Effects of sewage sludge and municipal compost application on bio availability of soil nutrients in onion (Allium cepa L.) cultivation

Degradation of water, soil and environmental resources by the excessive use of chemical materials encour aged researchers to focus on organic farming in recent years. Despite all positive effects associated with the consumption of municipal compost and sewage sludge on the physical and chemical properties of the soil, there are still many concerns about their environmental and health issues. The aim of this study was to investigate the heavy metal accumulation in the soil and plant using different amounts of municipal compost and sewage sludge in a field trial after five years.

The field study was conducted in Rudasht Irrigation and Drainage Research Station, Isfahan, Iran. A randomized complete block design with three replications within treatments was employed. The crop rotations were onion, wheat, sugar beet, corn, and onion, so there were two stages of onion cultivation. The treatments were 0 (as control), 25 ton ha-1 municipal compost, 50 ton ha-1 municipal compost, 15 ton ha-1 sewage sludge and 30 ton ha-1 sewage sludge applied per planting during five years.

Application of municipal compost and sewage sludge, especially at higher levels, increased the soil organic matter twice. In addition, the concentration of phosphorus (P), potassium (K), iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), and lead (Pb) in the soil increased significantly. Concentrations of available lead (Pb) and cadmium (Cd) in the first onion planting stage in the soil were not significant, but in the second onion planting stage the lead (Pb) concentration in 50 ton ha-1 municipal compost increased significantly (from 1.9 mg kg-1 in control treatment to 5.62 mg kg-1 in 50 ton ha-1 municipal compost treatment). Application of organic fertilizers increased the essential elements (e.g., N, P, K, Zn, Mn, and Cu) in onion shoot. The highest amounts of nutrient elements in onion shoot were detected in 50 ton ha-1 municipal compost,
which was not significantly different than 30 ton ha-1 sewage sludge. In the first stage, the absorption of copper (Cu) by plant shoot was lower than the second stage as a result of increasing concentration of other elements such as phosphorus (P) and competition between various elements. The highest copper (Cu) concentration in shoots was observed in 15 ton ha-1 sludge (5.4 mg kg-1), which reached to 3.76 mg kg-1 after five years in the second stage. Application of organic fertilizers at both stages increased zinc (Zn) concentration in the onion bulb. The highest zinc (Zn) concentrations in bulbs were measured in 50 ton ha-1 compost and 30 ton ha-1 sewage sludge treatments (3.05 and 3.87 mg kg-1 for the first stage and 4.25 and 4.50 mg kg-1 for the second stage, respectively). However, the concentrations of other nutrients such as N, K, Cu and Fe in the onion bulb increased at the second stage. The highest concentrations of N (0.18%) and Fe (0.81 mg kg-1) in the bulb were measured in 50 ton ha-1 compost and the highest concentration of Cu (0.52 mg kg-1) was in 30 ton ha-1 sludge. The concentrations of lead (Pb) and cadmium (Cd) in the plant were undetectable by atomic absorption in different treatments.

Long-term consumption of municipal compost and sewage sludge increased the soil organic matter and im proved the physical and chemical properties of the soil. Furthermore, these organic resources increased the concentration of nutrient elements, especially micronutrients including zinc (Zn) and iron (Fe), in the plant. One of the main concerns about such organic resources is the possibility of soil and plant contamination by heavy metals. The results of this study showed that the multi-year consumption of these materials did not increase the heavy metal elements such as lead (Pb) and cadmium (Cd) in the soil or plant. Considering the low amount of organic matters in the soil of most parts of the country and the limited availability of various organic resources, using such organic matters in agriculture would be one of the best methods for recycling these materials and would be very important to have sustainable agriculture.