J Soils Sediments DOI 10.1007/s11368-015-1342-9
SOILS, SEC 1 • SOIL ORGANIC MATTER DYNAMICS AND NUTRIENT CYCLING • RESEARCH ARTICLE
Effects of phosphate on trace element accumulation in rice (Oryza sativa L.): a 5-year phosphate application study Fei Dang 1 & Wen-Xiong Wang 2 & Huan Zhong 3 & Shenqiang Wang 1 & Dongmei Zhou 1 & Yu Wang 1
Received: 20 October 2015 / Accepted: 15 December 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Purpose Phosphate (P) fertilizers are being widely used to increase crop yield, especially in P-deficient soils. However, repeated applications of P could influence trace element bioaccumulation in crops. The effects of 5-year P enrichment on trace element (Cu, Zn, Cd, Pb, As, and Hg) accumulation in Oryza sativa L. were thus examined. Materials and methods Two paddy soils with different initial P availabilities were amended with and without P fertilizer from 2009 to 2013. Trace elements and P levels in rice and soils were analyzed. Results and discussion In soil initially with limited P, P amendment enhanced grain Pb, As, and Hg concentrations by 1.8, 1.5, and 1.4-fold, respectively, but tended to decrease the grain Cd level by 0.73-fold, as compared to the control. However, in soil initially with sufficient P, P amendment tended to reduce accumulation of all examined elements in rice grain.
Responsible editor: Leo Condron * Dongmei Zhou
[email protected] * Yu Wang
[email protected]
1
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, People’s Republic of China
2
Division of Life Science, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong
3
State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing 210023, China
Conclusions Phosphate amendment in initially P-limited and P-sufficient soils had different effects on trace element availability in soil (as reflected by extractable element) and plant physiology (growth and metal translocation), resulting in contrasting patterns of trace element accumulation in rice between the two types of soils. Our study emphasized the necessity to consider the promoting effects of P on Pb, As, and Hg accumulation in grain in initial P-deprived soil. Keywords Bioaccumulation . Chemical extraction . Metal . P amendment . Paddy soil
1 Introduction Phosphorus is often the limiting macronutrient for plant growth and development because of its low availability and mobility in soils. To overcome this problem, phosphate (P) is widely applied to improve a sustainable crop production (Withers et al. 2014). However, P fertilizer is also reported to be an important source of non-essential elements (especially toxic metal/metalloid, e.g., cadmium, arsenic, lead) (Chen et al. 2007; Jiao et al. 2012) into agricultural soils, and much attention has been paid to the release of trace element in accompany with P fertilization. Yet the potential effects of P itself on the availability, bioaccumulation, and distribution of trace elements (especially non-essential elements) within plant are largely unknown. The effects of P on plant response to trace elements are complex, involving both physiological process and chemical reaction in soil: (1) On one hand, phosphate amendment could increase plant productivity, which should in principle reduce trace element concentrations by somatic growth dilution (SGD). Indirect evidence of SGD can be found from the deficiency of micronutrients such as Zn and Fe when the crops
J Soils Sediments
were under fast growth (e.g., leading to a lower tissue concentration) induced by high P supply (Lambert et al. 1979; Misson et al. 2005). However, the effects of P-stimulated growth on the bioaccumulation of non-essential trace elements such as Cd, As, Pb, and Hg from soils to plants under natural conditions are largely unknown. Their responses to amended P may differ from those essential trace elements, which are likely to be under active physiological regulation, e.g., reduced uptake rates and/or changed metal allocation patterns (Hall 2002). Furthermore, the initial P availability in soils may largely affect the response of plant growth to P amendment and thus may play a role in bioaccumulation of trace elements. (2) On the other hand, P could modify trace element mobility and availability in bulk soil via surface complexation, ion exchange, and precipitation (Bolan et al. 2003). In contaminated soils, P has been widely used to immobilize metals and reduce their phytoavailability (Hettiarchchi et al. 2000). However, the potential effects of P amendment on the phytoavailability of trace elements (especially non-essential elements) in paddy soils with the baseline metal concentrations have received little attention. In this study, we aimed at answering two fundamental questions about the effects of P on trace element accumulation in rice plants: (1) How P amendment could affect plant growth and phytoavailability of trace elements (Cu, Zn, Cd, Pb, As, and Hg) and thus modify accumulation of elements in rice plants; (2) Are the effects of amended P on trace element bioaccumulation dependent on initial P availability in soils? To answer the questions, two paddy soils with contrasting initial soil P availability were amended with or without P for 5 years, which may subsequently result in differences in plant growth and trace element availability. Both mass- and concentration-based accumulation of trace element in plants were quantified to further explore the changes in element bioaccumulation due to modified growth rates. Besides, both essential (Cu and Zn) and non-essential (Cd, Pb, As, and Hg) trace elements were considered in this study, considering that they may respond differently to P amendment. Among these trace elements, contamination of Cd, As, Hg, and Pb in rice has been of utmost concerns in China.
2 Materials and methods 2.1 Soils and study sites Two paddy soils, namely Pdeficient (hereafter referred to initial P-deficient soil) and Psufficient (hereafter referred to initial Psufficient soil), were collected between 0 and 20 cm depth in profile from Yixing city, Jiangsu Province, China in 2009. No clear point-source contamination was noted. According to the Soil Classification Working Group, Pdeficient and Psufficient were Wushan and Huangni soil, respectively. Soils were air-dried
and sieved to 5 mm. After that, soils were analyzed for total and Olsen phosphate (molybdate blue colorimetric assay), pH (1:10 to water) and organic matter content (acid dichromate, Waikley-Black method). The two soils differed dramatically in initial Olsen-P levels (Table 1). Specifically, trace element concentrations, which were environmental relevant, were generally comparable between the two soils. Then the effects of amended P on trace element accumulation in rice were investigated in both soils with different P availability. The long-term P enrichment experiment was subsequently conducted in Nanjing Institute of Soil Science, Chinese Academy of Sciences, using open-air chambers (polyvinyl chloride) with 6 kg soil in each chamber. In total, four treatments were used in this study, with three replicates per treatment: For BPsufficient − control^ or BPdeficient − control^ treatment, initial Psufficient or Pdeficient soil was watered with P-free deionized water. As for BPsufficient + P^ or BPdeficient + P^ treatment, initial Psufficient or Pdeficient soil was watered with monocalcium phosphate (1.65 g Ca(H2PO4)2, analytical reagent from Sigma) twice per year. The P fertilizing rate was 1 g P2O5 chamber−1 year−1, which was within the recommended value for rice (Dobermann et al. 1996). Thus, Ca(H2PO4)2 was the only phosphate source, effectively eliminating any potential metal input due to commercial P fertilizer (Table 1). Nitrogen and potassium fertilizers were applied basally in the form of urea at a rate of 1.1 g N chamber−1 and as KCl at a rate of 1 g K2O chamber−1, respectively. From 2009 to 2013, the rice Oryza sativa L. was cultured from May to November each year, flooded (5 cm in depth) throughout the experimental period, except for mid-season aeration. The shoots were harvested at grain maturity without removing the roots (approximate 120 days after planting), which is common in farming practices. This may explain the increased soil organic matter concentrations at the end of experiment compared to that in 2009 (Table 1). 2.2 Sampling and analysis Sampling of soil and rice plants was conducted in November 2013 after 5 years of P amendment and cropping. Soils were collected from 0 to 20 cm with a 3-cm diameter soil core at four locations within each chamber. The sampled soils were freeze-dried, mixed thoroughly, and sieved to 2 mm until further analysis. Aboveground plants were separated into unhusked grain and straw, rinsed with deionized water thoroughly, and oven-dried at 50 °C. Roots were hand-sampled, cleaned in ultrasonic bath to remove the adsorbed soil particles, and then freeze-dried. All plant tissues were ground to 20 mg kg−1; Li et al. 2011) in Pdeficient + P and Psufficient + P treatments (Table 1 and Fig. 1). 3.2 Trace element availability and bioaccumulation under P amendment Relative variation (%, (elevated − control) / control × 100) in trace element availability in soils or total element content in plant after 5-year P amendment are depicted in Fig. 3. The effects of amended P on trace element availability in soils (as reflected by the extractable element concentrations, Fig. 3b) depended on soils and trace elements. For Pdeficient soil, P amendment increased markedly the extractable Cu and As but reduced the extractable Cd (p < 0.05). For Psufficient soil, P amendment enhanced the extractable As but decreased the extractable Cd (p < 0.05). Neither soil showed significant differences in extractable Zn and Hg under P enrichment.
Fig. 2 Tissue biomass of rice Oryza sativa L. grown in soils after 5 years of P enrichment (+P) compared to those in unamended soils (control). Pdeficient and Psufficient are two soil types with different initial P availability. Data are means ± SD (n = 3). Single and double asterisks indicate significant difference at p < 0.05 and p < 0.01 between Pdeficient − control and P deficient + P or between Psufficient − control and P sufficient + P’, respectively
Fig. 3 Relative effects of 5-year P enrichment on total element content in rice plant (a) or on extractable element concentrations in soils (b) after 5year P amendment. Calculated as (elevated − control) / control × 100. Total element content in plant calculated as grain element concentration × grain mass + straw element concentration × straw mass + root element concentration × root mass. Single and double asterisks indicate significant differences at p < 0.05 and p < 0.01 between the control and P amendment treatments, respectively
Mass-based element accumulation in whole plant (μg plant−1, which described the amount of trace elements taken up by plant from the ambient environment, referred as total element content in plant hereafter) is depicted in Fig. 3a. Phosphate amendment had differential effects on total element content in plant between soils. For Pdeficient soil, P amendment substantially enhanced total element content in plant: total Cu, Zn, As, and Hg in plant increased by 120, 80, 140, and 120 % for Cu, Zn, As, and Hg, respectively (Fig. 3a, p < 0.05). Furthermore, total P in plant had strong correlations with those of Cu, Zn, As, and Hg (Pearson’s r = 0.93, 0.97, 0.94, and 0.98, respectively) (Table 2). Total Cd and Pb within plant did not show significant difference following P amendment in Pdeficient soil (p > 0.05, Fig. 3a). In contrast to Pdeficient soil, total element contents in plant were less affected by P amendment in Psufficient soil (Fig. 3a). Most trace elements showed a marginal decrease in total element content in plant. Only total Cd in plant was significantly reduced (p < 0.05) and correlated with P in Psufficient soil (Pearson’s r = −0.96, Table 2). Concentration-based element accumulation (i.e., mg kg−1 dw) in grain and other plant tissues are shown in Figs. 4 and 5.
J Soils Sediments Table 2 Pearson correlation coefficients (r) of P with the total element content per plant (μg plant−1) or with grain trace element concentrations (mg kg−1 or μg Hg kg−1) Cu
Zn
Total element content per plant Pdeficient 0.93** 0.97** −0.33 Psufficient −0.46
Cd
Pb
As
Hg
0.04 −0.96**
−0.69 −0.67
0.94** −0.02
0.98** 0.38
0.89* −0.48
0.83* −0.13
0.97** −0.17
Grain trace element concentrations 0.32 −0.82* Pdeficient 0.35 −0.08 −0.77 Psufficient −0.37 *p < 0.05; **p< 0.01 (significant correlation)
Grain in Pdeficient + P was elevated in Pb (1.8-fold), As (1.5fold), and Hg (1.4-fold) relative to the Pdeficient − control (p < 0.05). Moreover, P concentration correlated significantly with Cd, Pb, As, and Hg in grain (Pearson’s r = −0.82, 0.89, 0.83, and 0.97, respectively) (Table 2). However, in Psufficient soil, no significant differences in grain element concentrations were observed (i.e., Psufficient + P versus Psufficient + control). Although the average grain Cd decreased by 27 and 44 % (p > 0.05) as grain P increased for Pdeficient and Psufficient soil, respectively, the decrease was not significant. In addition, responding to P stimulation of rice growth, the resultant whole-plant concentration of trace elements exhibited a decline pattern following P amendment (Fig. 5), e.g., reducing by 8 % (As)—78 % (Pb) in Pdeficient soil and 4 % (Hg)—59 % (Cd) in Psufficient soil, respectively. And element concentrations in straw or root generally decreased significantly following P amendment. Translocation factor of trace element from root to straw (TF root-s traw, calculated as Trace element straw /Trace elementroot) was less affected by amended P, with the exception of Pb and As in Pdeficient or Hg in Psufficient soil (Fig. 6a, p < 0.05). In contrast, P amendment produced more variations in TFstraw-grain, e.g., Zn, Cd, Pb, As, and Hg in Pdeficient soil increased considerably, and Cu, Zn, and Pb in Psufficient soil were also enhanced (Fig. 6b, p < 0.05).
Fig. 4 Trace element concentrations in rice grains after 5 years of P enrichment (+P) in Pdeficient and Psufficient soils. Control: without P amendment. Data are means ± SD (n = 3). Single and double asterisks indicate significant differences at p < 0.05 and p < 0.01 between the control and P amendment treatments, respectively
4 Discussion Our results indicated that phosphate amendment produced soil-specific (i.e., Pdeficient and Psufficient) and trace elementspecific (Cu, Zn, Cd, Pb, As, and Hg) bioaccumulation in rice plants. Given that amended P did not modify trace element concentrations in soil relative to the control (Fig. 5), the observed variations in trace element bioaccumulation (massbased or concentration-based) under P amendment should be associated with differences in initial P availability in soils, as well as variations in trace element availability in soils and physiological changes in plant under P amendment, as discussed below.
4.1 Phosphate effects on mass-based element accumulation Our results demonstrated that the effects of P amendment on element bioaccumulation (i.e., mass-based element accumulation) were element-dependent and soil-dependent: P amendment enhanced total Cu, Zn, As or Hg significantly in plant in Pdeficient soil, but tended to decrease most trace element contents in plant in Psufficient soil (Fig. 3a). The observed soilspecific effects of P amendment on element bioaccumulation could be most likely associated with the differences in initial soil P availability as the distinct characteristics between Pdeficient and Psufficient soil (Table 1 and Fig. 1). This could be attributed to the following: (1) in initial Pdeficient soil where Plimited growth, P amendment resulted in higher plant growth and larger biomass than that in initial Psufficient soil (Fig. 2). For instance, root biomass in Pdeficient + P was 3.0-fold higher than that in Pdeficient − control (p < 0.05), but comparable between Psufficient − control and Psufficient + P (Fig. 2). Consequently, the evident increase in root biomass could maximize the interaction at the root-soil interface, allowing more nutrient acquisition (Péret et al. 2011) and coincident trace element uptake from Pdeficient soil. (2) The lower initial P availability in Pdeficient soil resulted in higher P uptake by plants in response to P amendment when compared to those in Psufficient soil
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Fig. 5 Trace element concentrations in different rice tissues and soils after 5 years of P enrichment (+P) in Pdeficient and Psufficient soils. Control: without P amendment. Data are means ± SD (n = 3). Note that the unit of Hg concentration is μg kg−1. Single and double asterisks indicate significant differences at p < 0.05 and p < 0.01 between the control and P amendment treatments, respectively
(Fig. 3a). High P uptake in Pdeficient soil may facilitate plant uptake of trace elements, as evidenced by the positive relationship between P and trace element contents in plant (e.g., Zn, As, and Hg, Table 2). It has been reported that P amendment could also meet the demand for P-involving energy-requiring metabolic and transport processes (Plaxton and Carswell 1999) and balance the anion/cation ratio (Misson et al. 2005). Besides, a large proportion of P was stored as vacuolar orthophosphate, polyphosphate, and phytate (Veneklaas et al. 2012) within plant cells, which were primarily the sink and source for phosphorus (Panigrahy et al. 2009), but also the important binding sites for metals (Wang et al. 1991; Sarret et al. 2002; Haydon and Cobbett 2007; Vogel-Mikuš et al. 2010; Gupta et al. 2014). Therefore, P amendment may increase metal sequestration within plant cells and thus trace element uptake in Pdeficient soil. Meanwhile, the element-specific effects of P amendment on element bioaccumulation could be partially explained by the variation in trace element availability in soils under P amendment. The residual P in soils, which was not taken up by plant but adsorbed by iron/aluminum (hydro)oxides or precipitated as Ca-P minerals in soil (Beauchemin et al. 2003), could modify trace element availability in soils. For Pdeficient soil, the increased available Cu and As levels (Fig. 3b), resulting from amended P-induced Cu desorption (Bolan et al. 2003) and competition for soil sorption sites between amended P and As (Smith et al. 2002; Bolan et al. 2013),
Fig. 6 Translocation factors (TFs) of P and trace elements from root to straw (a) and from straw to grain (b) in rice after 5 years of P enrichment (+P) in two soils (Pdeficient and Psufficient). Control: without P amendment. Data are means ± SD (n = 3). Single and double asterisks indicate significant differences at p < 0.05 and p < 0.01 between the control and P amendment treatments, respectively
could facilitate Cu and As uptake by plants and consequently total Cu and As content in plant (Fig. 3a). However, this was not observed in Psufficient soil, and the difference in trace element availability between soils in response to P amendment remained further investigation. For Psufficient soil, the decreased extractable Cd was probably attributed to the Pinduced Cd immobilization in soil (Bolan et al. 2003; Cao et al. 2009) and thus led to the largest decline in total Cd in plant as compared to other trace elements tested in this study (Fig. 3a). Earlier, a similar pattern of decline in metal bioavailability (e.g., Pb, Zn) was also reported under P enrichment in metal contaminated sites, mainly resulting from the conversion of more labile species to stable phosphate minerals in bulk soil (e.g., pyromorphite-like mineral) (Laperche et al. 1997; Cao et al. 2002; Bolan et al. 2003; Chen et al. 2007). However, the changes in extractable trace elements in bulk soil alone were insufficient to explain the variations in massbased element accumulation following P amendment. For example, the enhanced accumulation of Zn and Hg under P amendment in Pdeficient soil (1.8- and 2.2-fold, respectively) cannot be attributed to the comparable extractable Zn and Hg concentrations between Pdeficient + P and Pdeficient − control
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(Fig. 3b). Enhanced Zn bioaccumulation at moderate Zn application rate, despite comparable available concentrations in soil, was also noted previously (Loneragan et al. 1979). In this case, it appears to be an indirect effect of P in stimulating growth and biomass, which would increase the effectiveness of root in obtaining trace element from soil, as discussed previously. 4.2 Phosphate effects on concentration-based element accumulation Phosphate effects on trace element concentrations in various tissues were further explored. Our results demonstrated that P stimulation of plant growth diluted trace element in plant, and reduced the whole-plant concentrations of trace elements, consistent with somatic growth dilution (SGD, Fig. 5). Besides growth dilution, the variation in total element content in plant could also be responsible for element-specific variation in whole-plant concentrations. For instance, the decrease in total Cd in plant (Fig. 3a) and SGD allowed for a 63 % decline in whole-plant Cd concentration in Pdeficient soil (Fig. 5). Trace element concentrations in grain also varied among elements. Variations in the concentrations of essential elements (e.g., Cu, Zn) in response to P amendment in both soils were much smaller than those of non-essential trace elements (e.g., As, Cd, Hg, Pb), probably due to the tighter physiological regulation of essential elements (Clemens 2006). The decline pattern of grain Cd concentration (albeit not statistically significant) was probably attributed to the coincident effects of decreased total Cd content in plant and SGD, as stated previously and elsewhere (Sarwar et al. 2010). Of particular interest in our study was the concentrations of Hg, As, and Pb in rice grain in Pdeficient soil, which was significantly higher following P amendment even considering the P amendment-stimulated plant growth and the consequent somatic growth dilution. Concentrations of those trace elements were highly coupled with the increasing concentration of P in rice grain (Table 2). At the physiological level, the increasing grain element concentrations were partially associated with the P-enhanced total As and Hg content in plant (Fig. 3a) and P-increased translocation from straw to grain for Pb, As, and Hg (Fig. 6b). Meanwhile, the more evident variations in TFstraw-grain than TFroot-straw might indicate that P mainly affected the distribution of trace elements in grain. Recent studies showed that the node played an important role in distributing trace elements (e.g., Cu, Zn, Cd, Mn) to rice grain (Yamaji and Ma 2014). However, the molecular mechanisms underlying the apparent Bsynergetic effect^ between P and As/Pb/Hg in rice grain from Pdeficient soil remain largely unknown at present. For As, the grain dimethylarsinic acid concentrations of four rice lines also tended to increase under P amendment in flooded soils (Wu et al. 2011). To our
knowledge, this study is the first to report that long-term P amendment increased accumulation of Hg, As, and Pb in rice grain in initially P-limited environment relative to the control, which is suggestive of many research avenues. Trace element concentrations in both control soils (i.e., Pdeficient − control and Psufficient − control) increased from 2009 to 2013 (Table 1). It was plausible that irrigation water, nitrogen and potassium fertilizers, and atmospheric deposition caused the significant direct input of trace elements, which were not specially monitored in this study. Trace element input from P fertilizer during a 5-year application (Table 1) resulted in concentrations of
