S. Patzold et Gw. Brummer, FATE, SORPTION, AND LEACHING OF THE HERBI CIDE DIURON AFTER ANNUAL APPLICATION IN AN ORCHARD SOIL (ORTHIC LUVISOL), Zeitschrift fur Pflanzenernahrung und Bodenkunde, 160(2), 1997, pp. 165-170
A three years field trial was conducted in an apple orchard on a loess
site (Orthic Luvisol) with annual applications of the herbicide diuro
n. An accumulation of biologically effective residues of the herbicide
was not observed, although a carry-over of herbicide residues in a ra
nge of 46-77 mu g/kg dry soil in the 0-5 cm layer occurred between dif
ferent years. After. application in May, the residues were rapidly deg
raded during the following summer months. Disappearance of diuron resi
dues from the 0-5 cm layer conformed to first-order kinetics during th
e first six months after application. The DT50-values ranged from 14 t
o 30 days. As the degradation rate decreased with time, the level of r
emaining residues on a long-term scale was best represented by two com
bined exponential functions for the fast and-slow degradation rate. Th
e distribution coefficient between adsorbed and dissolved residues inc
reased with time, indicating decreasing mobility of herbicide residues
. The distribution coefficients and the-extractable residue contents w
ere strongly negatively correlated. A few days after a diuron applicat
ion in May 1994, about 15% of the applied amounts could be analyzed in
subsoil samples from 30-60 cm depth (6%) and 60-90 cm depth (9%), whi
le high concentrations of diuron remained in the 0-5 cm soil layer. Th
is vies the result of a heavy rainfall (20 mm) and a rapid movement of
seepage water into the subsoil due-to preferential transport in soil
macropores like earthworm channels and shrinkage cracks, which frequen
tly occurred in this untilled loess soil. Different transport patterns
were observed during the winter months in 1992/93 and 1993/94, when o
nly low amounts of the diuron applied in spring were leached down to 3
0-60 cm depth. In winter time, the remaining low concentrations of ext
ractable residues were strongly adsorbed and therefore remained in the
upper soil layers.