Interactions between nitrogen application and soil properties and their impacts on the transfer of cadmium from soil to wheat (Triticum aestivum L.) grain

Geoderma 357 (2020) 113923

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Interactions between nitrogen application and soil properties and their impacts on the transfer of cadmium from soil to wheat (Triticum aestivum L.) grain ⁎

Syed Tahir Ata-Ul-Karim, Long Cang, Yujun Wang , Dongmei Zhou

T

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Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing, Jiangsu 210008, PR China

A R T I C LE I N FO

A B S T R A C T

Handling Editor: Junhong Bai

Rapid urbanization, industrialization, and agricultural intensification have triggered soil and environmental pollution through excessive nitrogen (N) application and cadmium (Cd) contamination. There is still contradiction regarding the effect of N application and soil properties on grain Cd concentration in wheat (Triticum aestivum L.). Grain Cd concentration does not only rely on crop genotypic characteristics and soil geochemical properties but also to a large extent on N fertilizer application. Assessing and identifying the factors governing Cd phytoavailability in soil-plant systems is thus crucial. For this purpose, we developed and validated quantitative relationships of wheat grain Cd concentration with soil geochemical properties related to Cd phytoavailability (pH, Eh, CEC, EC), and N application rates and to investigate interactions between N application rates (0–300 kg ha−1), soil geochemical properties, and grain Cd concentration with data acquired from four field trials conducted in China during 2017–2018 using two wheat cultivars (Annong-1124 and Ningmai-15). The results indicated that N application rates significantly influenced soil properties (pH, Eh, CEC, and EC) and played a decisive role in the transfer of Cd from soil to wheat grain by increasing soil acidification, soil salinity, oxidation reactions, and exchange capacity. Soil geochemical properties were ranged from 5.49 to 6.02 (pH), 55.07 to 103.73 mV (Eh), 164.57 to 258.63 2μS.cm−1 (EC), 19.2 to 23 cmol.kg−1 (CEC), while grain Cd was ranged from 0.16 to 0.27 mg.kg−1. Quantitative relationships were highly significant for both wheat cultivars with R2 > 0.83. Validation results indicated a solid performance of quantitative relationships (RMSE < 3%, RRMSE < 8%, RE < 7%, and R2 > 0.90) and confirmed their robustness as reliable predictors for assessing wheat grain Cd concentration. The finding of this work will further assist agronomist to assess the possible risk of elevated Cd content in these wheat cultivars arising from regional crop and soil management strategies, especially related to N. The findings will help to improve food security and soil sustainability without compromising the food safety-related risks.

Keywords: Cadmium Phytoavailability Nitrogen fertilizer Soil pH Soil Eh Soil EC Soil CEC

1. Introduction The world cropland is facing unprecedented challenges accredited to a drastic increase in population, industrialization, and urbanization. The annual global loss of fertile and productive land reached approximately 25 billion tons (Schmidt-Bleek, 2009). Agriculture and industry are the two prime causes of this soil depletion and degradation. The ever-growing competition for a better share of the natural resources between agriculture and industry lead to the exploitation of natural resources succeeded by soil and environmental contamination with heavy metals (Tóth et al., 2016). The steady accumulation of heavy metals in soil through agricultural and industrial activities make it the

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major sink for heavy metals released into the environment. Cadmium (Cd) due to its high solubility and toxicity is considered as the most hazardous of all the potentially toxic heavy meals in croplands (Jaishankar et al., 2014). Unfortunately, Cd does not undergo microbial or chemical degradation unlike organic contaminants and persists in soils after inception (Kirpichtchikova et al., 2006). Soil being the foundation of the food chain, plays a vital role in food security and safety by determining potential food quantity and quality (Tóth et al., 2016). Yet, the significantly alarming Cd accumulation in croplands has posed food safety implications, potential health risks, and detrimental effects on agro-ecosystem. Therefore, it is indispensable to examine the Cd transfer and accumulation in the soil-plant system.

Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (D. Zhou).

https://doi.org/10.1016/j.geoderma.2019.113923 Received 23 February 2019; Received in revised form 18 August 2019; Accepted 21 August 2019 0016-7061/ © 2019 Elsevier B.V. All rights reserved.

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application rates in China (400 kg ha−1) has crossed the internationally recognized ceiling (225 kg ha−1) for the safe use of fertilizer (Delang, 2017). Yet, no attempt has been made to investigate the impacts of N application on Cd uptake in wheat grains in China including Yangtze River Reaches, one of the world's most heavily N fertilized region and most intensively cultivated, densely populated, economically developed, and Cd-polluted region of China (Ata-Ul-Karim et al., 2017; Chen et al., 2018). Unclear and complex interactions of wheat grain Cd concentration, soil properties, and N management with contradictory results calls for a comprehensive investigation to better understand the fundamental ecophysiological and biochemical mechanisms affecting Cd uptake in wheat. The study endeavored to investigate the effect of N application rates on soil properties (pH, EC, Eh, and CEC) and wheat grain Cd concentration, to establish the relationships between soil properties and grain Cd concentration, and to investigate the interactions among grain Cd concentration, N application rates, and soil properties in wheat. The results will be informative in quantifying Cd-soil-plant interactions and predicting grain Cd in wheat by providing simple algorithms and articulating N management strategies aiming to improve crop productivity and soil sustainability without compromising the food safetyrelated risks.

Soil-to-plant transfer of Cd is the primary exposure pathway in term of soil contamination, and even a very low concentration of Cd forms highly eco-toxic trace elements, hazardous to plants, humans, and animals (Qian et al., 2010). Soil as a key natural resource and critical component of agriculture serves various indispensable tasks to conciliate basic societal necessities. However, Cd contamination owing to its toxicity, persistence, and concealment alters plant morpho-physiological and biochemical functions and makes Cd a major concern for croplands globally (Ran et al., 2016; Rizwan et al., 2016). Croplands in China are also being progressively degraded due to rapid industrialization, urbanization, and agricultural intensification. Approximately one-sixth of China's total cropland is now contaminated, and heavy metals account for 82% of the total contaminated soils, among which Cd ranks the first (Li et al., 2014; Mahar et al., 2016). Therefore, maintaining the fertility, productivity, and sustainability of soil is imperative to enhance agricultural productivity both in developed and emerging nations. Quantification of Cd-soil-plant interactions and the factors affecting Cd phytoavailability is of utmost importance. The extent of Cd phytoavailability, transport, and concentration in wheat grain is significantly influenced by plant characteristics such as cultivars differences in root to shoot ratio and xylem to phloem transfer of Cd, soil properties including pH, redox potential (Eh), cation exchange capacity (CEC), organic matter (OM) content, electrical conductivity (EC), and agronomic practices, particularly nitrogen (N) management (Fairbrother et al., 2007). pH due to its strong effects on solubility and speciation of Cd plays the most important role in determining Cd phytoavailability (Grant et al., 1995). OM due to its chelating ability also affect Cd phytoavailability in the soil, however, studies indicated that the retaining power of OM for Cd is predominately through its CEC property instead of chelating ability (Haghiri, 1974; Adriano, 2001). Eh plays an important role in the uptake of the predominant form (Cd2+) of Cd by crops, as its uptake is suppressed under reduced conditions (Honma et al., 2016), while EC being significantly influenced by fertilizer salts plays an essential role in Cd phytoavailability (Gao et al., 2011). Excessive use of N fertilizer also results in soil degradation through irrevocable changes in the nature and properties of soil by adding heavy metals (e.g. Cd, Pb, As, Cr, Zn, Fe) and fertilizer salts (Atafar et al., 2010; Molina et al., 2009), decomposing OM, threating microbial community, compacting soil, acidifying soil, decreasing soil fertility/quality, strengthened pesticides use, over-exploitation of natural resources, and environment (soil/ water/air) pollution (Singh, 2018). Consequently, an in-depth investigation regarding the association and interactions among soil properties, N fertilizer, and wheat grain Cd concentration is imperative to better understand and reveal the underlying mechanisms. Compendium of scientific literature has reported the relationships between wheat grain Cd concentration and soil properties, yet these findings were contradicting, as few studies reported the impact of soil properties on grain Cd concentration while other reported large differences in wheat grain Cd concentration of even same cultivar grown at different sites categorically stating that the lack of relationships between grain Cd concentration and soil properties were attributed to agronomic practices, particularly N application (Adams et al., 2004; Li et al., 2013). Besides, strong and positive association between Cd phytoavailability and wheat grain concentration under varied N rates has also been previously reported in Sweden, Australia, Canada, United Kingdom, United States, and New Zealand (Li et al., 2011). The N

2. Materials and methods 2.1. Experimental site and design Four multi-N rates (0, 180, 240, and 300 kg.N.ha−1) experiments were conducted during the 2017–2018 wheat growing season on four non-adjoining plots within a field contaminated with Cd in Tongling City (N 31°01′, E117°53′), Anhui Province, China. Two wheat (Triticum aestivum L.) cultivars, Annong-1124 (AN-1124) [Experiment 1 and 3] and Ningmai-15 (NM-15) [Experiment 2 and 4] used in this study were two of the most widely cultivated wheat cultivars in the region. . The soil properties of the experimental site are shown in Table 1. The site located in Yangtze River Reaches is categorized by a subtropical-temperate climate with cold winter and hot summer. The site receives mean annual precipitation, temperature, and humidity of 1852.4 mm, 16.5 °C, and 78%, respectively. Rice, wheat, and rape crops are cultivated in a pattern of rice-wheat/rice-rape rotation. The soil was classified as an anthrosol (first qualifier, hydragric; second qualifier, stagnic) according to World Reference Base. The sowing and harvesting dates in all experiments were 5 November and 20 May. Randomized complete block design with four replications was used in all experiments. The size of every plot was 5 m × 5 m. The planting density was approximately 180 × 104 plants ha−1 with an inter-row spacing of 25 cm in all experiments. N fertilizer was applied as urea before sowing (50%) and at the jointing stage (50%). Each plot received 135 kg.P2O5.ha−1 and 190 kg.K2O.ha−1 before sowing. In order to obtain potential yield, experiments were conducted with suitable crop management practices except for N application rates. 2.2. Soil and plant analysis Between four to six soil samples were collected from the topsoil layer (0–20 cm depth) from each established plot after making plots at heading, and at harvesting. The soil samples were then combined to a

Table 1 Basic soil properties of the top 20 cm depth of the soil used in this study. pH

6.01

CEC cmol g.kg−1 19.6

OM g.kg−1

33.55

Total N g·kg−1

Total P g.kg−1

Total K g.kg−1

Total Zn mg.kg−1

Total Cu mg.kg−1

Total Cd mg.kg−1

1.74

0.92

18.5

187

59.7

0.51

2

Sand (0.02–2 mm) % 8.92

Silt (0.002–0.02 mm) % 63.2

Clay (< 0.002 mm) % 27.88

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and 6.02 and between different experiments there was not much variation. In this study, soil Eh ranged from 55.1 to 104 mV with minor variances between experiments (Exp. 1 and 3 and Exp. 2 and 4). Soil EC ranged from 165 to 259 μS.cm−1, while CEC ranged from 19.2 to 23 cmol.kg−1 with minor differences between experiments. There was an obvious difference in Cd concentration between two cultivars, ranging from 0.21 to 0.27 mg.kg−1 for AN-1124 and 0.16 to 0.22 mg.kg−1 for NM-15. The lowest pH, EC, and CEC were observed for experiments with AN-1124 while lowest Eh and grain Cd concentration were observed for experiments with NM-15 and vice versa. The wheat dry mass and grain yield increased proportionately to the N application rates and refer to the two Figs. S1 and S2.

single composite sample to determine of soil chemical properties. Subsamples were collected in a grid pattern to increase the precision of field sampling and to increase the accuracy of the test. This composite sample was then air dried, ground, and passed through a 2-mm stainless steel sieve prior to analysis. Wheat plants were harvested from an area of 1m2 from three different locations/plot at harvesting. Theacquired wheat grains were then air-dried. Soil pH was measured at a soil to deionized water (without CO2) ratio of 1:2.5 (w/v) using a glass electrode (Orion Star A211, Thermo Scientific, USA). Soil Eh was measured at a soil to deionized water (without CO2) ratio of 1:2.5 (w/v) with an Eh meter (Horiba D-52, Kyoto, Japan). Soil EC was measured at a soil to deionized water (without CO2) ratio of 1:5 (w/v) with an EC meter. Soil CEC was measured using the method of displacing exchangeable cations on soil particles with NH4+ followed by NH4+determination. Grain samples were digested in HNO3 and H2O2 (9,1, V/V) mixture (Ge et al., 2019). The inductively coupled plasma mass spectroscopy (ICP-MS) was used to determine Cd concentration in grains. Wheat grain flour (GBW10035) certified standard reference by China National Center for Standard Material as well as blank (acid mixture without sample) were used for quality control in the digestion and analysis processes. The method detection limit for grain Cd was 0.003 mg kg−1.

3.2. Relationships among pH, Eh, EC, CEC, and wheat grain Cd concentration under varied N rates The relationships among soil properties, N rates, and wheat grain Cd concentration showed a strong association between them and can be utilized for estimating wheat grain Cd concentration. The grain Cd concentration was articulated as a function of soil pH, Eh, EC, CEC, and N rates (Fig. 2). The results indicated that the wheat grain Cd concentration increased with decreasing soil pH and for increasing soil Eh, EC, CEC, and N application rates. All the relationships developed in the present study were highly significant (P = 0.001) and explained the variation in Cd concentration, accurately under different N rates and soil properties. The relationships were more robust for AN-1124 as compared to NM-15, all with R2 values > 0.90, except grain Cd relations with N rates in NM-15 where R2 value was 0.84. The R2 values of relationships for predicting grain Cd concentration in NM-15 were ranged from 0.84 to 0.93 while they ranged from 0.91 to 0.98 for AN1124. The most robust relationships for NM-15 were observed for soil Eh and EC while for AN-1124 strongest relationships were observed for soil Eh and CEC. These relationships elucidated that grain Cd concentration was influenced by soil properties and N rates, and can be applied for estimating wheat grain Cd concentration.

2.3. Statistical analysis The data (Exp. 1–4) for soil properties (pH, EC, Eh, CEC), and grain Cd concentration under varied N rates prior to be subjected to analysis of variance (ANOVA) using GLM procedures in IBM SPSS Version19.0 (IBM Corporation, Armonk, New York) were tested for homogeneity of variance (Levene's test) and normality (Shapiro-Wilks test). The analysis resulted P value > 0.05 for homogeneity of variance and < 0.05 for normality test indicated that data can be subjected to one-way ANOVA. The differences between treatment means were assessed using least significant difference (LSD) test at 95% level of significance. The linear regressions between grain Cd concentration and soil properties (average of three sampling) were conducted using Microsoft Excel 2010 (Microsoft Corporation, Redmond, WA, USA). The thin plate spline regression using MATLAB (The MathWorks, Inc., Natick, Massachusetts) was performed to establish the interactions (response surface plots) between grain Cd concentration, soil properties, and N application rates as well as for grain Cd concentration and among different soil properties. The data acquired from experiments 1–4 (data from randomly selected three repeats) were used for establishing models, while data acquired from the fourth repeat were used for partial validation of the models. To test the applicability of models for other wheat cultivars, the models developed for AN-1124 were validated with data from NM-15 experiments (Exp. 2 and 4) and vice versa for Exp. 1 and Exp. 3. The root mean square error (RMSE), predicted root mean square error (PRMSE), relative error (RE), and coefficient of determination (R2) between observed and predicted values were employed to assess the fitness of the models based on soil pH, Eh, EC, CEC, and N rates for their grain Cd prediction accuracy using Microsoft Excel 2010. The higher value of R2 and lower values of RMSE, RRMSE, and REP indicates the higher precision and accuracy of the model in predicting grain Cd concentration.

3.3. Validation of models The validation results revealed that the newly developed models predicted grain Cd concentration of both wheat cultivars reliably and accurately, and have potential to predict grain Cd concentration in these cultivars arising from regional crop and soil management strategies, particularly related to N management. The performances of the models were estimated by comparing the RMSE, RRMSE, RE, and R2 values for both cultivars. The soil pH, Eh, EC, CEC, and N rate based models developed for AN-1124 (Exp. 1–3) showed good agreement between the observed and predicted values with RMSE (1.14–1.93%), RRMSE (1.95–7.18%), RE (1.70–3.12%), and R2 (0.87–0.98). The soil pH, Eh, EC, CEC, and N rate based models developed for NM-15 (Exp. 2–4) also showed good agreement between the observed and predicted values with RMSE (0.64–2.42%), RRMSE (1.27–8%), RE (1.13–6.84%), and R2 (0.81–0.98) (Table 2). The newly developed models based on soil pH, Eh, EC, CEC, and N rates provided an enhanced accuracy and stability in estimating grain Cd concentration of both wheat cultivars, with a simplified and applicable formulation. The validation results obtained in the present study well supported our hypothesis that soil properties and N application interactions (variation of soil properties due to N application) can be utilized for predicting grain Cd concentration in these wheat cultivars. The lower RMSE, RRMSE, RE values, and higher R2 exhibited a good agreement between the observed and predicted values.

3. Results 3.1. Effect of different N application rates on soil properties, wheat dry mass, grain yield, and grain Cd concentration Soil pH, Eh, EC, CEC, and wheat grain Cd concentration were strongly affected by N management, and N rates exhibited a significant effect (P < 0.05) on soil properties and wheat grain Cd concentration (Fig. 1). Soil pH declines while Eh, EC, CEC, and grain Cd concentration increases with increasing N application rates. Soil pH ranged from 5.49

3.4. Interactions among pH, Eh, EC, CEC, wheat grain Cd concentration, and N application rates The response surface plots were generated to investigate the 3

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Fig. 1. Changes in soil pH (a, b); soil Eh (c, d); soil EC (e, f); soil CEC (g, h); and grain Cd concentration (i, j) in AN-1124 and NM-15 wheat cultivars, respectively under varied N application rates (0–300 kg.N.ha−1) in four field experiments conducted during 2017–2018 wheat growing season. The error bars indicate standard deviation (n = 3).

higher grain Cd concentration in both wheat cultivars and vice versa. The highest values of grain Cd concentration in wheat cultivars were observed in the upper right corners (N-pH, N-CEC interactions) upper left to upper right corners (N-Eh interactions), and upper left corner (NEC interactions) of the plots, respectively, corresponding with higher N rates, Eh, CEC, EC and lower pH values, while the lowest grain Cd concentration was observed in the left corners (N-pH, N-CEC interactions) and lower right corners (N-Eh, N-EC interactions) of the plots, which corresponds with lower N application rates, CEC, EC, Eh, and

interactive effects of N rates and soil properties (pH, CEC, EC, and Eh) on wheat grain Cd concentration. The interactions of N rates (x-axis), soil properties (y-axis), and grain Cd concentration at the z-axis are shown in Figs. 3 and 4. Response surface plots represent the simultaneous and interactive effects of N rates and soil properties on wheat grain Cd concentration. The effect of N fertilizer and soil properties on grain Cd concentration was obvious and their interactive effects on grain Cd concentration were dependent on each other. Increasing N application rates, CEC, EC, and Eh and decreasing soil pH resulted in

4

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Fig. 2. Relationships between average grain cadmium concentration (mg kg−1) and soil properties in two wheat cultivars under varied N application rates (kg.N.ha−1) and (a) soil pH; (b) soil Eh; (c) soil EC; (d) soil CEC; and (e) N application rates. Triangles represents the averages of three replicates for the cultivars AN1124 and NM-15, respectively. The error bars indicate standard deviation (n = 3).

55.07 to 99.37 mV, 170 to 259 μS.cm−1, and soil pH decreased from 5.54 to 6.02 as a consequence of increased N supply from 0 to 300 kg.ha−1, the grain Cd concentration in NM-15 was ranged from 0.16 to 0.22 mg.kg−1. Additionally, if N, soil pH, CEC, Eh, and EC are fixed at 0 kg.ha−1, 6, 19 cmol.kg−1, 57 mV, and 170 μS.cm−1, the grain Cd concentration ranged from 0.16 to 0.21 mg.kg−1 for NM-15 and AN-

higher values of soil pH. The results illustrated that when soil CEC, Eh, and EC increased from19.2 to 23 cmol.kg−1, 55.90 to 103.73 mV, 164.37 to 250 μS.cm−1, and soil pH decreased from 6.0 to 5.49 owing to increasing N supply from 0 to 300 kg.ha−1, the grain Cd concentration in AN-1124 was ranged from 0.21 to 0.27 mg.kg−1. Similarly when CEC, Eh, and EC were increased from 19.4 to 23 cmol.kg−1,

Table 2 Validation of the grain prediction models, (A) validation of models developed for AN-1124 with data of NM-15 and vice versa. (B) Partial validation with data of repeat four. AN-1124

A

NM-15

Factor

RMSE (%)

RRMSE (%)

RE (%)

R2

RMSE (%)

RRMSE (%)

RE (%)

R2

pH Eh EC CEC N rate

1.14 1.71 1.52 1.17 1.93

4.81 7.18 6.39 4.93 6.13

1.9 2.74 1.93 1.7 3.12

0.96 0.93 0.96 0.87 0.87

0.69 0.64 1.08 1.11 2.42

3.69 3.46 5.78 5.98 1.27

1.28 1.13 2.02 2.1 6.84

0.9 0.92 0.81 0.93 0.96

AN-1124

B

NM-15 2

Factor

RMSE (%)

RRMSE (%)

RE (%)

R

pH Eh EC CEC N rate

0.47 0.61 1.62 1.08 0.92

1.95 2.56 6.74 4.52 3.82

1.58 1.93 2 2.88 2.26

0.96 0.89 0.92 0.98 0.98

5

RMSE (%)

RRMSE (%)

RE (%)

R2

0.48 0.5 2.5 0.94 0.98

2.52 2.62 8 4.89 5.12

1.24 2.88 4.9 1.86 1.85

0.98 0.95 0.96 0.98 0.9

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Fig. 3. Response surface showing the interaction of N fertilizer rates (x-axis) with soil pH, CEC, EC, and Eh (y-axis) on average grain Cd concentration (z-axis) in AN1124. The different panel show pH (a); CEC (b); EC (c); and EH (d), in AN-1124, respectively.

Fig. 4. Response surface showing the interaction of N fertilizer rates (x-axis) with soil pH, CEC, EC, and Eh (y-axis) on average grain Cd concentration (z-axis) in NM15. The different panel show pH (a); CEC (b); EC (c); and EH (d), in NM-15, respectively.

1124, respectively.

3.5. Interactions among pH, Eh, EC, CEC, and wheat grain Cd concentration under varied N rates To explore the simultaneous interaction of soil properties (pH-Eh, pH-EC, pH-CEC, Eh-EC, Eh-CEC, and EC-CEC) on wheat grain Cd 6

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Fig. 5. Response surfaces showing the interaction of soil pH with soil Eh (a), EC (b), and CEC (c) and the effects of soil Eh with CEC (d) and EC (e), and effect of soil EC with CEC (f) on average grain Cd concentration in AN-1124, respectively under varied N rates field experiments.

Fig. 6. Response surfaces showing the interaction of soil pH with soil Eh (a), EC (b), and CEC (c) and the effects of soil Eh with CEC (d) and EC (e), and effect of soil EC with CEC (f) on average grain Cd concentration in NM-15, respectively under varied N rates field experiments.

pH-Eh, pH-EC, pH-CEC, CEC-Eh, CEC-EC, and EC-Eh on grain Cd concentration were significant (P < 0.05) and contingent on each other. Decreasing soil pH with increasing soil Eh, EC, and CEC, as well as with the simultaneous increase in soil CEC, Eh, and EC, resulted in higher wheat grain Cd concentration. The highest values of grain Cd

concentration under different N rates, response surface plots were generated. The interactions of soil properties (x-axis and y-axis) and grain Cd concentration (z-axis) are shown in Figs. 5 & 6. The interactions between pH-Eh, pH-EC, pH-CEC, CEC-Eh, CEC-EC, and EC-Eh on grain Cd concentration in wheat were plotted. The interactions between 7

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cultivars in present study was attributed to the difference in the physiological processes associated with the differences in N use efficiency of two cultivars (Figs. S1 & S2), and were in agreement with previous reports on spring, winter, and durum wheat cultivars (Mitchell et al., 2000; Li et al., 2013). Additionally, differences in Cd concentration between two wheat cultivars might also be associated to phloemmediated Cd transport to the grain (Hart et al., 1998). Exchangeable Cd is radially transported towards the stele and loaded into the xylem of the vascular bundles, from where it is translocated upwards in the xylem sap by the transpiration stream and to a lower extent by the root pressure and distributed among shoot organs during pre-anthesis growth period (Yan et al., 2018). In contrast, Cd in vegetative tissues can be remobilized and allocated to developing grains during post-anthesis grain period (Uraguchi and Fujiwara, 2013). Although grain protein contents were not measured in this study, however, according to breeder information AN-1124 have slightly higher protein content than NM-15. AN-1124 in this study also showed higher grain Cd concentration as compared to NM-15 (Fig. 1). Previous studies indicated that Cd is bound to proteins in mature grain of wheat (He et al., 2002) and grain protein may represent a sink for Cd (Gao et al., 2011). Therefore, it is plausible that grain protein might also have served as a sink for Cd in this study. Consequently, N fertilizer is likely to play an important role in affecting protein and Cd contents of grain (Gao et al., 2011). Although, N is simultaneously important owing to its impacts on soil pH and uptake of nutrient and metal from soil liquid phase (Stritsis and Claassen, 2013). Yet, soil pH due to its effect on soil surface charge properties, Cd retention ability in soil solid phase, and hydrolysis of metal cations are considered as the most crucial factor for controlling the transfer of Cd in soil-plant systems (White and Brown, 2010). The robust grain Cd-pH relations (P = 0.001) under varied N rates (Fig. 2) were potentially associated with acidification, increased ionic strength of soil solution, and ensuing ion exchange reactions altering the Cd release and in soil solution by hindering the Cd adsorption to metal binding sites/soil colloids (Grant et al., 1999), ultimately making it readily phytoavailable. Besides, these results were in agreement with reports stating that Cd phytoavailability under N application is strictly related to the Cd solubility regulated by soil sorption processes and soil pH (Lorenz et al., 1997). The increasing urea supply and low nitrate adsorbability on soil colloids can easily increase ionic strength of soil solutions (Mitchell et al., 2000), which results in solubilization of exchangeable Cd in soil solution. Additionally, chemically similar divalent cations such as Zn2+, Cu2+, Pb2+, and Ca2+ present in soil might also have increased the Cd in soil solution due to the competition for binding sites in soils, for uptake and translocation within plants thus displacing Cd from sorption sites in soil, resulting in increased grain Cd concentration in this study. In contrast, the lower Cd phytoavailability under higher pH in this study leads to lower grain Cd concentration due to strong binding of Cd with soil colloids, increased charge density, deprotonation of specific adsorption sites, and due to the high affinity of soil for Cd at pH 6 (Loganathan et al., 2012). The soil CEC is dependent on clay %, type/nature of clay (smectite > fine mica > kaolinite), amount of OM, nutrient availability, and soil pH (Hazelton and Murphy, 2007). The changes in soil CEC and strong grain Cd-CEC relations (Figs. 1 & 2) were associated to the concentration of free salts, formation of energy-rich phosphorylated N compounds and soil acidification due to N application as well as due to the dissolution and distribution of fertilizer (N, P, and K) in soil plough layer (Haghiri, 1974; Radulov et al., 2011). Kaolinite (low activity clay) is most commonly occurring clay in Chinese soils, particular that of southeast China. The variation in soil pH not only results in dissociation of H+ ions from the clay minerals especially kaolinite but also in variation of number of negative charges on the colloids, thereby changing/ increasing CEC. That's why when a soil has a high CEC resulting from OM content, it is said to be pH dependent. The rise of CEC in this study might also be attributed to the retention of H+ ions by soil adsorptive complex as a consequence of NH4+oxidation as well as to the retention

concentration were observed in the upper left corners (pH-Eh, pH-EC, pH-CEC, CEC-Eh, and EC-Eh) and upper right corners (CEC-EC) of the plots, which corresponds with lower soil pH and higher soil Eh, EC, and CEC values, while the lowest values of grain Cd concentration were observed in the lower right corners (pH-Eh, pH-EC, pH-CEC, CEC-Eh, and EC-Eh) and lower right corners (CEC-EC) of the plots, which corresponds with higher soil pH and lower values of soil Eh, EC, and CEC. Together with a decrease in soil pH from 6 to 5.5, there were increases in Eh (55.90 to 103.73 mV), EC (164.37 to 250 μS.cm−1), and CEC (19.2 to 23 cmol.kg−1), the grain Cd concentration in AN-1124 was ranged from 0.21 to 0.27 mg.kg−1. Similarly, grain Cd concentration in NM-15 was ranged from and 0.16 to 0.22 mg.kg−1, with the decrease in soil pH from 6.02 to 5.54 and increases of Eh (55.1 mV to 99.4), EC (170 to 259 μS.cm−1), and CEC (19.4 to 23 cmol.kg−1). In addition, if soil pH, Eh, EC, and CEC were fixed at 6, 57 mV, 170 μS.cm−1, and 19 cmol.kg−1, respectively, the grain Cd concentration ranged from 0.16 to 0.21 mg.kg−1 for NM-15 and AN-1124, respectively. 4. Discussion Soil is the fundamental indicator of agricultural productivity, yet the deterioration of productive and fertile croplands calls for a further agricultural intensification by triggering injudicious N application to ensure global food security and to keep cropland productive, consequently compromising the sustainability of this finite and fragile natural resource. Despite relatively low level of Cd (0.02 ± 0.03 mg Cd/ kg N in (NH4)2SO4 and 1.12 ± 0.10 mg Cd/kg N in KNO3; (Atafar et al., 2010)) in N fertilizers, N increases Cd phytoavailability by enhancing its mobility and accumulation near the soil plough layer owing to its effects on rhizosphere composition and soil chemical reactions (Mitchell et al., 2000). Nitrogen application has been reported to increase dry mass accumulation, leaf area expansion, and protein content through its impacts on crop growth and to increase the activity of Cd in soil-plant systems by influencing Cd speciation, complexation, desorption, phytoavailability, and transportation by affecting soil properties and crop growth (Smith et al., 2015). The increasing N rates were strongly linked to a decrease in soil pH and increased soil Eh, EC, CEC, and wheat grain Cd concentration (Fig. 1) and was consistent with previous reports (Haghiri, 1974; Srivastava and Srivastava, 1992; Wångstrand et al., 2007; Loganathan et al., 2012; Jönsson and Asp, 2013). The increased wheat grain Cd concentration in both wheat cultivars (Fig. 1) was strongly and positively related with increasing Eh, EC, CEC, and decreasing pH under varied N rates (Fig. 2) which is likely to have increased Cd absorption by roots as Cd2+ from the soil solution. The increasing rates of N (urea) in our study decreased soil pH and might have increased Cd phytoavailability, solubility of exchangeable and water-soluble Cd, and ionic strength of soil solution by acidifying the rhizosphere pH via H+ extrusion and nitrification process (Hinsinger et al., 2003). The increased ionic strength of the soil solution and desorption of Cd from exchange sites/soil colloids via ion exchange in this study were attributed to NH4+ from urea application (Lorenz et al., 1997), which is likely to have resulted in higher competition between Cd and cations in the electrolyte (Jat et al., 2017). The lower soil pH in this study was probably also attributed to organic acid production as a consequence of OM decomposition under excessive N application rates. The association between grain yield and grain Cd concentration under varied fertilizer rates with contradictory results have been previously reported. Gray et al. (2002, 2019) reported a positive correlation between grain Cd concentration and yield while Williams and David (1976) reported that the relationships between concentration of Cd in soil and wheat grain were independent of yield. However, this study has not considered the association between grain yield and grain Cd concentration, as the evidence in support of it was not conclusive. The increase in grain Cd concentration was strongly related to N rates (Fig. 2), while the variations in grain Cd concentration of two wheat 8

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of applied Ca2+, NH4+, and K+ ions by soil colloid (Radulov et al., 2011). In additions to acidifying the rhizosphere pH via H+ extrusion and nitrification process, the urea application also results in decomposition of soil OM which in this study mainly come from plant root and plant debris. The decomposition of OM derived from dead plant tissues due to urea application might have influenced the concentration of free salts, which in turn have a significant impact on soil CEC due to the formation of energy-rich phosphorylated N compounds. Cd transport into the cell is anticipated to reduce generally due to its bounding on cation exchange sites in mucilage excretions of plants root tips or on sites in root cell walls (Wångstrand et al., 2007). However, low rhizosphere pH and competition with other cation may have reduced such Cd sequestration in our study. The differences of grain Cd concentration in two cultivars in our study might be attributed to the difference of their roots CEC and the valence of the cation, as the differences in the ability of plants to take up cations from soil are largely controlled by CEC (Haghiri, 1974). The decomposition of OM due to excessive N application might also result in the changes in CEC in this study. The changes in soil EC and robust grain Cd-EC relations (Fig. 2) were attributed to increased soil salinity due to the addition of fertilizer salts, nitrification of urea, and decomposition of soil OM under varied N rates and have unanimity with previous reports. Soil salinity associated with fertilizer salts is considered as a key factor for higher Cd concentration in edible plant parts by enhancing Cd desorption from the soil (Khaledian et al., 2017). The potassium (KCl) and phosphorus (Ca (H2PO4)2·H2O) fertilizers applied in this study were also responsible for influencing soil EC and increasing wheat grain Cd concentration by adding salt ions in soil solution. Consequently, chloride (Cl−) from KCl after being released in soil solution form relatively strong soluble chloroCd complexes and resulted in reduced Cd sorption and a greater Cd phytoavailability (Zhao et al., 2004). Increased grain Cd concentration due to Cl− component of KCl in this study was in agreement with previous studies on wheat (Gao et al., 2010). Additionally, Ca2+ from phosphorus fertilizer due to its similar ionic radius as that of Cd2+ has increased the affinity of Ca2+ containing soil colloids/sorption sites for Cd2+, and increased Cd concentration in soil solution by inhibiting Cd sorption by soil colloids due to competitive effects between Ca2+ and Cd2+ (Zhao et al., 2014), consequently increasing wheat grain Cd concentration. Soil N is unique as its molecular state varies depending on soil Eh, i.e. under oxidized state, most of mineral N is present as NO3−, NO2−, while reduced conditions favors mineral N presence as NH4+. Soil Eh is known to have obvious influence on the transformation of N as it determines N loss as N2O and N2 or its uptake as NH4+ or NO3− by plants. The changes in soil Eh and robust grain Cd-Eh relations (Fig. 2) were attributed to the significant influence of soil Eh on the phytoavailability of predominant form of Cd (Cd2+), which constitutes 40–90% of the soil solution Cd (Honma et al., 2016). This study was conducted on paddy fields which have significant variation in Eh on drying (aerobic condition), hence enhancing Cd phytoavailability through oxidation of CdS to Cd2+ and SO42−, which has a much higher solubility than CdS formed under flooding conditions. The decline of soil pH due to excessive N application in this study also influenced aforementioned Cd speciation through solution activity of Cd and Cd distribution between soil and solution phases (Loganathan et al., 2012). In contrast, increased sorption of Cd at elevated pH/lower Eh values under lower N rates in our study reduced the solution concentration and thus decrease Cd phytoavailability. Soil redox potential is primarily controlled by microbial activity, which is a function of soil temperature, water levels, and soil C and nutrient supply (Fiedler et al., 2007). Excessive N application causes nutrient imbalance and soil degradation due to loss of equilibrium of a stable soil, by increasing the activity of decomposing microbes, the rate of OM decomposition, by affecting the soil microbial community, and land associated microbial transformations. Prediction of Cd mobility, phytoavailability, and accumulation in crops is of simultaneous interest for the crop, soil, and environmental

scientists. The newly developed models have accurately predicted grain Cd concentration in both wheat cultivars and have potential to be employed in breeding programs for screening wheat cultivars with higher Cd uptake and in decision support systems for making crop and soil management decision. Newly developed models being developed on low to moderate Cd contaminated agricultural soils have advantages over previous models as they were developed on heavily polluted soils with industrial waste or pot experiment (with toxic and no Cd treatments), hence have the potential to be applied for predicting wheat grain Cd concentration in low to moderate Cd contaminated agricultural fields. These models might also overcome the issue of faulty extrapolation as a consequence of differences in several processes in soil-plant systems (soil chemistry to soil-plant transfer mechanisms) under high and low Cd concentrations. 5. Conclusions Nitrogen fertilizer application by significantly influencing soil geochemical properties (pH, Eh, CEC, and EC) increased soil acidification, soil salinity, oxidation reactions, and exchange capacity and played a decisive role in Cd transfer from soil to wheat grain. The results will be informative in estimating Cd-soil-plant interactions and predicting grain Cd in both wheat cultivars arising from regional soil management strategies, particularly related to N management by providing simple algorithms and articulating crop and soil N management strategies for preventing excessive Cd transfer from soils to wheat grain without compromising food security, food safety, and soil sustainability. N management decisions according to soil properties and wheat cultivars used in this study will prevent land degradation and will have direct positive impacts on soil productivity. However, systematic investigations with other wheat cultivars grown in the region as well as those grown in different regions are imperative to extrapolate the present results, to reveal the underlying processes for suggesting appropriate measures, and for their broader application to other crops and field conditions. Acknowledgments This research was financially supported by the China Postdoctoral Science Foundation (2018M630617), Chinese Academy of Sciences (2018PC0067), Earmarked Fund for China Agriculture Research System, and the demonstration project of typical heavy metal pollution farmland restoration in Jiangsu Province. Declaration of competing interest The authors declare no competing interests. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2019.113923. References Adriano, D.C., 2001. Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability and Risks of Metals, 2nd edition. Springer, New York, pp. 867. Adams, M., Zhao, F., McGrath, S., Nicholson, F., Chambers, B., 2004. Predicting cadmium concentrations in wheat and barley grain using soil properties. J. Environ. Qual. 33, 532–541. Atafar, Z., Mesdaghinia, A., Nouri, J., Homaee, M., Yunesian, M., Ahmadimoghaddam, M., Mahvi, A.H., 2010. Effect of fertilizer application on soil heavy metal contamination. Environ. Monit. Assess. 160, 83–89. Ata-Ul-Karim, S.T., Zhu, Y., Liu, X., Cao, Q., Tian, Y., Cao, W., 2017. Comparison of different critical nitrogen dilution curves for nitrogen diagnosis in rice. Sci. Rep. 7. Chen, H., Tang, Z., Wang, P., Zhao, F., 2018. Geographical variations of cadmium and arsenic concentrations and arsenic speciation in Chinese rice. Environ. Pollut. 238, 482–490. Delang, C.O., 2017. China’s Soil Pollution and Degradation Problems. Routledge, London.

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