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Accumulation of heavy metals in edible crops is amongst major international concerns today. While consuming Lenjan variety of rice is very popular in Iran, limited evidence exists on its safety. Amid increasing public concern about the safety of locally grown and imported rice in the market, a field study was carried out to investigate uptake and translocation of Cd, Pb, Ni, and Zn by a local variety of rice crop (Oryza sativa) exposed to contaminated water. At harvest time and in paddy fields, 41 soil and plant samples were collected from four locations of Lenjan, central Iran; irrigated from Zayandeh Rood River. In the laboratory, different parts of the plant were milled, digested via acid digestion method, and then analysed for Cd, Pb, Ni and Zn using atomic absorption spectrophotometry. The results showed that average concentrations of Cd, Pb, Ni and Zn were 1.07, 17.22, 1.73 and 13.75 mg/kg in the plant’s stem; and 1.27, 12.32, 1.099 and 19.39 mg/kg in its grain, respectively. In general, both in the plant’s stem and grain, the Cd and Pb concentrations were much higher than the FAO/WHO standard and labelled as harmful for consumers. Moreover, among the studied heavy metals, Ni transported very weakly, while Cd and Zn conveyed most easily into the plant’s stem and grain. Of course, Pb was the least mobile metal. However, it had highly accumulated in the plant’s stem and grain.

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The monitoring of water quality is an important tool for sustainable development and provides important information for water management. Forty-eight groundwater samples were collected from northern Malayer, western Iran, to assess their nutrient status. Seventy-five percent of the water samples showed nitrate (NO À 3 ) concentrations above the human-affected value (13 mg l -1 NO À 3 ), while more than 12.5% exceeded the maximum acceptable level (50 mg l -1 NO À 3 ) according to World Health Organization (WHO) regulations. The samples were classified into four groups based on chloride (Cl À ) and NO À 3 concentrations. Several factors indicated that the NO À 3 and some Cl À were derived from agricultural sources. The correlation between NO À 3 and Cl À was consistent with a manure source, due to the practice of adding salt to animal feed, and suggests that denitrification is not appreciable at most locations. The aquifer plays a role in the decrease of contamination risk from phosphorous (P) through the high calcium (Ca 2+ ) ion concentrations that cause P to precipitate. The relatively low levels of potassium (K + ) in groundwaters may be a consequence of its tendency to be fixed by clay minerals. Pollution of groundwaters appeared to be affected by the application of fertilizers, irrigation practice, the solubility of mineral phases, and the discharge of domestic sewage. Application of high NO À 3 groundwater irrigation can minimize the requirement for inorganic fertilizers and reduce the cost of cultivation.

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SummaryPotassium (K + ) directly released from primary K-bearing minerals can contribute to plant nutrition. The objective of this research was to assess short-term K + release and fixation on a range of intensively cropped calcareous soils. Potassium sorption and desorption properties and the contributions of exchangeable-K + (EK) and nonexchangeable-K + (NEK) pools to K + dynamics of the soil-solution system was measured using a modified quantity-to-intensity (Q : I) experiment. Release and fixation of K + were varied among soils. The relation between the change in the amount of NEK during the experiment and the initial constrain was linear, and soil ability for K + release and fixation (b) for all soils varied from 0.041 to 0.183, indicating that 4% to 18% of added K + converted to NEK when fixation occurred. The equilibrium potential buffering capacity (PBC) for K + derived from Q : I experiments had significant correlation (r = 0.75, p < 0.01) with b, indicating that PBC depends not only on exchange properties but also on release and fixation properties. The depleted soils showed higher b value than the other soils, indicating much of the added K + was converted to NEK in case of positive constraint. The range of the amount of EK which was not in exchange equilibrium with Ca (E min ) in the experimental conditions was large and varied from 0.68 to 9.00 mmol kg -1 . On average, E min amounted to 64% of EK. This fraction of EK may not be available to the plant. The parameters obtained from these short-term K + release and fixation experiments can be used in plant nutrition.

Organic residues play a vital role in maintaining soil fertility in arid and semi-arid regions. Knowledge of the nutrient release from organic residues will help in optimizing nutrient efficiency in agricultural crop production systems. This study was conducted to assess the continuous release of phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) from eight different organic residues (sheep manure, beet, waste material, rape, poultry manure, sunflower, potato and garlic) using successive extractions with distilled water. The residues were shaken for 1 h and equilibrated for longer intervals (1, 3.5, 18, 42, 72, 192, 768, 1248, 1608, 1968 and 2328 h) with successive extractions. Cumulative nutrient release curves by different organic residues versus time showed an initial rapid phase followed by a slow phase. The average percentage of nutrients released and the average release rate of nutrients found using a power model was different and were in the order K 4 P 4 Ca 4 Mg and Ca 4 Mg 4 P 4 K, respectively. According to MINTEQ software, the partitioning of total P between HPO 4 27 and H 2 PO 4 7 is sensitive to the type of organic residue. On average (pooled over all extractions), between 13% (rape) to 40% (poultry) and 27% (poultry manure) to 79% (beet) of total P in aqueous solutions present as HPO 4 27 and H 2 PO 4 7 , respectively. Among the various organic residues and in the first stage of release, sunflower residue, beet residue, poultry manure and garlic residue had the highest rate constant for Ca, Mg, P and K, respectively, indicating the high potential availability of these nutrients in the early stage of application to soils.