Investigation of contaminants transport to soil depth from Soybean planting fertigated by wastewater sludge

Produced sludge from wastewater treatment plants is a rich source of nutrient elements (nitrogen, phosphorus, potassium) needed for agriculture. Also, due to having large amounts of organic matters, it affects on soil properties. On the other hand, application of sludge with no criterion may cause some health risks and also, soil and water resources pollution may be followed. In this regards, the effect of different amounts of sewage sludge used as organic fertilizer in compare with chemical fertilizers on transport of minerals below the root zone and yield and yield components in soybean cultivation were studied. Therefore, 15 cylindrical plastic lysimeters with a height of 100 cm and 60 cm diameter were constructed and drainage PVC pipes with geotextile filter were installed for all of them. The sludge of Ekbatan housing complex wastewater treatment plant after processing and digestion was taken place and used. Treatments applied base on statistical completely randomized design, include three levels of sewage sludge fertilizer (10, 25 and 50 tons/ha), chemical fertilizers (150 kg/ha ammonium phosphate and 50 kg/ha urea) and control without using fertilizer in three replications. According to the results, the highest mean amount of nitrate in drainage water of lysimeters was 57.17 mg/lit and treatment related to the use of sludge was 50 tons/ha. The lowest mean value of nitrate obtained from the control and was 3.51 mg/lit. The highest rate of drainage water salinity (EC) at the beginning of the planting season was 5.04 dS/m, of 50 tons/ha applied sludge fertilizer. Accumulation of heavy metals in the soil at the end of the planting season was measured in soil saturation extract samples. The results showed the most increase of some metal elements in treatment that used 50 tons of sludge per hectare (up to 0.22 mg/lit Zn, 0.7 mg/lit Mn, 0.09 mg/lit Ni and 0.73 mg/lit Fe). Application of sewage sludge could not significantly increase the amount of Pb, Cu and Cd concentrations in surface soil of lysimeters and the amount of these elements added to soil via sewage sludge application was negligible. The most yield of soybean was 277.85 gr grains which obtained from use of 50 tons/ha of manure sludge. The greatest weight of 1000 grains and the number of pods, respectively, with values of 127.42 gr and 856 in number, measured in this treatment too. So, it can be concluded that application of sewage sludge has a significant impact on soybean yield with no sensitivity on plants production.