Phyto- Availability of Cadmium to Rice and Wheat in Contaminated Soils Treated With Organic Amendments

Abstract

Cadmium (Cd) pollution is an emerging concern in many industrialial areas, mainly where raw effluents released from industry and cities. In many developing countries like Pakistan these effluents are used to raise grain crops and vegetables despite someof theses effluents having higher levels of metals such as Cd. In this thesis we investigate Cd availability and accumulation in wheat and rice following a series of four experiments. The first experiment was a hydroponic study executed to explore the effect of Cd on growth and micronutrient accumulation in wheat and rice. Cadmium was applied in three levels i.e. 5, 10 and 15 µM and nutrient solution was Hoagland. Results indicated that growth of both wheat and rice was significantly affected at higher Cd (15 µM) level. Increasing Cd levels significantly reduced gas exchange characteristics including photosynthetic rate and transpiration rate of both crops. Root and shoot Cd contents were highest, whereas micronutrient (Cu, Fe, Mn and Zn) concentration was minimum in 15 µM Cd treated plants of both crops. On the basis of solution culture experiment it was concluded that, Cd has significant effect on growth, physiological and micronutrient contents of wheat and rice especially at 15 µM Cd application. The second experiment was a pot study to evaluate the effectivesness of different organic amendments on reducing Cd availability to rice and wheat in an artificially Cd contaminated soil. Amendments used were cotton sticks biochar (CSB), wheat straw biochar (WSB), rice husk biochar (RHB), farm yard manure (FYM), poultry manure (PM) and press mud (PrMd) @ 2% (w/w). The results revealed that addition of organic amendments significantly improved growth and reduced shoot, root and grain Cd and AB-DTPA extractable soil Cd in wheat-rice cropping. The maximum Cd reduction in soil and plant was recorded with the application of PrMd and RHB in both crops whereas the highest Cd concentration was observed in unamended Cd applied soil. On the basis of this study, it was concluded that, addition of organic amendments especially PrMd and RHB improved growth and yield of both crops and reduced Cd uptake. The third trial was accomplished to study the role of selected organic materials on Cd phyto-availability and growth of wheat and rice in raw effluent irrigated farmer’s field at village Suraj Miani, suburbs of Multan city. Treatments were control; RHB; FYM; PM and PrMd @ 10 Mg ha-1 randomized in 9 m2 sized plots. The results indicated that RHB and PrMd showed higher gas-exchange characteristics and lower plant and AB-DTPA extracable soil Cd. The soil (0-15 cm) Cd was lowered by 33 and 32% in post-wheat whereas 48% in post-rice by the application of RHB and PrMd respectively over control. The economics of treatments showed that except RHB, addition of amendments gave higher benfit cost ratio over control. Among amendments FYM was found more economical followed by PM and PrMd. The fourth experiment was performed in pots to study the effect of organic acids on Cd availability to wheat and rice. Organic acids viz. oxalic acid (OA), citric acid (CA), EDTA and tartaric acid (TA) were applied @ 2 mM before sowing of wheat. Rice seeedlings were transplanted to similar (post-wheat) pots to observe the long term effectiveness of applied organic acids. The results showed that Cd in root, shoot and grain/paddy of wheat and rice was increased by addition of organic acids and found highest for EDTA as compared to rest of treatments. The post-harvest AB-DTPA extractable soil Cd was also higher with EDTA treatment. Over all, Cd stress reduced growth and yield of both wheat and rice in natural and Cd spiked growth medium. This shows that Cd stress is inversely related with growth, yield and micronutrient uptake, whereas Cd uptake and concentration is directly related with Cd concentration in growth medium. Application of organic amendments especially PrMd and RHB improve growth, yield and photosynthetic attributes of both crops in artificially (spiked) Cd contaminated soil and raw sewage irrigated field as a result of less Cd uptake and translocation. Addition of organic acids especially EDTA formed soluble chelates with Cd thus increased Cd phytoavailability and uptake by wheat and rice.