Levels of nitrate in locally grown lettuce and spinach.

Abstract

The major factors contributing to the accumulation of nitrate nitrogen (N-N03) in cultivated lettuce and spinach cultivars and the consequential negative effects likely to accrue on consumer health following their consumption were thoroughly reviewed and The major factors contributing to the accumulation of nitrate nitrogen (N-N03) in cultivated lettuce and spinach cultivars and the consequential negative effects likely to accrue on consumer health following their consumption were thoroughly reviewed and assessed. Results of studies carried out both locally and abroad are also discussed. This research study revealed that plant nitrate-nitrogen (N-N03) concentrations prevailing in locally cultivated crops were moderately high, with values ranging from 1659mg/kg ± 10 15 for lettuce cultivars to 1667mg/kg ± 1435 in fresh spinach. The major contributing factors likely to contribute excessive accumulation of nitrate at the local level were closely monitored. The N-N03 levels in the soil profile within the root zone showed ample variation with the N-N03 concentrations ranging from 88mg/l ± 48 for the first 15cm (top) layer and 95mg/l ± 51 for the bottom layer respectively. The N-N03 concentrations in water used to irrigate these crops were found ranging from 77mg/l ± 43 for lettuce irrigation and 98mg/l ± 57 for spinach irrigation respectively. The correlation obtained was Soil Average (N-N03) = 0.26 (Irrigation water N-N03) + 71 (R2 =6% P= 0.2032), indicating that the contribution of irrigation water to nitrate accumulation in both lettuce and spinach was small if not insignificant. Regression analysis confirmed that soil N-N03 was the major contributing factor with Plant N-N03) = 2.76 (Soil N-N03) + 1329 (R2 =32% P=0.1858) when compared with (Plant N-N03) = 0.35 (Water N-N03) + 1569 (R2 =10% P=0.0185); whereas using multiple regression the results were even more sustained i.e. for lettuce the relationship observed followed (Plant N-N03) - 1370 + 1.98 (Ave Soil N-N03) - 0.325 (Water NXlll N03 ) (R2 =13% P=0.1013), whilst for spinach the relation observed was (Plant NN03) 1089 + 2.1 (Average Soil N-N03) - 0.09(Water N-N03 ) (R2 =33% P=0.2132). Other factors such as planting density and maturation stage surprisingly gave the following correlations, chiefly that (Plant N-N03) = -7.87 (Maturation days) + 2078 (R2 =17% P=0.0014) and that (Plant N-N03) = 0.009 (Planting Density) + 1374 (R2 13% P=0.014). This was highly indicative that the plants were competing for water and nutrients and the competition was high. This stresses the need of taking into consideration other elements tied to the local scenario e.g. geology, hydrology, meteorology, cultivation and irrigation practices before drawing any conclusions. Fall in precipitation and availability of nutrients at the third stage of maturity and other factors that may adversely affect the efficiency of the plants' internal systems, in pmiicular the regime surrounding nitrate reductase, are likely to be the cause of the progressive accumulation of nitrate in locally cultivated lettuce and spinach. It would therefore be pertinent to state the results obtained and the conclusions that derive from the statistical evaluations apply to lettuce and spinach under local circumstances. Lastly, compliance status for each sampled plants was reviewed within the local and regional regulatory framework, mostly vis-a-vis permissible levels of nitrate-nitrogen that should be present in lettuce and spinach intended for human consumption (EC Regulation 563 of 2002). Statistical analysis indicates that the problem of N-N03 in plant material mainly presents itself to the north of the islands, especially when farmers resorted to the use of the nitrate-polluted perched aquifer and high fertilization rates. In the case of fields in the open, nitrogen management may be somewhat difficult. However cultivation in greenhouses warrants a different and more stringent approach. Technical assistance is needed for ensuring that farmers maintain compliance and quality control of their products through self-monitoring.