Study of the Effect of Municipal Solid Waste Compost and Nitrogen on Spinach Yield, Soil and Spinach Nitrate Content and Evaluation of Its Risk Aversion for Human Health

Spinach (Spinacea oleracea L.) is one of the most important vegetables that is rich in minerals and vitamins, and according to the nutritional value of this vegetable in the household basket. Increasing the yield per unit area with emphasis on the health of the product from the position is nececery. In this regard, combining organic fertilizers with chemical fertilizers can increase the yield of this plant. The results of many researches show that urban waste compost has the ability to be used in agricultural lands and can be an affordable organic fertilizer that can replace part of chemical fertilizers and has a special position in sustainable agriculture. Among the nutrients required by plants, nitrogen is the most limiting factor for plant growth and the key nutrient in achieving optimal performance in most crops, especially in dry areas. The most important method of providing nitrogen required in agriculture is the use of nitrogen fertilizers. Therefore, the most widely used fertilizer among chemical fertilizers is nitrogen fertilizers. Despite the role of nitrogen fertilizers as a tool to achieve the maximum production per unit area, the management of fertilizer consumption should be in a way that does not cause the accumulation of pollutants such as nitrates in the consuming organs of the products.This research was conducted with the aim of field evaluation of urban waste compost in combination with nitrogen on spinach yield and nitrate content of the product and soil.

In order to investigate the effect of different levels of urban waste compost and nitrogen in spinach cultivation, a factorial field experiment based on a randomized complete block design with 9 treatments and 3 replications under a calcareous soil in the research farm of the research and education center for agriculture and natural resources, and it was implemented in Tehran province in 2014. The waste compost factor included three levels of urban waste compost consumption (0, 10 and 20 tons per hectare) and the nitrogen factor included three levels of pure nitrogen consumption (zero, 100 and 200 kg per hectare). Each experimental plot consisted of six crop lines with a length of three meters and a distance between the rows of 60 cm. Planting operations were carried out in the second half of September with Varamin broadleaf spinach variety. Harvesting took place on November 30. The data obtained from the experiment were analyzed using F test and SAS software version 9.1. The average of the data was also performed using Duncan's multi-range test at the five percent probability level.

The results showed that the highest fresh yield of spinach was 67.3 ton.ha-1 due to the use of 10 ton.ha-1 of MSWC whit 200 kg.ha-1 nitrogen. Nitrate concentration in spinach and soil was increased by use of MSWC and nitrogen. Data showed that the highest nitrate concentration in spinach was 3200 mg.kg-1 fresh weight due to the application of 20 tons per hectare of MSWC and 200 kg.ha-1 nitrogen, which was more than the national standard of Iran. Assessing the health risk of nitrate in spinach showed that in the consumption of municipal waste compost at a rate of 10 tons per hectare with a consumption of 100 kg.ha-1 nitrogen, the nitrate content of the spinach product is 1900 mg. kg-1 fresh weight which was less than the maximum permissible standard for nitrate in national Iranian standard. The base of the results, the maximum daily intake of spinach was 137 g for the person and 47 g for the baby. Excessive amounts of these levels will endanger their health. Meanwhile, data showed that in 20 ton. ha-1 MSWC whit use of 200 kg.ha-1 nitrogen treatment, the maximum allowed consumption of spinach was only 81 grams per day for adults and 28 grams per day for a child. Therefore, to order to maintain the health of the consumer, use of 10 ton.ha-1 MSWC with 100 kg.ha-1 nitrogen is recommended.

The results of this research showed that in the same soil conditions as the place where this research was carried out, the consumption of more than 10 tons per hectare of municipal waste compost and the consumption of more than 100 kg of pure nitrogen caused an increase in the nitrate limit in spinach and increased the risk for consumption. Therefore, it is recommended to use 10 tons per hectare of municipal waste compost and 100 kg per hectare of pure nitrogen to produce spinach while respecting the maximum allowed nitrate concentration.