Major agricultural area of Pakistan lies in arid and semi-arid regions and climate changes have worsen the severity in temperature. Less rainfall and increasing demand of fresh water for non-agriculture purpose has caused water shortage for agriculture sector and have necessitated the use of untreated raw city effluents for agriculture which is steeply increasing. These effluents carry variety of non-point inorganic and organic pollutants, more dominantly abundance of heavy metals. Among these heavy metals, cadmium (Cd) is more important due to its highly mobile nature in contaminated soils and extremely toxic at relatively low concentrations for plants and animals compared to other metals.
The major sources of Cd entry into environment are industrial processes, phosphatic fertilizers, raw city effluents, sewage sludge and unlimited use of nickel-cadmium batteries etc. Cd toxicity causes unavoidable loses to plants by interrupting several biochemical and physiological attributes of plants including photosynthesis, respiration, chlorophyll content, nitrogen metabolism, stomatal conductance, and nutrient imbalance. Cd permissible limits of the grain crops for human consumption varies from 0.01 to 0.02 mg kg-1 of the dry matter. Cd is more toxic to humans than that of plants, being non-digestible and exalted accumulation in life regulating organs due to consumption of Cd contaminated food stuff. In humans Cd mainly affects the kidney and/ or renal cortex, liver, testes along with demineralization of bones. Besides this, it also affects the pulmonary, cardiovascular and muscular systems. Cd has also been proven as a human carcinogenic agent.
So, there is a dire need of different practices to breakdown the channel through which Cd enters into plants, by using soil applied amendments to immobilize Cd in soils and decreasing its concentration in plant parts. There are different strategies for in situ and ex situ remediation of Cd contaminated soils and sediments. These techniques include immobilization, phyto-extraction, phyto-stabilization, excavation, physical separation, and extraction of Cd. Among these, the use of inorganic amendments is practically feasible choice. In this remediation technique cheaper and easily available chemicals are used to decrease phyto-availability of toxic metals in contaminated soils. These amendments include calcium (Ca) and phosphorus (P) containing and many other inorganic amendments.
P is included in 17 essential elements that are required for plant growth and completion of plant reproduction cycle. P is involved for energy storage and transfer, maintenance and transfer of genetic code, and is the structural component of plant cell. Besides these, it is also an element that affects the bioavailability of other elements. The idea of remediation of metal polluted soils with phosphatic compounds has been shown as good strategy and has been proposed as an alternative to removal of contaminated soils. The phosphatic amendments lower the soluble concentration of metals through precipitation, specific adsorption or complexation. Phosphates induce change in pH of soil which ultimately influences the solubility of Cd in soils. It has been reported that soluble phosphorus containing amendments could increase soluble phosphorus in soil that results into the formation of cadmium phosphate at soil solution interphase.
Like phosphorus, Ca is also one of plant nutrients. Ca involves in plant cell elongation, participates in metabolic processes of other nutrients uptake, strengthens cell wall structure, participates in enzymatic and hormonal processes and helps in protecting the plant against heat stress. Moreover, different Ca compounds also play important role as amendments in metal contaminated soils. Among Ca containing amendments, gypsum is the best option for economic remediation of metals contaminated soils. Regular use of gypsum is essential to the sustainability of sewage irrigated soils because gypsum helps in immobilization of different heavy metals particularly Cd in such soils and reduces metal contamination. Gypsum induced immobilization of Cd is due to the increased negative charges (CEC) in charge variable soils, formation of strongly bound hydroxyl metal species, precipitation of metals as hydroxides, and sequestration due to enhanced microbial activity. Calcium released from gypsum followed by plant absorption could prevent the translocation of Cd from roots to shoots. In this way P and Ca containing amendments help in immobilization of Cd in contaminated fields.
Wheat-rice crop rotation is extensively practiced on large areas in Pakistan particularly in Punjab Province. Bio-accumulation of Cd in wheat and rice crops has serious implications to animal and human health, and to local and international cereal marketing. As redox potential in rice fields decreases due to puddling practice and water-logging conditions that may reduce pH of soils. Due to decrease in pH, Cd availability increases. But in the case of wheat due to oxidized condition of fields Cd mobility is not higher as in rice growing fields but continuous use of sewage sludge may cause too much increase in its concentration in soils and ultimately in wheat crop.
Thus, it is a supportive recommendation for the farmers to use gypsum as an amendment for Cd immobilization and for the production of Cd free food stuff from Cd contaminated soils due to high cost benefit ratio as compared to phosphates. No doubt phosphates have also been proven as efficient immobilizing agent but these are less economical compared to gypsum. So, the use of gypsum would be eco-friendly, economical and proficient approach for getting good quality production from Cd contaminated fields receiving raw city effluents for irrigation purposes.
This article is collectively authored by Muhammad Zia-ur-Rehman, Aqsa Fatima, Komal and Hinnan Khalid.