Microbial response to nitrogenous fertilizers and pesticides with reference to nitrogen transformations in tea soils

Abstract

Distinctly acidic (pH below 5.0) tea soils of Assam; receive annually a wide array of chemicals, as fertilizer, pesticides, weedicides etc, during production of the crop grown under the coverage of shade tree. Considering the unique ecosystem of the soils under tea. plantation, it is probable that the behaviour of soil microorganisms may vary from the common cultivable land of the region. Such studies may throw some light on the above aspects and generate useful information on nutrition of tea plants. Keeping this in view present work was undertaken to study some aspects of nitrogen transformation through nitrifying, denitrifying and nitrogen fixing microorganism in soil samples drawn from three different sections of tea plan-tation graded on the basis of tea yield (section I, poor; section II, medium; and section III, good). Soil samples from a virgin section was included in the study for comparison. Soil samples drawn from different section were analysed for organic carbon, available potash, nitrogen and pH. Total, nitrifying, denitrifying and nitrogen fixing bacterial counts were recorded during different month of the year. The respective functioning groups of bacteria encountered during population count were isolated, purified and characterized including their behaviour to various agricultural chemicals. A laboratory incubation studies for transformation of applied fertilizer nitrogen (ammonium sulfate, urea and potassium nitrate) in soil was conducted both in presence and absence of pesticides. A soil perfusion study was also run for low and high yielding soil section using solution of ammonium sulfate and urea in presence and absence of thiourea. Results showed that fertility status of different soil group varies in the decreasing order of III,II,IV and I respectively. Soil pH fluctuation during different months of the year was heighest in the soil drawn from the poor yielding section followed by II,III and IV section respectively. Virgin section maintained a stable and high soil pH as compared to soil...