible colloids.

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Highly reactive nanomineral assembly in soil colloids contributed to paddy soil carbon storage.

Graphical abstract

Abstract Mineral availability for carbon (C) binding is a key regulator of soil C storage, yet little is known about the highly reactive nanomineral assembly in the paddy soil colloids. Here, using high-resolution transmission electron microscopy (HRTEM), solid-state 27

Al and 29Si nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron 2

spectroscopy (XPS), we investigated the coordination nature of short-range-ordered (SRO) minerals in water-dispersible colloids that were isolated from the paddy soil under different six-year fertilization regimes. Our results showed that organic fertilization not only promoted the transformation of crystalline minerals to SRO phases in the bulk soils but also increased the concentrations of Fe, Al and Si in the soil colloids compared to chemical fertilization alone, and thus enhanced the accumulation of organic C in both the bulk soils and the soil colloids. The HRTEM images revealed that water-dispersible colloids in all soils, regardless of treatment, were composed of crystalline Fe nanominerals (with some Al/Si) and SRO-Al/Si nanominerals (with some Fe) associated with organic C. Furthermore, the combined results from the 27Al and 29Si NMR spectroscopy and XPS not only confirmed the presence of SRO-Al/Si nanoparticles as Si-rich allophane and phytolith but also demonstrated that organic fertilization promoted the transformation of aluminosilicates to SRO-Al/Si nanominerals in soil colloids. Together, these findings indicate that six-year organic fertilization promotes the formation of SRO minerals (e.g., ferrihydrite, Si-rich allophane and Fe-substituted allophane, as well as phytolith) in soils and modulates the assembly of organo-mineral complexes possibly by driving the biogeochemical cycles of Fe, Al, Si and specific organic ligands, thus contributing to the long-term storage of C in paddy soils. Keywords: Organic fertilization; Organo-mineral complexes; Water-dispersible colloids; Paddy soil carbon storage. 1. Introduction

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Soils represent an important terrestrial carbon (C) reservoir in the global C cycle and may even serve as a potential sink for future C sequestration from either the terrestrial vegetation or the atmosphere. Thus, the decomposition, transformation and stabilization of soil organic carbon (SOC) have profound influences on global climate change and food production (Heimann and Reichstein, 2008; Lal, 2004). Evidence has revealed that the persistence of SOC in terrestrial ecosystems mainly depends on the microbial accessibility to substrates rather than the chemical recalcitrance of organic matter (Schmidt et al., 2011). Therefore, the entombment of microbial-derived C in organo-mineral complexes, to a large extent, can be responsible for the long-term C storage in soils (Huang et al., 2018; Liang et al., 2017; Yu et al., 2017). However, the mechanisms underlying the formation of organo-mineral complexes in paddy soils are still poorly understood. Highly reactive short-range-ordered (SRO) minerals (e.g., ferrihydrite, allophane and imogolite) more readily form organo-mineral complexes than well-ordered minerals do (e.g., goethite, hematite and boehmite), and thus play a more important role in improving the soil C storage (Kramer and Chadwick, 2018; Lalonde et al., 2012; Torn et al., 1997; Yu et al., 2012, 2017). Paddy soils make up the largest anthropogenic wetlands on earth and exhibit a considerable C sequestration potential, which, to some extent, can be attributed to the inherent anoxic conditions (Kalbitz et al., 2013; KögelKnabner et al., 2010; Pan et al., 2003). Furthermore, the redox fluctuations induced by the frequent alternation of wetting and drying can promote the formation of SRO iron (Fe) oxy-hydroxides (e.g., ferrihydrite) by driving the biogeochemical cycle of Fe as it

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pertains to electron shuttling in paddy soils (Chacón et al., 2006; Ginn et al., 2017; Kögel-Knabner et al., 2010). The role of SRO-Fe oxy-hydroxides in the preservation of SOC in paddy soils has been well documented in the literature (Huang et al., 2017, 2018; Wissing et al., 2013, 2014; Zhou et al., 2009). Moreover, silicon (Si) is a beneficial element for rice plants, and thus the cycling of Si through the rice plants may drive the transformation of aluminosilicates to SRO phases (e.g., allophane, imogolite and phytolith) (Nguyen et al., 2019; Wen et al., 2014b; Yang and Zhang, 2018; Yu et al., 2012). Song et al. (2018) noted that Si played an important role in improving the stabilization of SOC by forming phytoliths, and thus displayed a great potential to mitigate climate change. Nguyen et al. (2019) reported that the accumulation of phytoliths was closely associated with the oxalate-extractable aluminum (Al), SOC and clay content in Vietnamse paddy soils. Additionally, aluminosilicates and Fe/Al oxyhydroxides combined with natural organic matter are the major components of soil colloids, which represent one of the most reactive interface components in soils, and thus are readily affected by anthropogenic management practices (Regelink et al., 2014; Thompson et al., 2006; Yu et al., 2017). Accordingly, the inherent flooding, puddling and drainage management practices may considerably intensify the dispersion and agglomeration of soil colloids during the cultivation of paddy rice (Henderson et al., 2012; Huang et al., 2018; Kögel-Knabner et al., 2010). Besides, organic fertilization has been shown to critically affect the concentrations of reactive elements (e.g., Fe, Al and Si) in water-dispersible colloids, which are often presumed to be precursors for the formation of SRO minerals (Wen et al., 2019; Xiao et al., 2016; Yu et al., 2017).

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Therefore, it is expected that mechanistic insights into the formation of organo-mineral complexes in paddy soils regulated by different fertilization regimes may be gained by exploring the coordination nature of SRO minerals in the water-dispersible colloids. In general, colloidal particles are defined as particles that range from 1 nm to 1 μm, which makes them sufficiently small to remain in suspension and capable of scattering light (Klitzke et al., 2008). Although selective extraction methods (e.g., acid ammonium oxalate, sodium-pyrophosphate and sodium-dithionite-citrate-bicarbonate) can be used to determine the chemical speciation of Fe, Al and Si of bulk soils, they are operationally impossible for the quantitative mineralogical analysis of soil colloids because of the small amount of colloids in soil (Henderson et al., 2012; Regelink et al., 2014; Xiao et al., 2016; Yu et al., 2017). X-ray diffraction (XRD) is the most widely used technique for determining the structure of organo-mineral complexes, but it can only give averaged structural information on the material examined (Kölbl et al., 2014; Li et al., 2016; Wissing et al., 2013, 2014). High-resolution transmission electron microscopy (HRTEM) combined with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) represents a promising technique that can provide detailed information on the morphology, composition and crystallinity of nanoparticles, and thus can help in understanding the variations in the organo-mineral complexes in soil colloids under different fertilization regimes (Li et al., 2016; Wen et al., 2014a; Xiao et al., 2016). Furthermore, high-resolution nuclear magnetic resonance (NMR) spectroscopy has been developed into a powerful tool for investigating the structure of solid materials, and thus solid-state 27Al and 29Si NMR spectroscopy can

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be combined to elucidate the transformation of aluminosilicates by studying the local coordination environments of Al and Si in soil colloids as affected by different fertilization regimes (Kang et al., 2010; Li et al., 2016; Wen et al., 2014b). Moreover, X-ray photoelectron spectroscopy (XPS) is one of the most important analytical techniques for exploring the surface chemistry of solid materials (depth of analysis < 10 nm), and thus has been extensively exploited to identify the elemental composition and chemical state of solid surfaces (Huang et al., 2018; Wen et al., 2019; Xiao et al., 2016; Zama et al., 2018). Therefore, a comprehensive understanding of the organomineral interactions in paddy soils may be achieved by a combination analysis of soil colloids using HRTEM, 27Al and 29Si NMR spectroscopy and XPS. Here we hypothesize that organic fertilization in paddy soils will promote the transformation of crystalline minerals to SRO phases and modulate the assembly of organo-mineral complexes, which contribute to long-term C storage. Therefore, the objectives of this study were: (i) to explore the effects of organic fertilization on the mineral availability for C binding by determining the concentrations of oxalateextractable Fe, Al and Si (Feo, Alo and Sio) of the bulk soils and the concentrations of the total Fe, Al and Si of the soil colloids; and (ii) to elucidate how organic fertilization influences the formation of SRO nanominerals and the assembly of organo-mineral complexes by studying the soil colloids using HRTEM, spectroscopy and XPS. 2. Materials and methods 2.1 Field site and experimental design

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27

Al and

29

Si NMR

A long-term field experiment located in Jintan City, Jiangsu Province, China (31° 39´N, 119° 28´E 3 masl) was established in October 2010 in an intensively cultivated paddy soil with a long-term annual rotation of summer rice (Oryza sativa L.) and winter wheat (Triticum aestivum L.). The mean annual temperature is 15.3 °C and the mean annual precipitation is 1063.6 mm, belonging to a typical subtropical monsoon climate. The soil is classified as Gleyic Stagnic Anthrosol with a silt loam texture (5% sand, 70% silt, and 25% clay) (IUSS Working Group WRB, 2015). The dominant clay minerals are kaolinite, illite and vermiculite (Huang et al., 2017). Before initiating the field experiment, the neutral paddy soil (pH = 7.3) had 13.5 ± 1.4 g kg-1 SOC, 1.6 ± 0.18 g kg-1 total N, 2.5 ± 0.24 g kg-1 Feo, 0.9 ± 0.07 g kg-1 Alo, 0.3 ± 0.02 g kg-1 Sio, 12.0 ± 1.1 g kg-1 dithionite-extractable Fe (Fed), 27.3 ± 1.4 g kg-1 total Fe, 45.5 ± 3.4 g kg-1 total Al, and 364.4 ± 23.8 g kg-1 total Si. The field experiment included five treatments: no fertilizer control (CK), chemical fertilizer (NPK), 50% chemical fertilizer plus manure (NPKM), 100% chemical fertilizer plus straw (NPKS), and 30% chemical fertilizer plus manure organicinorganic compound fertilizer (NPKMOI). All treatments were arranged in a randomized complete block design with four replicates. The plot size was 40 m2 (8 × 5 m). Each year, the NPK and NPKS treatments received 240 kg N ha-1, 120 kg P2O5 ha1

and 100 kg K2O ha-1, and 300 kg N ha-1, 120 kg P2O5 ha-1 and 100 kg K2O ha-1 in the

wheat and rice seasons, respectively. Each season, the NPKM and NPKMOI treatments received fifty and thirty percent of the levels of chemical fertilizer in the NPK treatment, respectively. In the NPKM treatment, pig manure compost containing 29.1% moisture,

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26.3% organic C, 2.3% N, 1.3% P and 1.0% K was applied at 6000 kg of pig manure compost ha-1 to each wheat and rice crop. In the NPKS treatment, rice straw containing 33.1% moisture, 45.6% organic C, 0.6% N, 0.1% P and 0.8% K was applied at 8000 kg of rice straw ha-1 to the wheat crop, while the wheat straw containing 30.7% moisture, 47.9% organic C, 0.5% N, 0.1% P and 1.1% K was applied at 5000 kg of wheat straw ha-1 to the rice crop. It is noteworthy that the manure organic-inorganic compound fertilizer was comprised of pig manure compost and a suitable amount of chemical fertilizer, containing 20.0% moisture, 20.0% organic C, 12.0% N, 4.0% P and 4.0% K, and was applied at 2000 kg of manure organic-inorganic compound fertilizer ha-1 to each wheat and rice crop in the NPKMOI treatment. Chemical fertilizers were applied in the form of urea, calcium superphosphate and potassium chloride. Urea was applied both as a basal fertilizer before planting and as a supplementary fertilizer at the tillering stage and at the panicle stage (4:3:3) in each cropping season, whereas other chemical and organic fertilizers were applied as basal fertilizers before planting the summer rice in June (for harvest in late October) and before planting the winter wheat in November (for harvest in late May). In general, paddy fields were continuously flooded before transplanting and alternately flooded and drained from the seedling stage to the maturity stage, during which intermittent irrigation was conducted on a weekly schedule. 2.2 Soil sampling The surface soil samples (0-20 cm) were collected from eight randomly selected points in each plot using a 5 cm internal auger after the rice harvest in 2016. The samples of the same plot were mixed together as a composite sample, placed in a plastic

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box and immediately transported to the laboratory. The field-moist soil samples were gently broken apart along the natural fractures and sieved to 5 mm, during which soil impurities, such as stones, organic debris and plant residues, were removed with forceps. Subsequently, the fresh soil samples were air dried and divided into two parts: one part was used for soil colloid extraction, while the other part was further sieved to 0.15 mm and used to determine the soil properties. 2.3 Soil colloid extraction Soil water-dispersible colloids were extracted following the procedure described by Schumacher et al. (2005). Briefly, the soil samples were suspended in deionized water at a 1:5 weight:volume ratio, shaken at 170 rpm for 8 h at 25 °C, and then centrifuged at 2500 g for 6 min (Eppendorf Centrifuge 5804R). Aliquots of the supernatant suspensions were either stored in the dark at 4 °C or freeze dried for further analysis. 2.4 Soil characteristics determination The soil pH was determined using a pH meter (PCM 90, TOKO, Tokyo, Japan) with a 1:5 soil:distilled water ratio. The SOC contents of the bulk soil samples were measured with an elemental analyser (NA 1110, CE Instruments, Milan Italy). The total C concentration determined in this way equaled the SOC concentration because the carbonate content was negligible (< 0.3 g kg-1). Quantification of the SRO minerals of the bulk soil samples and the applied organic fertilizers was performed by selective dissolution with acid ammonium oxalate (McKeague and Day, 1966). Of note, not only ferrihydrite but also some crystalline Fe oxides such as goethite and lepidocrocite could be extracted by the acid ammonium oxalate. The total Fe and Al concentrations in the

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soil samples and organic fertilizers were measured by a wet digestion method with a mixture of concentrated HNO3, HClO4 and HF (Zhou et al., 2007). The concentrations of Fe and Al in the extracts were determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Teledyne Leeman Labss, Hudson, NH 03051, USA). Soil and fertilizer samples were fused with sodium hydroxide at 650 °C and neutralized with dilute hydrochloric acid. The total Si concentrations were determined colorimetrically by the molybdate-ascorbic acid method (Mortlock and Froelich, 1989). The mineral elements and organic C concentrations of the soil colloids were determined following the procedure described by Yu et al. (2017). Briefly, the soil colloids were mixed with 10% (volume:volume) HNO3 at a 1:1 volume:volume ratio and digested at 150 °C for 2 h on a heating plate. After digestion, the mixture was filtered through a 0.45 μm filtration membrane. Finally, aliquots of the supernatant were measured on an ICP-AES (Teledyne Leeman Labss, Hudson, NH 03051, USA) to determine the concentrations of the total Fe, Al, Si and Ca in the soil colloids (mineral availability) and on a TOC/TN analyser (multi N/C 3000, Analytik Jena AG, Germany) to determine the concentrations of organic C in the soil colloids (SCOC). Of note, the concentration of a metal in water-dispersible colloids is a good proxy for the availability of a mineral, which is referred to as the mineral surfaces available for C binding (Wen et al., 2019; Yu et al., 2017). 2.5 Determination of C and Fe, Al and Si introduced by organic fertilizers The C or Fe, Al and Si introduced by organic fertilizers (i.e., Cinput, Feo, Alo and Sio and total Fe, Al and Si) were calculated as the product of the C or Fe, Al and Si

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concentration, application rate and application times divided by the product of the soil density (1.1 g cm-3), area and thickness (20 cm) (Davis et al., 2007). The changes in the SOC contents (ΔSOC) were calculated by comparing the SOC contents in 2016 after the rice harvest with that of the soil prior to the field experiment. Of note, organic fertilization hardly affected the rice yield and only marginally influenced the wheat yield compared with chemical fertilization alone in the rice-wheat cropping system (Huang et al., 2017). Therefore, we inferred that the root and stubble residues in soils amended with organic fertilizers should be approximately equal to those amended with chemical fertilizer alone. Humification coefficients are defined as the fraction of organic C inputs such as crop residues and organic amendments entering the SOC pool (Kätterer et al., 2014; Liang et al., 2016). To calculate the partial humification coefficient (PHC, the percentage of SOC change per unit of exogenous Cinput) of different organic fertilizers, the ΔSOC was adjusted (ΔSOCadjusted) based on that of the NPK treatment to eliminate the interference of the organic C input from the root and stubble residues. The PHC was calculated as the ΔSOCadjusted divided by the exogenous Cinput. 2.6 Fractionation of organic C binding to Fe/Al and calcium (Ca) The fractionation of Ca-bound and Fe/Al-bound organic C (Ca-OC and Fe/Al-OC) was conducted following the method proposed by Xu and Yuan (1993). Briefly, the soil samples were first suspended in a 1.8 g cm-3 NaI solution and dispersed at an energy output rate of 170 J min-1 for 5 min using a probe-type sonicator (Shanghai Zhixin, JVD-650). After dispersion, the suspension was centrifuged at 4000 rpm for 5 min

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(Eppendorf Centrifuge 5804R) and the supernatant was decanted to separate the soil’s light fraction. This procedure was repeated several times until no floating material remained. The heavy fraction was then sequentially extracted with 0.5 M Na2SO4, and 0.1 M NaOH and 0.1 M Na4P2O7 to obtain Ca-OC and Fe/Al-OC, respectively. The organic C in the extracts was suitably diluted and determined using a TOC/TN analyser (multi N/C 3000, Analytik Jena AG, Germany). 2.7 HRTEM analyses The HRTEM analyses were conducted to characterize the morphology and chemical composition of soil water-dispersible colloids using a JEOL JEM-2100F microscope at the Modern Analysis Centre of Nanjing University, China. The HRTEM specimens were carefully prepared by dropping soil water-dispersible colloids onto carbon-coated copper grids. Simultaneous HRTEM, SAED and EDS analyses were conducted on the same samples at an acceleration voltage of 200 keV combined with a 10 nm diameter beam at a constant current and count time (100 s) to obtain a good correlation between images and areas of chemical analysis (Li et al., 2016; Wen et al., 2014a; Xiao et al., 2016). To obtain valid results, the HRTEM analyses of the soil water-dispersible colloids were replicated two times for each fertilizer treatment. 2.8 Solid-state 27Al and 29Si NMR spectroscopy High-resolution

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Al and

29

Si NMR spectroscopy was applied to elucidate the

coordination structures of Al and Si in the freeze-dried soil colloids (Kang et al., 2010; Li et al., 2016; Wen et al., 2014b). Solid-state 27Al and 29Si magic angle spinning (MAS) NMR analyses were performed on a Bruker AVANCE III 400 spectrophotometer

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(Bruker, Fallandenat, Switzerland) at Larmor frequencies of 104.3 MHz and 79.5 MHz, respectively. The

27

Al and

29

Si MAS NMR spectra were acquired with single-pulse

excitations of 0.5 μs (π/18) and 4.5 μs (π/2), repetition times of 1 s and 150 s, and MAS probes of 4 mm spinning at 14 kHz and 7 mm spinning at 6 kHz, respectively. Meanwhile, special care was taken to ensure that the relaxation delays were long enough to obtain an acceptable signal-to-noise ratio and quantitative measurements. A single-pulse with a 1H high-power decoupling pulse program was applied to circumvent the Hartmann-Hahn mismatches. The chemical shifts of

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Al and

29

Si are externally

referenced as 1 M AlCl3 and pure tetramethylsilane (TMS) solutions, respectively. 2.9 XPS analyses The XPS analyses were conducted to quantify the organic functional groups and the chemical speciation of Al and Si at the freeze-dried soil colloid surfaces (Huang et al., 2018; Wen et al., 2019; Xiao et al., 2016; Zama et al., 2018). The C 1s, Al 2p3/2 and Si 2p3/2 spectra of the soil colloid samples were collected using XPS (UlVAC-PHI5000 Versa Probe, Japan) equipped with a monochromatic Al Kα X-ray source (1486.68 eV). The binding energy was corrected using the adventitious hydrocarbon C 1s spectrum at 284.8 eV. The freeze-dried soil colloids were fixed onto a metallic sample holder using a double-sided nonconducting tape and transferred to the analysis chamber, where the vacuum was better than 10-8 Pa. The spot size was approximately 0.5 mm in diameter, and the surface charge induced by the photo ejection process was balanced using a flood gun at 6 eV. The spectra were recorded at a detector resolution corresponding to 0.125 eV per channel to optimize the signal-to-noise ratio. The high-resolution XPS spectra

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were analyzed and de-convoluted using XPSPEAK software (Version 4.1, Hong Kong, China) with a Shirly background correction. The full width at half maximum (FWHM) values of the C 1s, Al 2p3/2 and Si 2p3/2 spectra of the soil colloids were fixed to be as consistent as possible (1.5 eV) and a 30% Gaussian-Lorentzian value was selected to optimize the spectra (Huang et al., 2018; Xiao et al., 2016; Zama et al., 2018). The binding energy of C 1s XPS was assigned to 283.6 eV for carbide/fullerenic, 284.6 eV for C=C, 285 eV for C-C/C-H, 286.2 eV for C-O, 286.7 eV for C-O-C, 287.8 eV for C=O/C(O)N and 289 eV for C(O)O (Huang et al., 2018; Wen et al., 2019). The binding energy of Al 2p3/2 XPS was assigned to 73.8 eV for allophane Al2O3/Al2O3nH2O, 74.5 eV for boehmite AlO(OH) and 75.4 eV for AlOx (Xiao et al., 2016), while that of Si 2p3/2 XPS was assigned to 101.7 eV for C-Si-O (SRO-Si structures), 102.6 eV for Si-Al (layered Si structures) and 103.4 eV for Si-O (SiO2) (Gostynski et al., 2017; Zama et al., 2018). The relative proportion of each functional group or coordination structure could be obtained by integrating the peaks at the binding energy of the specific excited states. 2.10 Statistical analyses The statistical analyses were performed by using the SPSS 19.0 software package for Windows (IBM, Armonk, NY). An analysis of variance (ANOVA) followed by Tukey's HSD post hoc test at P < 0. 05 was used to evaluate the differences between different fertilization treatments in the bulk soils and the soil colloids. Regression analyses were used to explore the relationships between the SRO minerals and the SOC in the bulk soils and between the mineral elements and organic C in the soil colloids.

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3. Results 3.1 Soil C storage The six-year application of different fertilizers increased (P < 0.05) the SOC content by 9.1%-29.4% compared to the no fertilizer control (Fig. 1a). The NPKM treatment showed the highest increase and the NPK treatment showed the lowest increase, whereas no significant differences were observed between the NPKM and NPKS treatments or between the NPK and CK treatments. Similarly, different fertilizer treatments increased (P < 0.05) the SCOC concentration compared to the control (Fig. 1b). The SCOC concentration increased more with the NPKM treatment (43.1%) than with the other treatments (13.2%-17.7%), whereas no significant differences were found among the NPKS, NPKMOI and NPK treatments. In all treatments, Fe/Al-OC was the dominant C fraction, accounting for 47.1%55.2% of the total SOC, whereas Ca-OC contributed less than 3.0% to the total SOC (Fig. 1c, d). Different fertilizer treatments increased (P < 0.05) the Fe/Al-OC content by 14.0%-51.5% but decreased the Ca-OC content by 22.6%-47.4%, compared to the control. The NPKM treatment showed the highest influence and the NPK treatment showed the lowest influence, whereas no significant differences were found between the NPK and NPKMOI treatments. The exogenous C inputs (Cinput) were 6.1, 11.1 and 1.7 g kg-1 for the NPKM, NPKS and NPKMOI treatments, respectively, which were not able to fully account for the changes in the SOC content (ΔSOC) in the NPKM and NPKMOI treatments (Table 1). The ΔSOCadjusted (i.e., the SOC derived directly from the organic fertilizers) accounted

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for 37.0%, 26.7% and 12.5% of the total ΔSOC in the NPKM, NPKS and NPKMOI treatments, respectively. The partial humification coefficients (PHC) of the pig manure compost and the organic-inorganic compound fertilizer were 60.4% and 53.2%, respectively, and were higher (P < 0.05) than that of the crop straw (20.6%). 3.2 SRO minerals and mineral availability Different fertilizer treatments increased (P < 0.05) the concentrations of Feo, Alo and Sio by 7.6%-19.5%, 10.6%-23.9%, and 5.5%-24.2%, respectively, compared to the control (Fig. 2a, b and c). The NPKM treatment showed the highest increase, whereas the NPK treatment showed the lowest increase. Furthermore, the SOC content was significantly positively correlated with Feo (R2 = 0.715, P < 0.001), Alo (R2 = 0.608, P < 0.001) and Sio (R2 = 0.718, P < 0.001) (Fig. 2d, e and f). Moreover, the NPKM treatment increased the SRO minerals by 2.6 g kg-1 compared to the initial soil, whereas the absolute amount of SRO minerals introduced by the pig manure compost itself was negligible (0.12 g kg-1) (Fig. S1). In addition, the NPK treatment decreased (P < 0.05) the soil pH, whereas no significant differences were found among the CK, NPKM and NPKMOI treatments (Fig. S2). Compared to the control, the application of organic fertilizers increased (P < 0.05) the concentrations of Fe, Al and Si in soil colloids by 83.9%-151.4%, 122.3%-201.4% and 72.6%-137.8%, respectively, and decreased the concentration of Ca in soil colloids by 32.6%-49.5%, whereas chemical fertilization alone hardly affected the concentrations of mineral elements in soil colloids (Fig. 3). The NPKM treatment showed the highest influence and the NPKMOI treatment showed the lowest influence,

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whereas no significant differences were found between the NPKS and NPKMOI treatments. Furthermore, although different fertilizer treatments increased (P < 0.05) the Al/Si ratio of the soil colloids compared to the control, it was less than 0.6 in all treatments (Fig. S3). Moreover, the available minerals in the soil colloids only accounted for a small portion of the bulk soil SRO minerals (5.0%-13.4%), and less than 0.4% of the total soil minerals (Table S1). The percentages of colloidal Fe and Al in soils were higher under NPKM, NPKS and NPKMOI than under NPK and CK, whereas no significant differences were found in the percentages of SCOC in soils across all treatments. In addition, the SCOC concentration was significantly positively correlated with the colloidal Fe (R2 = 0.568, P < 0.001), Al (R2 = 0.488, P < 0.001) and Si (R2 = 0.499, P < 0.001), but negatively correlated with the colloidal Ca (R2 = 0.419, P < 0.001) (Fig. 4a, b, c and d). 3.3 Colloidal nanoparticles characterized by HRTEM The HRTEM images indicated the presence of nanoparticles with contrasting electron densities (region 1 and region 2) in the soil colloids in all treatments (Fig. 5a and Figs. S4-S12). The SAED patterns combined with the corresponding EDS elemental maps and the relative proportions of elements further revealed that the nanoparticles in region 1 were amorphous or SRO minerals dominated by Si, Al and O (with some Fe), whereas those in region 2 were crystalline minerals dominated by Fe and O (with some Al/Si) (Fig. 5b and c). However, no significant differences were found in the composition of the soil colloids across all treatments (Fig. 5d and Figs. S4S12).

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3.4 Coordination nature of Al and Si in soil colloids characterized by 27Al and 29Si NMR The

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Al NMR spectra showed major signals at 1.3 ppm (Fig. 6a), which were

usually assigned to the octahedrally coordinated Al derived from monomer Al. However, they may also be attributed to the octahedral Al in a “nanotube-like” coordination environment (Li et al., 2016; Yucelen et al., 2011). The peak located at 67.7 ppm was typically assigned to the tetrahedral Al derived mainly from the wellcharacterized crystalline minerals (Kang et al., 2010; Wen et al., 2014b). The NPKM treatment substantially increased the relative signal intensity of the octahedral Al compared to the NPK treatment, whereas no significant differences were observed among the other treatments. The 29Si NMR spectra showed two major signals at -17.3 ppm and -92.4 ppm (Fig. 6b), which were typically ascribed to SiC derived from the phytolith and polymerized Si derived from the Si-rich allophane and layered Si structures (Carduner et al., 1990; Levard et al., 2012; Li et al., 2016). 3.5 Chemical speciation of C, Al and Si quantified by XPS Different fertilizer treatments showed obvious effects on the XPS spectra of the soil colloids (Fig. 7 and Figs. S13-S16). The C 1s peak-fitting results showed that the major organic functional groups were C=C, C-O, C-C/C-H and C=O/C(O)N, whereas the other organic functional groups such as carbide, C-O-C and C(O)O were less than 10% in all treatments (Table S2). Different fertilizer treatments increased the percentage of the C-C/C-H functional group compared to the control, but no consistent trend was found among other organic functional groups. Furthermore, the results of the Al 2p3/2 peak-fitting indicated that fertilization increased the relative proportion of Al2O3-nH2O

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at the expense of AlO(OH) compared to the control. Moreover, the Si 2p3/2 peak-fitting results revealed that different fertilizer treatments increased the percentage of C-Si-O at the expense of Si-O and/or Si-Al compared to the control. The NPKM treatment showed the highest influence on the chemical speciation of both Al and Si, whereas the NPK treatment showed the lowest influence. 4. Discussion 4.1 C sequestration potential promoted by SRO minerals in paddy soils Six-year rice-wheat rotation even without any fertilization substantially increased the SOC content compared to the initial soil (Table 1), indicating a positive feedback loop for the long-term C sequestration in our studied paddy soil. The positive impact of rice-wheat rotation on SOC sequestration has been reported in the Indo-Gangetic plains (Brar et al., 2013). Using different SOC sequestration scenarios, Pan et al. (2003) estimated that China’s paddy soils have an easily attainable SOC sequestration potential of 0.7 Pg and may ultimately sequester 3.0 Pg under favourable conditions. The C sequestration coefficient (i.e., PHC) of organic fertilizers was as high as 20.6%-60.4% in our studied paddy soil (Table 1); however, that reported in the literature was less than 17% (Fan et al., 2014; Hua et al., 2014; Liang et al., 2016; Song et al., 2012; Zhao et al., 2016), further confirming that there may be still a considerable C sequestration potential for our studied paddy soil. Furthermore, the ΔSOCadjusted for the NPKM, NPKS and NPKMOI treatments (3.7, 2.3 and 0.9 g kg-1, respectively) were significantly lower than ΔSOC for the CK and NPK treatments (4.7 and 6.3 g kg-1, respectively), indicating that root and stubble residues as well as rhizodeposits were superior to

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exogenous organic amendments for SOC sequestration in the rice-wheat cropping system. This is consistent with the findings of Rasse et al. (2005), who reported that root-derived C was retained in soils much more efficiently than the above-ground inputs. Moreover, evidence has shown that the soil C sequestration efficiency depended mainly on the edaphic factors rather than the exogenous C inputs (Liang et al., 2016; Schmidt et al., 2011). Recently, mineral availability for C binding has been thought to be a key regulator of soil C storage (Song et al., 2018; Wen et al., 2019; Yu et al., 2017). Our previous results showed that the seasonal variation of wetting and drying in the ricewheat cropping system enhanced the redox transformation of Fe oxides and improved the accumulation of Feo possibly by forming organo-Fe complexes, which, to a large extent, may be responsible for the positive feedback loop of C sequestration in our studied paddy soil (Huang et al., 2017, 2018). Compared to chemical fertilization alone, six-year organic fertilization further promoted the formation of SRO minerals, which were positively correlated with the SOC content (R2 = 0.715, P < 0.001; R2 = 0.608, P < 0.001; and R2 = 0.718, P < 0.001 for Feo, Alo and Sio, respectively) (Fig. 2). Highly reactive SRO minerals have been shown to promote the preservation of organic C in soils, sediments and peatlands by forming organo-mineral complexes (Kramer and Chadwick, 2018; Lalonde et al., 2012; Riedel et al., 2013; Torn et al., 1997), which could not only protect SRO minerals from being transformed to their crystalline counterparts but also improve the accumulation of organic C (Huang et al., 2018; Wen et al., 2019; Yu et al., 2017). Kramer and Chadwick (2018) found that C retained by reactive minerals accounted for 3%-72% of

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organic C at the global scale. Furthermore, a substantial amount of Fe/Al-OC enhanced by organic fertilization (Fig. 1d) indicated that the chemical retention of organic ligands by binding to oxyhydrates might play a vital role in the sequestration of exogenous organic amendments in our studied paddy soil (Huang et al., 2018; Song et al., 2012; Zhou et al., 2009). Of note, the SRO minerals introduced by the pig manure compost were negligible (Fig. S1b), indicating that the increase in SRO minerals in the organically fertilized soils resulted mainly from enhanced mineral transformations rather than exogenous addition. Moreover, although organic fertilization improved the redox transformation of Fe oxides compared to chemical fertilization alone, the Feo/Fed ratio was less than 0.4 under the present conditions (Huang et al., 2017, 2018), indicating a large potential for future C sequestration in our studied paddy soil (Kölbl et al., 2014; Pan et al., 2003; Zhou et al., 2009). Therefore, organic fertilization promoted the formation of SRO minerals, which in turn enhanced the sequestration of exogenous organic amendments possibly by forming organo-mineral complexes through coprecipitation and/or adsorption, and soil aggregation in our studied paddy soil (Huang et al., 2018; Lalonde et al., 2012; Yu et al., 2017). 4.2 Mineral availability in response to six-year organic fertilization in paddy soils The mineral availability for C binding has been suggested as a key regulator of soil C storage (Song et al., 2018; Wen et al., 2019; Yu et al., 2017). Six-year organic fertilization increased the mineral availability of Fe, Al and Si compared to chemical fertilization alone (Fig. 3), which may preserve organic C possibly by forming organomineral complexes as indicated by the positive correlations between the available

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minerals and SCOC (R2 = 0.568, P < 0.001; R2 = 0.488, P < 0.001; and R2 = 0.499, P < 0.001 for Fe, Al and Si, respectively) (Fig. 4a, b and c). Although the available minerals in the water-dispersible colloids only constituted a small portion of the reactive minerals in the bulk soils (Table S1), they have been shown to play a key role in binding and stabilizing newly added organic C in soils (Wen et al., 2019; Yu et al., 2017). Evidence has shown that long-term organic fertilization enhanced the production of root and microbial exudates (e.g., organic acids), which in turn increased the mineral availability for C binding and promoted the formation of organo-mineral complexes, thus contributing to C sequestration in arable soils (Huang et al., 2018; Li et al., 2016; Wen et al., 2018; Yu et al., 2017). The presence of SRO-Al/Si nanominerals in soil water-dispersible colloids under all treatments (Fig. 5 and Figs. S4-S12) may be attributed to the long-term cultivation of rice, which is a typical Si accumulator plant and plays an important role in Si cycling in soil-plant systems (Nguyen et al., 2019; Song et al., 2018; Yang and Zhang, 2018). Our combined results of the 27Al and 29Si NMR spectra (Fig. 6) further confirmed the accumulation of Si in phytolith and Si-rich allophone, indicating that plant-induced Si cycling promoted the formation of highly reactive SRO-Al/Si nanominerals in soil water-dispersible colloids (Kögel-Knabner et al., 2010; Song et al., 2018; Yu et al., 2017). The biogeochemical cycle of Si has a profound influence on C sequestration in diverse terrestrial ecosystems (Nguyen et al., 2019; Song et al., 2018; Yang and Zhang, 2018). Therefore, the production of Si-rich nanominerals may further enhance the C sequestration potential of our studied paddy soil. Furthermore, six-year organic

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fertilization not only increased the concentration of Si in soil colloids but also promoted the formation of Si-rich allophane compared to chemical fertilization alone (Fig. 3 and Table S2), indicating that organic fertilization might promote the formation of Siassociated organo-mineral complexes in paddy soils by increasing the availability of Si for C binding and enhancing the assembly of SRO-Al/Si nanominerals. This is consistent with the findings of Yang and Zhang (2018), who reported that both agricultural activities and plant species critically influenced the weathering of silicate minerals, and thus the translocation and redistribution of Si in the soil profiles. Together, our combined results from the HRTEM, 27Al and 29Si NMR spectra and XPS indicated that long-term rice cultivation promoted the formation of highly reactive SRO-Al/Si nanominerals, and organic fertilization further enhanced the organo-Si associations, which contributed to long-term C storage in paddy soils (Nguyen et al., 2019; Song et al., 2018; Yang and Zhang, 2018). Similarly, water-dispersible colloids in all soils, regardless of treatment, contained SRO-Fe nanoparticles (e.g., ferrihydrite) (Fig. 5 and Figs. S4-S12). One possible reason is that redox fluctuations induced by frequent alternation of wetting and drying can drive the biogeochemical cycle of Fe in paddy soils, because Fe oxides might act as alternative electron acceptors when paddy fields inundate (Ginn et al., 2017; Huang et al., 2018; Kögel-Knabner et al., 2010). Thus, the anaerobic microbial respiration in paddy soils might stimulate the reductive dissolution of crystalline Fe oxides due to a large demand for alternative electron acceptors, making the subsequent oxidative precipitation of Fe2+ and organic ligands as organo-SRO-Fe nanoparticles possible

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(Huang et al., 2018; Kögel-Knabner et al., 2010; Wissing et al., 2013). This is consistent with the findings of Ginn et al. (2017), who noted that frequent shifts in the redox conditions promoted the formation of rapidly reducible SRO-Fe oxides in a tropical forest soil. Furthermore, the accumulation of SRO-Fe minerals in arable soils was usually accompanied by shifts in the Fe redox bacterial community (Ding et al., 2015; Wen et al., 2018). Moreover, the dissolution and precipitation of Fe oxides in redoxdynamic soils might substantially influence the dispersion and coagulation of soil colloids, and thus the liberation of colloid-borne highly reactive minerals (Henderson et al., 2012; Regelink et al., 2014; Thompson et al., 2006). In addition, natural organic matter, especially labile organic compounds and quinone moieties, might act as a C source for dissimilatory Fe(III)-reducing bacteria and/or an electron shuttling compound for abiotic reductive dissolution of Fe oxides, and thus might play an important role in stimulating Fe reduction in soil ecosystems (Chacón et al., 2006; Kögel-Knabner et al., 2010; Thompson et al., 2006; Wen et al., 2018). Therefore, longterm rice cultivation likely enhanced the redox transformation of Fe oxides and promoted the formation or the release of organo-Fe nanoparticles from the bulk soil to colloidal fraction, which improved the C sequestration potential in our studied paddy soil. However, we have to acknowledge that we only analyzed a few crystalline and non-crystalline Fe minerals from each treatment, and they are not necessarily representative of the bulk soil. Furthermore, although six-year organic fertilization promoted the formation of SRO-Fe minerals in the bulk soil, we cannot conclude this for the colloids.

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The mineral availability of Ca was lower in the organically fertilized soils than in the inorganically fertilized soil (Fig. 3), indicating that organic fertilization could more efficiently accelerate soil decalcification than chemical fertilization alone. Globally, organic C retained by reactive minerals increased with an increasing effective moisture (i.e., mean annual precipitation after correcting for potential evapotranspiration), whereas the inorganic C content decreased with an increasing effective moisture, indicating that climate-driven soil decalcification is accompanied by the formation of reactive minerals (Kramer and Chadwick, 2018). Furthermore, paddy soil formation on calcareous marine sediments was characterized by an accelerated decalcification and a pronounced accumulation of modern organic C by reactive minerals relative to pedogenesis in non-flooded systems on the same initial parent material (Kölbl et al., 2014; Wissing et al., 2013, 2014). Although Ca might have a synergistic effect on C sequestration to ferrihydrite by forming Ca, Fe and organic C ternary organo-mineral complexes (Rowley et al., 2018; Sowers et al., 2018), the relatively higher affinity of Fe/Al over Ca for organic ligands may be, to some extent, responsible for the decalcification in our studied paddy soil (Figs. 1 and 4). Moreover, the cycling of Si through plants might accelerate soil decalcification, because after Si is absorbed by the rice plants, Ca-bearing particles might be no longer protected by the undisturbed mineral assembly (Kellermeier et al., 2010; Wissing et al., 2014; Yang and Zhang, 2018). As discussed above, organic fertilization promoted the release of organo-mineral nanoparticles from the bulk soil to colloidal fraction, and thus might expose more unprotected fresh surfaces for decalcification in our studied paddy soil.

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Six-year organic fertilization not only accelerated the soil decalcification but also alleviated the soil acidification compared to chemical fertilization alone (Fig. S2). Evidence has shown that long-term organic fertilization altered the local coordination environment of Al and promoted the formation of nanominerals (e.g., allophane and imogolite), which in turn alleviated the soil acidification in the red soil of southern China (Wen et al., 2014b). However, long-term overuse of N fertilizer has been shown to significantly accelerate soil acidification in major Chinese croplands (Guo et al., 2010). Although chemical fertilization alone significantly decreased the soil pH compared to the control, it hardly affected the soil decalcification in our studied paddy soil (Fig. 3). This might be due to the protective role of the undisturbed mineral assembly as discussed above. Furthermore, without the exogenous organic supply, the relatively lower pH might reduce the colloidal dispersibility by provoking the polymerization of available minerals around the CaCO3 particles (Kellermeier et al., 2010). This result also challenges the long-standing conceptual view that acidification accelerates soil decalcification (Guo et al., 2010; Kögel-Knabner et al., 2010; Kölbl et al., 2014; Wissing et al., 2013, 2014). Collectively, six-year organic fertilization enhanced the mineral availability for C retention by promoting the formation of SRO nanominerals, which not only accelerated the soil decalcification but also alleviated the soil acidification. 4.3 Organo-mineral associations in the soil colloids Highly reactive SRO nanominerals (e.g., ferrihydrite, allophane and imogolite) have been shown to be more likely to form organo-mineral complexes through anion

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and inner-sphere ligand-exchange reactions than crystalline minerals (e.g., goethite, hematite and boehmite), which could not only promote the preservation of SOC but also contribute to the stabilization of their geometries (Huang et al., 2018; Lalonde et al., 2012; Song et al., 2018; Yu et al., 2017). Six-year organic fertilization facilitated the formation of SRO minerals compared to chemical fertilization alone, which in turn might decrease the reactivity and bioavailability of organic matter by forming stable organo-mineral complexes, thus contributing to long-term SOC storage (Wen et al., 2014a, b; Xiao et al., 2016; Yu et al., 2017). The previous results have shown that forming aromatic-SRO-Fe complexes at the aggregate surfaces combined with soil aggregation following long-term organic fertilization promoted the preservation of SOC in our studied paddy soil (Huang et al., 2018). Similarly, the selective enrichment of aromatic organic ligands at the colloid surfaces (25.8%-48.4%) could also be confirmed by the present study (Table S2), which, to a large extent, might be attributed to the formation of Fe nanominerals in soil water-dispersible colloids. The role of SROFe oxides in the selective preservation of aromatic organic compounds in soils has been well documented in the literature (Huang et al., 2017, 2018; Riedel et al., 2013; Wen et al., 2019). Furthermore, the relative percentage of O-alkyl organic ligands (i.e., C-O and CO-C) ranged between 25.7% and 30.0% at the colloid surfaces (Table S2) and reached up to approximately 45% in the bulk soil samples (Huang et al., 2018), indicating that the long-term cultivation of rice might preferentially preserve the labile organic compounds such as carbohydrates and proteins. Although different fertilizer treatments

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significantly increased the SOC content compared to the control, they hardly affected the chemical composition of SOC in our studied paddy soil (Huang et al., 2017, 2018), further indicating that the accumulation of labile organic compounds might have occurred in the course of paddy soil formation. This might be attributed to the cycling of Si through the rice plants and the formation of Si-rich nanominerals (i.e., phytolith and Si-rich allophane) in our studied paddy soil. Filimonova et al. (2016) reported that the peculiar microporous tortuous structure of allophane aggregates might impose certain criteria for the preferential accumulation of labile organic compounds (e.g., carbohydrates and amino acids) in an allophanic Andosol. Similarly, long-term organic fertilization in the red soil region of southern China promoted the formation of allophane and imogolite, which preferentially retained cellulose rather than lignin (Wen et al., 2014a; Yu et al., 2012). Moreover, long-term rice cultivation likely promoted the formation of SROAl/Si/Fe nanominerals in soil colloids in all treatments (Fig. 5 and Figs. S4-S12), further emphasizing the role of organo-mineral associations in paddy soil C storage. Coprecipitation of Fe, Al and Si following long-term rice cultivation might promote the formation of Fe-substituted allophane, which has been shown to readily form highly resistant organo-mineral complexes by enhancing the ligand exchange, electrostatic interactions, and the electron accepting properties of substituting Fe3+ species (Filimonova et al., 2016). In addition, SRO-Fe nanominerals might also act as cementing agents, binding to the SRO-Al/Si nanominerals, and can be released under reducing conditions (Henderson et al., 2012; Regelink et al., 2014; Thompson et al.,

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2006). Therefore, the reductive dissolution of SRO-Fe nanominerals might facilitate the mobilization of SRO-Al/Si nanominerals, making the selective preservation of specific organic compounds possible (Filimonova et al., 2016; Huang et al., 2018; Riedel et al., 2013; Yu et al., 2012). However, from the present study we cannot evaluate the importance of the various above-listed organo-mineral complexes in soils amended with organic fertilizers. 5. Conclusions Our results provide direct evidence that six-year organic fertilization promotes the formation of SRO minerals (e.g., ferrihydrite, Si-rich allophane and Fe-substituted allophane, as well as phytolith) in soils and modulates the assembly of organo-mineral complexes. Thus, it contributes to the long-term SOC sequestration in the rice-wheat rotation of paddy soil. Our findings not only highlight the structural importance of soil minerals for C retention induced by the biogeochemical cycles of Fe, Al and Si but also provide insights into the organo-mineral associations regulated by organic fertilization, which may represent a practical tool for managing the global C cycle. Therefore, future research should examine the formation of some specific mixed nanominerals at the redox interfaces, for example, in the rice rhizosphere, and their environmental implications. Acknowledgements We are grateful to the anonymous reviewers and to the editor (Baoliang Chen) for their constructive comments. This work was supported by National Natural Science Foundation of China (41671294, 41907083 and U1710255-3), National Key

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Captions Figure 1. Effects of six-year different fertilizer treatments on the concentrations of organic carbon in the bulk soil (SOC, a) and soil colloids (SCOC, b), and the amount of calcium bound organic carbon (Ca-OC, c) and iron/aluminum-bound organic carbon (Fe/Al-OC, d) in the rice-wheat cropping system. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using one-way ANOVAs followed by Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively. Figure 2. Effects of six-year different fertilizer treatments on the concentrations of oxalate extractable iron minerals (Feo, a), aluminum minerals (Alo, b) and silicon minerals (Sio, c), and the relationships between soil organic carbon (SOC) and Feo (d), Alo (e) and Sio (f) in the rice-wheat cropping system. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using one-way ANOVAs followed by Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively. Figure 3. Effects of six-year different fertilizer treatments on the concentrations of iron

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(Fe), aluminum (Al), silicon (Si) and calcium (Ca) in the soil colloids. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using one-way ANOVAs followed by the Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively. Figure 4. Relationships between soil colloid organic carbon (SCOC) and colloidal iron (Fe, a), aluminum (Al, b), silicon (Si, c) and calcium (Ca, d) in the rice-wheat cropping system. Figure 5. High resolution transmission electron microscopy (HRTEM) images of soil colloids from the control soil and the relative proportions of iron (Fe), aluminum (Al) and silicon (Si) in the selected two regions for all treatments. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); (c) energy dispersive X-ray spectroscopy (EDS) images; and (d) the relative proportions of Fe, Al and Si of the two regions for all treatments. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively.

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Figure 6. Effects of six-year different fertilizer treatments on the coordination environments of aluminum (Al) and silicon (Si) in the soil colloids characterized by solid-state 27Al (a) and 29Si (b) NMR spectroscopy. Figure 7. X-ray photoelectron spectroscopy (XPS) peak-fitting spectra (C 1s, a; Al 2p3/2, b; and Si 2p3/2, c) of soil colloids from the control soil. Table 1. The exogenous carbon inputs (Cinput), change in the soil organic carbon content (ΔSOC), adjusted change in the soil organic carbon content (ΔSOCadjusted) and partial humification coefficient (PHC) as affected by different fertilizer treatments in the ricewheat cropping system from 2010 to 2016. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using oneway ANOVAs followed by the Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively. Figure S1. Total Fe, Al and Si and oxalate extractable Fe, Al and Si in the rice straw, pig manure compost and organic-inorganic compound fertilizer (a), and total Fe, Al and Si and oxalate extractable Fe, Al and Si introduced by organic fertilizers after six-year application (b). Figure S2. Effects of six-year different fertilizer treatments on the soil pH in the ricewheat cropping system. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using one-way ANOVAs

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followed by Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organicinorganic compound fertilizer, respectively. Figure S3. Effects of six-year different fertilizer treatments on the aluminum to silicon (Al/Si) ratio of soil colloids in the rice-wheat cropping system. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using one-way ANOVAs followed by Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively. Figure S4. High resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPK treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S5. High resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPKM treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated

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by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S6. High resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPKS treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S7. High resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPKMOI treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S8. Replicated high resolution transmission electron microscopy (HRTEM) images of soil colloids from the control soil. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S9. Replicated high resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPK treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray

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region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S10. Replicated high resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPKM treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S11. Replicated high resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPKS treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S12. Replicated high resolution transmission electron microscopy (HRTEM) images of soil colloids in the NPKMOI treatment. (a) TEM image at 100 nm; (b) HRTEM images at 5 nm and selected area electron diffraction (SAED) pattern of the two regions indicated by the white squares (black region is completely crystalline, whereas the gray region remains amorphous); and (c) energy dispersive X-ray spectroscopy (EDS) images. Figure S13. X-ray photoelectron spectroscopy (XPS) peak-fitting spectra (C 1s, a; Al 2p3/2, b; and Si 2p3/2, c) of soil colloids in the NPK treatment.

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Figure S14. X-ray photoelectron spectroscopy (XPS) peak-fitting spectra (C 1s, a; Al 2p3/2, b; and Si 2p3/2, c) of soil colloids in the NPKM treatment. Figure S15. X-ray photoelectron spectroscopy (XPS) peak-fitting spectra (C 1s, a; Al 2p3/2, b; and Si 2p3/2, c) of soil colloids in the NPKS treatment. Figure S16. X-ray photoelectron spectroscopy (XPS) peak-fitting spectra (C 1s, a; Al 2p3/2, b; and Si 2p3/2, c) of soil colloids in the NPKMOI treatment. Table S1. Percentage of iron (Fe), aluminum (Al), silicon (Si) and carbon (C) from colloids in different fertilized soils. Values are means and standard errors (n = 4). Significant differences between the fertilizer treatments were determined using oneway ANOVAs followed by the Tukey’s HSD post hoc tests at P < 0.05, where conditions of normality and homogeneity of variance were met. Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively. Table S2. The relative percentages (%) of different chemical speciations of carbon (C), aluminum (Al) and silicon (Si) in soil colloids as affected by different fertilizer treatments quantified using X-ray photoelectron spectroscopy (XPS). Treatments CK, NPK, NPKM, NPKS and NPKMOI represent control, chemical fertilizer, 50% chemical fertilizer plus manure, 100% chemical fertilizer plus straw and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer, respectively.

46

47

48

49

50

51

52

53

Table 1

Cinput (g kg-1)

ΔSOC (g kg-1)

ΔSOCadjusted (g kg-1)

PHC (%)

CK

/

4.7±0.5 d

/

/

NPK

/

6.3±0.7 c

/

/

NPKM

6.1

10.0±1.1 a

3.7±0.4 a

60.4±6.7 a

NPKS

11.1

8.6±0.8 b

2.3±0.5 b

20.6±4.2 b

NPKMOI

1.7

7.2±0.6 c

0.9 ±0.2 c

53.2±9.5 a

54

Highlights 

Organic fertilization promoted the formation of short-range-ordered minerals in paddy soils.



Organic fertilization enhanced the mineral availability for carbon binding in waterdispersible colloids.



Highly reactive nanomineral assembly in soil colloids contributed to paddy soil carbon storage.

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