INFLUENCE OF HERBICIDES ON TRANSFORMATIONS OF UREA NITROGEN IN SOIL

The influence of 28 formulated herbicides on transformations of urea nitrogen in soil was studied by determining the effects of 5 and 50 mg active ingredient kg-1 soil of each herbicide on the amounts of urea hydrolyzed and the amounts of nitrite and nitrate produced when samples of two coarse-textured soils and two fine-textured soils were incubated aerobically at 20-degrees-C for various times after treatment with urea. The herbicides studied were alachlor, amitrole, atrazine, bentazon, bifenox, bromoxynil, chloramben, chlorpropham, 2,4-D amine, 2,4-D ester, dalapon, dicamba, dinitramine, dinoseb, diuron, EPTC, glyphosate, linuron, monuron, paraquat, pendimethalin, picloram, prometryn, propanil, propham, simazine, trifluralin and vernolate. When the 28 herbicides studied were applied at the rate of 5 mg active ingredient kg-1 soil, they did not retard urea hydrolysis in the four soils used or nitrification of urea nitrogen in the two fine-textured soils, but six of them (amitrole, chlorpropham, 2,4-D amine, dinoseb, propham and propanil) retarded nitrification of urea nitrogen in the twocoarse-textured soils. When the herbicides were applied at the rate of 50 mg active ingredient kg-1 soil, two of them (alachor and dinoseb) retarded urea hydrolysis in both of the coarse-textured soils, and six of them (bromoxynil, 2,4-D ester, dicamba, linuron, diuron and glyphosate) retarded urea hydrolysis in one of these soils. All of the herbicides tested retarded nitrification of urea nitrogen in one of the coarse-textured soils when they were applied at the rate of 50 mg active ingredient kg-1 soil, but only four of them (propanil, propham, amitrole and dinoseb) markedly retarded nitrification of urea nitrogen in all four of the soils used when they were applied at this rate. ANOVA and correlation analyses indicated that the effects of the herbicides tested on nitrification of urea nitrogen in soils were significantly influenced by incubation time, soil texture and soil organic-carbon content. The work reported indicates that the 28 herbicides studied are not likely to substantially affect nitrification when they are applied to soils in conjunction with. urea fertilizers.