Effects of straw amendment on selenium aging in soils: Mechanism and influential factors

Science of the Total Environment 657 (2019) 871–881

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Effects of straw amendment on selenium aging in soils: Mechanism and influential factors Dan Wang a, Ming-yue Xue a, Ying-kun Wang a, De-zhi Zhou a, Li Tang a, Sheng-yan Cao a, Yu-hong Wei a, Chen Yang a, Dong-Li Liang a,b,⁎ a b

College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling, Shaanxi 712100, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The dynamical changes of fractionation and speciation of DOM bound Se in soils were investigated. • Straw amendment accelerated selenium retention and the reduction of hydrophilic acid bound Se. • FA and HON bound Se fraction reserved available Se while HA fraction fixed Se into soil solid phase. • EX-FA-Se indicates the soil Se availability and maintains the balance of Se within soil pools. • The acidic and phenolic functional groups were involved in DOM and Se interaction.

a r t i c l e

i n f o

Article history: Received 10 October 2018 Received in revised form 30 November 2018 Accepted 3 December 2018 Available online 04 December 2018 Editor: Filip M.G. Tack Keywords: Selenium Straw amendment DOM Aging Availability

a b s t r a c t Soil dissolved organic matter (DOM) alters heavy metal availability, but whether straw amendment can manipulate soil selenium (Se) speciation and availability through DOM mineralization remains unclear. In this study, allochthonous maize straw and selenate were incubated together in four different soils for 1 y. The transformation and availability of DOM associated Se (DOM-Se) was investigated during aging. Results indicated that soil solution and soil particle surfaces were dominated by hexavalent hydrophilic acid-bound Se (Hy-Se). The amount of fulvic acid bound Se in soil solution (SOL-FA-Se) was higher than humic acid bound Se in soil solution (SOLHA-Se), except in krasnozems, and mainly existed as hexavalent Se (Se(VI)). Tetravalent Se (Se(IV)) was the main valence state of FA-Se adsorbed on soil particle surfaces (EX-FA-Se) after 5 w of aging. The proportion of soil-available Se (SOL + EX-Se) decreased with increasing straw rate. However, under an application rate of 7500 kg·hm−2, soluble Se fraction (SOL-Se) reduction was minimal in acidic soils (18.7%–34.7%), and the organic bound Se fraction (OM-Se) was maximally promoted in alkaline soils (18.2%–39.1%). FA and HON could enhance the availability of Se in the soil solution and on particle surfaces of acidic soil with high organic matter content. While Se incorporation with HA could accelerate the fixation of Se into the solid phase of soil. Three mechanisms were involved in DOM-Se aging: (1) Reduction, ligand adsorption, and inner/outer-sphere complexation associated with the functional groups of straw-derived DOM, including hydroxyls, carboxyl, methyl, and aromatic

Abbreviations: SOL-Hy-Se, hydrophilic acid bound Se in soil solution; SOL-FA-Se, fulvic acid bound Se in soil solution; SOL-HA-Se, humic acid bound Se in soil solution; SOL-HON-Se, hydrophobic organic neutral bound Se in soil solution; EX-Hy-Se, hydrophilic acid bound Se adsorbed on soil particle surface; EX-FA-Se, fulvic acid bound Se adsorbed on soil particle surface; EX-HA-Se, humic acid bound Se adsorbed on soil particle surface; EX-HON-Se, hydrophobic organic neutral bound Se adsorbed on soil particle surface; OM-Hy-Se, hydrophilic acid bound Se in soil solid phase; OM-FA-Se, fulvic acid bound Se in soil solid phase; OM-HA-Se, humic acid bound Se in soil solid phase; OM-HON-Se, hydrophobic organic neutral bound Se in soil solid phase. ⁎ Corresponding author at: College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China. E-mail address: [email protected] (D.-L. Liang).

https://doi.org/10.1016/j.scitotenv.2018.12.021 0048-9697/© 2018 Elsevier B.V. All rights reserved.

872

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

phenolic compounds; (2) interconnection of EX-FA-Se between non-residual and residual Se pools; and (3) promotion by soil electrical conductivity (EC), clay, OM, and straw application. The dual effect of DOM on Se aging was highly reliant on the characteristics of the materials and soil properties. In conclusion, straw amendment could return selenium in soil and reduce soluble Se loss. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Selenium (Se) is a metalloid and variable element that exists mainly as Se(VI), Se(IV), Se(0), and organic/inorganic Se(−II) (Tamás et al., 2010). There is a narrow gap between Se deficiency and toxicity to living organisms, which is highly dependent upon the applied level and Se speciation in soils (Natasha et al., 2018). Most Se species combine with soil components in the liquid or solid phases, whereas Se(0) is incorporated into the soil residual pool (Wang et al., 2012). Water soluble (SOL-Se) and exchangeable (EX-Se) Se could be directly absorbed by plants, which is so-called available Se and mainly exists as inorganic Se(VI) and Se(IV) (Wang et al., 2017). Organic matter (OM) and bound Se (OM-Se) are made up of hydrophilic, hydrophobic, and humus compounds, among which hydrophilic acid-bound Se (Hy-Se) and fulvic acid-bound Se (FA-Se) could easily be mineralized into available Se. Thus, OM-Se is referred to as potential available Se (Qin et al., 2012; Supriatin et al., 2015), and the transformation of OM-Se in soil is crucial for evaluating Se bio-availability. Straw amendment not only alleviates the environmental pollution introduced by incineration but also enhances the available nutrient contents in soil. Thus, this technique is considered an environment-friendly agronomic strategy (Yemadje et al., 2017). Straw amendment coordinated with efficient cultivation and fertilization may also accelerate straw decomposition, promote seasonal crop growth, improve soil physical construction (e.g., porosity), change soil humus composition and enhance the activity of microorganisms and enzymes (Witt et al., 2000). Great amounts of Se were reduced from soil solution (92%–97% of the initial soluble Se) in seleniferous soil and transform Se(VI) and Se(IV) into Se(−II) without apparent changes in EX-Se after straw input (Arbestain, 1998). Zhang and Frankenberger (2003) also found that rice straw amendment assisted in reduction of Se(VI) to Se (0) and diminished Se availability. However, recent research demonstrated that increasing the DOM derived from straw returning can ameliorate soil Se availability and retard Se volatilization (Moreno-Jiménez et al., 2013). Soil dissolved organic matter (DOM) is a key factor affecting heavy metal mobility and availability (Kaiser et al., 2002). Straw decomposition mostly produces humic acid (HA) and aromatic compounds with phenolic, C_C, and C_O functional groups, which can form stable OM-bound heavy metals to reduce their availability (Kim et al., 2018). However, rapid degradation of OM can dissolve insoluble heavy metals, thereby improving their solubility and availability (Vink et al., 2017). The effect of straw amendment on heavy metal availability correlated with the solubility of released OM and its chelating functional groups (Albers et al., 2009). So the composition and functional groups of straw-derived DOM could be modified during exogenous Se aging, via different combination patterns. As a result, DOM released from straw decomposition bidirectionally affects Se availability in soil. Straw derived-DOM is composed of aromatic and aliphatic organic substances containing 25%–50% FA and HA. While the rest of its components include polyphenols, fatty acids, low-molecular weight free amino acids, and glycolic acids (Li et al., 2011). The associative ability of each DOM fraction could cause varied Se availability. According to the guidelines for isolation of OM recommended by International Humic Substances Society (IHSS), DOM can be fractionated into Hydrophilic acid (Hy), Fulvic acid (FA), Humic acid (HA), and Hydrophobic Organic Neutral materials (HON) (Thurman and Malcolm, 1981). Among

these four fractions, Hy is the dominant component, followed by FA, HON and HA (Ren et al., 2015; Supriatin et al., 2015). The Hy fraction contains low-molecular weight hydrophilic organic acids and amino acids and has strong chelating ability for heavy metals (Ren et al., 2015). In addition, the large amounts of amino and carboxyl groups and moderate amounts of aromatic C and phenol groups possessed by Hy promote the formation of Hy-Se, which could be mineralized to inorganic Se or directly absorbed by plants (Schneider et al., 2016). The FA fraction is the most important fraction in DOM, and the Se associated with this fraction can easily be mineralized to Se(VI), Se(IV), and the low-molecular weight organo-seleno compounds preferred by plants (Coppin et al., 2006; Bruggeman et al., 2007). The HON fraction includes aromatic hydrophobic neutral compounds, and Se incorporated with this fraction is highly mobile (Vink et al., 2017). Compared with the above fractions, the HA fraction is dominated by highmolecular weight aromatic OM with stable structures (Kim et al., 2018), leading to its preferential incorporation with Se(IV) (Martin et al., 2017; Peel et al., 2017). During aging, the FA and HA fractions diminish with time, whereas Hy and HON become dominant (Vink et al., 2017). However, the transformation of DOM-Se fractions with time is not fully explored and the contribution of DOM adsorbed on soil particle surfaces was not considered in the transformation of Se species. This is significant as OM primarily contributes to heavy metal availability in an exchangeable form (Zeng et al., 2011). Different soil physico-chemical properties, such as pH and OM content, could also influence the transformation of both the DOM fraction and associated Se species (Wang et al., 2017). Clarifying the dynamic transformation of Se species associated with DOM is the foundation for understanding the effect of straw amendment on Se availability. In this study, four distinct agricultural soils were selected, spiked with selenate, and amended with different levels of maize straw for 1 y (52 w) to illustrate (1) the binding capacity of different DOM fractions in the soil solution, soil particle surfaces, and soil solid phase; (2) the transformation of different DOM-bound Se species; and (3) the effect of straw-derived DOM on Se availability. Corresponding mechanisms were to be identified, as well as the main influential factors. 2. Materials and methods 2.1. Experimental materials Farmland soil samples were collected from the four provinces of Shaanxi (Lou soil- Eum-Orthic Anthrosol), Jiangxi (KransnozemsHaplic Acrisol), Jiangsu (Black grid soil- Hydragric Fluvisal) and Hebei (Fluvo-aquic soil- Haplic Cambisols). Samples were obtained from the surface layer (0–20 cm depth) with disparate physico-chemical properties (Table 1). Soil samples were air-dried, homogenized, and ground to pass through 0.25 and 0.149 mm sieves for physical and chemical analysis and 2 mm sieve for aging incubation. All the detailed analysis methods for physico-chemical properties of the soils were mentioned in a previous study (Wang et al., 2017). The Lou soil and fluvo-aquic soil were alkaline soils while the krasnozems and black grid soil were acidic soils. The maximum OM content was found in black grid soil (51.35 g·kg−1) and minimum in fluvo-aquic soil (6.40 g·kg−1). The greatest EC value was 263.0 s·m−1 (black grid soil) and minimum

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

873

Table 1 Physico-chemical property of the four different experimental soils. Soil types

pH

Carbonate Organic content (g kg−1) matter (g kg−1)

Amorphous Clay CEC (cmol EC (s Amorphous m−1) iron (g kg−1) aluminum (g kg−1) (%) kg−1)

Silt (%)

Ambient total Se Ambient (mg kg−1) available Se (mg kg−1)

Fluvo-aquic soil (Hebei) Lou soil (Yangling) Black grid soil (Jiangsu) Krasnozems (Jiangxi)

8.34 8.14 6.82 5.77

30.30 118.00 0.66 0.58

8.12 23.34 26.20 8.70

52.7 57.8 61.6 62.0

0.222 0.139 0.360 0.516

6.40 8.53 51.35 11.95

146.0 178.5 263.0 64.0

was 64.0 s·m−1 (krasnozems). More than 30% particles were clay in Lou soil and krasnozems but the minimum was only 11.1% in black grid soil. The maize straw was collected from farmland at Northwest A&F University. After water rinsing, the straw samples were air-dried, homogenized, and ground to pass through a 2-mm sieve for the incubation experiment. The water content of maize straw was 19%, Se content was 0.079 mg·kg−1, organic C was 431.46 g·kg−1, total N was 7.58 g·kg−1, C:N was 56.9. The cellulose, hemicellulose and lignin contents were 35.8%, 27.6% and 15.2%, respectively, in maize straw. 2.2. Experimental design Samples of each soil (2000 g equivalent dry mass) were spiked with 140 ml 15.7 mg Se l−1 inside PVC containers to apply the same Se concentration of 1 mg Se kg−1 soil. Three straw application rates were selected as 0, 7500 and 15,000 kg·hm−2. Selenate solution was spiked into soil and then straw was thoroughly mixed with soil and homogenized. Control samples were also prepared by spraying the same amount of distilled water into soils. All the treatments had triplicates and were maintained at 70% water holding capacity, during the whole incubation process. Urea was used to adjust the C:N ratio to 25:1. All soils were incubated under room temperature, and distilled water was added to maintain soil moisture every 2 or 3 days for 1 y. 100 g soils were sampled at different aging periods (0, 1, 2, 5, 9, 14, 26 and 52 weeks). Each sampling soil was air-dried, passed through 0.149 mm sieve for Se speciation analysis and 2 mm for DOM functional groups investigation. 2.3. Determine fractionation and speciation of DOM bound Se in soil The sequential extraction and DOM isolation method were applied for DOM bound Se fractionation and speciation analysis, according to Qin et al. (2012), Supriatin et al. (2015) and Thurman and Malcolm (1981). Briefly, soil samples were added into 100 ml centrifuge tube and extracted by different solutions (solid/liquid = 1:10) for each sequential extraction step. The Se fractions were divided into soluble (SOL), exchangeable (EX), organic matter-bound (OM), and residual (RES) fractions. The soil suspension was centrifuged at 4000 rpm for 20 min after each extraction. Then, DOM isolation was applied to all the three above mentioned extraction solutions. After being filtered through 0.45 μm microfiber filters (Millipore), 10 ml of extracted solution was analyzed for SOL-Se/ EX-Se/OM-Se, representing the sum of hydrophilic acid bound Se (HySe), fulvic acid bound Se (FA-Se), hydrophobic organic neutral bound Se (HON-Se) and humic acid bound Se (HA-Se) in each soil phase. The rest of the solution was firstly acidified by 6 M HCl to maintain pH to 1.0–1.5 overnight, then the HA was precipitated and isolated after centrifuging. The precipitated HA was redissolved by 0.1 M KOH for HA-Se (IV/VI) analysis. The Hy-Se, FA-Se and HON-Se were fractionated by XAD-8 resin (AMBERLITE XAD™, USA); the solution to resin ratio was 1:5 and FASe and HON-Se was adsorbed on resin after being shaken for 1 h at 220 rpm. The Hy-Se was isolated and analyzed for Se(VI), Se(IV) and Se(-II). Then 0.1 M KOH was applied for FA-Se desorption from resin, and analyzed for Se (VI) and Se (IV). The HON-Se was achieved through subtraction (HON-Se = Seextracted in solution − Hy-Se − FA-Se − HA-Se).

1.08 1.20 2.43 1.76

0.39 0.40 1.58 1.31

18.8 39.6 11.1 36.3

0.044 0.005 0.007 0.008

The whole extraction and isolation process was conducted for the samples gathered from weeks 0 and 52, the rest of them were determined with SOL-Se and EX-Se extraction and DOM isolation only. The three Se species (Se(VI), Se(IV), Se(-II)) were measured and calculated according to Wang et al. (2012). The Se(IV) concentration was directly detected from the extract. The Se(VI) concentration was measured after Se(VI) reduced to Se (IV) in 100 °C water bath with 6 M HCl solution (1:1), then subtracting the directly measured Se(IV) concentration from the total concentration of Se (VI + IV) detected after reduction. The Se (-II) concentration was determined by 5% K2S2O8 oxidation and same reduction process mentioned above to Se (VI) and total Se measured in solution deducts Se (IV + VI) was the Se (-II) contents. Total Se in soil was digested by HNO3:HClO4 (3:2) at 170 °C until clear and reduced to Se(IV) for analysis (Table S1). The Se concentration mentioned above was analyzed by an atomic fluorescence spectrophotometer (AFS-9780, Beijing Titan Instruments Co., Ltd.) with hydride generation according to the Chinese National Standard Method GB/T 5009.93-2017. The detailed extraction process was presented in Fig. S1. 2.4. Qualitative analysis of DOM functional groups by ATR-FTIR Three time points were selected for functional group of DOM analysis, including the beginning (0 w) and end (52 w) of aging as well as week 9 when the thriving straw decomposition period was finished (Cao et al., 2015). Both SOL-DOM and EX-DOM was extracted and the functional groups were identified by ATR-FTIR (Bruker Tensor 27, Germany). The control samples were also extracted for deduction as background. The spectra were recorded at 25 °C over a range of 4000–400 cm−1 with a resolution of 2 cm−1 and 64 scans. 2.5. Quality control and statistical analysis Multiple quality assurance and control means were conducted during sample preparation and chemical analysis, including replicates, certified reference materials for instrumental calibration, reagent blanks, and detection limit verification. The certified reference materials were analyzed with each batch of samples. The measured Se concentration in limestone soil (GBW07404, standard Se = 0.64 ± 0.18 mg kg−1) was 0.67 ± 0.01 mg kg−1, which was highly comparable with their certified values. The recovery of Se for sequential extraction was calculated based on the sum of Se fractions to the total Se content, which was 95.3–103.6%. Statistical analyses were conducted in SPSS 21.0 for one-way ANOVA (at significance level α = 0.05), ATR-FTIR data was analyzed by OMINIC 8.2, RDA analysis was conducted with Canoco 4.5 and diagraph analyses were applied by Origin Pro 9.2. 3. Results 3.1. Dynamic changes in the SOL-DOM-Se fraction during aging Compared with that in non-straw amendment treatments, more SOL-DOM-Se was transformed from Se(VI) into lower valence states after straw amendment. Owing to straw application and soil properties, the transformation duration and strength differed. SOL-Hy-Se was

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

dominated by Se(VI) during the entire aging period (minimum of Se(VI) in SOL-Hy-Se fraction at beginning and end of incubation were 65.66% and 64.76%, respectively) and gradually reduced to Se(IV) and Se(−II) (Fig. 1). The reductive amounts were positively correlated with the straw quantity. During the first 5 w of aging, the Se(VI) in the soil solution was mostly reduced to Se(−II) instead of other valence states, which might be related to the stimulation of microorganic activity in the first 5 w of straw decomposition (Yemadje et al., 2017); thus, the SOL-HySe(−II) contents peaked during this period and then gradually decreased, nearly disappearing after 14 w. In addition, the maximum production and duration of the SOL-Hy-Se (−II) fraction was found in black grid soil (205.44 μg·kg−1, under a straw application rate of 7500 kg·hm−2 at week 5), which contained the maximum autochthonous OM. By contrast, although SOL-Hy-Se (IV) showed continuous growth, its content in soil solution was not comparable with those of other valence states. The maximum SOL-HySe(IV) contents in the four soils showed the order black grid soil (5.83 μg·kg−1) N krasnozems (10.95 μg·kg−1) N Lou soil (13.33 μg·kg−1) N fluvo-aquic soil (37.21 μg·kg−1). Se(IV) began to increase from week 9 to week 26 and then decreased slightly in alkaline soils during aging; by contrast, this decrease occurred dramatically in acidic soils. The SOL-FA-Se and SOL-HA-Se fractions existed in the form of hexavalent Se (Figs. 2 and 3), and the dynamic change trend of SOLFA-Se with aging was consistent with that of SOL-Hy-Se. SOL-HA-Se contents increased with increasing Se(IV) and then declined after 9 w of incubation. Aging effects coordinated with increasing straw amounts could accelerate the reduction and incorporation of SOL-HA-Se into the soil solid phase. 3.2. Dynamic changes in the EX-DOM-Se fraction during aging The speciation transformation of DOM bound Se adsorbed onto soil particle surfaces (EX-DOM-Se) was significantly different from that of

SOL-Hy-Se content (µg kg -1 )

800

(a) Lou soil

0

7500

15000

Se(VI)

400 0 16

Se(IV)

8 0 210

DOM-Se in soil solution (Figs. 4 and 5). After Se(VI) had completely disappeared in the EX-Hy-Se fraction at week 9, EX-Hy-Se(IV) became the major form in soil. However, more EX-Hy-Se(-II) existed within the first 5 w of aging, similar to that in soil solution, which indicates that organoseleno compounds (Se(−II)) degraded by microorganisms were adsorbed on soil particle surfaces from the soil solution. Soils with more autochthonous OM are preferred by microorganism; hence, krasnozems generated the highest EX-Hy-Se(-II) (229.62 μg·kg−1), followed by black grid soil (204.20 μg·kg−1), and fluvo-aquic soil (107.98 μg·kg−1) was the minimum. The major production of EX-Hy-Se(−II) was found under an application rate of 15,000 kg·hm−2 straw in alkaline soils; in acidic soils 7500 kg·hm−2 straw amendment was sufficient for maximum production of EX-Hy-Se(−II). Similar to the soil solution, solid soil components show much higher relative affinity in acidic soils than in alkaline soils. Nevertheless, EX-Hy-Se(IV) showed distinct dynamic patterns in the four soils. The concentration of EX-Hy-Se(IV) increased with increasing straw added to Lou soil and krasnozems, and this increase occurred continuously until week 26. The maximum EX-Hy-Se(IV) content was found in black grid soil (5th week) and fluvo-aquic soil (9th week) under 7500 kg·hm−2 straw treatment, and this amount remained constant until the end of aging. Fluvo-aquic soil adsorbed most of the EXHy-Se(IV) (226. 38 μg·kg−1) on its particle surfaces, which was 6-fold higher than the minimum amount of EX-Hy-Se(IV) (37.70 μg·kg−1) observed in black grid soil. The EX-FA-Se fraction mainly existed in the hexavalent state at the first 5 w of aging (only the first week for krasnozems) and was dominated by EX-FA-Se(IV) afterwards. Straw amendment accelerated the transformation of EX-FA-Se, and EX-FA-Se(IV) contents were the highest under the 15,000 kg·hm−2 straw application. In fluvo-aquic soil, the maximum reduction was achieved at an application rate of 7500 kg·hm−2 straw. Hexavalent Se disappeared within 9 w in acidic soils but was sustained for N14 w in alkaline soils, indicating a more rapid reduction rate for EX-FA-Se in the former rather than in the latter

Se(-II)

140

800

SOL-Hy-Se content (µg kg -1 )

874

(b) Krasnozems

15000

Se(VI)

400 0 16

Se(IV)

8 0 210

Se(-II)

140

0

0 012 5

9

14

26

012 5

52

9

14

0

7500

15000

Se(VI)

400 0 16

Se(IV)

8 0 210

Se(-II)

140

800

SOL-Hy-Se content (µg kg-1)

(c) Black grid soil

26

52

Time (w)

Time (w)

SOL-Hy-Se content (µg kg-1)

7500

70

70

800

0

(d) Fluvo-aquic soil

0

7500

15000

Se(VI)

400 0 40

Se(IV)

20 0 210

Se(-II)

140 70

70

0

0 012 5

9

14

26

Time (w)

52

012 5

9

14

26

52

Time (w)

Fig. 1. Concentration variations (μg·kg−1) of SOL-Hy-Se with different valent states including Se(IV), Se(VI) and Se(-II) in soil solution from (a) Lou soil (b) Krasnozems (c) Black grid soil and (d) Fluvo-aquic soil under different straw application rates during 52 w incubation.

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

875

Fig. 2. Concentration variations (μg·kg−1) of SOL-FA-Se with different valent states including Se(IV) and Se(VI) in soil solution from (a) Lou soil (b) Krasnozems (c) Black grid soil and (d) Fluvo-aquic soil under different straw application rates during 52 w incubation.

soils. EX-HA-Se was not gathered from this isolation process when precipitation was applied. 3.3. Variation of the distribution of DOM-Se species in soils during aging Through sequential extraction and DOM isolation, the distribution of DOM-Se species before incubation and after aging (weeks 0 and 52) in the three non-residual soil pools (soil solution, particle surface, and solid phase) and residual soil pool was determined; results are presented in Fig. 6. At the beginning of aging, Hy-Se was the major fraction in the three non-residual soil pools, followed by FA-Se, except in krasnozems, which contained more SOL-HON-Se than SOL-FA-Se. The proportion of DOMSe in the three soil pools could be ranked as follows: SOL-(Hy, FA, HON, HA)-Se N EX-(Hy, FA, HON)-Se N OM-(Hy, FA, HON, HA)-Se. By the end of aging (week 52), the proportion of low-molecular weight DOM-Se (Hy-Se and FA-Se) decreased with straw amendment in alkaline soils. The greatest decline in SOL-Hy-Se was found in fluvo-aquic soil (37.9%–43.2%), while that in SOL-FA-Se was found in Lou soil (10.8%–15.0%). By contrast, the proportion of aromatic SOL-HON-Se increased with aging, and the maximum growth was found in black grid soil (21.3%–30.2%). Owing to the stable structure of SOL-HA-Se, its proportion to total Se significantly decreased as it moved from the soil solution to the solid phase. EX-Hy-Se increased in the soil particle surface in fluvo-aquic soil and krasnozems (5.9%–6.9%) but decreased in Lou soil and black grid soil. The EX-FA-Se fraction fluctuated during the aging process in all soils. The proportion of EX-HON-Se generally decreased with increasing straw amendment except in black grid soil. No significant difference was found among straw application rates, and EX-HON-Se increased

by 8.7%–10.8% in total Se. The proportion of OM-Se was highly dependent on soil pH. The proportion of OM-Hy-Se and OM-FA-Se in total Se increased by 6% in alkaline soils, whereas that of aromatic OM-HASe and OM-HON-Se was enhanced by 2%–5% in acidic soils. By the end of aging, the proportion of SOL-DOM-Se to total Se declined with increasing straw amendment. The maximum reduction was found in fluvo-aquic soil and the minimum was observed in black grid soil. However, amendment with 7500 kg·hm−2 straw preserved SOL-DOM-Se in acidic soils, and the reduction rate was 18.7% and 34.7% in black grid soil and krasnozems, respectively, which was relatively less than that observed under the two other straw application rates (the reduction of SOL-DOM-Se N21% in black grid soil and N36% in krasnozems under both 0 and 15,000 kg·hm−2 straw rates). EX-DOM-Se decreased in Lou soil and krasnozems in conjunction with more straw input. By contrast, the maximum growth of this fraction was found under an application rate of 7500 kg·hm−2 in black grid soil and fluvo-aquic soil. The proportion of OM-Se in the soil solid phase was positively correlated with the straw application rate in acidic soil. Conversely, the greatest OM-Se proportion was found under 7500 kg·hm−2 straw input in Lou soil (18.2%) and fluvo-aquic soil (39.1%). Straw amendment also promoted the formation of the RES-Se fraction in soils. The maximum RES-Se fraction was found under 15,000 kg·hm−2 straw input for all four tested soils. 3.4. Effects of different DOM functional groups on the transformation of Se species ATR-FTIR results for soils sampled at weeks 1, 9, and 52 showed absorption bands of SOL-DOM occurring at 2993.91–3040.61 cm−1 (aliphatic hydroxyl group, \\OH), 1683.29–1922.60 cm−1 (saturated and

876

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

Fig. 3. Concentration variations (μg·kg−1) of SOL-HA-Se with different valent states including Se(IV) and Se(VI) in soil solution from (a) Lou soil (b) Krasnozems (c) Black grid soil and (d) Fluvo-aquic soil under different straw application rates during 52 w incubation.

unsaturated aliphatic carboxyl groups,\\C_O), 1346.56–1398.67 cm−1 (methyl group,\\CH3) and 1061.06–1236.22 cm−1 (aromatic\\C\\O) (Table 2). After background adjustment, non-straw amendment treatment only revealed one peak between 965.06 and 994.35 cm−1 , which was assigned to outersphere adsorption of Se(VI) (Liu et al., 2018). The peak absorbance of aromatic C\\O groups increased with increasing incubation time in acidic soils, indicating the transformation of aliphatic DOM-Se to aromatic DOM-Se. By contrast, no apparent transformation pattern was found for aromatic C\\O groups in alkaline soils, where Se is predominantly combined with aliphatic hydroxyl groups. Table 3 shows the peak absorbance of EX-DOM. Most of the functional groups of EX-DOM were similar to those of SOL-DOM, although one new peak at 2332.7–2368.98 cm−1 (\\C`/_C_) was found. In addition, the absorption of methyl groups was displaced from 1375 to 1460 cm−1. These results clearly illustrated the high isomerization of EX-DOM-Se, structural rearrangement of hydrocarbon molecules, and transference of \\CH3 and _C_ when Se was coupled to EX-DOM (Caporale et al., 2018). The dominant peak of aromatic C\\O at 1150–1220 cm−1 indicated a phenol and ether C\\O was the preferential binding site for Se. The EX-DOM extracted from non-straw amendment soil also revealed peaks for carboxyl, hydroxyl, phenol, and ether C\\O groups, indicating the intermediate role of EX-DOM on Se transformation from outer-sphere absorption to inner-sphere absorption. _C_ was converted to\\C` in EX-DOM from Lou soil and black grid soil with higher OM content compared with the two other soils within the same pH range. This transformation verified that soil solid OM facilitated DOMSe conversion into more stable fractions (Li et al., 2017).

3.5. Availability of DOM-Se and its main influencing factors The Redundancy analysis (RDA) of the variance of soil DOM-Se as well as available Se before and after aging was conducted to determine the correlation between DOM-Se species and available Se with influential factors (Fig. 7). The soil physico-chemical properties and straw application rates were also studied to elucidate factors influencing Se availability. After 1 yr of aging, the FA-Se in soil solution (SOL), particle surface (EX), and solid phase (OM) was significantly correlated with soil available Se; this finding confirmed that FA-Se is the immediate source of available Se in the three soil phases. Moreover, the minimum inclined angle between EX-FA-Se and available Se confirmed the supplementation from EX-FA-Se to available Se. Several soil properties affect Se availability; in particular, soil pH, EC, clay contents, and straw application rates played vital roles on selenate aging in different soils (the red background). The clay contents contributed to Se adsorption and fixation in soil particle surface and solid phase as EX-Se and OM-Se (the first quadrant with blue background). But the mobility of Se was highly dependent on the EC, CEC, OM content and amorphous Fe/Al (the fourth quadrant with red background), which might coincide with the outer-sphere electrostatic adsorption of Se (VI) (Yu et al., 2018).

4. Discussion 4.1. The speciation transformation of DOM-Se during aging The speciation transformation of DOM-Se was primarily depended on the composition of straw derived DOM. Maize straw contained

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

(a) Lou soil

0

7500

15000

40 0 100

Se(IV)

50 0 210

80

Se(VI)

Se(-II)

140

EX-Hy-Se content (µg kg -1 )

EX-Hy-Se content (µg kg -1 )

80

70

(b) Krasnozems

7500

15000

Se(VI)

40 0 Se(IV)

100 50 0 210

Se(-II)

140

0 012 5

9

14

26

52

012 5

9

14

Time (w) (c) Black grid soil

0

7500

15000

Se(VI)

40 0 Se(IV)

100 50 0 210

26

52

Time (w)

Se(-II)

140 70

80

EX-Hy-Se content (µg kg-1)

EX-Hy-Se content (µg kg-1)

0

70

0

80

877

(d) Fluvo-aquic soil

0

7500

15000 Se(VI)

40 0 Se(IV)

100 50 0 210

Se(-II)

140 70

0

0 012 5

9

14

26

52

Time (w)

012 5

9

14

26

52

Time (w)

Fig. 4. Concentration variations (μg·kg−1) of EX-Hy-Se with different valent states including Se(IV), Se(VI) and Se(-II) on soil particles from (a) Lou soil (b) Krasnozems (c) Black grid soil and (d) Fluvo-aquic soil under different straw application rates during 52 w incubation.

large amounts of lignin, cellulose and hemicellulose, which could be rapidly decomposed and release low-molecular weight organic acids in the Hy fraction. The latter was dominated by oxalic acid (carboxyl + hydroxyl) (Cao et al., 2015). These organic acids constituted a large quantity of reductive functional groups, such as hydroxyl groups, carboxyl (C_O) groups, methyl groups, and C`/_C_ (Tables 2 and 3), which could directly reduce or supply an electron shuttle to microorganisms to reduce exogenous Se(VI) through detoxification or respiration (Bauer and Blodau, 2006). Soluble or adsorbed Se(VI) could be converted into lower valence states, including Se(IV), Se(−II) (Figs. 1 and 4), and Se(0) (Fig. 6), which coincided with the results observed by Fan et al. (2018) and Zhang and Frankenberger (2003). Moreover, a higher input of straw, which meant more acidic reductive functional groups, promoted the reduction of available Se in the four soils (Table S2) into OM-Se and RESSe fractions. This was more apparent in alkaline soils than in acidic soils. Unlike Hy fraction, FA consists of low-molecular weight aliphatic carbon chains and smaller degrees of aromatic rings (Kang et al., 2011), and HA is dominated by aromatic C\\O and highly aromatized (Zhang et al., 2018). The reductive ability of HA is greater than that of FA because the former contains more aromatic rings with stronger reducing capacity than aliphatic compounds (Bauer and Blodau, 2006). Thus it is more receptive to electrons than FA (Kim et al., 2018). Therefore, a clear transformation from aliphatic to aromatic DOM is found in acidic soils with large amounts of HA. However, the direction of Se transformation contrasted in soils with difference acidities under different OM levels. Reduced Se was incorporated into the soil solid phase in krasnozems through complexation and precipitation in the forms of Se(IV), Se(0), and Se(−II, undissolved selenide, e.g., FeSe). Inorganic Se in black grid soil was coupled to HON compounds or reduced to Se(−II, organo-seleno compounds), which could be preserved in the soil solution or adsorbed onto the soil particle surface (Zhang et al., 2018). Additionally, aromatic lignin compounds

were the major products at the late stage of straw decomposition (Yemadje et al., 2017); thus, the greatest straw application rate 15,000 kg·hm−2 resulted in the highest proportion of RES-Se by the end of aging. Se(VI) remained the predominant species in the soil solution because the reduction of Se(VI) to Se(IV) was the rate-limiting step, and the rate of this step was only 1/4 that of the reduction from Se(IV) to Se(0) (Zhang and Frankenberger, 2003). A dramatic valence shift occurred in the EX-DOM-Se adsorbed on soil particle surfaces, which was attributed to the effects of microorganisms and organic acids. EX-Hy-Se was transformed to Se(−II) through microorganic respiration during the early aging period (0–5 w), during which microorganic activity was stimulated (Wang et al., 2018). This result was in conflict with the finding that Se(IV) was the main valence state in EX-Se form (Wang et al., 2012; Li et al., 2016). Since the microorganic transformation of Se occurred at the early stage of aging, EX-Hy-Se and EX-FA-Se still existed as Se(IV) after 9 weeks of aging (Figs. 4 and 5), due to a large amount of reductive organic acids released at the straw decomposition stage (5–9 w, Cao et al., 2015). This resulted in Se(VI) being entirely converted into Se(IV) on the soil particle surface. 4.2. Mechanism of Se(VI) aging with straw amendment The aging effect on soil Se speciation/bioavailability could be attributed to the biotic or abiotic reactions of OM in the soil solution and solid phase (Li et al., 2017). Adsorption–desorption is the predominant mechanism influencing heavy metal mobility and availability (Zeng et al., 2011). In soil amended with straw, Se(VI) was initially adsorbed through outer-sphere electrostatic attraction. Hence, the Elovich model can well fit the aging process (Table S2), and the equilibrium time (14 w, Fig. S2) was consistent with the previous finding of 109 d (Wang et al., 2017). Favorito et al. (2018) discovered that low-molecular weight DOM could inhibit Se(VI) adsorption and enhance its solubility under

878

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

Fig. 5. Concentration variations (μg·kg−1) of EX-FA-Se with different valent states including Se(IV) and Se(VI) on soil particles from (a) Lou soil (b) Krasnozems (c) Black grid soil and (d) Fluvo-aquic soil under different straw application rates during 52 w incubation.

acidic conditions. By contrast, neutralization of OH– by straw-released organic acid in alkaline soil could reduce competing ions and adsorb more Se (Li et al., 2018). This mechanism could explain the higher aging rate of Se in alkaline soils than in acidic soils after straw input (Table S2).

Aliphatic hydroxyl and aromatic carboxyl groups also play a significant role in DOM adsorption (Vink et al., 2017. Particularly in the black grid soil with high autochthonous OM content, the priming effect induced by crop decomposition may increase the mineralization of

100%

RE-Se OM-HON-Se OM-HA-Se

80%

OM-FA-Se OM-Hy-Se

60% EX-HON-Se EX-FA-Se 40%

EX-Hy-Se SOL-HON-Se

20%

SOL-HA-Se

SOL-FA-Se SOL-Hy-Se

0% W0

W52 0

W0

W52

7500 Lou soil

W0

W52

15000

W0

W52 0

W0

W52

7500 Krasnozems

W0

W52

15000

W0

W52 0

W0

W52

7500 Black grid soil

W0

W52

15000

W0

W52 0

W0

W52

7500

W0

W52

15000

Fluvo-aquic soil

Fig. 6. The proportion of different DOM bound Se fractions occupied total Se content in four selenate treated soils under different straw application rates at beginning (0 w) and end of incubation (52 w).

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

879

Table 2 ATR-FTIR results of different functional groups associated with Se and Se(VI) in SOL-DOM extracted from the four different soils at week 0, 9 and 52. Species

Bands position (cm−1)

Straw (kg/hm2−)

0

Time (W)

0

9

52

0

9

52

0

9

52

– – – – 982.26 – – – – 983.88 – – – – 983.97 – – – – 985.24

– – – – 980.87 – – – – 985.23 – – – – 982.73 – – – – 984.33

– – – – 984.96 – – – – 984.33 – – – – 980.35 – – – – 984.05

3019.01 1690.46 1387.81 1061.91 986.65 3017.53 1880.26 1358.81 1061.90 985.49 3014.90 1876.51 1346.56 1062.03 986.63 3017.84 1878.79 1355.76 1062.15 978.96

3017.36 1706.30 1398.67 1062.31 979.47 3013.56 1887.02 1363.24 1175.90 983.43 3015.26 1889.33 1351.48 1122.11 986.63 3016.43 1890.23 1362.34 1063.38 982.38

3019.46 1683.29 1388.76 1062.11 965.06 3017.06 1875.54 1344.01 1236.22 978.28 3016.23 1880.08 1357.26 1138.16 986.62 3014.56 1880.08 1350.16 1062.06 983.10

3040.61 1894.17 1362.79 1060.92 984.54 3013.78 1922.60 1361.35 1081.29 985.44 3013.07 1880.13 1360.91 1062.24 983.71 3013.92 1878.64 1360.68 1061.43 982.86

3030.55 1881.76 1382.41 1061.21 994.35 3003.27 1910.32 1363.43 1213.43 982.34 3008.37 1888.24 1367.87 1172.11 986.67 3015.32 1885.94 1373.42 1065.88 980.67

2993.91 1895.38 1360.92 1061.48 979.78 2994.95 1880.21 1360.42 1215.88 978.67 2995.23 1895.83 1360.97 1201.53 994.76 3013.56 1894.79 1359.76 1061.06 979.42

Lou soil

Krasnozems

Black grid soil

Fluvo-aquic soil

νO\ \H νC_O δCH3 ν + δC\ \O Se (VI) νO\ \H νC_O δCH3 ν + δC\ \O Se (VI) νO\ \H νC_O δCH3 ν + δC\ \O Se (VI) νO\ \H νC_O δCH3 ν + δC\ \O Se (VI)

7500

“existing OM” to release more labile DOM (Yemadje et al., 2017), thereby promoting the transfer of hydroxyl groups from the solid to the liquid phase and promoting competitive adsorption with Se (Constantino et al., 2017). Therefore, the proportion of SOL-DOM-Se in black grid soil at the end of aging remained higher than that in other soils. Based on DOM fractionation in the soil solution (Supriatin et al., 2015), this study further explored the idea that exchangeable DOM as OM is a major contributor to the ability of soils to retain heavy metals in an exchangeable form (Zeng et al., 2011). A previous study demonstrated that exchangeable Se is more relevant to plant Se content in soils with high OM content than in those with low OM content (Wang et al., 2018). In addition, DOM could improve anion availability through ligand-exchange reactions and compete with Se for (hydr)oxide

15,000

surfaces (Favorito et al., 2018). Among the different DOM fractions, FA is the most active in humus and can bind Se on soil particle surfaces via ligand-exchange reactions with carboxyl groups and hydroxyls (Caporale et al., 2018). Therefore, the variation of EX-FA-Se is an optimal indicator for Se availability (Fig. 7). Owing to electron-donor isolation in the ligand-exchangeable layer, EX-FA-Se could be preserved as Se(VI) to maintain balance between the soil solution and solid phase (Zhang et al., 2018). Besides adsorption, reduction, and ligand exchange, complexation predominantly determined the effect of DOM on metal availability (Bauer and Blodau, 2006; Zhang et al., 2018). Acidic functional groups, including carboxyl, carbonyl and hydroxyls, are known to complex with Se (Kim et al., 2018). The FA fraction contained a large number of acidic functional groups and is highly capable of combining with Se

Table 3 ATR-FTIR results of different functional groups associated with Se and Se(VI) in EX-DOM extracted from the four different soils at week 0, 9 and 52. Species Straw (kg/hm2−)

0

Time (W) Lou soil

Krasnozems

Black grid soil

Fluvo-aquic soil

Bands position (cm−1)

νO\ \H νC`/_C_ νC_O δCH3 ν + δC\ \O Se (VI) νO\ \H νC`/_C_ νC_O δCH3 ν + δC\ \O Se (VI) νO\ \H νC`/_C_ νC_O δCH3 ν + δC\ \O Se (VI) νO\ \H νC`/_C_ νC_O δCH3 ν + δC\ \O Se (VI)

7500

15,000

0

9

52

0

9

52

0

9

52

2933.89 – – – 1182.46 1006.70 – 2260.96 – – 1176.89 1003.88 – 2243.88 – – 1176.09 1003.97 – 2260.99 – – 1176.42 1005.24

2993.41 2269.78 – – 1184.74 1005.31 2938.74 2243.34 – – 1178.32 1005.66 2933.56 2268.98 – – 1179.34 1000.23 2946.31 2246.83 – – 1180.98 1004.23

– 2262.54 – – 1176.35 1004.27 2993.91 2242.98 – – 1175.88 1004.00 2994.95 2265.75 – – 1178.39 1000.35 3001.34 2243.96 – – 1181.51 1004.05

3014.98 2244.17 1876.90 1433.17 1176.26 1009.83 2932.41 2363.22 1895.79 1459.65 1175.94 1000.07 3012.21 2243.59 1909.99 1457.93 1176.70 1001.29 3015.77 2260.73 1876.83 1432.60 1176.40 1002.24

2934.33 2243.77 1833.78 1432.81 1178.23 1009.56 2995.47 2246.73 1880.54 1462.79 1180.87 990.36 3023.77 2268.98 1900.12 1448.33 1178.35 995.86 3013.21 2250.38 1883.21 1446.77 1188.73 1003.45

3015.38 2261.11 1894.97 1433.78 1179.89 1008.16 3013.28 2243.38 1886.64 1459.38 1176.22 1007.02 3015.19 2261.17 1894.30 1431.83 1176.16 1009.60 3014.18 2243.55 1894.36 1433.22 1181.78 1004.83

– 2243.78 1894.17 1433.11 1181.29 1004.54 – 2261.95 1922.60 1461.35 1176.92 1005.44 – 2244.03 1880.13 1460.91 1176.11 1003.71 2994.29 2260.87 1878.64 1440.13 1176.64 1002.68

2943.56 2268.98 1843.21 1433.83 1178.27 1003.76 2938.79 2251.65 1900.33 1483.49 1182.98 1000.34 2938.22 2265.98 1893.78 1454.97 1173.77 1001.11 3014.81 2234.33 1895.38 1463.22 1180.74 992.12

2993.91 2262.98 1895.38 1431.80 1175.88 1004.00 2994.95 2243.78 1880.21 1460.42 1176.50 978.67 2995.23 2260.68 1895.83 1433.91 1176.36 994.76 3013.56 2232.76 1894.79 1435.37 1176.60 979.42

880

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881

Consequently, the effect of straw amendment on Se aging was also influenced by soil pH, EC, clay, and OM under different soil textures (Fig. 7). As such, the observation could be expected that the most extensive Se reduction was found in the fluvo-aquic soil with the highest pH, because alkaline conditions facilitate DOM release (Li et al., 2018). By contrast, owing to the great buffering capability of the black grid soil with high OM content, straw amendment did not substantially elevate the aging rate (Table S2) but enhanced the proportion of HON-Se in the soil solution, thereby preserving the available Se. 5. Conclusions

Fig. 7. The contribution of different DOM bound Se fractions to available Se in soils and their influential factors. The red arrows with red words indicated all the environmental influential factors in soil referred as environment and the blue arrows with black italic words were all the DOM-Se fractions referred as species in RDA analysis. The background color in red indicated a closer relationship with available Se both for influential factors and Se fractions.

(Kang et al., 2011). The preferential combination of FA with Se resulted in a greater quantity of FA-Se than HA-Se in both soil solution (SOL) and solid phase (OM), thereby confirming its conspicuous role in Se availability. Se(VI) is mainly incorporated into soil via outer-sphere complexation, whereas Se(IV) is added to soil through inner-sphere complexation (Wang et al., 2017; Li et al., 2016). According to symmetry theory, the peak shift of Se(VI) (from 980 cm−1 in SOL-DOM to 1000 cm−1 in EX-DOM) indicates bidentate binuclear inner-sphere complexation during aging (Constantino et al., 2017; Favorito et al., 2018), which parallels the transformation of SOL-DOM-Se to EX-DOM-Se. Compared with that in SOL-DOM, the C\\H deformation vibration (methylation) in EX-DOM also contributed to Se complexation, and the aromatic functional groups immobilized more Se than in the soil solution (Se(IV) having one long-pair electron more than Se(VI)). Thus, EX-DOM is the key factor controlling Se availability, and the combination between EX-FA and Se determines Se availability in different soils.

Based on previous findings on Se aging, this study further explores the effect of allochthonous OM on exogenous Se retention in soils. The organic acids released from straw decomposition contained large amounts of functional groups with strong reducibility, which promotes the reduction of Se(VI) to Se(IV) and lower valence states (0 and –II). Besides transformation of Se species during the active microbial period (0–5 weeks) and straw decomposition period (5–9 weeks), adsorption, ligand-exchange reactions, and inner-outersphere complexation of DOM-Se also occurred in the soil solution, particle surface, and solid phase. Phenol hydroxyl, methyl, and carboxyl groups were involved in the above-mentioned reactions. EX-FA-Se connected the non-residual and residual Se pools through replenishment of the available Se pool. This process was highly influenced by soil pH, EC, clay, and OM contents. Meanwhile, straw application rates determined the effects of DOM on Se species transformation and the soil micro-environment. In this study, Se could be mostly preserved in the available or potentially available Se pool under 7500 kg·hm−2 application rate, which also mitigates the risk of Se leaching (compared with the 0 kg·hm−2 treatment) or losses to the residual pool (compared with the 15,000 kg·hm−2 treatment). Nevertheless, further verification of the related mechanisms under plant growth is necessary to evaluate the effect of straw amendment on Se speciation transformation and availability during aging. Acknowledgments The authors thank for the financial support provided by the National Natural Science Foundation of China (No. 41571454 to D.L. Liang). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.12.021.

4.3. The influential factors on Se availability

References

Soil pH was the most significant factor influencing Se availability according to RDA analysis. Having pH between 7 and 11 was the most suitable condition for microorganism reduction in soil (LipczynskaKochany, 2018), which explained the drastic reduction of Se(VI) in alkaline soil after straw amendment (Fig. 1). The straw application rate also was another important factor influencing Se availability, because the decomposition of straw could release organic acid to the soil micro-environment. This caused pH fluctuations, either increasing or decreasing, (Shi et al., 2018) and accelerated soil EC change, as well as microorganism respiration-induced transformation between electron donor and acceptor (reductive capability) (Stern et al., 2018). The higher the straw input, the more extensive the Se reduction to lower valence state and its incorporation into the soil solid phase (Fig. 6). Soil EC determined the outer-sphere adsorption and complexation of Se(VI) with soil particles (Wang et al., 2017). Whereas clay and OM contents affected the inner-sphere complexation of Se(VI) and its incorporation into the soil solid phase after reduction (Li et al., 2016).

Albers, C.N., Banta, G.T., Hansen, P.E., Jacobsen, O.S., 2009. The influence of organic matter on sorption and fate of glyphosate in soil-comparing different soils and humic substances. Environ. Pollut. 157, 2865–2870. https://doi.org/10.1016/j. envpol.2009.04.004. Arbestain, M.C., 1998. Effect of straw amendment and plant growth on selenium transfer in a laboratory soil-plant system. Revue Canadienne De La Science Du Sol 78, 187–195. Bauer, M., Blodau, C., 2006. Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments. Sci. Total Environ. 354, 179–190. https://doi.org/10.1016/ j.scitotenv.2005.01.027. Bruggeman, C., Maes, A., Vancluysen, J., 2007. The interaction of dissolved boom clay and gorleben humic substances with selenium oxyanions (selenite and selenate). Appl. Geochem. 22, 1371–1379. https://doi.org/10.1016/j.apgeochem.2007.03.027. Cao, Y., Zhang, H., Liu, K., Dai, Y., Lv, J., 2015. Organic acids variation in plant residues and soils among agricultural treatments. Agron. J. 107, 2171–2180. https://doi.org/ 10.2134/agronj15.0137. Caporale, A.G., Adamo, P., Azam, S., Rao, M.A., Pigna, M., 2018. May humic acids or mineral fertilization mitigate arsenic mobility and availability to carrot plants (Daucus carota L.) in a volcanic soil polluted by as from irrigation water? Chemosphere 193, 464–471. https://doi.org/10.1016/j.chemosphere.2017.11.035. Constantino, L.V., Quirino, J.N., Monteiro, A.M., Abrão, T., Parreira, P.S., Urbano, A., Santos, M.J., 2017. Sorption-desorption of selenite and selenate on Mg-Al layered double hydroxide in competition with nitrate, sulfate and phosphate. Chemosphere 181, 627–634. https://doi.org/10.1016/j.chemosphere.2017.04.071.

D. Wang et al. / Science of the Total Environment 657 (2019) 871–881 Coppin, F., Chabroullet, C., Martin-Garin, A., Balesdent, J., Gaudet, J.P., 2006. Methodological approach to assess the effect of soil aging on selenium behavior: first results concerning mobility and solid fractionation of selenium. Biol. Fertil. Soils 42, 379–386. https://doi.org/10.1007/s00374-006-0080-y. Fan, J., Zeng, Y., Sun, J., 2018. The transformation and migration of selenium in soil under different Eh conditions. J. Soil. Sediment., 1–13 https://doi.org/10.1007/s11368-0181980-9. Favorito, J.E., Eick, M.J., Grossl, P.R., 2018. Adsorption of selenite and selenate on ferrihydrite in the presence and absence of dissolved organic carbon. J. Environ. Qual. 47, 147–155. https://doi.org/10.2134/jeq2017.09.0352. Kaiser, K., Guggenberger, G., Haumaier, L., Zech, W., 2002. The composition of dissolved organic matter in forest soil solutions: changes induced by seasons and passage through the mineral soil. Org. Geochem. 33, 307–318. https://doi.org/10.1016/ S0146-6380(01)00162-0. Kang, J., Zhang, Z., Wang, J.J., 2011. Influence of humic substances on bioavailability of Cu and Zn during sewage sludge composting. Bioresour. Technol. 102, 8022–8026. https://doi.org/10.1016/j.biortech.2011.06.060. Kim, H.B., Kim, S.H., Jeon, E.K., Kim, D.H., Tsang, D.C., Alessi, D.S., Kwon, E.E., Baek, K., 2018. Effect of dissolved organic carbon from sludge, Rice straw and spent coffee ground biochar on the mobility of arsenic in soil. Sci. Total Environ. 636, 1241–1248. https://doi.org/10.1016/j.scitotenv.2018.04.406. Li, T., Di, Z., Yang, X., Sparks, D.L., 2011. Effects of dissolved organic matter from the rhizosphere of the hyperaccumulator sedum alfredii, on sorption of zinc and cadmium by different soils. J. Hazard. Mater. 192, 1616–1622. https://doi.org/10.1016/j. jhazmat.2011.06.086. Li, J., Peng, Q., Liang, D., Liang, S., Chen, J., Sun, H., Li, S., Lei, P., 2016. Effects of aging on the fraction distribution and bioavailability of selenium in three different soils. Chemosphere 144, 2351–2359. https://doi.org/10.1016/j.chemosphere.2015.11.011. Li, Z., Liang, D., Peng, Q., Cui, Z., Huang, J., Lin, Z., 2017. Interaction between selenium and soil organic matter and its impact on soil selenium bioavailability: a review. Geoderma 295, 69–79. https://doi.org/10.1016/j.geoderma.2017.02.019. Li, G., Khan, S., Ibrahim, M., Sun, T.R., Tang, J.F., Cotner, J.B., Xu, Y.Y., 2018. Biochars induced modification of dissolved organic matter (DOM) in soil and its impact on mobility and bioaccumulation of arsenic and cadmium. J. Hazard. Mater. 348, 100–108. https://doi.org/10.1016/j.jhazmat.2018.01.031. Lipczynska-Kochany, E., 2018. Humic substances, their microbial interactions and effects on biological transformations of organic pollutants in water and soil: a review. Chemosphere 202, 420–437. https://doi.org/10.1016/j.chemosphere.2018.03.104. Liu, J., Zhu, R., Liang, X., Ma, L., Lin, X., Zhu, J., He, H., Parker, S.C., Molinari, M., 2018. Synergistic adsorption of Cd(II) with sulfate/phosphate on ferrihydrite: an in situ ATRFTIR/2D-COS study. Chem. Geol. 477, 12–21. https://doi.org/10.1016/j. chemgeo.2017.12.004. Martin, D.P., Seiter, J.M., Lafferty, B.J., Bednar, A.J., 2017. Exploring the ability of cations to facilitate binding between inorganic oxyanions and humic acid. Chemosphere 166, 192–196. https://doi.org/10.1016/j.chemosphere.2016.09.084. Moreno-Jiménez, E., Clemente, R., Mestrot, A., Meharg, A.A., 2013. Arsenic and selenium mobilisation from organic matter treated mine spoil with and without inorganic fertilization. Environ. Pollut. 173, 238–244. https://doi.org/10.1016/j. envpol.2012.10.017. Natasha, Shahid, M., Niazi, N.K., Khalid, S., Murtaza, B., Bibi, I., Rashid, M.I., 2018. A critical review of selenium biogeochemical behavior in soil-plant system with an inference to human health. Environ. Pollut. 234, 915–934. Peel, H.R., Martin, D.P., Bednar, A.J., 2017. Extraction and characterization of ternary complexes between natural organic matter, cations, and oxyanions from a natural soil. Chemosphere 176, 125–130. https://doi.org/10.1016/j.chemosphere.2017.02.101. Qin, H.B., Zhu, J.M., Su, H., 2012. Selenium fractions in organic matter from Se-rich soils and weathered stone coal in selenosis areas of China. Chemosphere 86, 626–633. https://doi.org/10.1016/j.chemosphere.2011.10.055. Ren, Z.L., Tella, M., Bravin, M.N., Comans, R.N.J., Dai, J., Garnier, J.M., Sivry, Y., Doelsch, E., Straathof, A., Benedetti, M.F., 2015. Effect of dissolved organic matter composition

881

on metal speciation in soil solutions. Chem. Geol. 398, 61–69. https://doi.org/ 10.1016/j.chemgeo.2015.01.020. Schneider, A.R., Ponthieu, M., Cancèanc, CaConreux, A., Morvan, X., Gommeaux, M., Marin, B., Benedetti, M.F., 2016. Influence of dissolved organic matter and manganese oxides on metal speciation in soil solution: a modelling approach. Environ. Pollut. 213, 618–627. https://doi.org/10.1016/j.envpol.2016.03.010. Shi, H., Li, Q., Chen, W., Cai, P., Huang, Q., 2018. Distribution and mobility of exogenous copper as influenced by aging and components interactions in three Chinese soils. Environ. Sci. Pollut. Res. 25, 10771–10781. https://doi.org/10.1007/s11356-0181288-8. Stern, N., Mejia, J., He, S., Yang, Y., Ginder-Vogel, M., Roden, E.E., 2018. Dual role of humic substances as electron donor and shuttle for dissimilatory iron reduction. Environ. Sci. Technol. 52, 5691–5699. https://doi.org/10.1021/acs.est.7b06574. Supriatin, S., Weng, L., Comans, R.N., 2015. Selenium speciation and extractability in Dutch agricultural soils. Sci. Total Environ. 532, 368–382. https://doi.org/10.1016/j. scitotenv.2015.06.005. Tamás, M., Mándoki, Z., Csapó, J., 2010. The role of selenium content of wheat in the human nutrition. A literature review. Acta Universitatis Sapientiae, Alimentaria 3, 5–34. Thurman, E.M., Malcolm, R.L., 1981. Preparative isolation of aquatic humic substances. Environ. Sci. Technol. 15, 463–466. Vink, J.P., van Zomeren, A., Dijkstra, J.J., Comans, R.N., 2017. When soils become sediments: large-scale storage of soils in sandpits and lakes and the impact of reduction kinetics on heavy metals and arsenic release to groundwater. Environ. Pollut. 227, 146–156. https://doi.org/10.1016/j.envpol.2017.04.016. Wang, S.S., Liang, D.L., Wang, D., Wei, W., Fu, D.D., Lin, Z.Q., 2012. Selenium fractionation and speciation in agriculture soils and accumulation in corn (Zea mays L.) under field conditions in Shaanxi Province, China. Sci. Total Environ. 427–428, 159–164. https:// doi.org/10.1016/j.scitotenv.2012.03.091. Wang, D., Zhou, F., Yang, W.X., Peng, Q., Man, N., Liang, D.L., 2017. Selenate redistribution during aging in different Chinese soils and the dominant influential factors. Chemosphere 182, 284–292. https://doi.org/10.1016/j.chemosphere.2017.05.014. Wang, D., Dinh, Q.T., Thu, T.T.A., Zhou, F., Yang, W., Wang, M., Song, W., Liang, D., 2018. Effect of selenium-enriched organic material amendment on selenium fraction transformation and bioavailability in soil. Chemosphere 199, 417–426. https://doi.org/ 10.1016/j.chemosphere.2018.02.007. Witt, C., Cassman, K.G., Olk, D.C., Biker, U., Liboon, S.P., Samson, M.I., Ottow, J.C.G., 2000. Crop rotation and residue management effects on carbon sequestration, nitrogen cycling and productivity of irrigated rice systems. Plant Soil 225, 263–278. https://doi. org/10.1023/A:1026594118145. Yemadje, P.L., Chevallier, T., Guibert, H., Bertrand, I., Bernoux, M., 2017. Wetting-drying cycles do not increase organic carbon and nitrogen mineralization in soils with straw amendment. Geoderma 304, 68–75. https://doi.org/10.1016/j. geoderma.2016.06.023. Yu, Y., Wan, Y., Camara, A.Y., Li, H., 2018. Effects of the addition and aging of humic acidbased amendments on the solubility of Cd in soil solution and its accumulation in rice. Chemosphere 196, 303–310. https://doi.org/10.1016/j. chemosphere.2018.01.002. Zeng, F., Ali, S., Zhang, H., Ouyang, Y., Qiu, B., Wu, F., Zhang, G., 2011. The influence of pH and organic matter content in paddy soil on heavy metal availability and their uptake by rice plants. Environ. Pollut. 159, 84–91. https://doi.org/10.1016/j. envpol.2010.09.019. Zhang, Y., Frankenberger, W.T., 2003. Factors affecting removal of selenate in agricultural drainage water utilizing rice straw. Sci. Total Environ. 305, 207–216. https://doi.org/ 10.1016/S0048-9697(02)00479-5. Zhang, J., Yin, H., Chen, L., Liu, F., Chen, H., 2018. The role of different functional groups in a novel adsorption-complexation-reduction multi-step kinetic model for hexavalent chromium retention by undissolved humic acid. Environ. Pollut. 237, 740–746. https://doi.org/10.1016/j.envpol.2017.10.120.