The effects of cadmium and cow manure on nodulation and growth attributes of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is one of five cultivated species from the genus Phaseolus and it is the most important grain legume for direct human consumption (Broughton et al. 2003). This crop constitutes a traditional food for many people in Latin America, Africa, and Asia. It is high in protein, essential vitamins and minerals, carbohydrates, fiber, and it is a low fat food (Messina 1999). P. vulgaris has a unique property of symbiotically associating with Rhizobium leguminosarum and convert atmospheric nitrogen into a usable form to the plants. 

Rhizobium is the most well-known species of a group of bacteria that acts as the primary symbiotic fixer of nitrogen. These bacteria can infect the roots of leguminous plants, leading to the formation of lumps or nodules where the nitrogen fixation takes place. The symbiosis between Rhizobium and legumes are a cheaper and usually more effective agronomic practice for ensuring an adequate supply of N for legume-based crop and pasture production than the application of fertilizer-N (Zahran 1999). The symbiotic relationship between rhizobium bacteria and legume contribute at least 70 million tons of N per year (Brockwell et al. 1995). Typical environmental stresses faced by the legume nodules and their symbiotic partner (Rhizobium) may include photosynthate deprivation, water stress, salinity, soil nitrate, temperature, heavy metals, and biocides (Walsh 1995).

 Early studies of the effects of heavy metal on nitrogen fixation by legumes found little evidence that symbiotic N2-fixation was sensitive to heavy metal toxicity (Obbard and jones 1993; Obbard et al. 1993; Vigue et al. 1981). Cadmium toxicity is 2 to 20 times greater than any other heavy metals (Kabata-Pendias and Pendias 2001), and the limit of cadmium concentration in soil associated to biomass reduction for the majority of agricultural plants is reported to be between 5 and 15 mg Cd Kg-1 of soil (Simon 1998). The main sources of cadmium in the agricultural sector include sewage sludge, deposition from basemetal smelter emissions and uncontrolled application of Cd-rich phosphatic fertilizers (Robinson et al. 1998; Nicholson et al. 2003). Apart from the source of cadmium, the bioavailability of Cd in soils is a function of its solubility (Ernst 1996) with pH and organic matter content being the main controlling factors (Grey et al. 1998).

The toxic effects of cadmium on biological systems were reported by various authors (Mukherjee et al. 1984; Sharma et al. 1985). Presence of Cd in growth media reduces nodule formation and impairs nodule functioning in Phaseolus vulgaris (Vigue et al. 1981).There is also evidence that suggests that reduction in plant growth, nodule size and nitrogenase activity in white clover was due to Cd, Pb and Zn, when plants were grown in soils highly contaminated with these metals (Rother et al. 1983). 

One of the ways to recover and maintain the soil’s productivity has been shown to be the addition of organic wastes as organic amendment (Baran et al. 2001). However the use of organic wastes can lead to problems pertaining to their heavy metal content and their successive application result in heavy metal accumulation in soil. Basta et al. (2005) reported that the presence of significant quantities of humic substances in organic amendments adsorbed toxic metals temporarily, by formation of chelates (Adriano 2001) or other more complexes, which sorb them for longer periods of time (tordoff et al. 2000). However, Narwal and Singh (1998) suggested that the efficiency of the organic material in reducing Cd uptake was generally small. On the contrary, Hanc et al. (2008) found that addition of manure increased cadmium uptake by plants. The amount and type of organic matter present in the soil can also affect the metal solubility by complexation and chelation with metals. In Iran, application of cow manure is one of the most common practices used for cultivating plants. In addition, development of industrial manufactories and use of industrial waste leads to contamination of soil with heavy metals in important zones under cultivation of bean in Iran (Bahmani et al. 2012). Hence, the purposes of this experiment were to study the effects of cow manure and cadmium and their interactions on nodulation and growth Attributes of common bean (Phaseolus vulgaris L.) under greenhouse conditions.