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Zinc fractions in soils and uptake in winter wheat as affected by repeated applications of zinc fertilizer
Soil & Tillage Research 200 (2020) 104612
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Zinc fractions in soils and uptake in winter wheat as affected by repeated applications of zinc fertilizer
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Yu-Min Liu, Dun-Yi Liu, Qing-Yue Zhao, Wei Zhang, Xiu-Xiu Chen, Shi-Jie Xu, Chun-Qin Zou* College of Resources and Environmental Sciences, National Academy of Agriculture Green Development, Key Laboratory of Plant-Soil Interactions, Ministry of Education, China Agricultural University, Beijing, 100193, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Repeated Zn application Soil Zn fractions Zn uptake
This field study was conducted to determine the effect of repeated zinc (Zn) applications at different rates on the concentrations and availability of different forms of Zn in the calcareous soil, in relationship to their contributions to Zn uptake by wheat. Plot-based field experiment was established in 2009 in a winter wheat-summer maize rotation. In each growing season of wheat and maize, six levels of Zn (0, 2.3, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1) as ZnSO4·7H2O were applied at planting. Following the harvest of the winter wheat in 2010, 2012, 2014 and 2016, soil samples (0−30 cm) from 5 selected Zn levels except 2.3 kg Zn ha-1 were collected and subjected to a sequential extraction to examine the concentrations of Zn fractions including water soluble plus exchangeable Zn (Ex-Zn) and Zn as bounded to carbonate (Car-Zn), manganese oxide (MnO-Zn), iron oxide (FeOZn), or organic matter (OM-Zn), and residual (Res-Zn). Stepwise multiple regression was used to identify the contributions of each soil Zn fraction to variation in crop Zn uptake. The results showed that repeated Zn fertilization over the multiple years increased concentrations of all soil Zn fractions. However, the percentage of each Zn fraction to total Zn varied with Zn inputs. Increasing Zn input increased the percentages of Ex-, Carb-, MnO- and FeO-Zn to soil total Zn, whereas reduced that of OM- and Res-Zn. The FeO-, Ex-, and Res-Zn were selected by the stepwise regression procedure as the significant variables explaining the variation in wheat Zn uptake, with partial adjusted coefficient (R2) of 0.81, 0.03 and 0.04, respectively. Crop Zn uptake also increased linearly with soil DTPA-extractable Zn and reached a maximum at DTPA-extractable Zn concentration of 12 mg kg−1. In conclusion, repeated Zn fertilization increased the concentrations of Zn in exchangeable and adsorption forms, which in turn resulted in higher uptake of Zn by winter wheat. DTPA-extractable Zn is a good indicator of Zn availability for wheat. Fraction of FeO-Zn is an important Zn retention pool for determining crop uptake on the calcareous soil. These results emphasize the importance of Zn retention capacity in response to repeated application of Zn fertilizer.
1. Introduction Zinc (Zn) deficiency among humans is a worldwide problem, especially for those who rely on cereals as staple food (Welch and Graham, 2004). Low availability of Zn in soils and edible parts of crops is a key factor contributing to the widespread occurrence of Zn deficiency in humans (Cakmak et al., 1996). Low availability of Zn is frequently reported for grains of wheat (Triticum aestivum L.), which is the second to rice (Oryza sativa L.) as food grains for humans consumption worldwide (FAO, 2013). Among the developing countries such as China, India and others, average Zn concentration in wheat grain ranges from 20 to 35 mg kg−1 (Rengel et al., 1999; Cakmak et al., 2004; Chen et al., 2017; Huang et al., 2019), which is approximately 20–55 % lower than the target value (45 mg kg-1) to satisfy human requirements (Ortiz⁎
Monasterio et al., 2007; Cakmak et al., 2010). Therefore, it is of great importance to improve Zn availability in crops through agricultural management practices such as application of Zn fertilizer (Cakmak and Kutman, 2017). Application of Zn fertilizer such as ZnSO4 through soil or foliar application is an effective strategy to increase Zn uptake and thus correct Zn deficiency in crops (Alloway, 2009). Zinc fertilizer efficacy depends on various soil properties such as pH, CaCO3 content, and organic matter content (Noulas et al., 2018). Only 0.5 %–4 % of applied Zn is available for plants directly, whereas the most is immobilized by soil components (Singh and Sekhon, 1977; Singh et al., 2005; McLaren et al., 1991). The behaviors of Zn in soil and its availability to crops depend on the concentration and percentage of each Zn fraction to soil total Zn. In addition to occurring as free Zn or chelate complex in the
Corresponding author. E-mail address: [email protected] (C.-Q. Zou).
https://doi.org/10.1016/j.still.2020.104612 Received 19 June 2019; Received in revised form 19 February 2020; Accepted 19 February 2020 0167-1987/ © 2020 Published by Elsevier B.V.
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soil solution, Zn in soil may be adsorbed on surfaces of solid particles such as carbonate, metal oxides of iron (Fe), alumimium (Al), manganese (Mn), and organic matter. Its adsorption strength varies depending on the type of bonding as outer-sphere or inner-sphere complex (Shuman, 1979). While, it remains unclear how application of Zn fertilizer affects the concentration and availability of different Zn fractions in soil and which forms are more closely associated with crop uptake. The development of sequences of different chemical extractants to determine Zn speciation has provided a useful tool to better understand Zn form and behavior in soils in relation to uptake by crops (Tessier et al., 1979; D’Amore et al., 2005). Water soluble and exchangeable Zn is considered as the most directly available fraction for crop uptake. In contrast, Zn entrapped in the primary and secondary minerals is considered unavailable to plants, and Zn adsorption on the solid surface represents a reserve pool that may continuously supplement the plant available forms (Zou and Mo, 1993). Previous studies on the effect of Zn fertilizer application on soil Zn fractions and availability to plants revealed inconsistent results. For example, Gonzalez et al. (2008) reported that the application of Zn fertilizer to a calcareous soil increased the concentrations of most Zn fractions in soils with the organic matter bound Zn being the potential plant available pool. Lu et al. (2012a) reported that Zn application increased the concentrations of organic matter- and carbonate-bound Zn, which were identified as the potential pools of plant available Zn. These studies, however, only investigated the changes of soil Zn fractions in response to Zn application in a single growing season. More studies are required to understand the effect of repeated Zn application over multiple growing seasons on the concentration and availability of Zn fractions in soils in relation to crop uptake. Our hypothesis was that the repeated Zn applications would increase the percentage of available Zn fractions in soil and thereby increase crop Zn uptake. This is especially important for the high-yielding winter wheat production system in China as the repeated application of Zn fertilizer has been recommended as an effective strategy to increase its concentration in wheat grain to meet human requirements (Liu et al., 2017a). The objectives of this field study were (1) to determine how the repeated Zn applications affect the concentration and percentage of each Zn fraction to soil total Zn; (2) to assess the relationship between various soil Zn fractions and Zn uptake of winter wheat.
of winter wheat-summer maize since establishment, where summer maize was generally planted in late June and harvest in late September, and winter wheat was planted in October and harvested in June of the following year. The experimental design has been described previously (Liu et al., 2017a,b). Briefly, six Zn application rates at 0, 2.3, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1 as ZnSO4∙7H2O was arranged in a randomized complete block design. Each treatment had four replicate plots and the area of each plot was 75 m2. Zinc fertilizer treatments were continuously employed in each crop year for both winter wheat and summer maize seasons since establishment of the long-term experiment. Other studies from this long-term experiment have reported the effect of Zn fertilizer application on yield production and Zn accumulation in crops (Liu et al., 2017a,b), whereas the current study investigated the concentration and availability of soil Zn fractions as affected by the repeated Zn applications over seven crop years from 2009 to 2016. Specifically, soil samples were collected after harvest of winter wheat in 2011, 2012, 2014 and 2016, respectively, to monitor the changes of soil Zn fractions over time in response to repeated Zn application. Based on results from previous studies (Havlin et al., 2005; Liu et al., 2017a), Zn fertilizer at rates of 5−10 kg Zn ha−1 is frequently recommended to meet crop needs. Therefore, in this study, we have selected five Zn rates (0, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1) to span a range of Zn levels from null to excess applications. Same winter wheat cultivar (Triticum aestivum L., cv. Liangxing 99) and summer maize cultivar (Zea mays L., cv. Zhengdan 958) were used throughout the experimental period. The same treatments and amount of nitrogen (N), phosphorus (P), potassium (K) and zinc (Zn) fertilizers were conducted for winter wheat and summer maize. Zinc fertilizers were applied as aqueous solution of ZnSO4∙7H2O, which were sprayed onto the soil surface before planting in each growing season of winter wheat and summer maize. Following Zn application, a basal compound fertilizer (N-P2O5-K2O; 15-15-15; 500 kg ha−1) was subsequently broadcast onto the soil surface and incorporated into the soil (2−4 cm) by disk-plowing. At elongation stage of wheat and V6 stage of maize, all plots received topdressing of 150 kg N ha-1 as urea, which was hand applied to the soil surface. Based on readings of soil water content, irrigations were applied over the growing seasons to satisfy crop needs.
2. Materials and methods
Soil samples were collected for a total of four times, following the harvest of winter wheat in 2010, 2012, 2014 and 2016. In each plot, five soil samples (0−30 cm) were collected using a stainless steel auger followed as an ‘X’ route and combined as one composite sample. Soil samples were air-dried, ground and then sieved through a 16-mesh screen for further analysis. Soil Zn fractions were determined by a modified sequential extraction procedure (Tessier et al., 1979; Jalali and Khanlari, 2008). Briefly, water soluble plus exchangeable Zn (ExZn) was determined by soil extraction using 20 ml of 1 M NH4OAc with pH 7.0 for 30 min at 25 °C; Zn bound to carbonates (Car-Zn) was determined by soil extraction using 20 ml of 1 M NaOAc with pH 5 for 5 h at 25 °C; Zn bound to manganese oxides (MnO-Zn) was determined by soil extraction using 20 ml of 0.1 M NH2OH·HCl in 0.1 M HNO3 for 30 min at 25 °C; Zn bound to iron oxides (FeO-Zn) was determined by soil extraction using 20 ml of 0.04 M NH2OH·HCl in 25 % (v/v) acetic acid for 6 h in hot water (96 °C); and organic matter bound Zn (OM-Zn) was determined by soil extraction using 5 ml 0.1 M HNO3 and 10 ml 30 % H2O2 for 5 h in hot water (85 °C) with occasional agitation, and after cooling, 15 ml of 3.2 M NH4OAc in 20 % HNO3 for 30 min at 25 °C. Total Zn was determined after digestion using the HF−HClO4−HCl fusion method. Concentrations of Zn in the extracts or digested solution were measured using graphite furnace atomic absorption spectrophotometer (GFAAS, Analytik jena AG, ZEEnit 700P). Residual Zn fraction (Res-Zn) was calculated as the difference between the total Zn and the sum of the measured Zn fractions. For comparison with the sequential extraction, DTPA-extractable Zn in soil samples were also
2.3. Soil sampling and analyses
2.1. Site description This study was part of a long-term field experiment established in 2009 at Quzhou Experimental Station of China Agricultural University (36.9 °N, 115.0 °E). The map of the study area is provided in Figure S1. The research area belongs to the North China Plain and has a typical continental monsoon climate. The mean air temperature was 14.0 °C and annual precipitation was 437 mm over the seven years’ experimental period (2009–2016). The soil is classified as a gypsiric Fluvisols in FAO system (FAO, 2016). The initial soil (0−30 cm) had a silt loam texture (clay 79, silt 553 and sand 368 g kg−1) with pH 8.0, CaCO3 content 4.5 %, total Zn content 60.5 mg kg−1 and diethylene triamine pentacetatic acid (DTPA)-extractable Zn 0.45 mg kg−1. Based on the critical deficient level for the DTPA-extractable Zn (1.0 mg kg−1, Mortvedt, 1985), the experimental site was deficient in plant available Zn. Soil organic matter was analyzed by wet oxidation using a Vario Max CN instrument (Elementar, Langenselbold, Germany). Soil total Zn was determined by the HF−HClO4−HCl fusion method (Shuman, 1979) and DTPA-extractable Zn was measured as described by Lindsay and Norvell (1978). 2.2. Experimental design The experimental site has been managed as an annual crop rotation 2
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fractions firstly increased and then kept stable in response to the increasing total Zn input. The percentages of OM-Zn and Res-Zn decreased firstly with Zn inputs and then kept stable. The critical total input for the stable OM-Zn and Res-Zn was 253.3 and 192.3 kg Zn ha−1, respectively. Similar to Zn fractions, concentration of DTPA-extractable Zn increased linearly with total Zn input, with R2 of 0.95 (Fig. 3). The highest soil DTPA-extractable Zn concentration of 35 mg kg−1 was obtained at total Zn input of 467.3 kg Zn ha-1.
determined according to Lindsay and Norvell (1978). 2.4. Plant sampling and analysis At maturity stage of winter wheat in 2010, 2012, 2014 and 2016, wheat above-ground samples including straw and grain were randomly collected from two 0.5-m lengths of two adjacent rows in each plot. Both straw and grain samples were rapidly washed with deionized water and then dried at 65 °C to constant weight in oven. The ovendried samples were ground with a stainless steel grinder, and then digested using a mixture acid of HNO3-H2O2 in a microwave accelerated reaction system (CEM, Matthews, USA). Concentration of Zn in the digested solutions were determined by inductively coupled plasma atomic emission spectroscopy (ICPAES, OPTIMA 7300 DV, Perkin–Elmer, USA). Certified plant materials (IPE684 and IPE126, Wageningen University, Netherlands) were used for quality control of Zn analysis. Crop uptake of Zn was calculated as the sum of Zn content in straw and grain of wheat, which in turn was calculated by multiplying Zn concentration and dry weight in the respective part.
3.2. Crop Zn uptake The total Zn inputs over the experimental period from 2009 to 2016 in Zn treatments of 0, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1 were 0, 79.8, 159.6, 317.8 and 477.4 kg Zn ha−1, respectively (Table S1). The total Zn outputs increased with Zn rate and ranged between 3.3 and 10.1 kg Zn ha−1. The total Zn balance was -3.3, 74.6, 152.8, 308.9 and 467.3 kg Zn ha−1, for Zn treatments of 0, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1, respectively. The Zn recovery efficiency ranged between 1.4 % and 2.4 % and decreased with Zn rate. Over the experimental period, crop Zn uptake showed a clear increasing trend with the increasing total Zn input (Fig. 4). According to a non-linear regression analysis, the linear-plateau model had the best fit for the relationship between crop Zn uptake and total Zn input over the experimental period. The critical total Zn input for the maximal crop Zn uptake (978 g Zn ha−1) was 245 kg Zn ha−1.
2.5. Calculations and statistical analysis
Total Zn balance = Total Zn input − Total Zn output Total Zn input was calculated as the sum of Zn fertilizer application rate over the experimental period; Total Zn output was calculated as the sum of Zn uptake of wheat and maize over the experimental period. All samples of wheat and maize were collected and used for Zn concentration analysis.
3.3. Relationship between soil Zn availability and crop Zn uptake Stepwise multiple regression with all soil Zn fractions as variables revealed that FeO-Zn was the most important factor determining crop Zn uptake with a partial R2 of 0.81 (Table 1). The Ex-Zn showed a negatively minor impact with a partial R2 of only 0.03. In contrast, the Res-Zn showed a positively minor impact with a partial R2 of 0.04. Inclusion of other Zn fractions as input variables did not further improve the model. Overall, the model had a R2 of 0.88, which was confirmed by a positively linear relationship between the measured and predicted crop Zn uptake (Fig. 5). Over the experimental period, Zn uptake of wheat increased linearly with soil DTPA-extractable Zn, achieving the maximum value at approximately 12 mg kg−1 and was relatively stable thereafter (Fig. 6).
Zinc recovery efficiency = (Total Zn outputfertilizer − Total Zn outputcontrol)/Total Zn input × 100 %. Total Zn outputfertilizer refers to total Zn output in the 5.7, 11.4, 22.7, and 34.1 kg Zn ha−1 treatments, and Total Zn outputcontrol refers to crop Zn uptake in the unfertilized control. Data of soil Zn fractions, DPTA-extractable Zn and crop Zn uptake were subjected to rank transformation to meet the requirements for normality and homogeneity of residuals according to KolmogorovSmirnov test. The two-way analysis of variance (ANOVA) was performed using SPSS 20.0 (SPSS Inc. Chicago, IL, USA) to determine the main and interactive effects of Zn rate and growing season on concentrations of soil Zn fractions, DPTA-extractable Zn and crop Zn uptake. In the model, fertilizer rate and growing season were fixed factors and the plot replicate was random factor. When an effect was significant, treatment means were compared using Fisher’s protected least significant difference (LSD) test. Pearson correlation and multiple regression analysis (forward stepwise) were used to explore the relationships of DTPA-extractable Zn or various Zn fractions with crop uptake. Treatment means and standard errors calculated from untransformed data are presented.
4. Discussion 4.1. Soil Zn fractions as affected by repeated Zn application In the current study, Zn recovery efficiency ranged between 1.4 % and 2.4 % for a total Zn input of 0–477.4 kg Zn ha−1 over the seven years, which was consistent with results of earlier reports (Subramanian et al., 2009; Rupa et al., 2003). Compared to the short-term experiments which mainly investigated the effect of a single application of Zn fertilizer, the current study provided a better understanding of how different forms of Zn in soil in relation to crop Zn uptake were affected by repeated Zn fertilizer application over multiple growing seasons. In agreement with results from previous studies (Liu et al., 2017a), repeated applications as ZnSO4 has significantly increased Zn uptake of winter wheat in the current study. We also observed that Zn fertilizer application in the first growing season (2010) exhibited limited effect as only the highest rate (34.1 kg Zn ha−1) significantly increased Zn uptake over the unfertilized control. The ineffectiveness of one-time application of Zn fertilizer on yield or crop Zn uptake was also reported in other studies (Lu et al., 2012b), which was attributed to the poor Zn mobility and rapid adsorption of Zn on soil solids. The greater effect of Zn applications on crop uptake in later than the first growing season was in accordance with the increased concentrations of Zn fractions and DTPA-extractable Zn, confirming findings of previous studies which
3. Results 3.1. Soil Zn fractions and DTPA-extractable Zn Concentrations of all soil Zn fractions including Ex-, Car-, MnO-, FeO-, OM- and Res-Zn were affected by Zn rate, growing season and their interaction. Concentrations of all Zn fractions increased with total Zn inputs, with the increase being linear (R2 = 0.83–0.96) for Zn fractions of Car-, MnO-, FeO-, OM-, and Res-Zn, and being quadratic (R2 = 0.95) for Ex-Zn (Fig. 1). The percentage of each soil fraction to total Zn was highest for ResZn, ranging between 40 % and 70 % (Fig. 2). In contrast, the Ex-Zn fraction had the lowest percentage (0.3–3.9 %) and its percentage increased with total Zn input. The percentages of Car-, MnO-, and FeO-Zn 3
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Fig. 1. Concentrations of soil Zn fractions as affected by Zn fertilizer applications over the experimental period. Ex-Zn, Car-Zn, MnO-Zn, FeO-Zn, OM-Zn and Res-Zn represent water soluble plus exchangeable Zn, carbonate-bound Zn, manganese oxide-bound Zn, iron oxide-bound Zn, organic matter-bound Zn and residual Zn, respectively.
Fig. 2. Percentages of soil Zn fractions to total Zn as affected by Zn fertilizer applications over the experimental period. Ex-Zn, Car-Zn, MnO-Zn, FeO-Zn, OM-Zn and Res-Zn represent water soluble plus exchangeable Zn, carbonate-bound Zn, manganese oxide-bound Zn, iron oxide-bound Zn, organic matter-bound Zn and residual Zn, respectively. 4
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Fig. 3. Soil DTPA-Zn concentration as affected by Zn fertilizer applications over the experimental period.
Fig. 5. The positively linear relationship between the measured and predicted crop Zn uptake using stepwise multiple regression. The dash line is the 1:1 line.
Fig. 4. Wheat Zn uptake as affected by Zn fertilizer applications over the experimental period.
Fig. 6. Relationship between soil DTPA-extractable Zn and crop Zn uptake over the experimental period.
Table 1 Stepwise multiple regression model of soil Zn fractions to predict crop Zn uptake. Variables from Zn application rates in all growing season were used (n = 80).
investigated changes of soil Zn speciation and its relationship with crop Zn uptake in response to continuously repeated Zn applications over multiple growing seasons. While Zn application generally increased concentrations of all Zn fractions in soil, its effect on the percentage of each Zn fraction to soil total Zn differed. The sequential extraction procedure (Tessier et al., 1979) was used to partition Zn into forms, on the basis of Zn retention mechanisms in soils. The percentage of Ex-Zn to total Zn increased linearly with total Zn input showed that Zn fertilizer application was effective to increase the percentage of Ex-Zn, which was considered as the most directly plant available form (Lindsay and Cox, 1985; Reed and Martens, 1996). Increase of the percentage of Car-Zn with total Zn input was associated with high calcium carbonate content in the calcareous soils. Different responses between adsorption on Mn and Fe oxides or organic matter could be associated with the kinetics of the complexation. Li et al. (2009) reported that Zn adsorption on river sediments followed order of MnO > FeO > organic matter. Therefore, the applied Zn was more likely adsorbed on Mn oxide than Fe and organic matter and thus increased more percentage of MnO-Zn than FeOZn, whereas decreased the percentage of OM-Zn. Meantime, the percentages of all Zn fractions to total Zn except Ex-Zn kept stable when total Zn input increasing up to approximate 250 kg Zn ha−1. This illustrated that the Zn input rate reached a chemical equilibrium with main soil components in the calcareous soil (Kiekens, 1995).
Variable FeO-Zn
P < 0.001
Partial R2 0.81
Coefficient 49.8
Ex-Zn Res-Zn Constant
< 0.001 < 0.001
0.03 0.04
−111.5 14.3 −740.7
Model R2 0.88
Notes: Ex-Zn: exchangeable Zn, FeO-Zn: iron oxide Zn, and Res-Zn: residual Zn.
reported that the residual Zn fertilizer can be effective for multiple years (Gupta and Kalra, 2006). It should be noticed that a relatively large range of application rates (5.7–34.1 kg Zn ha−1) was used in the current study. For most field crops, rate of 10 kg ha−1 is recommended in clay and loam soils and 5 kg ha−1 in sandy soils (Havlin et al., 2005). The relatively higher rates were used in the current study mainly to test whether they can improve Zn concentrations in the grain of winter wheat to reach the healthy target by human consumption. This study showed that high Zn application rate of 34.1 kg Zn ha−1 (150 kg ZnSO4·7H2O) did not negatively affect crop growth or grain yield (data not shown), suggesting high rates of Zn application in the current study did not cause any phytotoxicity in winter wheat. Our previous studies also showed that the repeated high Zn application was effective to increase grain Zn concentration from 31 mg kg−1 up to 50 mg kg−1 for winter wheat (Liu et al., 2017a). To our knowledge, the current study is the first study that
4.2. The availability of Zn fractions to crop Zn uptake Previous studies which attempted to relate different forms of Zn to 5
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bioavailability in crop grains. The relationships of different forms of Zn and crop Zn uptake can be used as an important reference for Zn fertilizer management.
plant availability revealed ambiguous results (Iyengar et al., 1981; Norouzi et al., 2014). In this study, the FeO bound Zn was identified as the most important fraction contributing to crop Zn uptake in the regression model, suggesting Zn adsorption on Fe oxides can be considered labile pools that play a significant role in supplying Zn for crops. In contrast, other studies suggested FeO-Zn as non-plant available fraction due to the formation of inner-sphere complex (Ghanem and Mikkelsen, 1988). Different results between studies highlight the importance of soil properties such as pH on determining the availability of Zn fractions to crop uptake. Similar to our studies, Pradhan and Kanwar (1990) found that soil FeO-Zn was positively associated with grain Zn uptake of rice grown in a pot experiment. Han et al. (2011) also reported that FeO-Zn is one of the main fractions controlling Zn uptake by soybeans grown on 11 representative soils in Northeast China. The importance of FeO-Zn in crop uptake could be explained by previous observation of Stanton and Burger (1967) that Zn adsorption on Fe oxides was potentially plant available in spite of the strong affinity. While many studies had reported a positive correlation of Ex-Zn with crop uptake (Iyengar et al., 1981; Han et al., 2011; Norouzi et al., 2014), our study with the multiple stepwise regression model reveals Ex-Zn fraction had even a minor effect on crop uptake negatively. It is likely the small proportion of Ex-Zn had weakened its effectiveness, especially at high rates of Zn fertilizer application. In addition, the exchangeable Zn could also be converted to the adsorption forms on metal oxides, especially for calcareous soils with high pH (Havlin et al., 2005). Several studies also reported positive correlation between the OM-Zn and crop uptake, highlighting the importance of soil organic matter in the process of Zn retention (Nogueira et al., 2010; Norouzi et al., 2014). This, however, is not the case in the current study as the soil was low in organic matter content (10 g kg−1). These findings suggest that the plant availability of Zn fractions are highly dependent on soil properties. The minor but significant correlation of Res-Zn with crop uptake highlights the retention capacity of the soil, confirming the residual effect of Zn fertilizer could last for multiple growing seasons. Similar to other studies (Hamilton et al., 1993; Han et al., 2011), soil DTPA-extractable Zn showed a positive correlation with crop uptake, suggesting it can be used as a reliable index of predicting Zn availability on calcareous soils with high pH. Han et al. (2011) also reported a good correlation between soil DTPA-extractable Zn and uptake by soybean grown on major soil types in northeast China, with a similar coefficient as reported in the current study. Our study reveals the crop Zn uptake in response to DTPA-extractable Zn was linearplateau, suggesting that high DTPA-extractable Zn can’t continuously increase crop Zn uptake. The effect of DTPA-extractable Zn on crop uptake would reach the maximum at approximate 12 mg kg−1. Therefore, this level of DTPA-extractable Zn can be used as a critical concentration level above which Zn fertilizer should not be recommended.
Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was funded by the National Natural Science Foundation of China (31672240), the 973 Project (2015CB150402), and the Innovative Group Grant of National Natural Science Foundation of China (31421092). We thank Dr. Bruce Jaffee from USA for improving the English of the manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.still.2020.104612. References Alloway, B.J., 2009. Soil factors associated with zinc deficiency in crops and humans. Environ. Geochem. Health 31 (5), 537–548. Cakmak, I., Kutman, U.B., 2017. Agronomic biofortification of cereals with zinc: a review. Eur. J. Soil Sci. 69, 172–180. Cakmak, I., Yilmaz, A., Kalayci, M., Ekiz, H., Torun, B., Ereno, B., Braun, H.J., 1996. Zinc deficiency as a critical problem in wheat production in Central Anatolia. Plant Soil 180, 165–172. Cakmak, I., Torun, A., Millet, E., Feldman, M., Fahima, T., Korol, A., Nevo, E., Braun, H.J., Ozkan, H., 2004. Triticum dicoccoides: an important genetic resource for increasing zinc and iron concentration in modern cultivated wheat. Soil Sci. Plant Nutr. 50, 1047–1054. Cakmak, I., Pfeiffer, W.H., McClafferty, B., 2010. Review: biofortification of durum wheat with zinc and iron. Cereal Chem. 87, 10–20. Chen, X.P., Zhang, Y.Q., Tong, Y.P., Xue, Y.F., Liu, D.Y., Zhang, W., Deng, Y., Meng, Q.F., Yue, S.C., Yan, P., Cui, Z.L., Shi, X.J., Guo, S.W., Sun, Y.X., Ye, Y.L., Wang, Z.H., Jia, L.L., Ma, W.Q., He, M.R., Zhang, X.Y., Kou, C.L., Li, Y.T., Tan, T.S., Cakmak, I., Zhang, F.S., Zou, C.Q., 2017. Harvesting more grain zinc of wheat for human health. Sci. Rep. UK 7, 7016. D’amore, J.J., Al-Abed, S.R., Scheckel, K.G., Ryan, J.A., 2005. Methods for speciation of metals in soils. J. Environ. Qual. 345, 1707–1745. FAO, 2013. FAO Statistical Yearbook: World Food and Agriculture. FAO. FAO, 2016. http://www.fao.org/faostat/en/#home. Ghanem, S.A., Mikkelsen, D.S., 1988. Sorption of zinc on iron hydrous oxides. Soil Sci. 146, 15–21. Gonzalez, D., Obrador, A., Lopez-Valdivia, L.M., Alvarez, J.M., 2008. Effect of zinc source applied to soils on its availability to navy bean. Soil Sci. Soc. Am. J. 72, 641–649. Gupta, U.C., Kalra, Y.P., 2006. Residual effect of copper and zinc from fertilizers on plant concentration, phytotoxicity, and crop yield response. Commun. Soil Sci. Plant Anal. 37, 2505–2511. Hamilton, M.A., Westermann, D.T., James, D.W., 1993. Factors affecting zinc uptake in cropping systems. Soil Sci. Soc. Am. J. 57 (5), 1310–1315. Han, X., Li, X., Uren, N., Tang, C., 2011. Zinc fractions and availability to soybeans in representative soils of Northeast China. J. Soils Sediments 11 (4), 596–606. Havlin, J.L., Beaton, J.D., Tisdale, S.L., Nelson, W.L., 2005. Soil Fertility and Nutrient Management, 7th edn. Pearson Prentice Hall, Upper Saddle River, NJ. Huang, T., Huang, Q., She, X., Ma, X., Huang, M., Cao, H., He, G., Liu, J., Liang, D., Malhi, S.S., Wang, Z., 2019. Grain zinc concentration and its relation to soil nutrient availability in different wheat cropping regions of China. Soil Till. Res. 191, 57–65. Iyengar, S.S., Martens, D.C., Miller, W.P., 1981. Distribution and plant availability of soil zinc fractions. Soil Sci. Soc. Am. J. 45 (4), 735–739. Jalali, M., Khanlari, Z.V., 2008. Effect of aging process on the fractionation of heavy metals in some calcareous soils of Iran. Geoderma 143 (1-2), 26–40. Kiekens, L., 1995. Zinc. In: Alloway, B.J. (Ed.), Heavy Metals in Soils, 2nd edn. Blackie Academic and Professional, London, pp. 284–305. Li, Y., Wang, X.L., Huang, G.H., Zhang, B.Y., Guo, S.H., 2009. Adsorption of Cu and Zn onto Mn/Fe oxides and organic materials in the extractable fractions of river surficial sediments. Soil Sediment Contam. 18 (1), 87–101. Lindsay, W.L., Cox, F.R., 1985. Micronutrient soil testing for the tropics. Fertil. Res. 7, 169–200. Lindsay, W.L., Norvell, W.A., 1978. Development of a DTPA soil test for zinc iron manganese and copper. Soil Sci. Soc. Am. J. 42, 421–428. Liu, D.Y., Zhang, W., Pang, L.L., Zhang, Y.Q., Wang, X.Z., Liu, Y.M., Chen, X.P., Zhang, F.S., Zou, C.Q., 2017. Effects of zinc application rate and zinc distribution relative to
5. Conclusions Repeated applications of Zn fertilizer over multiple years have greatly increased Zn uptake by winter wheat and concentrations of all Zn fractions in the soil, with the increase being more evident at higher rates. Over the experimental period, there was a linear or quadratic increasing relationship between concentration of Zn fractions and total Zn input. Zinc application also increased the percentage to total Zn of Ex-, Car-, MnO- and FeO-Zn fractions, whereas had no effect or even reduced the percentages of OM- and Res-Zn. Crop Zn uptake increased linearly with soil concentrations of DTPA-extractable Zn, and reached the maximum at 12 mg kg−1, which can be considered as the maximum level for recommendation of Zn fertilizer application. The stepwise regression procedure had selected FeO-Zn as the most important variable affecting Zn uptake of winter wheat, suggesting the bond of Zn on Fe oxides dominated Zn availability for the calcareous soil if repeated applications of Zn fertilizer are used for improvement of Zn 6
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by some chemical methods from rice growing soils of North-Western Himalayas. Plant Soil 126 (1), 149–153. Reed, S.T., Martens, D.C., 1996. Copper and zinc. In: Sparks, D.L. (Ed.), Methods of Soil Analysis. Part 3-Chemical Methods. Soil Sci. Soc. Am. J. Inc., Madison, Wisconsin. Rengel, Z., Batten, G.D., Crowley, D.E., 1999. Agronomic approaches for improving the micronutrient density in edible portions of field crops. Field Crop. Res. 60, 27–40. Rupa, T.R., Srinivasa Rao, C., Subba Rao, A., Singh, M., 2003. Effects of farmyard manure and phosphorus on zinc transformations and phyto-availability in two alfisols of India. Bioresour. Technol. Rep. 87 (3), 279–288. Shuman, L.M., 1979. Zinc manganese and copper in soil fractions. Soil Sci. 127, 10–17. Singh, B., Sekhon, G.S., 1977. Effect of soil properties on adsorption and desorption of zinc by alkaline soils. Soil Sci. 126, 366–369. Singh, B., Natesan, S.K.A., Singh, B.K., Usha, K., 2005. Improving zinc efficiency of cereals under zinc deficiency. Curr. Sci. 88, 36–44. Stanton, D.A., Burger, R.D.T., 1967. Availability to plants of zinc sorbed by soil and hydrous iron oxides. Geofis Int. 1, 13–17. Subramanian, K.S., Tenshia, V., Jayalakshmi, K., Ramachandran, V., 2009. Biochemical changes and zinc fractions in arbuscular mycorrhizal fungus (Glomus intraradices) inoculated and uninoculated soils under differential zinc fertilization. Appl. Soil Ecol. 43, 32–39. Tessier, A., Campbell, P., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844–851. Welch, R.M., Graham, R.D., 2004. Breeding for micronutrients in staple food crops from a human nutrition perspective. J. Exp. Bot. 55 (396), 353–364. Zou, B.J., Mo, R.C., 1993. Transformation and availability of various forms of zinc in soils. Pedosphere 3, 35–44.
root distribution on grain yield and grain Zn concentration in wheat. Plant Soil 411 (1–2), 167–178. Liu, D.Y., Zhang, W., Yan, P., Chen, X.P., Zhang, F.S., Zou, C.Q., 2017b. Soil application of zinc fertilizer could achieve high yield and high grain zinc concentration in maize. Plant Soil 411, 47–55. Lu, X.C., Cui, J., Tian, X.H., Ogunniyi, J.E., Gale, W.J., Zhao, A.Q., 2012a. Effects of zinc fertilization on zinc dynamics in potentially zinc-deficient calcareous soil. Agron. J. 104 (4), 963. Lu, X.C., Tian, X.H., Zhao, A.Q., Cui, J., Yang, X.W., 2012b. Effect of Zn supplementation on Zn concentration of wheat grain and Zn fractions in potentially Zn-deficient Soil. Cereal Res. Commun. 40 (3), 385–395. Mclaren, R.G., Mclenaghen, R.D., Swift, R.S., 1991. Zinc application to pastures: effect on herbage and soil zinc concentrations. N. Z. J. Agric. Res. 34, 113–118. Mortvedt, J.J., 1985. Plant uptake of heavy metals in zinc fertilizers made from industrial by-products. J. Environ. Qual. 14, 424–427. Nogueira, T.A.R., Melo, W.J., Fonseca, I.M., Marcussi, S.A., Melo, G.M.P., Marques, M.O., 2010. Fractionation of Zn, Cd and Pb in a Tropical soil after nine-year sewage sludge applications. Pedosphere 20, 545–556. Norouzi, M., Khoshgoftarmanesh, A.H., Afyuni, M., 2014. Zinc fractions in soil and uptake by wheat as affected by different preceding crops. Soil Sci. Plant Nutr. 60, 670–678. Noulas, C., Tziouvalekas, M., Karyotis, T., 2018. Zinc in soils, water and food crops. J. Trace Elem. Med. Biol. 49, 252–260. Ortiz-Monasterio, J.I., Palacios-Rojas, N., Meng, E., Pixley, K., Trethowan, R., Peña, R.J., 2007. Enhancing the mineral and vitamin content of wheat and maize through plant breeding. J. Cereal Sci. 46 (3), 293–307. Pradhan, Y., Kanwar, B.B., 1990. Contribution of zinc fraction to available zinc extracted
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Zinc fractions in soils and uptake in winter wheat as affected by repeated applications of zinc fertilizer
T
Yu-Min Liu, Dun-Yi Liu, Qing-Yue Zhao, Wei Zhang, Xiu-Xiu Chen, Shi-Jie Xu, Chun-Qin Zou* College of Resources and Environmental Sciences, National Academy of Agriculture Green Development, Key Laboratory of Plant-Soil Interactions, Ministry of Education, China Agricultural University, Beijing, 100193, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Repeated Zn application Soil Zn fractions Zn uptake
This field study was conducted to determine the effect of repeated zinc (Zn) applications at different rates on the concentrations and availability of different forms of Zn in the calcareous soil, in relationship to their contributions to Zn uptake by wheat. Plot-based field experiment was established in 2009 in a winter wheat-summer maize rotation. In each growing season of wheat and maize, six levels of Zn (0, 2.3, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1) as ZnSO4·7H2O were applied at planting. Following the harvest of the winter wheat in 2010, 2012, 2014 and 2016, soil samples (0−30 cm) from 5 selected Zn levels except 2.3 kg Zn ha-1 were collected and subjected to a sequential extraction to examine the concentrations of Zn fractions including water soluble plus exchangeable Zn (Ex-Zn) and Zn as bounded to carbonate (Car-Zn), manganese oxide (MnO-Zn), iron oxide (FeOZn), or organic matter (OM-Zn), and residual (Res-Zn). Stepwise multiple regression was used to identify the contributions of each soil Zn fraction to variation in crop Zn uptake. The results showed that repeated Zn fertilization over the multiple years increased concentrations of all soil Zn fractions. However, the percentage of each Zn fraction to total Zn varied with Zn inputs. Increasing Zn input increased the percentages of Ex-, Carb-, MnO- and FeO-Zn to soil total Zn, whereas reduced that of OM- and Res-Zn. The FeO-, Ex-, and Res-Zn were selected by the stepwise regression procedure as the significant variables explaining the variation in wheat Zn uptake, with partial adjusted coefficient (R2) of 0.81, 0.03 and 0.04, respectively. Crop Zn uptake also increased linearly with soil DTPA-extractable Zn and reached a maximum at DTPA-extractable Zn concentration of 12 mg kg−1. In conclusion, repeated Zn fertilization increased the concentrations of Zn in exchangeable and adsorption forms, which in turn resulted in higher uptake of Zn by winter wheat. DTPA-extractable Zn is a good indicator of Zn availability for wheat. Fraction of FeO-Zn is an important Zn retention pool for determining crop uptake on the calcareous soil. These results emphasize the importance of Zn retention capacity in response to repeated application of Zn fertilizer.
1. Introduction Zinc (Zn) deficiency among humans is a worldwide problem, especially for those who rely on cereals as staple food (Welch and Graham, 2004). Low availability of Zn in soils and edible parts of crops is a key factor contributing to the widespread occurrence of Zn deficiency in humans (Cakmak et al., 1996). Low availability of Zn is frequently reported for grains of wheat (Triticum aestivum L.), which is the second to rice (Oryza sativa L.) as food grains for humans consumption worldwide (FAO, 2013). Among the developing countries such as China, India and others, average Zn concentration in wheat grain ranges from 20 to 35 mg kg−1 (Rengel et al., 1999; Cakmak et al., 2004; Chen et al., 2017; Huang et al., 2019), which is approximately 20–55 % lower than the target value (45 mg kg-1) to satisfy human requirements (Ortiz⁎
Monasterio et al., 2007; Cakmak et al., 2010). Therefore, it is of great importance to improve Zn availability in crops through agricultural management practices such as application of Zn fertilizer (Cakmak and Kutman, 2017). Application of Zn fertilizer such as ZnSO4 through soil or foliar application is an effective strategy to increase Zn uptake and thus correct Zn deficiency in crops (Alloway, 2009). Zinc fertilizer efficacy depends on various soil properties such as pH, CaCO3 content, and organic matter content (Noulas et al., 2018). Only 0.5 %–4 % of applied Zn is available for plants directly, whereas the most is immobilized by soil components (Singh and Sekhon, 1977; Singh et al., 2005; McLaren et al., 1991). The behaviors of Zn in soil and its availability to crops depend on the concentration and percentage of each Zn fraction to soil total Zn. In addition to occurring as free Zn or chelate complex in the
Corresponding author. E-mail address: [email protected] (C.-Q. Zou).
https://doi.org/10.1016/j.still.2020.104612 Received 19 June 2019; Received in revised form 19 February 2020; Accepted 19 February 2020 0167-1987/ © 2020 Published by Elsevier B.V.
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soil solution, Zn in soil may be adsorbed on surfaces of solid particles such as carbonate, metal oxides of iron (Fe), alumimium (Al), manganese (Mn), and organic matter. Its adsorption strength varies depending on the type of bonding as outer-sphere or inner-sphere complex (Shuman, 1979). While, it remains unclear how application of Zn fertilizer affects the concentration and availability of different Zn fractions in soil and which forms are more closely associated with crop uptake. The development of sequences of different chemical extractants to determine Zn speciation has provided a useful tool to better understand Zn form and behavior in soils in relation to uptake by crops (Tessier et al., 1979; D’Amore et al., 2005). Water soluble and exchangeable Zn is considered as the most directly available fraction for crop uptake. In contrast, Zn entrapped in the primary and secondary minerals is considered unavailable to plants, and Zn adsorption on the solid surface represents a reserve pool that may continuously supplement the plant available forms (Zou and Mo, 1993). Previous studies on the effect of Zn fertilizer application on soil Zn fractions and availability to plants revealed inconsistent results. For example, Gonzalez et al. (2008) reported that the application of Zn fertilizer to a calcareous soil increased the concentrations of most Zn fractions in soils with the organic matter bound Zn being the potential plant available pool. Lu et al. (2012a) reported that Zn application increased the concentrations of organic matter- and carbonate-bound Zn, which were identified as the potential pools of plant available Zn. These studies, however, only investigated the changes of soil Zn fractions in response to Zn application in a single growing season. More studies are required to understand the effect of repeated Zn application over multiple growing seasons on the concentration and availability of Zn fractions in soils in relation to crop uptake. Our hypothesis was that the repeated Zn applications would increase the percentage of available Zn fractions in soil and thereby increase crop Zn uptake. This is especially important for the high-yielding winter wheat production system in China as the repeated application of Zn fertilizer has been recommended as an effective strategy to increase its concentration in wheat grain to meet human requirements (Liu et al., 2017a). The objectives of this field study were (1) to determine how the repeated Zn applications affect the concentration and percentage of each Zn fraction to soil total Zn; (2) to assess the relationship between various soil Zn fractions and Zn uptake of winter wheat.
of winter wheat-summer maize since establishment, where summer maize was generally planted in late June and harvest in late September, and winter wheat was planted in October and harvested in June of the following year. The experimental design has been described previously (Liu et al., 2017a,b). Briefly, six Zn application rates at 0, 2.3, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1 as ZnSO4∙7H2O was arranged in a randomized complete block design. Each treatment had four replicate plots and the area of each plot was 75 m2. Zinc fertilizer treatments were continuously employed in each crop year for both winter wheat and summer maize seasons since establishment of the long-term experiment. Other studies from this long-term experiment have reported the effect of Zn fertilizer application on yield production and Zn accumulation in crops (Liu et al., 2017a,b), whereas the current study investigated the concentration and availability of soil Zn fractions as affected by the repeated Zn applications over seven crop years from 2009 to 2016. Specifically, soil samples were collected after harvest of winter wheat in 2011, 2012, 2014 and 2016, respectively, to monitor the changes of soil Zn fractions over time in response to repeated Zn application. Based on results from previous studies (Havlin et al., 2005; Liu et al., 2017a), Zn fertilizer at rates of 5−10 kg Zn ha−1 is frequently recommended to meet crop needs. Therefore, in this study, we have selected five Zn rates (0, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1) to span a range of Zn levels from null to excess applications. Same winter wheat cultivar (Triticum aestivum L., cv. Liangxing 99) and summer maize cultivar (Zea mays L., cv. Zhengdan 958) were used throughout the experimental period. The same treatments and amount of nitrogen (N), phosphorus (P), potassium (K) and zinc (Zn) fertilizers were conducted for winter wheat and summer maize. Zinc fertilizers were applied as aqueous solution of ZnSO4∙7H2O, which were sprayed onto the soil surface before planting in each growing season of winter wheat and summer maize. Following Zn application, a basal compound fertilizer (N-P2O5-K2O; 15-15-15; 500 kg ha−1) was subsequently broadcast onto the soil surface and incorporated into the soil (2−4 cm) by disk-plowing. At elongation stage of wheat and V6 stage of maize, all plots received topdressing of 150 kg N ha-1 as urea, which was hand applied to the soil surface. Based on readings of soil water content, irrigations were applied over the growing seasons to satisfy crop needs.
2. Materials and methods
Soil samples were collected for a total of four times, following the harvest of winter wheat in 2010, 2012, 2014 and 2016. In each plot, five soil samples (0−30 cm) were collected using a stainless steel auger followed as an ‘X’ route and combined as one composite sample. Soil samples were air-dried, ground and then sieved through a 16-mesh screen for further analysis. Soil Zn fractions were determined by a modified sequential extraction procedure (Tessier et al., 1979; Jalali and Khanlari, 2008). Briefly, water soluble plus exchangeable Zn (ExZn) was determined by soil extraction using 20 ml of 1 M NH4OAc with pH 7.0 for 30 min at 25 °C; Zn bound to carbonates (Car-Zn) was determined by soil extraction using 20 ml of 1 M NaOAc with pH 5 for 5 h at 25 °C; Zn bound to manganese oxides (MnO-Zn) was determined by soil extraction using 20 ml of 0.1 M NH2OH·HCl in 0.1 M HNO3 for 30 min at 25 °C; Zn bound to iron oxides (FeO-Zn) was determined by soil extraction using 20 ml of 0.04 M NH2OH·HCl in 25 % (v/v) acetic acid for 6 h in hot water (96 °C); and organic matter bound Zn (OM-Zn) was determined by soil extraction using 5 ml 0.1 M HNO3 and 10 ml 30 % H2O2 for 5 h in hot water (85 °C) with occasional agitation, and after cooling, 15 ml of 3.2 M NH4OAc in 20 % HNO3 for 30 min at 25 °C. Total Zn was determined after digestion using the HF−HClO4−HCl fusion method. Concentrations of Zn in the extracts or digested solution were measured using graphite furnace atomic absorption spectrophotometer (GFAAS, Analytik jena AG, ZEEnit 700P). Residual Zn fraction (Res-Zn) was calculated as the difference between the total Zn and the sum of the measured Zn fractions. For comparison with the sequential extraction, DTPA-extractable Zn in soil samples were also
2.3. Soil sampling and analyses
2.1. Site description This study was part of a long-term field experiment established in 2009 at Quzhou Experimental Station of China Agricultural University (36.9 °N, 115.0 °E). The map of the study area is provided in Figure S1. The research area belongs to the North China Plain and has a typical continental monsoon climate. The mean air temperature was 14.0 °C and annual precipitation was 437 mm over the seven years’ experimental period (2009–2016). The soil is classified as a gypsiric Fluvisols in FAO system (FAO, 2016). The initial soil (0−30 cm) had a silt loam texture (clay 79, silt 553 and sand 368 g kg−1) with pH 8.0, CaCO3 content 4.5 %, total Zn content 60.5 mg kg−1 and diethylene triamine pentacetatic acid (DTPA)-extractable Zn 0.45 mg kg−1. Based on the critical deficient level for the DTPA-extractable Zn (1.0 mg kg−1, Mortvedt, 1985), the experimental site was deficient in plant available Zn. Soil organic matter was analyzed by wet oxidation using a Vario Max CN instrument (Elementar, Langenselbold, Germany). Soil total Zn was determined by the HF−HClO4−HCl fusion method (Shuman, 1979) and DTPA-extractable Zn was measured as described by Lindsay and Norvell (1978). 2.2. Experimental design The experimental site has been managed as an annual crop rotation 2
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fractions firstly increased and then kept stable in response to the increasing total Zn input. The percentages of OM-Zn and Res-Zn decreased firstly with Zn inputs and then kept stable. The critical total input for the stable OM-Zn and Res-Zn was 253.3 and 192.3 kg Zn ha−1, respectively. Similar to Zn fractions, concentration of DTPA-extractable Zn increased linearly with total Zn input, with R2 of 0.95 (Fig. 3). The highest soil DTPA-extractable Zn concentration of 35 mg kg−1 was obtained at total Zn input of 467.3 kg Zn ha-1.
determined according to Lindsay and Norvell (1978). 2.4. Plant sampling and analysis At maturity stage of winter wheat in 2010, 2012, 2014 and 2016, wheat above-ground samples including straw and grain were randomly collected from two 0.5-m lengths of two adjacent rows in each plot. Both straw and grain samples were rapidly washed with deionized water and then dried at 65 °C to constant weight in oven. The ovendried samples were ground with a stainless steel grinder, and then digested using a mixture acid of HNO3-H2O2 in a microwave accelerated reaction system (CEM, Matthews, USA). Concentration of Zn in the digested solutions were determined by inductively coupled plasma atomic emission spectroscopy (ICPAES, OPTIMA 7300 DV, Perkin–Elmer, USA). Certified plant materials (IPE684 and IPE126, Wageningen University, Netherlands) were used for quality control of Zn analysis. Crop uptake of Zn was calculated as the sum of Zn content in straw and grain of wheat, which in turn was calculated by multiplying Zn concentration and dry weight in the respective part.
3.2. Crop Zn uptake The total Zn inputs over the experimental period from 2009 to 2016 in Zn treatments of 0, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1 were 0, 79.8, 159.6, 317.8 and 477.4 kg Zn ha−1, respectively (Table S1). The total Zn outputs increased with Zn rate and ranged between 3.3 and 10.1 kg Zn ha−1. The total Zn balance was -3.3, 74.6, 152.8, 308.9 and 467.3 kg Zn ha−1, for Zn treatments of 0, 5.7, 11.4, 22.7 and 34.1 kg Zn ha−1, respectively. The Zn recovery efficiency ranged between 1.4 % and 2.4 % and decreased with Zn rate. Over the experimental period, crop Zn uptake showed a clear increasing trend with the increasing total Zn input (Fig. 4). According to a non-linear regression analysis, the linear-plateau model had the best fit for the relationship between crop Zn uptake and total Zn input over the experimental period. The critical total Zn input for the maximal crop Zn uptake (978 g Zn ha−1) was 245 kg Zn ha−1.
2.5. Calculations and statistical analysis
Total Zn balance = Total Zn input − Total Zn output Total Zn input was calculated as the sum of Zn fertilizer application rate over the experimental period; Total Zn output was calculated as the sum of Zn uptake of wheat and maize over the experimental period. All samples of wheat and maize were collected and used for Zn concentration analysis.
3.3. Relationship between soil Zn availability and crop Zn uptake Stepwise multiple regression with all soil Zn fractions as variables revealed that FeO-Zn was the most important factor determining crop Zn uptake with a partial R2 of 0.81 (Table 1). The Ex-Zn showed a negatively minor impact with a partial R2 of only 0.03. In contrast, the Res-Zn showed a positively minor impact with a partial R2 of 0.04. Inclusion of other Zn fractions as input variables did not further improve the model. Overall, the model had a R2 of 0.88, which was confirmed by a positively linear relationship between the measured and predicted crop Zn uptake (Fig. 5). Over the experimental period, Zn uptake of wheat increased linearly with soil DTPA-extractable Zn, achieving the maximum value at approximately 12 mg kg−1 and was relatively stable thereafter (Fig. 6).
Zinc recovery efficiency = (Total Zn outputfertilizer − Total Zn outputcontrol)/Total Zn input × 100 %. Total Zn outputfertilizer refers to total Zn output in the 5.7, 11.4, 22.7, and 34.1 kg Zn ha−1 treatments, and Total Zn outputcontrol refers to crop Zn uptake in the unfertilized control. Data of soil Zn fractions, DPTA-extractable Zn and crop Zn uptake were subjected to rank transformation to meet the requirements for normality and homogeneity of residuals according to KolmogorovSmirnov test. The two-way analysis of variance (ANOVA) was performed using SPSS 20.0 (SPSS Inc. Chicago, IL, USA) to determine the main and interactive effects of Zn rate and growing season on concentrations of soil Zn fractions, DPTA-extractable Zn and crop Zn uptake. In the model, fertilizer rate and growing season were fixed factors and the plot replicate was random factor. When an effect was significant, treatment means were compared using Fisher’s protected least significant difference (LSD) test. Pearson correlation and multiple regression analysis (forward stepwise) were used to explore the relationships of DTPA-extractable Zn or various Zn fractions with crop uptake. Treatment means and standard errors calculated from untransformed data are presented.
4. Discussion 4.1. Soil Zn fractions as affected by repeated Zn application In the current study, Zn recovery efficiency ranged between 1.4 % and 2.4 % for a total Zn input of 0–477.4 kg Zn ha−1 over the seven years, which was consistent with results of earlier reports (Subramanian et al., 2009; Rupa et al., 2003). Compared to the short-term experiments which mainly investigated the effect of a single application of Zn fertilizer, the current study provided a better understanding of how different forms of Zn in soil in relation to crop Zn uptake were affected by repeated Zn fertilizer application over multiple growing seasons. In agreement with results from previous studies (Liu et al., 2017a), repeated applications as ZnSO4 has significantly increased Zn uptake of winter wheat in the current study. We also observed that Zn fertilizer application in the first growing season (2010) exhibited limited effect as only the highest rate (34.1 kg Zn ha−1) significantly increased Zn uptake over the unfertilized control. The ineffectiveness of one-time application of Zn fertilizer on yield or crop Zn uptake was also reported in other studies (Lu et al., 2012b), which was attributed to the poor Zn mobility and rapid adsorption of Zn on soil solids. The greater effect of Zn applications on crop uptake in later than the first growing season was in accordance with the increased concentrations of Zn fractions and DTPA-extractable Zn, confirming findings of previous studies which
3. Results 3.1. Soil Zn fractions and DTPA-extractable Zn Concentrations of all soil Zn fractions including Ex-, Car-, MnO-, FeO-, OM- and Res-Zn were affected by Zn rate, growing season and their interaction. Concentrations of all Zn fractions increased with total Zn inputs, with the increase being linear (R2 = 0.83–0.96) for Zn fractions of Car-, MnO-, FeO-, OM-, and Res-Zn, and being quadratic (R2 = 0.95) for Ex-Zn (Fig. 1). The percentage of each soil fraction to total Zn was highest for ResZn, ranging between 40 % and 70 % (Fig. 2). In contrast, the Ex-Zn fraction had the lowest percentage (0.3–3.9 %) and its percentage increased with total Zn input. The percentages of Car-, MnO-, and FeO-Zn 3
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Fig. 1. Concentrations of soil Zn fractions as affected by Zn fertilizer applications over the experimental period. Ex-Zn, Car-Zn, MnO-Zn, FeO-Zn, OM-Zn and Res-Zn represent water soluble plus exchangeable Zn, carbonate-bound Zn, manganese oxide-bound Zn, iron oxide-bound Zn, organic matter-bound Zn and residual Zn, respectively.
Fig. 2. Percentages of soil Zn fractions to total Zn as affected by Zn fertilizer applications over the experimental period. Ex-Zn, Car-Zn, MnO-Zn, FeO-Zn, OM-Zn and Res-Zn represent water soluble plus exchangeable Zn, carbonate-bound Zn, manganese oxide-bound Zn, iron oxide-bound Zn, organic matter-bound Zn and residual Zn, respectively. 4
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Fig. 3. Soil DTPA-Zn concentration as affected by Zn fertilizer applications over the experimental period.
Fig. 5. The positively linear relationship between the measured and predicted crop Zn uptake using stepwise multiple regression. The dash line is the 1:1 line.
Fig. 4. Wheat Zn uptake as affected by Zn fertilizer applications over the experimental period.
Fig. 6. Relationship between soil DTPA-extractable Zn and crop Zn uptake over the experimental period.
Table 1 Stepwise multiple regression model of soil Zn fractions to predict crop Zn uptake. Variables from Zn application rates in all growing season were used (n = 80).
investigated changes of soil Zn speciation and its relationship with crop Zn uptake in response to continuously repeated Zn applications over multiple growing seasons. While Zn application generally increased concentrations of all Zn fractions in soil, its effect on the percentage of each Zn fraction to soil total Zn differed. The sequential extraction procedure (Tessier et al., 1979) was used to partition Zn into forms, on the basis of Zn retention mechanisms in soils. The percentage of Ex-Zn to total Zn increased linearly with total Zn input showed that Zn fertilizer application was effective to increase the percentage of Ex-Zn, which was considered as the most directly plant available form (Lindsay and Cox, 1985; Reed and Martens, 1996). Increase of the percentage of Car-Zn with total Zn input was associated with high calcium carbonate content in the calcareous soils. Different responses between adsorption on Mn and Fe oxides or organic matter could be associated with the kinetics of the complexation. Li et al. (2009) reported that Zn adsorption on river sediments followed order of MnO > FeO > organic matter. Therefore, the applied Zn was more likely adsorbed on Mn oxide than Fe and organic matter and thus increased more percentage of MnO-Zn than FeOZn, whereas decreased the percentage of OM-Zn. Meantime, the percentages of all Zn fractions to total Zn except Ex-Zn kept stable when total Zn input increasing up to approximate 250 kg Zn ha−1. This illustrated that the Zn input rate reached a chemical equilibrium with main soil components in the calcareous soil (Kiekens, 1995).
Variable FeO-Zn
P < 0.001
Partial R2 0.81
Coefficient 49.8
Ex-Zn Res-Zn Constant
< 0.001 < 0.001
0.03 0.04
−111.5 14.3 −740.7
Model R2 0.88
Notes: Ex-Zn: exchangeable Zn, FeO-Zn: iron oxide Zn, and Res-Zn: residual Zn.
reported that the residual Zn fertilizer can be effective for multiple years (Gupta and Kalra, 2006). It should be noticed that a relatively large range of application rates (5.7–34.1 kg Zn ha−1) was used in the current study. For most field crops, rate of 10 kg ha−1 is recommended in clay and loam soils and 5 kg ha−1 in sandy soils (Havlin et al., 2005). The relatively higher rates were used in the current study mainly to test whether they can improve Zn concentrations in the grain of winter wheat to reach the healthy target by human consumption. This study showed that high Zn application rate of 34.1 kg Zn ha−1 (150 kg ZnSO4·7H2O) did not negatively affect crop growth or grain yield (data not shown), suggesting high rates of Zn application in the current study did not cause any phytotoxicity in winter wheat. Our previous studies also showed that the repeated high Zn application was effective to increase grain Zn concentration from 31 mg kg−1 up to 50 mg kg−1 for winter wheat (Liu et al., 2017a). To our knowledge, the current study is the first study that
4.2. The availability of Zn fractions to crop Zn uptake Previous studies which attempted to relate different forms of Zn to 5
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bioavailability in crop grains. The relationships of different forms of Zn and crop Zn uptake can be used as an important reference for Zn fertilizer management.
plant availability revealed ambiguous results (Iyengar et al., 1981; Norouzi et al., 2014). In this study, the FeO bound Zn was identified as the most important fraction contributing to crop Zn uptake in the regression model, suggesting Zn adsorption on Fe oxides can be considered labile pools that play a significant role in supplying Zn for crops. In contrast, other studies suggested FeO-Zn as non-plant available fraction due to the formation of inner-sphere complex (Ghanem and Mikkelsen, 1988). Different results between studies highlight the importance of soil properties such as pH on determining the availability of Zn fractions to crop uptake. Similar to our studies, Pradhan and Kanwar (1990) found that soil FeO-Zn was positively associated with grain Zn uptake of rice grown in a pot experiment. Han et al. (2011) also reported that FeO-Zn is one of the main fractions controlling Zn uptake by soybeans grown on 11 representative soils in Northeast China. The importance of FeO-Zn in crop uptake could be explained by previous observation of Stanton and Burger (1967) that Zn adsorption on Fe oxides was potentially plant available in spite of the strong affinity. While many studies had reported a positive correlation of Ex-Zn with crop uptake (Iyengar et al., 1981; Han et al., 2011; Norouzi et al., 2014), our study with the multiple stepwise regression model reveals Ex-Zn fraction had even a minor effect on crop uptake negatively. It is likely the small proportion of Ex-Zn had weakened its effectiveness, especially at high rates of Zn fertilizer application. In addition, the exchangeable Zn could also be converted to the adsorption forms on metal oxides, especially for calcareous soils with high pH (Havlin et al., 2005). Several studies also reported positive correlation between the OM-Zn and crop uptake, highlighting the importance of soil organic matter in the process of Zn retention (Nogueira et al., 2010; Norouzi et al., 2014). This, however, is not the case in the current study as the soil was low in organic matter content (10 g kg−1). These findings suggest that the plant availability of Zn fractions are highly dependent on soil properties. The minor but significant correlation of Res-Zn with crop uptake highlights the retention capacity of the soil, confirming the residual effect of Zn fertilizer could last for multiple growing seasons. Similar to other studies (Hamilton et al., 1993; Han et al., 2011), soil DTPA-extractable Zn showed a positive correlation with crop uptake, suggesting it can be used as a reliable index of predicting Zn availability on calcareous soils with high pH. Han et al. (2011) also reported a good correlation between soil DTPA-extractable Zn and uptake by soybean grown on major soil types in northeast China, with a similar coefficient as reported in the current study. Our study reveals the crop Zn uptake in response to DTPA-extractable Zn was linearplateau, suggesting that high DTPA-extractable Zn can’t continuously increase crop Zn uptake. The effect of DTPA-extractable Zn on crop uptake would reach the maximum at approximate 12 mg kg−1. Therefore, this level of DTPA-extractable Zn can be used as a critical concentration level above which Zn fertilizer should not be recommended.
Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was funded by the National Natural Science Foundation of China (31672240), the 973 Project (2015CB150402), and the Innovative Group Grant of National Natural Science Foundation of China (31421092). We thank Dr. Bruce Jaffee from USA for improving the English of the manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.still.2020.104612. References Alloway, B.J., 2009. Soil factors associated with zinc deficiency in crops and humans. Environ. Geochem. Health 31 (5), 537–548. Cakmak, I., Kutman, U.B., 2017. Agronomic biofortification of cereals with zinc: a review. Eur. J. Soil Sci. 69, 172–180. Cakmak, I., Yilmaz, A., Kalayci, M., Ekiz, H., Torun, B., Ereno, B., Braun, H.J., 1996. Zinc deficiency as a critical problem in wheat production in Central Anatolia. Plant Soil 180, 165–172. Cakmak, I., Torun, A., Millet, E., Feldman, M., Fahima, T., Korol, A., Nevo, E., Braun, H.J., Ozkan, H., 2004. Triticum dicoccoides: an important genetic resource for increasing zinc and iron concentration in modern cultivated wheat. Soil Sci. Plant Nutr. 50, 1047–1054. Cakmak, I., Pfeiffer, W.H., McClafferty, B., 2010. Review: biofortification of durum wheat with zinc and iron. Cereal Chem. 87, 10–20. Chen, X.P., Zhang, Y.Q., Tong, Y.P., Xue, Y.F., Liu, D.Y., Zhang, W., Deng, Y., Meng, Q.F., Yue, S.C., Yan, P., Cui, Z.L., Shi, X.J., Guo, S.W., Sun, Y.X., Ye, Y.L., Wang, Z.H., Jia, L.L., Ma, W.Q., He, M.R., Zhang, X.Y., Kou, C.L., Li, Y.T., Tan, T.S., Cakmak, I., Zhang, F.S., Zou, C.Q., 2017. Harvesting more grain zinc of wheat for human health. Sci. Rep. UK 7, 7016. D’amore, J.J., Al-Abed, S.R., Scheckel, K.G., Ryan, J.A., 2005. Methods for speciation of metals in soils. J. Environ. Qual. 345, 1707–1745. FAO, 2013. FAO Statistical Yearbook: World Food and Agriculture. FAO. FAO, 2016. http://www.fao.org/faostat/en/#home. Ghanem, S.A., Mikkelsen, D.S., 1988. Sorption of zinc on iron hydrous oxides. Soil Sci. 146, 15–21. Gonzalez, D., Obrador, A., Lopez-Valdivia, L.M., Alvarez, J.M., 2008. Effect of zinc source applied to soils on its availability to navy bean. Soil Sci. Soc. Am. J. 72, 641–649. Gupta, U.C., Kalra, Y.P., 2006. Residual effect of copper and zinc from fertilizers on plant concentration, phytotoxicity, and crop yield response. Commun. Soil Sci. Plant Anal. 37, 2505–2511. Hamilton, M.A., Westermann, D.T., James, D.W., 1993. Factors affecting zinc uptake in cropping systems. Soil Sci. Soc. Am. J. 57 (5), 1310–1315. Han, X., Li, X., Uren, N., Tang, C., 2011. Zinc fractions and availability to soybeans in representative soils of Northeast China. J. Soils Sediments 11 (4), 596–606. Havlin, J.L., Beaton, J.D., Tisdale, S.L., Nelson, W.L., 2005. Soil Fertility and Nutrient Management, 7th edn. Pearson Prentice Hall, Upper Saddle River, NJ. Huang, T., Huang, Q., She, X., Ma, X., Huang, M., Cao, H., He, G., Liu, J., Liang, D., Malhi, S.S., Wang, Z., 2019. Grain zinc concentration and its relation to soil nutrient availability in different wheat cropping regions of China. Soil Till. Res. 191, 57–65. Iyengar, S.S., Martens, D.C., Miller, W.P., 1981. Distribution and plant availability of soil zinc fractions. Soil Sci. Soc. Am. J. 45 (4), 735–739. Jalali, M., Khanlari, Z.V., 2008. Effect of aging process on the fractionation of heavy metals in some calcareous soils of Iran. Geoderma 143 (1-2), 26–40. Kiekens, L., 1995. Zinc. In: Alloway, B.J. (Ed.), Heavy Metals in Soils, 2nd edn. Blackie Academic and Professional, London, pp. 284–305. Li, Y., Wang, X.L., Huang, G.H., Zhang, B.Y., Guo, S.H., 2009. Adsorption of Cu and Zn onto Mn/Fe oxides and organic materials in the extractable fractions of river surficial sediments. Soil Sediment Contam. 18 (1), 87–101. Lindsay, W.L., Cox, F.R., 1985. Micronutrient soil testing for the tropics. Fertil. Res. 7, 169–200. Lindsay, W.L., Norvell, W.A., 1978. Development of a DTPA soil test for zinc iron manganese and copper. Soil Sci. Soc. Am. J. 42, 421–428. Liu, D.Y., Zhang, W., Pang, L.L., Zhang, Y.Q., Wang, X.Z., Liu, Y.M., Chen, X.P., Zhang, F.S., Zou, C.Q., 2017. Effects of zinc application rate and zinc distribution relative to
5. Conclusions Repeated applications of Zn fertilizer over multiple years have greatly increased Zn uptake by winter wheat and concentrations of all Zn fractions in the soil, with the increase being more evident at higher rates. Over the experimental period, there was a linear or quadratic increasing relationship between concentration of Zn fractions and total Zn input. Zinc application also increased the percentage to total Zn of Ex-, Car-, MnO- and FeO-Zn fractions, whereas had no effect or even reduced the percentages of OM- and Res-Zn. Crop Zn uptake increased linearly with soil concentrations of DTPA-extractable Zn, and reached the maximum at 12 mg kg−1, which can be considered as the maximum level for recommendation of Zn fertilizer application. The stepwise regression procedure had selected FeO-Zn as the most important variable affecting Zn uptake of winter wheat, suggesting the bond of Zn on Fe oxides dominated Zn availability for the calcareous soil if repeated applications of Zn fertilizer are used for improvement of Zn 6
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