Alachlor and Metribuzin Herbicide on N₂-fixing Bacteria in a Sandy Loam soil

Herbicides interact with soil organisms and their metabolic activities and may alter the physiological and biochemical behavior of soil microbes. Some microbial groups are capable of using applied pesticide as a source of energy and nutrients to multiply, whereas the pesticide may be toxic to other organisms. Laboratory experiment was conducted to investigate the effect of two selective systemic herbicides viz., alachlor and metribuzin, at their recommended field rates (2.0 and 0.4 kg a.i. ha⁻¹, respectively) on the growth and activities of non symbiotic N₂-fixing bacteria in relation to mineralization and availability of nitrogen in a sandy loam soil of India. Both the herbicides, either singly or in a combination, stimulated the growth and activities of N₂-fixing bacteria resulting in higher mineralization and availability of nitrogen in soil. The single application of alachlor increased the proliferation of aerobic non-symbiotic N₂-fixing bacteria to the highest extent while that of metribuzin exerted maximum stimulation to their N₂-fixing capacity in soil. Both the herbicides, either alone or in a combination, did not have any signifcant difference in the stimulation of total nitrogen content and availability of exchangeable NH₄⁺ in soil while the solubility of NO₃⁻ was highly manifested when the herbicides were applied separately in soil. The effect of combined application of the herbicides was more or less at par with their single application.