Biochar retards Al toxicity to maize (Zea mays L.) during soil acidification: The effects and mechanisms

Journal Pre-proof Biochar retards Al toxicity to maize (Zea mays L.) during soil acidification: The effects and mechanisms

Ren-Yong Shi, Ni Ni, Jackson Nkoh Nkoh, Ying Dong, Wen-Rui Zhao, Xiao-Ying Pan, Jiu-Yu Li, Ren-Kou Xu, Wei Qian PII:

S0048-9697(20)30958-X

DOI:

https://doi.org/10.1016/j.scitotenv.2020.137448

Reference:

STOTEN 137448

To appear in:

Science of the Total Environment

Received date:

30 November 2019

Revised date:

29 January 2020

Accepted date:

18 February 2020

Please cite this article as: R.-Y. Shi, N. Ni, J.N. Nkoh, et al., Biochar retards Al toxicity to maize (Zea mays L.) during soil acidification: The effects and mechanisms, Science of the Total Environment (2018), https://doi.org/10.1016/j.scitotenv.2020.137448

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© 2018 Published by Elsevier.

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Biochar retards Al toxicity to maize (Zea mays L.) during soil acidification: The effects and mechanisms Ren-Yong Shia, Ni Nib,c, Jackson Nkoh Nkoha,d, Ying Donga, Wen-Rui Zhaoa,d, Xiao-Ying Pana,d, Jiu-Yu Lia, Ren-Kou Xua,d,, Wei Qiana

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science,

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a

Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, China Nanjing Institute of Environmental Science, Ministry of Ecology and Environment,

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b

Key Laboratory of Pesticide Environmental Assessment and Pollution Control,

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c

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Nanjing 210042, P. R. China

College of Advanced Agricultural Sciences, University of Chinese Academy of

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d

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Ministry of Ecology and Environment, Nanjing 210042, P. R. China

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Sciences, Beijing 100049, P. R. China

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Corresponding author. Tel.: +86 25 86881183; fax: +86 25 86881000. E-mail address: [email protected] (R.-K. Xu). 1

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ABSTRACT Biochar can effectively alleviate the Al phytotoxicity in acidic soils due to its alkaline nature. However, the longevity of this alleviation effect of biochar under re-acidification conditions is still unclear. In the present study, the maize root growth responding to the simulated re-acidification of two acidic soils amended by peanut

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straw biochar or Ca(OH)2 was investigated to evaluate the long-term effect of biochar on alleviating Al toxicity in acidic soils. Compared with Ca(OH)2 amendment, the

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application of biochar significantly retarded Al toxicity to plant during soil

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re-acidification. When 4.0 mmol L-1 HNO3 was added, the maize seedling root

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elongation in an Oxisol with biochar was 99% higher than that in the Oxisol with

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Ca(OH)2. Also, the Evans blue uptake and Al content in the root tip in the biochar

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treatment were 60% and 51% lower than those in the Ca(OH)2 treatment. The retarding effect was mainly attributed to the slow decrease in soil pH during

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acidification and the release of dissolved organic carbon (DOC) in the soils amended by biochar. The slower decrease in soil pH resulting from the increased pH buffering capacity after biochar application inhibited the increase of soluble and exchangeable Al during re-acidification. The increased DOC after biochar application decreased the toxic soluble Al speciation at the same pH value and total Al concentration in soil solution. Therefore, given the re-acidification of soils, biochar presented a longer-term effect on alleviating Al toxicity of acidic soil than liming. Keywords: Biochar, Aluminum phytotoxicity, Soil re-acidification, pH, Dissolved organic carbon 2

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1. Introduction The earth soils contain abundant aluminum (Al) in the form of silicates, oxides and hydroxides. As the most abundant metallic element in the earth soil, Al accounts for ~8% of the total mineral soil (Cardiano et al., 2017). Most Al in soils are nontoxic to living organisms due to their inactivity (Hagvall et al., 2015). As the soil was acidified, the inactive Al in soils was released into soil solution forming various toxic

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Al species (e.g. Al3+, Al(OH)2+, Al(OH)2+) (Wang et al., 2016; Pavlů et al., 2019). For

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example, the activity of Al3+, the main toxic Al species, increases 1000-fold for every

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unit decrease in soil pH (Kopittke and Blamey, 2016). The free Al3+ is highly reactive

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and preferentially binds with the negative donors on cell surfaces, such as carboxyl

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and phosphate. The active Al in soil solution, even at a low concentration (< 50 μM),

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would attack the apical meristem of young seedlings root, which inhibits the root elongation and causes cell death. Due to the interrupted growth of plant roots by the

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Al phytotoxicity, the whole metabolism of the plant is disturbed and the crop yield would be sharply decreased (Yamamoto, 2019; de Sousa et al., 2019). The phytotoxicity of Al is, therefore, considered as the key constraint for crop productivity in acidic soils. Acid soils with high levels of active Al comprise about 40% of arable land on the earth, which are mainly distributed in tropical and subtropical regions with intense leaching, as well as temperate regions with humid and cold climates (von Uexküll and Mutert, 1995; Kopittke et al., 2016). Given the expanding population and serious land degradation, improving the crop productivity on these Al-toxic soils is particularly 3

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critical to ensure sustained global food security. Ameliorating acid soils, especially alleviating the toxicity of Al, has attracted extensive attentions from scientists. Traditionally, lime and manures have been widely applied to alleviate Al toxicity in acidic soils through neutralizing soil acidity (Fontoura et al., 2019; Zhang et al., 2019; Shi et al., 2019a). Recently, it has been discovered that biochars produced from the oxygen-limited pyrolysis of biomass contained various alkaline substances, such as

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the carbonates (CaCO3), carbonyl (COO-) and phosphate (PO43-) (Yuan et al., 2011;

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Fidel et al., 2017). These alkaline substances would neutralize soil acidity and

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increase soil pH after biochar application into acid soils (Shi et al., 2019a). Apart from

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the liming effect, the high-silicon biochar effectively immobilized the Al3+ in the

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AlCl3 solution at pH 4.3 through the complexation of Al with the organic carboxyl

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and hydroxyl groups and the surface adsorption and co-precipitation of Al onto silicate particles (Qian and Chen, 2013; Dai et al., 2017). Based on 111 independent

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statistical analysis studies, Jeffery et al. (2017) demonstrated that the application of biochars to acidic soils elicited a 25% average increase in crop yield through alleviating Al phytotoxicity.

Unfortunately, the rate of soil acidification has been greatly accelerated throughout the world in recent years due to various anthropogenic activities, including acid deposition, excessive application of ammonium-base fertilizers as well as crop removal (Guo et al., 2010; Zhu et al., 2018a; Cecchini et al., 2019; Cho et al., 2019; Meng et al., 2019). Zhu et al. (2018b) predicted that the soil pH of typical farmland in China would decrease by 1.1-2.5 units after 30 years if appropriate measures were not 4

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taken. Consequently, the relative crop yield losses resulting from Al phytotoxicity will increase from approximately 4% in 2010 to 24% in 2050 (Zhu et al., 2019). Under intensive agricultural production mode, the ameliorated soils would encounter the risk of re-acidification. However, there are few comparative studies on the effects of different amendments during re-acidification.

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Our previous studies showed that the application of biochar effectively slowed the decline in soil pH during acidification compared with lime, which was mainly

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caused by the protonation of the carboxyl groups (-COO-) on the surface of biochars

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(Shi et al., 2017, 2018a, b). Biochar can persist in soils over long periods of time

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under field conditions due to its highly aromatic nature. The carboxyl groups on the

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surface of biochar increased with the surface chemical and biochemical oxidation (de

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la Rosa et al., 2018). However, as the critical result of soil re-acidification, the role of biochar in alleviating the Al toxicity to plant during soil re-acidification remains

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unknown. The phytotoxicity of Al in acidic soils was determined by the concentration and species of Al in soil solution. Generally, the concentration of active Al in soils increased with the decline in soil pH. Due to the increased resistance of soil to acidification after biochar addition, the application of biochar may retard the release of active Al from the soil during re-acidification. Additionally, biochar application led to an increase in dissolved organic carbon (DOC) of soil (Liu et al., 2019). Due to great binding affinity with Al3+ (Hagvall et al., 2015; Cardiano et al., 2017), the increased DOC could enhance the dissolution of Al in soils through forming the soluble organic-complexed Al (Zhang et al., 2019). These soluble Al organic 5

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complexes (Al-DOM) are supposed to be non-toxic (Drabek et al., 2005; Kopittke et al., 2016). Therefore, we hypothesized that biochar has the potential to retard the toxicity of Al to plant during soil acidification. The present study aims to (i) investigate the effect of biochar on alleviating the toxicity of Al to plant during soil re-acidification; (ii) elucidate the mechanisms of

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biochar on alleviating Al toxicity during soil re-acidification. As a major food crop, maize is planted in acid farmland soils around the world (Pan et al., 2019; de Sousa et

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al., 2019). The Al toxicity to maize roots during soil re-acidification was investigated

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by an exposure experiment using a slurry system containing typical acidic soils

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amended by peanut straw biochar, and the treatment with lime was set as control. The

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results obtained from this study will provide important references for the alleviation

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of Al phytotoxicity in acidic soils using crop straw biochars, especially under the intensive agricultural systems.

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2. Materials and methods 2.1 Soil samples

Two acidic soils, collected from Guangdong Province in South of China, were used in the present study. The Ultisol was derived from granite (23° 10′ N, 113° 30′ E) and the Oxisol was derived from basalt (20° 19′ N, 110° 13′ E). Both soil samples were collected from the top layer (0-15 cm). After air-drying, the soil samples were ground to pass through a 2-mm sieve and stored in plastic bags until use for incubation experiment. Another portion of the dried soil samples was ground to pass through a 0.25-mm sieve for the determination of the soil basic properties. The pH of 6

Journal Pre-proof Ultisol and Oxisol was 4.56 and 5.42, respectively. The exchangeable Al3+ was 3.03 and 0.55 cmol kg-1 in the Ultisol and Oxisol, respectively. Other basic properties of the studied soils are listed in Table S1. The measurement details of the soil basic properties can be found in our previous report (Shi et al., 2018a). 2.2 Biochar and the amended soils

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The peanut (Arachis hypogaea L.) straw was collected from local farmlands, and then was air-dried and ground to pass through a 2-mm sieve. The ground straw sample

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was placed into ceramic crucibles with close-fitting lids and then pyrolyzed in the

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muffle furnace (12D-16, Shanghai Yizhong Electricity Furnace Inc., Shanghai, China)

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under limited oxygen supply. After raising the temperature to 400C at a rate of ~20C

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min-1, it was maintained for 3 h. The biochar sample was cooled over-night to room

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temperature and ground to pass through a 0.25-mm sieve. The biochar sample was analyzed for the pH, alkalinity, cation exchange capacity (CEC), the carbonate and the

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carboxyl contents. Details on sample analysis were reported in our previous report (Shi et al., 2018b).

The amended soils by biochar or Ca(OH)2 were prepared by an incubation experiment. Peanut straw biochar was mixed thoroughly with the selected soils at the rate of 30 g kg-1, which was similar to the application rates (72 t ha-1) under field conditions. Parallelly, the Ultisol and Oxisol were ameliorated by Ca(OH)2 to increase the soil pH to the same value of the biochar treatments (pH~ 6.40). Each amended treatment was in triplicate. The amount of Ca(OH)2 added was estimated depending on the target soil pH and soil pH buffering capacity (pHBC) with the equation (1): 7

Journal Pre-proof mCa(OH)2 =pHBC×(pHtar -pHini )×msoil ×M/2

(1)

where the mCa(OH)2 (g) is the amount of Ca(OH)2 added, pHtar is the target soil pH, pHini is the initial soil pH, msoil (g) is the dry weight of the soil sample, and M is the molar mass of Ca(OH)2 (Shi et al., 2018b). Details of the incubation process can be found in our previous report (Shi et al., 2019b). The amended soil samples were

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air-dried and ground to pass through a 0.25-mm sieve for the next root elongation experiment.

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2.3 The response of maize seedling to Al toxicity during soil re-acidification

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The maize (Zea mays L.) of Donghai 11 was used in the experiment due to the

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high sensitivity to Al toxicity. The maize seeds were exposed to 10% H2O2 for 20 min

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to disinfect and then rinsed with deionized water. The disinfected maize seeds were

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soaked in deionized water for 4 h before dark germination. After 4 days of cultivation at 25C in hydroponic solution, the maize seedlings with a root of ~3 cm were

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selected for the subsequent root elongation experiment. Before the root elongation experiment, a simulated re-acidification experiment was conducted by adding HNO3 into the amended soil samples. In brief, 10.0 g amended soil samples were weighted into 500-mL polypropylene beakers, and 300 mL various concentrations of HNO3 solution (0 to 4.0 mM) was added (Qian et al., 2016; Shi et al., 2018a). The NO3- ion is adsorbed by soils through the electrostatic mechanism, thus, it will not initiate the release of OH- from the soils to inhibit soil acidification (Xu and Ji, 2001). The suspension was magnetically stirred for 24 h and then equilibrated for 6 d, in which the samples were magnetically stirred for 5 min 8

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each day. Each treatment was in triplicate. The pH of the equilibrated suspension was determined by an Orion pH meter with a glass electrode and a calomel reference electrode with a double stage salt bridge of lithium chloride (Shi et al., 2018b). Then, ten selected maize seedlings were planted into the suspensions and cultured in a Percival incubator (Percival 136NL, 179 Percival Scientific, Perry, IA,

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US) at 25 C in the dark. The maize seedlings were harvested after 48 h of exposure and the relative elongation of the roots was calculated with the equation (2): L

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Relative root elongation (%)= L i ×100% 0

(2)

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where L0 (cm) is the root elongation in each treatment without HNO3 addition. Li

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(cm) is the root elongation in each treatments with various HNO3 concentrations (i = 0

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to 4 mM) (Dong et al., 2019a).

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2.4 Evans blue staining and the Al content in the root tips In addition to the root elongation, the cell death of root tips (1 cm) and their Al

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content were analyzed using the harvested maize seedling roots. The cell death was determined spectrophotometrically using Evans blue staining to indicate the loss of plasma membrane integrity (Dong et al., 2019a). Three maize seedling root tips in each plastic cup were submerged in 1 mL of Evans blue solution for 30 min and then rinsed with deionized water to remove the redundant free dye. The stained root tips were homogenized with 1.5 mL of determining buffer containing 1% sodium dodecyl sulfate and 50% ethanol. The Evans blue dye trapped in the root tips was extracted for 15 min at 50C. After filtering through a 0.45-μm filter, the extracted solution was determined using an ultraviolet-visible spectrophotometer (UV-3000, Mapada 9

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Instruments, Shanghai, China) at 600 nm. The relative uptake amount of Evans blue was calculated with the equation (3): Abs

Relative Evans blue uptake (%)= Abs i ×100% 0

(3)

where Abs0 is the absorbancy in each treatment without HNO3 addition. Absi is the absorbancy in each treatments with various HNO3 concentrations (i =0 to 4 mM).

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Another three maize seedling root tips were cut and rinsed with deionized water. The Al in these root tips were extracted by 1 M HCl for 24 h and measured by

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inductively coupled plasma atomic emission spectrometry (ICP-AES, VISTA-MPX,

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Varian, USA).

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2.5 Determination of the physicochemical properties of the soil samples after maize

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seedlings cultivation

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The pH of suspension after maize seedlings cultivation was determined using the same method before cultivation. The residual soil and solution in the suspension were

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separated through centrifuging at 4500 rpm (3644 g) for 5 min. The supernatants were filtered through a 0.45-μm Millipore filter membrane (Merck Millipore, Billerica, MA, USA). After being acidified to pH< 1.0, the total Al in the solutions was determined by spectrophotometry with 8-hydroxyquinoline at pH 8.3 (Shi et al., 2018b). The DOC in the solutions was measured using a total carbon and nitrogen analyzer (Multi C/N 3100, Analytik Jena AG, Germany). The Al speciation in the solutions were calculated by Visual MINTEQ ver. 3.1 using the pH value and the concentration of Al and DOC in the solution. The Gaussian DOM model was used to evaluate Al binding to DOM in the solution (Alleoni et al., 2010). The residual soil samples were 10

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oven-dried at 50C and ground to pass through a 0.25-mm sieve. Soil exchangeable acidity (Al3+ and H+) was extracted with 1.0 M potassium chloride and titrated by 0.01 M NaOH. Soil exchangeable base cations were extracted with 1.0 M ammonium acetate, and K+ and Na+ concentrations were measured by flame photometry (Sherwood M410, Sherwood Scientific Ltd, Cambridge, UK) while Ca2+ and Mg2+

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were determined using atomic absorption spectrometry (nov AA350, Analytik Jena AG, Jena, Germany). The effective cation exchange capacity (ECEC) was calculated

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by the sum of the equivalent charges for exchangeable Al3+, H+, K+, Na+, Ca2+ and

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Mg2+.

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2.6 Statistical analysis

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The obtained data were statistically analyzed using SPSS 20.0 (IBM Inc.,

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Armonk, NY, USA). A one-way analysis of variance (ANOVA) with the least significant difference (LSD) post hoc test was conducted to test the significant

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differences among the treatments at P < 0.05. 3. Results and discussion

3.1 The effect of biochar on root elongation of maize and Al phytotoxicity during soil re-acidification Both lime and biochar application amended the acidic soils. They corrected soil acidity through their inorganic alkalinity. In addition, the abundant organic functional groups on the surface of biochar also contributed to the amelioration of soil acidity. As the amount of HNO3 added into the Ultisol and Oxisol increased, the root elongation of maize was significantly inhibited due to soil acidification (Fig .1). When soils are 11

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acidified to a certain extent by the input of acid, the plant growth, especially the root elongation, would be inhibited (Yamamoto, 2019). However, the significant difference between the biochar amendment and Ca(OH)2 amendment was observed in terms of the negative effects of acid input on plant growth (Fig. 1). Compared with Ca(OH)2 treatment, the addition of biochar significantly improved the maize seedling root

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elongation when the same amount of HNO3 was added (Fig. 1). For example, when 4.0 mM HNO3 was added, the maize seedling root elongation in the Oxisol with

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biochar was 99% higher than that in the Oxisol with Ca(OH)2 (Fig. 1). In the Ultisoil,

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when the maize root elongation was suppressed to ~40% of the initial state, 2.0 mM

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HNO3 was needed for the treatment with Ca(OH)2, while 2.5 mM HNO3 was needed

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for the treatment with biochar. These results demonstrated that compared with

the plant root growth.

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Ca(OH)2 amendment, the biochar mitigated the inhibition of soil re-acidification on

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(Insert Figs. 1, 2 and 3 near here)

The inhibited root growth by soil acidification was generally caused by Al toxicity which induced cell death of the root tip. The Evans blue staining was used to evaluate the integrity of the root tip plasma membrane after 48 h exposure to the acidified soil slurry. As shown in Fig. 2, the uptake of Evans blue dye by the maize root tips increased with the increase in HNO3 inputs, which illustrated the loss of root tip plasma membrane integrity and root cell death. Both the free proton and Al3+ in the soil solution can cause the cell death of the root tip, while the toxicity of Al is more serious than that of protons (Ikka et al., 2007). The change trends of Evans blue 12

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uptake by maize root tips were also in agreement with the results of Al concentrations in the root tips in all the treatments (Fig. 3). It has been reported that the Evans blue displayed strong adsorption by the root cell as the root tips were destroyed by Al toxicity (Qian et al., 2016; Dong et al., 2019a). Obviously, these results confirmed that the inhibition of root growth by adding HNO3 is primarily attributed to the damaging

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effect of Al toxicity to root tip cells. Compared with the Ca(OH)2 treatments, both the Evans blue uptake by root tips and Al concentration on root tips were decreased in the

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biochar amended Ultisol and Oxisol at higher concentrations of HNO3 (Figs. 2 and 3),

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which was consistent with the result of root elongation (Fig. 1). For instance, when

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4.0 mM HNO3 was added into the modified Oxisol, compared with those in Ca(OH)2

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amendment, the Evans blue uptake by root tips and the content of root tips Al in the

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biochar amendment were reduced by 60% and 51%, respectively (Figs. 2 and 3). Similarly, when the relative Evans blue input reached ~300% and the Al content in the

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root tips reached ~1.70 mmol kg-1, the Ultisol modified by biochar required 25% more acid than that modified by Ca(OH)2. Qian et al. (2016) have reported that the wheat root elongation and cell integrity were significantly reduced when the Al concentration in the root tips was at ~1.5 mmol kg-1 fresh root, which was consistent with the results in the present study (Fig. 3). The application of biochar retarded the Al toxicity to plant root during soil re-acidification and thus alleviated the inhibition of re-acidification to plant growth. Generally, the improvement effect of amendments on acidic soils would fading with the inputs of acid, resulting in the re-acidification of amended soils (Cornelissen et al., 2018). The performance of amendments in 13

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retarding Al toxicity to plant during soil re-acidification determined the effective duration of the amendments on acidic soils. The above results indicated that biochar amendment performed a long-term effect on alleviating the Al phytotoxicity in acidic soils compared with Ca(OH)2 amendment. 3.2 Effects of biochar on soil pH and Al release during soil re-acidification

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The soluble Al in soil solution is responsible for Al phytotoxicity in acidic soils. Soil pH is the key factor determining the activity of soil soluble Al (Kopittke et al.,

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2016). When soils were acidified, minerals in soils dissolved, releasing Al into the soil

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solution. As shown in Fig. 4, with the addition of HNO3, the pH of the Oxisol with

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biochar addition decreased slower than that with Ca(OH)2. When the acid input was

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lower than 2 mM, the soluble Al contents in all treatments was very low. There were

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no significant differences in the soil pH, soluble Al contents and the root elongation between the lime and biochar treatments (Figs. 1 and 4). When 4 mM HNO3 was

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added into the Oxisol, the pH of the soil with biochar was 0.3 pH units higher than that with Ca(OH)2 (Fig. 4). Due to the higher soil pH, the soluble Al in the biochar treatment was 54% lower than that in the Ca(OH)2 treatment, which alleviated the Al phytotoxicity. It has been reported that the application of biochar increased the soil pH buffering capacity (pHBC) through the protonation of the carboxyl on the surface of biochar, and thus slowed the decrease of soil pH resulting from the acid inputs. When soil pH decreased from 7.0 to 4.5, the carboxyl groups on the biochar surface combined with the free protons followed by the release of base cations and the decrease in soil ECEC (Shi et al., 2018b). In this study, the pHBC of Oxisol increased 14

Journal Pre-proof by 33% (from 31.48 mmol kg-1 pH-1 to 41.77 mmol kg-1 pH-1) after biochar application. The decrease in soil ECEC during acidification verified the protonation of variable charge sites including the carboxyl groups (Fig. S1). Therefore, the protonation of carboxyl on the biochar surface retarded the decrease in soil pH with the inputs of HNO3, which inhibited the release of Al from the Oxisol with biochar

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during re-acidification. In contrast, there was no significant difference in the decrease of soil pH between

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the Ultisol with biochar and Ca(OH)2 during soil acidification (Fig. 4), although the

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addition of biochar increased the soil pHBC by 15% (from 24.35 to 27.99 mmol kg-1

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pH-1). Accordingly, the difference in soluble Al between the biochar treatment and

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Ca(OH)2 treatment was also negligible (Fig. 4). Apart from the soil pHBC, soil pH,

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especially rhizosphere pH, is influenced by the living plant roots, which release H+ or OH- to adapt environmental constraints (Sun et al., 2019; Hinsinger et al., 2003). The

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soil slurry pH decreased significantly after maize cultivation, especially in the slurry with a neutral pH (Fig. S2). The pH of the slurry without HNO3 addition decreased from 6.07 to 5.35 and from 6.21 to 5.38 in the Ultisol with biochar and Ca(OH)2, respectively (Fig. S2). Because the activity of root was inhibited, the slurry pH before and after plant cultivation was almost similar at the low pH. The decrease in slurry pH caused by maize cultivation was also observed in the Oxisol at higher pH levels (Fig. S2). The disturbance of soil pH by plant root activity partially counteracted the contribution of biochar to inhibiting the decrease in soil pH during acidification by increasing the soil pHBC. Compared to the treatment with Ca(OH)2, although biochar 15

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application did not significantly reduce the concentration of soluble Al in the Ultisol (Fig. 4), it effectively alleviated the toxicity of Al to plant root during soil re-acidification (Figs. 1, 2 and 3). Therefore, there were other mechanisms for biochar on retarding the occurrence of soil Al toxicity during soil re-acidification. (Insert Fig. 4 near here)

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3.3 Effects of biochar on soil DOC and the soluble Al species In addition to the concentration of soluble Al, the Al speciation in soil solution is

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critical for the Al phytotoxicity in acidic soils (Kopittke et al., 2016). Al toxicity to

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plants qualitatively decreases in the following order: polymer Al13, Al3+, monomeric

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Al-OH (Al(OH)2+, Al(OH)2+ and Al(OH)4-), Al-F complexes (AlF2+, AlF2+, AlF30 and

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AlF4-), and the Al-SO4 complexes (AlSO4+ and Al(SO4)2-). Al organic complexes

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(Al-DOM) are supposed to be non-toxic (Kopittke et al., 2016; Drabek et al., 2005). The soil organic matter could strongly influence the mobility and toxicity of Al due to

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its higher binding affinity with Al3+. The alleviation of Al toxicity has been observed in the acid soils with high DOC (Alleoni et al., 2010; Zhang et al., 2019). As shown in Fig. 5, the application of biochar significantly increased the DOC in the solution compared with the Ca(OH)2 amendment. The contents of DOC in the Ultisol and Oxisol with biochar at low pH was 36% higher than those in the soils with Ca(OH)2, respectively (Fig. 5). The increase in soil DOC after biochar application was also observed elsewhere (Yang et al., 2019; Smebye et al., 2016). The high content of DOC in the peanut straw biochar (12.75 g kg-1) contributed to the increased DOC in the soils with the biochar. Due to its strong binding capacity with Al, the increased 16

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DOC can alter the species of soluble Al in the soil solution thereby increasing the distribution of Al-DOM and thus alleviate the toxicity of Al to plant (Zhang et al., 2019). Here, to evaluate the contribution of the increased DOC to the distribution of Al speciation in soil solution, the Al speciation was calculated by Visual MINTEQ, in which the pH and the Al content in the solution were set to the same value between

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the biochar treatment and Ca(OH)2 treatment. When 1.5 mM and 4 mM HNO3 were added into the Ultisol and Oxisol with biochar, the root elongation in the biochar

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treatments was significantly longer than that in the Ca(OH)2 treatments (Figs. 1 and 4).

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Here, the pH of Ultisol and Oxisol were 4.2 and 4.4 (Fig. 4). At the low pH level, the

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Al3+, Al-DOM, and AlOH2+ were the main components of the soluble Al in all the

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treatments (Fig. 6). Compared with the Ca(OH)2 amendments, the Al-DOM with

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no-toxicity were respectively increased by 40.9% and 37.5% in the Ultisol and Oxisol amended by biochar, while the total contents of toxic Al species (Al3+, AlOH2+ and

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Al(OH)2+) were reduced by 21.1% and 10.3%, correspondingly (Fig. 6). Therefore, the increased DOC after biochar application also contributed to the alleviation of Al toxicity to the maize root at low pH through increasing the Al-DOM distribution while decreasing the concentration of toxic Al species, especially in the Ultisol. (Insert Figs. 5 and 6 near here) 3.4 Effects of biochar on the soil exchangeable Al3+ The active Al in soil solution was impacted by the exchange reactions between soil solution and the soil exchange sites, thus the exchangeable Al characterized the potential of Al toxicity in a soil (Collignon et al., 2012). In all the treatments, the 17

Journal Pre-proof exchangeable Al3+ increased with the concentration of HNO3 and with decreasing soil pH (Figs. 4 and 7). Compared with the Ca(OH)2 treatments, the increase in soil exchangeable Al3+ during acidification was inhibited by the application of biochar in both soils (Fig. 7). Soil pH is the key factor determining the content of exchangeable Al3+ in soils. As discussed above, the incorporation of biochar increased the soil

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pHBC and thus buffered the decrease in soil pH with HNO3 inputs. Consequently, the pH value of the soils amended with biochar was higher than those amended with

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Ca(OH)2 at the same HNO3 added level (Fig. 4). The increased soil pH reduced the

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Al3+ on the soil exchangeable sites (Fig. 7). When 4.0 mM HNO3 was added, the pH

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of the Oxisol with biochar was increased by 0.3 pH units compared to that with

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Ca(OH)2, while the exchangeable Al3+ was decreased by 39.5% (Figs. 4 and 7).

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Although there was no significant difference in the decrease of soil pH during soil acidification between the Ultisol with biochar and that with Ca(OH)2, the

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exchangeable Al3+ in the Ultisol with biochar was significantly lower than that in the Ca(OH)2 treatment. This phenomenon was also observed in the Oxisol at the low soil pH (Fig. S3). Apart from the liming effect, the surface complexation of biochar with Al also contributed to the substantial reduction in soil exchangeable Al3+. The abundant oxygen-containing functional groups on the surface of biochar, such as carboxyl and hydroxyl, can complex with the Al3+ and hold them from exchange sites (Qian and Chen, 2013). Therefore, the soil exchangeable Al3+ in the soils amended with biochar was lower than that in the soils with Ca(OH)2 at the same pH value. In addition, the surface complexation is facilitated by the increased soil pH in the 18

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biochar treatment (Fig. 4). The high soil pH leads to the dissociation of oxygen-containing functional groups on the surface of biochar forming the organic anions, which is more readily to bind with Al (Wang et al., 2018; Yang et al., 2020). The retarded increase in soil exchangeable Al3+ during acidification indicated that the application of biochar decreased the potential Al toxicity in the process of soil

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re-acidification.

3.5 Practical implication and future perspective

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(Insert Fig. 7 near here)

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It has been widely accepted that biochar can effectively ameliorate soil acidity

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and alleviate Al phytotoxicity due to its alkaline nature (Dai et al., 2017). However,

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the longevity of this ameliorating effect of biochar is controversial (Major et al., 2010;

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Cornelissen et al., 2018). The alkalinity in the amended soils would be neutralized by the inputs of H+ derived from acid deposition and excessive application of

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ammonium-base fertilizers, resulting in the fading effects of ameliorant over time. Therefore, the longevity of the ameliorating effect was determined by the performance of ameliorant on alleviating Al toxicity in the process of re-acidification. The results in the present study indicate that compared with the lime amendment, the application of biochar showed the longer-term effects on alleviating Al phytotoxicity in acidic soils under re-acidification conditions. The alleviated Al phytotoxicity by biochar at the same concentration of HNO3 was primarily attributed to the reduced soluble Al and exchangeable Al, the increased soluble base cations (Shi et al., 2018b) as well as the variation of Al species. The oxygen-containing functional groups on the biochar 19

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surface and the DOC in biochar are critical to the longevity of the alleviating effect of biochar to Al toxicity during soil re-acidification. However, this comparative study was depended on the short-term root elongation experiment with simulated acidification in the laboratory. The long-term field studies with proper control treatments are needed to evaluate the longevity of the alleviating effect of biochar to

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soil Al toxicity in the future. The increased DOC in the soils amended with biochar would be leached by rainfalls under long-term field conditions (Dong et al., 2019b;

groups

was

fairly stable

in

amended soils.

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functional

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Liu et al., 2019). While the biochar solid phase containing oxygen-containing The

content of

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oxygen-containing functional groups on the biochar surface even would be increased

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due to the chemical or microbiological ageing under field conditions (Mia et al. 2017).

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Therefore, we speculated that under the long-term field conditions, the oxygen-containing functional groups on the biochar surface will be primarily

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responsible for the long-term alleviating Al toxicity during re-acidification rather than the increased DOC after biochar application. Compared to the biocahr, the lime would be more easily consumed by soil acidity. The retarding effects of bichars on soil re-acidification will be more significant under the field conditions. This hypothesis will be verified under field conditions with biochars derived from different crop straws in the future study. 4. Conclusions Compared with liming, the application of biochar significantly retarded the toxicity of Al to plant during re-acidification. This was primarily attributed to the 20

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decreased soluble Al and exchangeable Al as well as the altered Al species by biochar application. The higher soil pH at the same amount of acid input, which resulted from the increased soil pHBC by biochar, led to the lower content of soluble Al and exchangeable Al. The increased DOC after biochar application promoted the forming of Al-DOM with non-toxicity while decreasing the concentration of toxic Al (Al3+ and

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Al(OH)2+) in soil solution. These results indicated that compared with the lime amendment, the application of biochar presented a superior long-term effect on

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alleviating Al phytotoxicity in acidic soils under re-acidification conditions. Therefore,

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the biochars derived from crop residues are excellent substitutes for traditional liming

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Supplementary data

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to ameliorate acidic soils.

version.

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Supplementary data associated with this article can be found in the online

Acknowledgements

This study was supported by the Natural Science Foundation of Jiangsu Province, China (Grant Number: BK20191103), the National Natural Science Foundation of China (Grant Number: 41907019).

Reference Alleoni, L.R., Cambri, M.A., Caires, E.F., Garbuio, F.J., 2010. Acidity and aluminum 21

Journal Pre-proof

speciation as affected by surface liming in tropical no-till soils. Soil Sci. Soc. Am. J. 74, 1010–1017. Cardiano, P., Cigala, R.M., Crea, F., Giacobello, F., Giuffrè, O., Irto, A., Lando, G., Sammartano, S., 2017. Sequestration of Aluminium (III) by different natural and synthetic organic and inorganic ligands in aqueous solution. Chemosphere 186,

of

535–545. Cecchini, G., Andreetta, A., Marchetto, A., Carnicelli, S., 2019. Atmospheric

ro

deposition control of soil acidification in central Italy. Catena 182, 104102.

-p

Cho, S., Dinwoodie, G., Fu, Y., Abboud, S., Turchenek, L., 2019. An assessment of

re

long-term soil acidification trends in Alberta, Canada. Ecol. Indic. 98, 712–722.

lP

Collignon, C., Boudot, J.P., Turpault, M.P., 2012. Time change of aluminium toxicity

na

in the acid bulk soil and the rhizosphere in Norway spruce (Picea abies (L.) Karst.) and beech (Fagus sylvatica L.) stands. Plant Soil 357, 259–274.

Jo ur

Cornelissen, G., Nurida, N.L., Hale, S.E., Martinsen, V., Silvani, L., Mulder, J., 2018. Fading positive effect of biochar on crop yield and soil acidity during five growth seasons in an Indonesian Ultisol. Sci. Total Environ. 634, 561–568. Dai, Z., Zhang, X., Tang, C., Muhammad, N., Wu, J., Brookes, P.C., Xu, J., 2017. Potential role of biochars in decreasing soil acidification-A critical review. Sci. Total Environ. 581, 601–611. de la Rosa, J.M., Rosado, M., Paneque, M., Miller, A.Z., Knicker, H., 2018. Effects of aging under field conditions on biochar structure and composition: Implications for biochar stability in soils. Sci. Total Environ. 613, 969–976. 22

Journal Pre-proof

de Sousa, A., Saleh, A.M., Habeeb, T.H., Hassan, Y.M., Zrieq, R., Wadaan, M.A.M., Hozzein, W.N., Selim, S., Matos, M., AbdElgawad, H., 2019. Silicon dioxide nanoparticles ameliorate the phytotoxic hazards of aluminum in maize grown on acidic soil. Sci. Total Environ. 693, 133636. Drabek, O., Mladkova, L., Boruvka, L., Szakova, J., Nikodem, A., Nemecek, K., 2005.

of

Comparison of water-soluble and exchangeable forms of Al in acid forest soils. J. Inorg. Biochem. 99, 1788–1795.

ro

Dong, Y., Wang, H., Chang, E., Zhao, Z., Wang, R., Xu, R., Jiang, J., 2019a.

soils

through

an

investigation

re

cultivating

-p

Alleviation of aluminum phytotoxicity by canola straw biochars varied with their of

wheat

seedling

root

lP

elongation. Chemosphere 218, 907–914.

na

Dong, X., Singh, B.P., Li, G., Lin, Q., Zhao, X., 2019b. Biochar has little effect on soil dissolved organic carbon pool 5 years after biochar application under field

Jo ur

condition. Soil Use Manage. 35, 466–477. Liu, C., Wang, H., Li, P., Xian, Q., Tang, X., 2019. Biochar's impact on dissolved organic matter (DOM) export from a cropland soil during natural rainfalls. Sci. Total Environ. 650, 1988–1995. Fontoura, S.M.V., de Castro Pias, O.H., Tiecher, T., Cherubin, M.R., de Moraes, R.P., Bayer, C., 2019. Effect of gypsum rates and lime with different reactivity on soil acidity and crop grain yields in a subtropical Oxisol under no-tillage. Soil Till. Res. 193, 27–41. Fidel, R.B., Laird, D.A., Thompson, M.L., Lawrinenko, M., 2017. Characterization 23

Journal Pre-proof

and quantification of biochar alkalinity. Chemosphere 167, 367–373. Guo, J.H., Liu, X.J., Zhang, Y., Shen, J.L., Han, W.X., Zhang, W.F., Christie, P., Goulding, K.W.T., Vitousek, P.M., Zhang, F.S., 2010. Significant acidification in major Chinese croplands. Science 327, 10087–1010. Hagvall, K., Persson, P., Karlsson, T., 2015. Speciation of aluminum in soils and

of

stream waters: The importance of organic matter. Chem. Geol. 417, 32–43. Hinsinger, P., Plassard, C., Tang, C., Jaillard, B., 2003. Origins of root-mediated pH

-p

review. Plant Soil 248, 43–59.

ro

changes in the rhizosphere and their responses to environmental constraints: A

re

Ikka, T., Kobayashi, Y., Iuchi, S., Sakurai, N., Shibata, D., Kobayashi, M., Koyama,

lP

H., 2007. Natural variation of Arabidopsis thaliana reveals that aluminum

na

resistance and proton resistance are controlled by different genetic factors. Theor. Appl. Genet. 115, 709–719.

Jo ur

Jeffery, S., Abalos, D., Prodana, M., Bastos, A.C., Van Groenigen, J.W., Hungate, B.A., Verheijen, F., 2017. Biochar boosts tropical but not temperate crop yields. Environ. Res. Lett. 12, 053001. Kopittke, P.M., Blamey, F.P.C., 2016. Theoretical and experimental assessment of nutrient solution composition in short-term studies of aluminium rhizotoxicity. Plant Soil 406, 311–326. Kopittke, P.M., Menzies, N.W., Wang, P., Blamey, F.P.C., 2016. Kinetics and nature of aluminium rhizotoxic effects: A review. J. Exp. Bot. 67, 4451–4467. Liu, C., Wang, H., Li, P., Xian, Q., Tang, X., 2019. Biochar's impact on dissolved 24

Journal Pre-proof

organic matter (DOM) export from a cropland soil during natural rainfalls. Sci. Total Environ. 650, 1988–1995. Major, J., Rondon, M., Molina, D., Riha, S.J., Lehmann, J., 2010. Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol. Plant Soil 333, 117–128.

of

Meng, C., Tian, D., Zeng, H., Li, Z., Yi, C., Niu, S., 2019. Global soil acidification impacts on belowground processes. Environ. Res. Lett. 14, 074003.

ro

Mia, S., Dijkstra, F. A., Singh, B., 2017. Long-term aging of biochar: a molecular

-p

understanding with agricultural and environmental implications. Adv. Agron. 141,

re

1–51.

lP

Pavlů, L., Borůvka, L., Drábek, O., Nikodem, A., 2019. Effect of natural and

regions

in

na

anthropogenic acidification on aluminium distribution in forest soils of two the

Czech

Republic.

J.

Forestry

Res.

1–8.

Jo ur

https://doi.org/10.1007/s11676-019-01061-1. Pan, X.Y., Baquy, M.A., Guan, P., Yan, J., Wang, R., Xu, R.K., Xie, L., 2019. Effect of soil acidification on the growth and nitrogen use efficiency of maize in Ultisols. J. Soil Sediment. https://doi.org/10.1007/s11368-019-02515-z. Qian, L, Chen, B., 2013. Dual role of biochars as adsorbents for aluminum: The effects of oxygen-containing organic components and the scattering of silicate particles. Environ. Sci. Technol. 47, 8759–8768. Qian, L., Chen, B., Chen, M., 2016. Novel alleviation mechanisms of aluminum phytotoxicity via released biosilicon from rice straw-derived biochars. Sci. Rep. 25

Journal Pre-proof

6, 29346. Shi, R.Y., Hong, Z.N., Li, J.Y., Jiang, J., Baquy, M.A.A., Xu, R.K., Qian, W., 2017. Mechanisms for increasing the pH buffering capacity of an acidic Ultisol by crop residue-derived biochars. J. Agric. Food Chem. 65, 8111–8119. Shi, R.Y., Li, J.Y., Jiang, J., Kamran, M.A., Xu, R.K., Qian, W., 2018a. Incorporation

of

of corn straw biochar inhibited the re-acidification of four acidic soils derived from different parent materials. Environ. Sci. Pollut. Res. 25, 9662–9672.

ro

Shi, R.Y., Hong, Z.N., Li, J.Y., Jiang, J., Kamran, M.A., Xu, R.K., Qian, W., 2018b.

-p

Peanut straw biochar increases the resistance of two Ultisols derived from

re

different parent materials to acidification: a mechanism study. J. Environ. Manag.

lP

210, 171–179.

na

Shi, R.Y., Li, J.Y., Ni, N., Xu, R.K., 2019a. Understanding the biochar's role in ameliorating soil acidity. J. Integr. Agr. 18, 1508–1517.

Jo ur

Shi, R.Y., Liu, Z.D., Li, Y., Jiang, T., Xu, M., Li, J.Y., Xu, R.K., 2019b. Mechanisms for increasing soil resistance to acidification by long-term manure application. Soil Till. Res. 185, 77–84.

Shi, R.Y., Ni, N., Nkoh, J.N., Li, J.Y., Xu, R.K., Qian, W., 2019c. Beneficial dual role of biochars in inhibiting soil acidification resulting from nitrification. Chemosphere 234, 43–51. Smebye, A., Alling, V., Vogt, R.D., Gadmar, T.C., Mulder, J., Cornelissen, G., Hale, S. E., 2016. Biochar amendment to soil changes dissolved organic matter content and composition. Chemosphere 142, 100–105. 26

Journal Pre-proof

Sun, X., Li, Z., Wu, L., Christie, P., Luo, Y., Fornara, D. A., 2019. Root-induced soil acidification and cadmium mobilization in the rhizosphere of Sedum plumbizincicola: evidence from a high-resolution imaging study. Plant Soil, 436, 267–282. von Uexküll, H. R., Mutert, E., 1995. Global extent, development and

of

economic-impact of acid soils. Plant Soil, 171, 1−15. Wang, L., Butterly, C. R., Tian, W., Herath, H. M., Xi, Y., Zhang, J., Xiao, X., 2016.

-p

soil. J. Soil Sediment. 16, 1933−1943.

ro

Effects of fertilization practices on aluminum fractions and species in a wheat

re

Wang, Y., Cheng, P., Li, F., Liu, T., Cheng, K., Yang, J., Lu, Y., 2018. Variable charges

lP

of a red soil from different depths: acid-base buffer capacity and surface

na

complexation model. Appl. Clay Sci. 159, 107−115. Xu, R.K., Ji, G.L., 2001. Effects of H2SO4 and HNO3 on soil acidification and

Jo ur

aluminum speciation in variable and constant charge soils. Water, Air, Soil Pollut. 129, 33−43.

Yang, Y., Wang, Y., Peng, Y., Cheng, P., Li, F., Liu, T., 2020. Acid-base buffering characteristics of non-calcareous soils: Correlation with physicochemical properties and surface complexation constants. Geoderma 360, 114005. Yang, X. Y., Chang, K. H., Kim, Y. J., Zhang, J., Yoo, G., 2019. Effects of different biochar amendments on carbon loss and leachate characterization from an agricultural soil. Chemosphere 226, 625−635. Yamamoto, Y., 2019. Aluminum toxicity in plant cells: Mechanisms of cell death and 27

Journal Pre-proof inhibition of cell elongation. Soil Sci. Plant Nut. 65, 41−55. Yuan, J.H., Xu, R.K., Zhang, H., 2011. The forms of alkalis in the biochar produced from crop residues at different temperatures. Bioresour. Technol. 102, 3488−3497. Zhang, J., Hu, A., Wang, B., Ran, W., 2019. Al (III)-binding properties at the molecular level of soil DOM subjected to long-term manuring. J. Soil Sediment.

of

19, 1099−1108.

ro

Zhu, Q., de Vries, W., Liu, X., Hao, T., Zeng, M., Shen, J., Zhang, F., 2018a.

-p

Enhanced acidification in Chinese croplands as derived from element budgets in

re

the period 1980–2010. Sci. Total Environ. 618, 1497–1505.

lP

Zhu, Q., Liu, X., Hao, T., Zeng, M., Shen, J., Zhang, F., De Vries, W., 2018b.

na

Modeling soil acidification in typical Chinese cropping systems. Sci. Total Environ. 613, 1339–1348.

Jo ur

Zhu, Q., Liu, X., Hao, T., Zeng, M., Shen, J., Zhang, F., de Vries, W., 2019. Cropland acidification increases risk of yield losses and food insecurity in China. Environ. Pollut. 113145. https://doi.org/10.1016/j.envpol.2019.113145.

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Fig. 1 Change trends in relative root elongation of maize with the increase of HNO3 inputs in the soils containing Ca(OH)2 or biochar during simulated re-acidification. Fig. 2 Change trends of Evans blue uptake by maize root tips with the increase of HNO3 inputs in the soils containing Ca(OH)2 or biochar during simulated re-acidification.

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Fig. 3 Change trends of Al on maize root tips with the increase of HNO3 inputs in the soils containing Ca(OH)2 or biochar during simulated re-acidification.

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Fig. 4 Change trends in soil pH (red lines) and soluble Al (black lines) with the

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increase of HNO3 inputs in the soils containing Ca(OH)2 or biochar.

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Fig. 5 The content of dissolved organic carbon in the soil solutions.

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Fig. 6 The species of Al in the solution of Ultisol at pH=4.20, c(Al)=37.41 μM and

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Oxisol at pH 4.40, c(Al) = 79.63 μM. Different letters on the pillars show significant differences among the treatments (P < 0.05). The red letters mean the Al-DOM, and

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the black letters mean the sum of Al(OH)2+, AlOH2+, and Al3+. Fig. 7 Change trends in the soil exchangeable Al3+ with the increase of HNO3 inputs in the soils containing Ca(OH)2 or biochar during simulated acidification.

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Journal Pre-proof Declaration of competing interest The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

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Highlights

Biochar retarded the Al toxicity to plant during soil re-acidification

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Biochar decreased the concentration of soil active Al during soil re-acidification

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Biochar altered the distribution of Al speciation in soil solution

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Biochar presented a long lasting effect on alleviating Al toxicity in acidic soils

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 7