The Lysimeter Concept - American Chemical Society

Chapter 3

Lysimeter Study of Imidacloprid After Seed Treatment of Sugar Beet in Two Crop Rotations

Downloaded by STANFORD UNIV GREEN LIBR on September 29, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0699.ch003

E . Hellpointner Agricultural Center Monheim, Institute for Metabolism Research and Residue Analysis, Bayer A G , Building 6660, 51368 Leverkusen, Germany

According to German Lysimeter Guideline the leaching behavior of [imidazolidine-4,5- C] imidacloprid (insecticide) and its degradation products is investigated under practice-relevant field conditions. Imida cloprid treated sugar beet seed was sown in two undisturbed soil monoliths (1.0 m surface area, 1.1 m depth) of sandy loam on April 1991 and 1994. The application rate was 1.3 mg active ingredient/pellet (9 pellet/m ) corresponding to about 120 g/hectare each. In the other years cereals were grown. Data/results were gathered during the five years on 1) weather, 2) occurance, volumes, radioactive residues of leachate, 3) yields, residues in the harvested crops, 4) distribution of residues in the soil monoliths at the end of the study, 5) material balance and calculation of portion mineralized to CO . This study confirms the low mobility of imidacloprid and its degradation products in soil under practice-relevant field conditions. Following the use of imidacloprid in the recommended manner the possibility of contamination of deeper soil layers and groundwater can be excluded. 14

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Plant protectants can be examined for their leaching potential in soils by various labo ratory studies. In case of the systemic insectice imidacloprid some physical-chemical properties (i.e. solubility in water and Koc-values) and its moderate degradability in soil indicated a leaching potential. On the other hand, column leaching and field studies as well as a 2 year lysimeter study (1) showed no leaching of imidacloprid into deeper soil layers. Therefore, the objective of this lysimeter study was to get further data on the potential long-term leaching behavior of imidacloprid and its degradation products by investigating its use as a seed dressing in a sugar beet/cereal crop rotation in which sugar beet was grown twice in 6 years.

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©1998 American Chemical Society

In The Lysimeter Concept; Führ, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

41 Materials, Methods and Test Conditions 14

Test substance. The study was conducted with [imidazolidine-4,5- C] imidacloprid (Figure 1). The specific radioactivity was 4.59 MBq/mg (124 μά/ιη£) and the radio chemical purity determined by TLC and HPLC was >98%.


Reference standards. Many reference standards covering possible transformation products of the test substance were available. However, metabolism and leaching studies of imidacloprid in soil indicated that the parent compound is expected to be the relevant residue in soil, together with the bound residues. Information on the lysimeter. This semi-field study running under GLP and according to official Guidelines (2) was conducted with two undisturbed soil monoliths (1.0 m surface area; 1.1 m depth) of sandy loam (Eutric Cambisol) taken from field plot A X X of the Experimental Farm Laacherhof of Bayer A G , Monheim (FRG) in April 1990. Initially, they were installed above ground in the "lysimeter hall" of the Metabolism Institute. Then 11 months after 1 application of [ C] imidacloprid, the two lysimeter systems were transferred into a newly built lysimeter facility and installed at ground level. Details of test system, type of soil and location of the Bayer lysimeter facility have already been published (5). The leachate was collected in stainless steel containers which were emptied every 2 weeks (more frequently, if necessary). The sample volume was determined gravimetricaUy. The net radioactivity (TRRaikaiine) was determined by liquid scintillation counting (LSC) of each of three 10 mL aliquots after addition of 100 μΐ. 1 M NaOH to ensure retention of dissolved C-carbonates. The TRRecidic was determined by L S C of each of three 10 mL aliquots after addition of 100 μL of 18% HC1 to release of C 0 from the solution. The arithmetic means were used for calculations. Sum fractions (i.e. annual leachates, AL) were prepared by mixing 10% of all the individual leachates which were analyzed by radio-TLC (for methods see later). No soil sampling was conducted during the period that the leachates were collected. In May 1996 the five top soil layers each 10 cm deep were taken from the complete area of the lysimeters and mixed in a dust-tight concrete mixer for about 1 hour. Five subsamples from each mixed layer were taken and five 1 g portions of each were combusted for determination of C-residues. Five soil cores of the deeper layers were taken using an electric soil corer ( 0 = 8 cm ) and five 1 g portions of each were combusted in order to determine the C-residues. The total weight/layer was calculated from the weight of the cored portion. The soil corer did not reach the gravel in the tray. An individual sample of about 550 g was taken after the lysimeter was lifted out of the unit in November 1996. The gravel was extracted by 200 mL acetonitrile/0.1 N HC1 1:1 (v+v) for 2 hours and the extract was radioassayed (3 aliquots each). The extractable radioactivity in the soil was determined to a depth of 80 cm. Two 25 g aliquots/layer each containing the mean C-residue of the respective layer were hot-extracted in a ®Soxtec apparatus with 40 mL of methanol (1 hr reflux, 0.5 hr rinsing). An aliquot of 500 μL (in the case of top soil layers) or 1,000 μΐ, (in the case of lower soil layers) of the crude extracts was spotted by Linomat I V (Camag Co.) in bands of 1 cm directly onto silica gel 60 F 2 5 4 pre-coated T L C plates (Merck Co.). The solvent systems ethylacetate/2-propanol/water 65/23/12 (v+v+v) and chloroform/ 2

st

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f f ^ V ^ N ^ M u

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* C-label

Figure 1. Structure of Test Substance


43 methanol/acetic acid/water 65/25/3.5/3.5 (v+v+v+v) were used for separation. Detection and evaluation was performed by radio-TLC-Scanner RITA 68,000 (Raytest Co.; lower limit of determination «2 Bq/appl. volume or /spot), later by Bio-Imaging Analyzer B A S 2000 (Fuji Co.; lower limit of imidacloprid determination about 0.005 μg/L leachate or 0.005 μg/kg soil). Application of test compound. On April 3, 1991 each 25 μΐ, of a dichloromethane solution of [ C]imidacloprid was applied directly to sugar beet pellets lying in three rows in the soil monoliths. The rate was 1.3 mg a.i./pellet with 9 pellets/m corresponding to 117 g a.i./hectare. The 2 sugar beet treatment corresponding to 126 g a.i./hectare was applied similarly on April 27, 1994. In the first treatment 53.95 M B q / lysimeter was applied (=100% in the testing period of 3 years) and in the second 58.72 MBqflysimeter. In total 112.67 M B q (=100% for the last 24 months of study and the total testing period of 5 years plus 1 month) was applied to each lysimeter. 14

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Cultural practices and cropping. A l l the agronomic and maintenance activities (fertilization, crop protection and other measures) within the lysimeters and the surrounding field plot (3) were conducted and documented by a gardener. The main goal of the activities, which were performed in compliance with good agricultural practice, was obtaining crop growing conditions and yields which would be expected on a local farm. Data about cropping, vegetation periods and crop yields are presented in Table I. Test conditions. The relevant climatic data were recorded at the weather station of the Bayer Experimental Farm Laacherhof, Monheim, located about 1 km away. Specimen weather data from 1966-1995 are listed (Table II) in order to characterize the test location and to compare it with the conditions in the course of the lysimeter study. The conditions generally required for a lysimeter study (2) were maintained during the period from April 1991 to April 1996. Cumulative curves of precipitation & irrigation and the measured leachate portions are shown in Figure 2. The combination of using a seed treatment (point source), the method of treating the sugar beet pellets (a.i. not incorporated) and the sandy soil used in the lysimeters were considered to produce a worst case situation for testing the leaching potential of imidacloprid. As usual for field testing conditions, the weather was quite different from year to year. Nevertheless, precipitation & irrigation on average exceeded by 12 mm/a the value of 800 mm/a required for lysimeter studies (2). The total leachate volumes (Figure 2) varied in the expected range, each reflecting the actual crop growing and weather conditions. The duplicates were closely comparable. Within the total testing period of 5 years plus 1 month the leachate amounted to 26% of total precipitation plus irrigation. Leachate volumes as well as crop yields harvested from the lysimeters indicated good crop growing conditions and a normal water balance in the soil monoliths. Results and Discussion Leachates. The net radioactive residues (TRR^unc) measured in all individual leachates and expressed as μg a.i.-equivalent/L are shown in Figure 3. In general, the


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Cropping, Yields and C-Residues in Crops Table I: < Crop

Elapsed time *

Crop yield (fwt.)

[days]

[days]

[kg/τηη

C harvested/ Total C residue (ai. equivalent) lysimeter [mg/kgfwt] [Voapplf

7.4

0.72

4.2

0.41

0.8

0.04

0

1st (target): 224 sugar beet - tubers

2

2nd: wheat 265 - grains

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- leaves Downloaded by STANFORD UNIV GREEN LIBR on September 29, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0699.ch003

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Period of growth

0.011

489 0.006

- straw

0.7

0.02

chaff

0.3

0.05

0.7

The Lysimeter Concept - American Chemical Society

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