INFLUENCE OF SOIL-MOISTURE ON LONG-TERM SORPTION OF DIURON AND ISOPROTURON BY SOIL

Long-term sorption of diuron and isoproturon by a clay loam soil was i nvestigated for nine weeks at two herbicide doses (0.6 or 3 mg kg(-1)) and two soil moisture contents (35 or 62% w/w, i.e. 3.16 or 1 kPa) by measuring changes in herbicide concentrations in the soil solution sa mpled by means of glass microfibre filters in presence of sodium azide (200 mg litre(-1)) which inhibited biodegradation for more than four weeks. After the first day equilibration period, where adsorption main ly occurred (>70% adsorbed), herbicide concentrations in the soil solu tion decreased (about 50% for diuron; up to 38% for isoproturon) for t wo weeks but equilibration required about one month. Small amounts of herbicides were sorbed during this process (<10% of the initial (24-h) adsorption). These were similar for both herbicides, although diuron was initially more adsorbed. Values of the partition coefficients of h erbicides between soil and soil solution were increased (75-125% for d iuron; 29-67% for isoproturon). High soil moisture enhanced sorption s peed for both herbicides and increased final sorption only for diuron. Sodium azide inhibited long-term sorption of the more stable diuron a nd this effect was reversed by low temperature only at the low soil mo isture. Sodium azide action might be complex (competition, effect on s oil micro-organisms) and was not elucidated.