Metal uptake via phosphate fertilizer and city sewage in cereal and legume crops in Pakistan

Abstract

Crop irrigation with heavy metal-contaminated effluents is increasingly common worldwide and necessitates management strategies for safe crop production on contaminated soils. This field study examined the phytoavailability of three metals (Cd, Cu, and Zn) in two cereal (wheat, maize) and legume (chickpea, mungbean) crops in response to the application of either phosphatic fertilizer or sewage-derived water irrigation over two successive years. Five fertilizer treatments, i.e. control, recommended nitrogen (N) applied alone and in combination of three levels of phosphorus (P), half, full and 1.5 times of recommended P designated as N0P0, N1P0, N1P0.5, N1P1.0, and N1P1.5, respectively. Tissue concentrations of Cd, Cu, Zn, and P were determined in various plant parts, i.e., root, straw, and grains. On the calcareous soils studied while maximum biomass production was obtained with application of P at half the recommended dose, the concentrations of metals in the crops generally decreased with increasing P levels. Tissue metal concentrations increased with the application of N alone. Translocation and accumulation of Zn and Cu were consistently higher than Cd. And the pattern of Cd accumulation differed among plant species; more Cd being accumulated by dicots than monocots, especially in their grains. The order of Cd accumulation in grains was maize > chickpea > mungbean > wheat. Mungbean and chickpea straws also had higher tissue Cd concentration above permissible limits. The two legume species behaved similarly, while cereal species differed from each other in their Cd accumulation. Metal ion concentrations were markedly higher in roots followed by straw and grains. Increasing soil-applied P also increased the extractable metal and P concentrations in the post-harvest soil. Despite a considerable addition of metals by P fertilizer, all levels of applied P effectively decreased metal phytoavailability in sewage-irrigated soils, and applying half of the recommended dose of P fertilizer was the most feasible solution for curtailing plant metal uptake from soils. These findings may have wide applications for safer crop production of monocot species when irrigating crops with sewage effluent-derived waters.

Bioaccumulation of Potentially Toxic Elements in Cereal and Legume Crops: A Review

“…It has been studied that P application to the soil decreases the phytoavailability of Cd from contaminated soil . Application of half the recommended dose of P, as practiced by farmers, seems to be an economical option for the production of cereal and legume crops on sewage effluent irrigated soils …”

“…The Cd content of P fertilizers varies depending upon the source of the raw rock phosphate used in its manufacturing process. Murtaza et al reported that application of P fertilizer reduced soil solution Cd and its mobility by precipitating the Cd in the low solubility products such as Cd 3 (PO 4 ) 2 , Cd 2 PO 3 , CdCO 3 , or Cd(OH) 2 which tend to be recalcitrant and generally unavailable to cereals (wheat, maize) and legumes (chickpea, mung bean). The reduction of Cd in plant tissues may be due to P‐induced sorption of Cd in the soil .…”

“…The reduction of Cd in plant tissues may be due to P‐induced sorption of Cd in the soil . Phosphorus amendments to contaminated soils decrease the availability of Cd and Pb due to the formation of insoluble metal‐phosphate compounds …”

“…Unfortunately, application of the untreated wastewater also results in the deposition of various PTEs and salts in soils over time . Thus, growing food/fodder crops on soils irrigated with the metal/metalloid‐contaminated wastewater can lead to bioaccumulation of toxic metals within plants and ultimately to human food chain . The concentration of metal(loid)s in plant tissues and the quantity of plants consumed as food determines the levels in the human body and associated health risks.…”

“…[4][5][6][7] Thus, growing food/ fodder crops on soils irrigated with the metal/metalloidcontaminated wastewater can lead to bioaccumulation of toxic metals within plants and ultimately to human food chain. [8] The concentration of metal(loid)s in plant tissues and the quantity of plants consumed as food determines the levels in the human body and associated health risks. Some metal(loid)s have known metabolic functions but this is not the case for As, Cd, Hg, and Pb.…”