Influence of Two Insecticides, Chlorpyrifos and Quinalphos, on Arginine Ammonification and Mineralizable Nitrogen in Two Tropical Soil Types PRAMILA MENON, ² MADHUBAN GOPAL,* ,§ AND RAJENDER PRASAD # Department of Zoology, University of Delhi, Delhi 7, India; Division of Agricultural Chemicals, IARI, New Delhi 12, India; and Indian Agricultural Statistics Research Institute, New Delhi 12, India Effects of seed treatments with chlorpyrifos [5 g of active ingredient (ai) kg -1 of seed] and quinalphos (6.25 g of ai kg -1 of seed) and standing crop treatments with chlorpyrifos (800 g of ai ha -1 ) and quinalphos (1000 g of ai ha -1 ) on arginine deamination and mineralizable nitrogen were monitored, in the sandy loam and loamy sand soils of two tropical semiarid fields, for three consecutive crop seasons. The arginine ammonification activity of rhizospheric microbes was inhibited after seed treatment with chlorpyrifos and quinalphos and their principal metabolites, 3,5,6-trichloro-2-pyridinol (TCP) and 3,5,6-trichloro-2-methoxypyridine (TMP) and 2-hydroxyquinoxaline and quinoxaline-2-thiol, respectively. Quinalphos produced transient inhibitions, whereas chlorpyrifos and its metabolites (TCP and TMP) exerted a greater inhibition in both loamy sand and sandy loam soils. Arginine ammonification by nonrhizospheric microbes was stimulated by standing crop treatments with both pesticides. In the loamy sand soil, the parent compounds stimulated rhizospheric N-mineralization, whereas the metabolites were inhibitory. However, nonrhizospheric N-mineralization was inhibited by both chlorpyrifos and quinalphos and stimulated by their metabolites. A higher magnitude of inhibition of arginine deamination in the loamy sand than in the sandy loam soil could be due to greater bioavailability of the pesticides in the former, resulting from lesser sorption of the pesticides due to alkalinity of the soil and its low content of clay and organic carbon. Although both pesticides affected mineralizable nitrogen, seed treatment with quinalphos and standing crop treatment with quinalphos and chlorpyrifos produced the most significant effects. The recommended doses of the pesticides not only efficiently controlled whitegrubs, which increased pod yields, but also left no residues in harvested kernels. They also caused no long-term inhibition of ammonification, which could have been of significant concern during the short crop period in semiarid areas where nitrogen determines plant productivity. KEYWORDS: Chlorpyrifos; quinalphos; arginine deaminase; mineralizable nitrogen; semiarid; whitegrubs; ammonification INTRODUCTION Arginine deaminase catalyzes the mineralization of nitrog- enous compounds in soil to release ammonium and nitrate, which are the principal sources of nitrogen for plants. As a function of nitrogen metabolism, arginine ammonification reflects the potential activity of microbes rather than the fluctuations of microbial populations (1) because it correlates strongly with the amidohydrolase or amidase activity of soil bacteria and fungi, the carbon content of soil (2, 3), and microbial biomass and respiration as an index of oxygen consumption (1, 4-8). The rate of ammonium production is inversely proportional to the C N -1 ratio of an amino acid and has been correlated to N mineralization in soils (1, 9, 10). A blanket application of agrochemicals, namely, metolachlor, pendimethalin, chlorpyrifos, quinalphos, monocrotophos, phor- ate, carbofuran, thiram, benomyl, and carbaryl, for the protection of oilseeds from whitegrubs (Holotrichia and Maladera spp.) and other pests in the semiarid regions masks the properties of soils, which are important for crop production, impairs soil “health” (11-15), and affects the sustainability of agricultural systems (16-18). Soil enzymes are indicators of biological equilibrium (8), fertility (11, 17-19), and changes in the biological status of soil due to pollution (16, 20). Both processes of N-mineralization, namely, ammonification and nitrification, are sensitive to pesticides (21). In agronomical terms, however, the inhibition of ammonification more than nitrification is a * Address correspondence to this author at the Division of Agricultural Chemicals, Lal Bahadur Shastri Building, IARI, New Delhi 110012, India (telephone 91-11-25848706; fax 91-11-25733062; e-mail madhubangopal@ hotmail.com or pmenonc2d@hotmail.com). ² University of Delhi. § IARI. # Indian Agricultural Statistics Research Institute. 7370 J. Agric. Food Chem. 2004, 52, 7370-7376 10.1021/jf049502c CCC: $27.50 © 2004 American Chemical Society Published on Web 10/30/2004