Effects of Neem Leaf Extract on the Soil Properties, Growth, Yield, and Inorganic Nitrogen Contents of Lettuce

While lettuce offers essential human nutrients, it also contains anti-nutrients, particularly nitrate (NO3-). The use of neem leaf extract as a natural nitrification inhibitor has proven itself promising to remediate lettuce tissue NO3- content. This study evaluated the effects of neem leaf extract on soil properties, soil nitrification, lettuce growth, yield, and NO3- content. Five nitrification inhibitor treatments were evaluated: (i) no inhibitor (control), (ii) nitrapyrin, and three rates of neem leaf extract based on the dry weight of the raw material: (iii) 1 g kg(-1) soil (Neem1), (iv) 2 g kg(-1) soil (Neem2), and (v) 4 g kg(-1) soil (Neem4). Neem leaf extract generally increased soil concentrations: P (47.6-55.8 mg kg(-1)), K (45.8-62.7 mg kg(-1)), Ca (129-164 mg kg(-1)), and Mg (29.0-35.7 mg kg(-1)) compared with the control (50.6 mg P kg(-1), 35.3 mg K kg(-1), 123 mg Ca kg(-1), and 24.8 mg Mg kg(-1)). Neem leaf extracts significantly increased soil NH4+-N concentrations (13.9-30.2 mg kg(-1)) and nitrification inhibition (12.5-70.5%), but significantly decreased soil NO3--N concentrations (6.4-13.2 mg kg(-1)) and net nitrification rates (0.08-0.23 mg N kg(-1) day(-1)) relative to the control (6.6 mg NH4+-N kg(-1), 14.7 mg NO3--N kg(-1), 0.26 mg N kg(-1) day(-1), and 0% nitrification inhibition). The neem leaf extracts significantly decreased shoot fresh weight (13.5-43.1 g plant(-1)), shoot dry weight (0.84-3.91 g plant(-1)), and root dry weight (0.14-0.27 g plant(-1)) compared with the control (52.3 g shoot fresh weight plant(-1), 5.36 g shoot dry weight plant(-1), and 0.35 g root dry weight plant(-1)). The significant decreases in the lettuce biomass in the neem extract treatments paralleled the significant decreases in the shoot's tissue NO3--N contents and significant increases in tissue NH4+-N content and soil Al concentrations.

Automated summary

The use of neem leaf extract as a nitrification inhibitor shows promise in reducing nitrate content in lettuce tissue, but it also negatively affects lettuce growth and yield.