Abstract
Nitrogen is an essential element for plant growth and development; however, due to environmental pollution, high nitrate concentrations accumulate in the edible parts of these leafy vegetables, particularly if excessive nitrogen fertilizer has been applied. Consuming these crops can harm human health; thus, developing a suitable strategy for the agricultural application of nitrogen fertilizer is important. Organic, inorganic, and liquid fertilizers were utilized in this study to investigate their effect on nitrate concentrations and lettuce growth. The results of this pot experiment show that the total nitrogen concentration in soil and the nitrate concentration in lettuce increased as the amount of nitrogen fertilizer increased. If the recommended amount of inorganic fertilizer (200 kg·N·ha−1) is used as a standard of comparison, lettuce augmented with organic fertilizers (200 kg·N·ha−1) have significantly longer and wider leaves, higher shoot, and lower concentrations of nitrate.
Highlights
Nitrogen is an essential element required for successful plant growth
This experimental result is in agreement with the results of previous studies [39,40,41] in that the organic fertilizer-amended lettuce accumulated lower nitrate concentrations compared with conventional lettuce
There was a 13%–34% decrease in the dry weight (DW) compared with organic fertilizer applications only, the additional application of liquid fertilizers increased the estimated leaf area (LA) (15%–23%) and decreased the nitrate concentrations (4%–9%). These results show that lettuce grown in soils treated with a combination of organic fertilizers and liquid fertilizers had thinner stems, larger leaves, and lower nitrate concentrations compared with the plants that grew in soil treated with organic fertilizers only
Summary
Inorganic nitrogen compounds (i.e., NH4+, NO2‒, and NO3‒) account for less than 5% of the total nitrogen in soil [1], they are the main form of the element absorbed by most plants. For an organic agricultural system, continuous application of manure increases the nitrogen (N), phosphorus (P), potassium (K), calcium, and magnesium content in soil [2,3]. Once organic fertilizers are applied to soils and mineralization begins, inorganic nitrogen is released and absorbed by plants. The rate of mineralization is controlled by several factors, including agricultural management, microorganism, soil properties, temperature, and water content [4,5,6], as well as the type of organic fertilizer [7]. Many models have been developed to predict the release of nitrogen in applied organic fertilizers [6,8]
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