Breakdown of azadirachtin A in a tropical soil amended with neem leaves and animal manures

A field investigation was conducted to assess the breakdown of azadirachtin A in a tropical coastal savanna soil amended with neem leaves (NL) combined with poultry manure (PM) or cow dung (CD) using gas chromatography. Samples in polythene bags 15 cm long and 4.8 cm in diameter were randomly placed to a depth of 14 cm in the soil, and azadirachtin A concentration was assessed on days 0, 14, 28, 42, 56, 70, and 84. Azadirachtin A degradation in the soil followed first-order reaction kinetics with different half-lives obtained for varying combinations of the amendments. Higher neem amendment levels of 100 g gave shorter half-lives of azadirachtin A than the lower levels of 50 g. Within the 50 g NL group the additions of the poultry manure and the cow dung gave significantly shorter (P < 0.05) half-lives of azadirachtin A than the sole neem amendment, whereas in the 100 g NL group only additions of 10 g CD and 10 g PM were significantly less (P < 0.05) than the sole neem amendment. Different changes resulting from the kind and quantity of animal manure added were observed in the half-lives of azadirachtin A. The 100 g NL group had significantly higher (P < 0.05) moisture content, which, coupled with the likely differences in microbial biomass, could be the major factor responsible for variations in the half-life of the compound. Therefore, the quantity of the neem leaves applied and the addition of animal manure affected the breakdown of azadirachtin A in the soil amended with neem leaves.