Effect of pest-controlling neem and mata-raton on bean growth, soil N and soil CO2 emissions

Abstract

Extracts of plants such as neem (Azadirachta indica A. Juss.) and mata-raton (Gliricidia sepium (Jacquin)) are used to control pests. However, certain components of neem, such as azadirachtin, can exert a negative effect on fungi and nitrifying bacteria, and, in turn, can impact the C and N cycles in soil. Nutrient cycling might thus be inhibited and affect the sustainability of an agricultural system in which plant extracts are used to control pests. Here, we investigated the effect of neem extract on microbial activity and N mineralization in soil. We studied the effect of neem and mata-raton leaf extracts on bean growth (Phaseolus vulgaris L.), nodule formation by Rhizobium, soil CO2 emissions and soil N dynamics. Four treatments were applied: (1) “neem treatment”: extracts of neem leaves, (2) “mata-raton treatment”: extracts of mata-raton, (3) “chemical treatment”: a chemical insecticide, lambda cyalothrin, and (4) “control”: untreated plants. Our results show that in non-amended soil the number of nodules in the neem treatment was 18 for beans cultivated. This nodule number was 2.1 times lower compared with the soil treated with lambda cyalothrin (chemical treatment). In manure-amended soil, the number of nodules was 28 in the neem treatment. This nodule number was 1.6 times lower than in the mata-raton treatment. This indicated that neem extracts inhibited Rhizobium in soil and nodule formation in bean. In the manure-amended soil, the emission of CO2 was 1.9 times lower in the neem-treated soil than in the other treatments. The increase in the concentration of NO 3 − was 1.03 mg N kg− soil day− in the neem treatment and 4.1 times lower compared with the other treatments. As such, microbial activity was inhibited by the neem extracts when added to the manure-amended soil. It was found that application of neem leaf extract inhibited microbial activity and reduced nodule formation in bean, but lambda cyalothrin or leaf extracts of Gliricidia sepium did not.