Effects of neem leaf extract on inorganic nitrogen transformation in sandy soil

The inhibitory effects of neem leaf extract on fertilizer nitrogen (N) transformation in soil have not been fully recognized. This study therefore aimed to evaluate the effects of fi ve nitri fi cation inhibitors on the status of soil inorganic N, urea hydrolysis, and nitri fi cation: 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; iv) 2 g kg –1 soil; v) 4 g kg –1 soil. Neem leaf extract in all rates increased urea hydrolysis rate on days 5 − 15. In contrast, nitrapyrin decreased urea hydrolysis on days 5 − 10, compared to the control. As for nitri fi cation, neem leaf extract showed both stimulatory and inhibitory effects. The stimulation ap-peared on day 3, whereas inhibition occurred and peaked on days 5 − 15. Nitrapyrin showed inhibitory effects on days 10 − 15, reaching its peak on day 15. Increasing rates of neem leaf extract brought about increases in stimulation and inhibition of urea hydrolysis and nitri fi cation. The results conclusively stated that the neem leaf extract had generally faster, stronger, and longer nitri fi cation inhibition than nitrapyrin. The stimulation in the early phase and the inhibition in the later phase of the incubation of neem leaf extract were more pronounced with higher application rates.