Phosphorus–arsenic interactions in variable-charge soils in relation to arsenic mobility and bioavailability
Graphical abstract
Phosphate (H2PO4) ions compete for As(V) adsorption by soil particles (a), and for As(V) absorption by plant roots (b).
Introduction
Certain hazardous elements including metal(loid)s in soil and water are becoming a serious problem in many countries, threatening human and animal health, and causing toxicity to plants and microorganisms. For example, significant detrimental impacts of arsenic (As) on human health have been recorded in some endemic areas of Bangladesh, India, Chile, and China, and millions of people are potentially at risk from As poisoning (Bhattacharya et al., 2012, Mahimairaja et al., 2005). In these countries, As reaches food chain through the use of potable water and plant uptake resulting from the irrigation of As-rich water. Historically, arsenical compounds have been used in many countries including Australia and New Zealand in cattle/sheep dipping vats for treating ticks in animals, resulting in significant As accumulation in pasture soils (Levot, 2007, McLaren et al., 1997, Sarkar et al., 2007). Unlike organic contaminants, metal(loid)s do not undergo microbial or chemical degradation and persist for a long time in soils after their introduction (Adriano et al., 2004, Ribeiro et al., 2013). With greater public awareness of the implications of contaminated soil and water on human and animal health, there has been increasing interest amongst the scientific community in the development of technologies to remediate metal(loid)-contaminated soil and water.
Remediation of metal(loid)-contaminated soils can be managed by manipulating the bioavailability of metal(loid)s. Phosphate (Pi) compounds have often been used to increase the immobilization of metal(loid)s such as lead (Pb) and zinc (Zn), thereby reducing their mobility and bioavailability in soil (Bolan et al., 2003, Song et al., 2009). Alternatively soil remediation can be achieved by metal(loid)s' mobilization processes that include phytoremediation and chemical washing. Metal(loid)s' mobilization can be enhanced by the addition of soil amendments such as chelating (e.g., EDTA for Cu removal) and desorption (e.g., Pi for As removal) agents (Cao et al., 2003, Lestan et al., 2008).
Large quantities of phosphate fertilizers are used as a nutrient source in agricultural and pasture soils. Phosphorus influences As uptake by plants by two processes. Firstly, it is well known that arsenate [As(V)] acts as a Pi analog and is taken up by plants via a Pi transporter system (Meharg and Macnair, 1992). While P deficiency can enhance As uptake, high concentration of Pi in soil solution can inhibit the uptake of As (Lei et al., 2012, Pigna et al., 2009). Secondly, Pi is more strongly adsorbed to soil than As(V) and hence competes for sorption sites, thereby facilitating the desorption of As to soil solution and its subsequent uptake by plants (Ravenscroft et al., 2001). Thus the influence of P on As mobilization depends on the charge characteristics of soils.
Although a number of studies have examined the potential value of P compounds in the immobilization of Pb and Zn in contaminated soils (Cao et al., 2009, Kumpiene et al., 2008), there has been limited work on the value of P compounds in the mobilization of As in variable-charge soils, thereby increasing its bioavailability (Mkandawire et al., 2004). Furthermore, there has been only limited work comparing the effect of Pi on the uptake of As between solution culture and soil systems (Table 1). In this study, As–P interaction in relation to As mobility and bioavailability has been examined in soils which varied in their variable-charge components.
Section snippets
Soil samples
Eight surface (0–15 cm) non-allophanic and allophanic soils from New Zealand, which vary in their variable-charge characteristics, were used to examine the effect of P on As adsorption and desorption (Table 2). Subsequently two of these soils (allophanic Patua and non-allophanic Manawatu) were used to examine the effect of P on As bioavailability. The soils contain different amounts of a number of variable-charge components, such as organic matter, allophane, kaolinite, and iron (Fe) and
Arsenate adsorption and desorption
Arsenate adsorption increased with an increase in the anion adsorption capacity of soils as measured by the phosphate retention (PR) test. Arsenate adsorption ranged from 25 to 37 mg kg− 1 and from 256 to 465 mg kg− 1 in non-allophanic and allophanic soils, respectively. However, the addition of P resulted in a decrease in As adsorption, and the P-induced decrease in As adsorption increased with an increase in PR (Fig. 1a). Phosphate addition decreased As(V) adsorption by 11.2–17.8% and 87.4–92.3% in
Discussion
A number of studies have reported the P-induced As mobility and bioavailability in soil (Davenport and Peryea, 1991, Cao and Ma, 2004, Chen et al., 2002, Peryea, 1991). However, this is one of the first integrated studies in which the relationship between anion retention capacity of soil and P-induced As mobility and bioavailability has been examined in variable-charge soils. This study also compared the P-induced As bioavailability between a soil system and a solution culture. The potential
Conclusions
The results reported in this study indicated that the effect of P on the mobilization and bioavailability of As varied between allophanic and non-allophanic soils and also between soil system and solution culture. The P-induced plant uptake of As in solution culture and soil system can be attributed to competition of P for both As adsorption by soil particles and absorption by plant roots (Fig. 5). In the solution culture system, P addition competed for As absorption by plant roots thereby
Acknowledgments
We would like to thank Massey University Research Foundation for the award of the Postdoctoral Fellowship. We would also like to thank Dr Chris Anderson for his help in As analysis. The Postdoctoral fellowship program (PJ008650042012) at the National Academy of Agricultural Science, Rural Development Administration, Republic of Korea, supported Dr Kunhikrishnan's contribution.
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