The effect of different sources of nitrogen on dry matter and nitrate and nitrite accumulation in potato tuber (Solanum tuberosum L.)

The accumulation of nitrate and nitrite in food and their effects on health is an issue that is still being studied and discussed (Jun et al., 2013), including the toxic effects of nitrate, nitrite formed from nitrate reduction by bacterial enzymes (Mensinga et al., 2003). Nitrite in the stomach can lead to N-nitrosamine compounds that have proven health hazards (Jeffrey et al., 2012). Nitrate accumulation is often dependent on the amount and type of nutrients in the soil, which is now due to the indiscriminate use of chemical fertilizers and reduce nitrate to nitrite during storage threatens the health of human society (Zhou et al., 2000). Therefore, study of the effect of nitrogen from manure and chemical sources on yield and tuber quality is the main goal of this research in order to improve dry matter accumulation and quality
A field experiment was conducted in a randomized complete block design with three replications in the Ardabil area during 2013. The experimental treatments included: a control treatment; application of 326 kg ha-1, 652 kg ha-1 and 978 kg ha-1 of chemical urea fertilizer; application of 21.1 ton ha-1, 42.3 ton ha-1 and 63.5 ton ha-1 of sheep manure; and application of 27.6 ton ha-1, 55.2 ton ha-1 and 82.9 ton ha-1 of cow manure. The fertilizing was done based on application of 150, 300 and 450 Kg of nitrogen per hectare, respectively. Nitrite and nitrate concentrations in tubers were determined using the diazo method with a spectrophotometer in accordance with standard methods of the Soil and Water Research Institute, at harvest time, 20 and 40 days after harvest
In general, the manures were remarkably superior to chemical fertilizer with respect to tuber yield. Application of 42.3 ton ha-1 and 63.5 ton ha-1 of sheep manure increased tuber yield to 42.37 ton ha-1 and 44.78 ton ha-1, respectively. In contrast, as the application amounts of the manures and chemical fertilizer increased, it led to a decrease in percentage of tuber dry matter. Treatment with 27.6 ton ha-1 of cow manure produced the highest percentage of dry matter in tuber. The effect of number of days after harvest was not significant on accumulation of nitrite, whereas it was significant on the accumulation of nitrate. In general, as the days after harvest increased, so did the accumulation of nitrite. Thus, nitrate concentrations measured in the last stage (40 days after harvest) was 1.63 mg kg-1 dry weight, about 5.8 and 1.2 fold higher compared with the first and second period, respectively. The highest application of chemical fertilizer produced higher accumulations of nitrate (879.90 mg kg-1) and nitrite (3.62 mg kg-1) than all the other treatments. In general, as the amounts of nitrogen increased in the fertilizer treatments, so did the accumulation of nitrate and nitrite; while the accumulations were lower with application of the manures than with application of chemical fertilizer. Accumulation of nitrate in the soil regardless of the type of nitrogen fertilizer had close relationship with the increased consumption. Most soil nitrate accumulation was observed with the application of sheep manure (63.5 ton ha-1); while the lowest levels of nitrate accumulation was observed in plots where cow manure (27.6 ton ha-1) and sheep manure (21.1 ton ha-1) were applied and had no significant difference with control plots
Increased application of both manures and chemical fertilizer led to an increase in soil nitrate, a decrease in the percentage of tuber dry matter and accumulation of nitrate and nitrite in potato tuber; the accumulations were lower with the application of the manures than with application of chemical fertilizer. The highest accumulation of nitrate was found to be in 40 days after harvest. Therefore, by reducing the use of nitrogen fertilizers, especially chemical fertilizers, and by optimizing storage conditions, the accumulation of nitrate and nitrite in potato tubers can be decreased.