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Side-effects of sequentially-applied pesticides on non-target soil microorganisms: field experiments
Sod Bwl. Btockm. Vol. 22, No. 3. pp. 367-373. 1990 Pnnted ,n Great Bnrain. All rights reserved
0038-0717 90 53.lM + 0.00 Copynghr C, 1990 Pergamon Press plc
SIDE-EFFECTS OF SEQUENTIALLY-APPLIED PESTICIDES ON NON-TARGET SOIL MICROORGANISMS: FIELD EXPERIMENTS Evr SCHUSTER*and D. SCHROEDER Department
of Soil Science, University
of Trier.
(Acceppled 20 Augusr
Postfach 3825. 5500 Trier.
F.R.G.
1989)
plant protection system consisting of 7 pesticide treatments was investigated for its side-effectson the soil microflora. Measurements of microbial activity were based upon microbial biomass,
Summary-A
dehydropenase activity. ammonification and nitritication. Investigations were carried out over 2 yr in a field experiment with four replications. Successive applications of the pesticides caused only slight and short-lived side-etkts on the soil microtlora. These usually disappeared before the next treatment was carried out. The results appeared to be significantly influenced by weather conditions. Thus, side-etkts were less in 1986 despite a more rapid sequence of pesticide treatments compared to the year before.
INTRODCCTION
Pesticides The chemicals applied were typical for a plantprotection system associated with intensive cultivation of ccrcals. It consisted of four herbicides, three fungicides. one insecticide and one growth regulator. The nine chemicals were applied at normal field rates in a sequence of seven treatments (two tank mixtures included). A detailed description is given in Table I. The application was carried out with a motor syringe supplied with teejet nozzles of type 1103 LP yielding a syringe pressure of 0.13 MPa. The chemicals were sprayed at a rate of 400 I. solution ha-‘.
The use of chemicals. including pesticides, has become an integral and economically essential part of modern agriculture. Pesticides arc often applied several times during one growing season and a part of the materials applied always reaches the soil. As they arc designed to be biologically active we arc concerned that continuous inputs of pesticides might affect the soil microflora and so impair soil fertility. Much rcscarch work has been done on testing the side-cffccts of single applications of chemicals. In general, observed effects have been minor and shortlived (&eaves, 1987). However, we do not know much about the more realistic situation where different active substances come together in the soil. There might arise additional stress from the use of mixtures and sequences of pesticides because of the accumulation of toxic agents or the formation of new compounds (from pesticide combinations) which can bc more toxic than the original substances. Our aim was to investigate whether applications of various pesticides in quick succession can adversely affect the activity of the soil microflora.
Weuther condirions Weather conditions for the 2yr experimental period are described in terms of monthly temperature and rainfall and their deviations from long-standing mean values in Fig. I. The data were provided by the weather station Trier-Petri&erg. Further details are probably not appropriate because the measuring station was too far away (ca 25 km) from our field site. The temperatures largely corresponded to the long-standing mean values in both investigation periods. The winter months January and February were colder than average, but there was no soil sampling during that time. The values of monthly rainfall vary around the long-standing mean value. There was a tendency toward more precipitation from March until August 1985. whereas the winter 1985-1986 was drier than normal. March and April 1986 were excessively moist, whereas the following months were drier than the long-standing mean value.
MATERIALS AND METHODS Tar
plots
A complete block design of 2 by IO m plots with four replications was used. The field site was cu 25 km northeast from Trier (western Germany, Mosel valley) near Fiihrcn in the “Wittlicher Senke”. The soil was primarily a Parabraunerde (Glcyic Luvisol) and had been under agricultural cultivation for a long time. The organic matter content averaged 0.85% and the soil pH (CaCI,) averaged 6.5 for all plots. The textural composition was 16.4% sand, 66.7% silt and 16.9% clay. The standing crops were wintcrwheat (1985) and spring barley (1986).
*Present
address: Department
Mcinchen.
of Soil 8050 Freising Weihenstephan,
Science. F.R.G.
Soil sampling
procedure
Thirty to 40 subsamples of topsoil (O-S cm), taken with a special soil-sampling auger, were combined to give one mixed sample of each plot taken for analysis. The soil was thoroughly mixed and, if necessary, allowed to dry to a moisture content of 4@-60% before sieving (< 2 mm). Airdrying of the WHC,,
TU
367
Ew SCHUSTER
368 Table
I. Propcrc~es and
and D. SCHR~DER dosage
of the apphed
pcwcldcs ACI!K
ProductIon Common
Aretlt
Dmoxbacetate
Hoechst
Dm~trobutylphenylacctatc
lsoproturon
Hoechst
.V..V-Dtmethyl-N-[t(
Dtchlorprop
BASF
Dtchlorphcnoxyproplonic
Chlormcquat
BASF
Chlorcholmchloride
Prochloraz+
Schcnng
~V.propyl-N-trichlorophcnony-ethylcarb~moyl~m~dazole
Arelon C-86
DP
Cycocel Sportak
Alpha
name
subrt
Trade-name
company
Carbendazim Baylcton
Captafol+
DF
Chmucal
name
(0.1 49
I-mcthylcthyI)phenyI]carbamidc and
Mcthoxycarbonylamlno-benrimldazoic Bayer
Triadimefon
,V-(1.1.2.2.tetrachlorcthyl-thio)_cyclohe~-J
PInmor
Pinmicarb
Schcring
Dimethyl-2-dimethyl
Roundup
Clyphosate
BASF
N-Phosphonomcthyl-glycine
A nalylical
hlicrobial biomass. The amount of carbon in the physiologically-active microbial biomass was dctcrmined (Anderson et al., 1978). It was found that 400mg glucose IOOg-’ soil was needed for biomass mcasurcmcnts. This quantity of glucose was thoroughly mixed into the soil (two rcplicatcs). Amcndcd soils wcrc then poured into glass cylinders and connected to an “Ultragus U3SB” CO,-analyser (Wiisthoff, Bochum) for automatic analysis of CO? production rates. Incubation was at 22’C for a maximum of 5 h. The maximum initial rates of respiration were dctcrmincd and the values obtained entered into the equation s = 40.04 _Y+ 0.37
ha
50
2.5
60
JO
56
05
8
I5
.J-dicarbonmlde
65
40
6
amino-J-pynmidinyldlmelhylcarbamate
5 36
03 4.0
where x = mg microbial C per unit soil and _V= ml CO: per unit soil h-’ with I mg CO: day-’ = 0.021 ml CO: h-’ (22’C; 101.3 kPa).
Dehychgenase actiriry was determined according to A. Thalmann (Dissertation). A 5 g sample of soil was saturated with 5 ml of a 0.6% solution of 2.3.5-triphenyltetrazolium chloride (TTC) buffered with trishydroxyaminomcthan at a pH of 7.6 and mixed thoroughly with a vortex shaker. The samples were then kept in sealed tubes at 27‘C for 24 h. Triphcnyltctruzolium formazan (TPF), produced by the reduction of XC, was extracted with 25 ml acetone and its concentration dctcrmincd photometrically at 547 nm. Anvnonifiufim W;LSdctcrmincd by the rclcasc of ammonia and nitrate (Beck. 1983). Subsamplcs (IO g) of soil wcrc filled into culture glasses and 3 ml distilled water wcrc carefully dropped onto the surfact (no mixing!). The glasses wcrc closed by Kapscnberg caps and kept for 2 weeks at 27 C. Then the samples wcrc shaken with a solution of I% KCI and tiltcrcd. The contents of ammonia and nitrate (photometrically; Cawsc, 1967) were dctcrmincd before and
Temperature
mm 808040zot
0 J’F’M’A’M’J’J’A
S
0
N
DIJ
I1 F
M
1985
Fig. I. Mean
monthly
temperature
‘)
30
30
Chlor-phenoxy-dimcthyl-triazolbutanonc
exterior of aggregates was avoided. The soil was stored at 4’C until analysis (period not exceeding 3 months). Soils were sampled I day before and 3 days after each pesticide treatment; the application dates for the two years of investigation (1985) and 1986) are given in Table 2.
Dosage (kg
A’H
J
J
A
S
O’N
1986
and rainfall and their deviations in 1985 and 1986.
from the longstanding
mean values
D
Soil microflora response to a sequence of pesticides
369
Table 2. Times of pesticide apphcarmn
(%)
Control Applicauon time Common name
Code
Are111+ Arelon U-t6 DP + CCC Sponak Alpha Corbel Bayleron DF Plnmor Roundup
HI HZ W FI F: F3 II H3
after the incubating. calculated as J = [(NH,-N,
1985 8 23 3 ?J II 28 2
Mar Apr May May Jun Jun AU8
60
I986
70
60
I
I
100 I
90
110
8 Apr 9 May I3 May lb
120 I
-
Hl
1 le Marl
May
23 May 9 Jun I Jul
The rate of ammonification
-
HZ/Wf2311wl
-
Fl
l3Moy)
-
F2
I24 May)
-
F3
(11 Jun )
+
11
(26 Jun)
-
Ii3
I2 Augt
was
+ NO,-NB) - (NH,-N,
The sum of all these (NA) NA = (NH,-N,,
.
+ NO?-N,)]iday
where _V= rate of ammonification as pg N IO g’ soil day- ‘, index 0 = before incubation, index B = after incubation. Measurements were done with two replications. Nitrifcation was determined in a manner analogous to ammonification (Beck, 1979). Instead of water, I ml of I% (NH&SO, solution was added to the soil sample and the conversion of NH, into NO, was measured and referred to as nitrification rate (NU). It has to be considered that: -the amount of added NH, might incrcasc during incubation by an additional rclcasc of NH, through ammonification; -a part of the NO, (and the NH, as well) might be assimilated by the soil micro0ora; and -a part of the NH, might be adsorbed or cvcn fixed to mineral particles. was calculated
(15Augl (26 Auql
(16 Scpl
Fig.
2a.
The
inlluence of’ a sequence of microbial biomass in 198s.
Control 70
80
90
100
pesticides
on
(%I 110
120
130
as
+ NO, - N,)
-(ad&d
NH,-N
+ NO,-N,
+ NH,-N,). -
Negative assimilated The NU by 10 g of exprcsscd calculated
130 1
HP/Wl9Moyl
values of NA indicate that more N is than released by ammonification. is the amount of NH, transformed to NO, soil during the incubation time of I4 days, as a percentage of added NH, and is as
NU (%) = [(NO)-N,
- NO,-N,)
x lOOO]/(added NH, Measurements
+ NA).day.
were done with two replications. Fig.
SloIistictrl uttulysis Data were statistically analysed using the software package SPSS on a Sperry main-frame computer. DifGrences of treatments were examined using the one-factorial analysis of variance, ONEWAY, by means of the Schetl? test. A 95% level of significance is designated by a “+” in all figures.
RESULTS
The microbial biomass increased from 25 to 80 and from 38 to 63 mg C 100 g-’ soil in the course of the years (I985 and 1986). respectively. The time of pesticide application was dependent on both the stage of development of the cereal and the weather
2b. The
influence of a sequence of microbial biomass in 1986.
pesticides
on
conditions. This led to a much shorter investigation period in 1986 with a rapid succession of pesticide applications as compared to the year before (Table 2). The microbial biomass was temporarily adversely affected by the pesticide treatments (Figs Za, b). This could be seen very clearly in 1985. where the application of Aretit + Arelon resulted in a drastic, 50% reduction of microbial biomass. During the following 6 weeks until the next treatment recovery was incomplete. Subsequent application of Dichlorprop + Cycocel caused a further loss of biomass, altogether resulting in a rather long-term depression of microbial biomass for 60days. At the end of May the microflora had recovered completely to the same level
370
EW SCHUSTER and D. Control
SCHR~DER
t-I.1
Control (%) 90
60
I
I
100
110
130
120
I
I
I
\\\\\\\.\
.
* nc
.\a
4
H2/W(23
-
Fl
l3May)
-
F2
i244May)
-
F3
(11 Junl
Fl
(3 May)
-
F2
(24Moyl
-
F3
111 Jun
+
I1
(2EJun)
-
11
(26
-
H3
(24~~1
*
H3
(2 4ugt
.
)
l16Ssp
Fig. ?a. The influcncc of a scqucnce of psticidcs dehydrogenasc activity in 1985.
(264~01
)
(16 kp)
on
Fig. 4n. The
influence
r
I
I
100
a
, 6
120
130
I
I
Ii1
(6
-
H2/W Fl F2 F3
l9May) 113 May) 116 Moyl
-
sequence in
Control(% 110
.
of
ammonification
Control (XI 90
Junl
l154ug)
.
(264ug)
60
4prl
-
(154ugl
70
(aMarI
60 I
70 1
EO 1
90 ,
of
pesticides
on
1985.
I
100
110 I
nprt
120 I Hl
+
130 1 I6 Apr)
-
HZ/WI9t4ay) I13Mayl Fl (16May) F2
(23 May)
-
F3
(23Moyl
11
(9Junl
-
II
(9
Junl
Ii3
I1 Jun)
-H3
II
Jut)
51.5
Fig. 3b. The influence of a sequence of pesticides on dehydrogenase activity in 1986.
Fig. 4b. The
as the control soil. In the following period, losses of biomass were short-lived and the control level was
DHA which was statistically significant only three times in every investigation period. The strong effects of the first treatment on biomass were not demonstrated by DHA though DHA normally proved to be the mom sensitive indicator for side-effects (Maas er al., 1986; B. Auspurg. unpublished Dissertation; Schuster et al., 1987). Further responses to pesticide application were similar to those of microbial biomass: there were only short-lived inhibitions of activity, followed by a quick recovery to the control level. so that side-effects had already disappeared before the next treatment was executed. The ammonification rate, which was determined in unamended soil. was very low (values between 0.4
reached again quickly. In 1986. despite the rapid succession of pesticide treatments, only minor effects were found. Reductions of biomass to a maximum of 10% could be statistically verified only in three cases and even the severe effect of Arctit + Arelon of the year before was not reproduced. In both years, no adverse effect could be seen after the complete pesticide sequence was finished. Dehydropenase activity (DHA) showed a very similar behaviour as microbial biomass (Figs 3a. b). In most casts the pesticides caused an inhibition of
influence
of
ammonification
a
sequence in
of
pesticides
on
1986.
Soil microflora mponx
to a sequence of pesticides
and 0.9 mg N kg-i
soil). Consequently, even small variations between treated and untreated soil led to large differences, if expressed as percentage of the control soil (Figs 4a. b). For this reason statistically significant effects could only be found in two cases each year. although there seemed to be strong sideeffects on the release of ammonia. In 1985 ammonification was stimulated after application of Sportak Alpha and Roundup by factors of SO and 30% respectively. In 1986 these effects were observed only by tendency (not statistically significant). In complete contrast to the year before, there was a strong inhibition of ammonification at the end of the observation period. Nitrilication was almost unaffected (Figs 5a. b). In 1985 the transformation of NH, to NO, was at first depressed by ca 15% but was stimulated shortly afterwards. Later in the year stimulative effects could be seen by tendency. In 1986 there was an initial depression of NO, formation, After mid-May. sideeffects on nitrilication were no longer noted.
371
Control t %) 60 I
70 1
60 I
90 1
100
110 1 -
120 I Hl
130 1 IeMorl
-
W2/Wf23Apprl
+
Fl
I3 May)
-
F2
(24M.a~)
-
F3
IllJUn)
-
It
(26Jun)
+
H3
il .
\\\\\\
l
(2Aug
1
IlSAug
I
l26Aug
1
DISCUSSION
The plant protection system used in our investigation caused only minor cffccts on microbial activitics. The microflora always rccovercd quickly, so that side-effects usually had already disappeared before the next pesticide trcatmcnt was carried out. Only the mixture of Arotit + Arclon induced inhibitions of microbial activity. The adverse effected of Aretit (dinoscbacctatc) wcrc dcscribcd by Malkomcs and collcagucs (Malkomcs and Pcstemer. 198 I. 1984; Malkomcs and Wohlcr. 1984). A. Wiedemann (unpublished Dissertation) and R. Auspurg (koc. cit.). Pacschkc and collcagucs (Paeschke and Hcitefuss, 1978; Passchkc (‘I 01.. 1978) attributed the toxicity of Arctit to its fungicidal ctfccts. When applied repeatcdly in consecutive years, the side-elTects decreased indicating an inductive adaptation of the soil fungi. This corresponds to the diminished effects observed in the second year of our investigation. We cannot really decide whcthcr there is one component of the pesticide mixture which is dominantly responsible for the side-effects. Results of Malkomes (1980) showed that both Arctit and Isoproturon reduced DHA. but the author ascribed the main effect to Aretit. Davis and Marsh (1980) and Tag-El-Din (1982) suggest that lsoproturon is microbiocidal only at higher concentrations (100 )lg g-l). Ncven ef al. (1975) and Mudd CI al. (1985) confirmed negligible effects of isoproturon when used at normal dosages. The increase of ammonification after Aretit + Arelon application might be due to the mineralization of killed microbes. Domsch and Schrijder (1986) showed that increased amounts of N,,, in the soil after the application of biocidal chemicals are at least partly explained by the mineralization of microbial cells, which provide easily available nutrients for the surviving population. Malkomes and Wohler (1983) found different reactions dependant on soil type: a less adsortive soil (loamy sand) showed decreased amounts of N,,, I month after the application of Aretit whereas a sandy loam soil showed significantly higher amounts.
(16Sapl
Fig.
%I. The
influence of a sequcncc of pesticides on nitrification in 198s.
Control TO
80
I
I
90
I b\\-.\
.
(%I
100 5
110
-
+
-
-
d -
Fig.
5b. The
I Hl
120
130
I
I l6Apr)
liZ/W(SMayl Fl (13Moy F2 (16Moy) F3 i23May
I1
l9Jun)
H3
(1
1 I
JutI
influence of a sequence of pesticides on nitritication in 1986.
Nitrification was not disturbed by Aretit. This is in agreement with other results of field experiments (Rankov. 1968; A. Wiedemann, lot. cit.), although laboratory experiments have sometimes shown an inhibition of nitrification which turned out to be concentration dependent (Sommer. 1970; Domsch and Paul, 1974; Johnen ef ol., 1977; Malkomes and Wohler. 1983). Little information is available for the other chemicals which showed only small and short-lived sideeffects in our investigation. Carbendazim, one of the active substances of Sportak Alpha, was degraded within 4 weeks by 60-80% (Sole1 ef al.. 1979). A. Wiedemann (lot. cit.) found no influence by Sportak Alpha on respiration and degradation of cellulose,
3-z
Ew SCHLSTER and D. SCHR~DER
even if applied at IO tnnes higher than normal concentration. Only DHA was inhibited in a laboratory experiment at the high dosage. Dichiorprop is rapidly degraded in soils (Kirkland and Fryer. 19’2; Hance. 1979: Thompson er ~1.. 1984). It seems 10 be non-toxic to microorganisms if applied at normal tieid rates (Sommer, 1970; S. Scholz und H. Kiinig. Dissertation: Marsh and Davies. 198 I; Xlaikomes. 1979). Simiiarily. the active ingredient of Roundup. Giyphosate. seems to be non-toxic. This is confirmed by L6nsj6 er al. (1980). Gomez and Sagardoy (1955) and Grossburd (1985). The toxicity of Buyieton DF appears to be dominantly determined by the active ingredient Captufoi which did not show grave side-effects when used at normal field rates (Atlas er nl.. 1978: A. Wicdemann, lot. cit.) In 1986. side-effects were less than in 1985 despite the more rapid sequence of pesticide treatments. This underlines the inRuence of weather conditions which may take erect in different ways. Ccrtainiy. time and intensity of rainfall determine the input into and distribution of chemicals within the soil. Indirect e&cts result from the efTccts of weather on plant cover and microbial activity. WC also know about the influcncc of soil tcmpcraturc and soil moisture on microbial activltv (Stott (81 [II.. 1986) which leads to a sc:tsonal variation and dynamic of microbi:li popui2linns. Dcpcnding upon lhc composition and slate of :lcti\ lly of the microtIora. chcmicais may thus cause v:lr>ing sid ,-ctrccls. E\cn ttic ditfcrcnt crop cuiturcs might conlributc to the ditycrcnt side-ctl?cts ohscrvcd in this study. B;lricy dovelops more quickly than wheat. FICIICC. the pl;lnt cover U;IS atrcatiy more dcnsc and t:lllcr when the pesticides wcrc appticd. reducing the input ot chcnuc:ils into the soil. These manifold ctl2crs of v;lrling cnvironmcnt;ll conditions ;lrc ohscrvcd in all invcstigtions that inctutlc replications in diffcrcnt years (Marsh and Dlvics. IYX I, Maikomcs, 1971); hl;likomcs and I’cslcmcr, IYX I ). The contradictory results of ammonitication contirm that this test has to be looked upon critically, ;IS mcntioncd above. Deviations from the control soil were rarely statistically significant and the observed ctfects might bc independent of pesticide treatments. This is also supported by the frequent and abrupt change between stimulations and inhibitions. The raw of ammomtication was slightly increased on the treated plots in 1985. Marsh er ~1. (1977). Van Schrrven er trl. (1970) and A. Wicdemann (lot. cir.) have explained such an effect as a stimulation of mineralization of soil nitrogen by pcsticidcs, howcvcr they orered no proof of this theory. It is our opinion that this obscrvarion is mainly attributed lo the problem of referring to a suitable “control plot” in field expcrimenrs. This topic is cxtensivciy discussed by Grossbard (197’)) and Anderson (1978) who assume the higher release of N,,, to be due lo an increased supply of organically-bound nitrogen in the form of uceds killed by herbicides etc. on the treated piors. Summarizing the obtained results we may establish that the sequential application of pesticides in the plant protection system under investigation did not show strong effects on the soil microflora. We did not
find results supporting our hypothesis that there might arise long-term inhibitIons of microbial activity when pesticides are applied in rapid succession. Evaiuatinp the side-effects using the model of Domsch er al. (1983) modified by Maikomes (1985) we may describe ail effects to be tolerable and most as even negligible. Nevertheless. we should bear in mind the shortcomings of present analytical methods as well as the possibility of additive stress arising from the simultaneous occurrence of natural and chemical stress situations. Furthermore. the freqrtencx of anthropogenic stress situations (e.g. number of pesticide treatments) should be taken into account when defining threshold values of tolerance. We must consider that most of the pesticides used in our investigation wcrc non-toxic to microorganisms. at least when used at recommended field rates. Assuming a pesticide treatment system consisting of more compounds as toxic as Arctit one can easily imagine iongcr-lasting side-erects devcioping which would then necessarily be assessed to be critical.
Anderson J. R. (107X) Pusticidc cti’cctson non-tar@ so11 microorganisms. In f’t~~ic~rd~* ~\/rmdM~~,q~~ (I. R. H111and S. J. L. Wright. Us). pp. 313 -353. Academic Press. London.
Domrch K. Ii. and Paul W. (lY74) Simularion and eapcrimcntat anlrlysls of the influence of herbicides on solI nitrification.. Arc,hf~ /ir ,tfikrohio/f~gw 97, 283 -301. Domsch K. H. and Schriidcr M. (1986) EintluB eininer Herbizide auf den mikrobiellen ‘Biomasse-Kohlenst&l und den MineralstickstotTgehalt dcs bodens. In DFGForschun~vhrrrchr Herhkide II. pp. 225-233. VCH Verlagsgesellschaft mbH, Wcinhcim. Domsch K. l-l.. Jagnow G. and Anderson T. H. (1983) An ecological concept for the avxcsmcn~ of side-ctTec& of agrochcmicals on soil microorganisms. Rcridur Rcritwx 86. 66 .lOS. Gomez M. A. and Sagardoy M. A. (IYNS) lnfluencc of glyphosatc herbicide on the microtlors and mesofaurw of a sandy soil in a semiarid rcgwn. Rw Lu~inocm~. Mwrohd. 27. 351 -357. Grcavcs M. P. (1987) Side-etl’cct testing: an alternalive approach. In Pr.rriciL E//kr.r WI Sod .\ficrulk,ra. (L. Somerville and M. P. Grcavcs. Eds). pp. l83-.lYO. Taylor & Francis. London. Grossbard E. (1979) The continuous monitoring of the rye straw. treated evolution of “CO from “C-labelled with herbicides. ~ncubattng undisturbed on the soil surface. In Slrcrn, Decuj, owf if.7 f$!fi,ct 011 nisposal und C’tilixrrirw (E. Grossbard. Ed.). pp. ?9Y-305. Wiley. Chichester.
Soil microflora
response to a sequence of pesticides
Grossbard E. (1985) Effects of glyphosate on the microflora. In The Herbicide Glyphosate (E. Grossbard and D. Atkinson. Eds), pp. 159-185. Butterworth. London. Hance R. J. (1979) Effect of pH on the degradation of atrazine. dichlorprop. linuron and propyzamide in soil. Pesticide Science 10, 83-86. Johnen B. G. and Davis P. I. (1977) Standorttests als lndikatoren fiir Nebenwirkungen van Pflanzenschutzmitteln auf Bodenmikroorganismen. Mirreitungen der Biologischen Bundesansralf (Braunschweig) 178. 227-218. Kirkland K. and Fryer J. 0. (1972) Degradation of several herbicides in a soil previously treated with MCPA. weed Research 12. 90-95. Ltinsjii H.. Stark J., Torstensson L. and Wessen 8. (1980) Glyphosate: decomposition and effects on biological processes in soil. ,lls~ Swedish Weed Conference Report (W’eeds Ried Conrrol) 21. 140-146. Maas G., Malkomes H. P. and Pestemer W. (1986) Beeinflussung bodenbiologischer Aktivitlten durch Herbizide allein und durch Pflanzenschutzmittel-spritzfolgen in Zuckerriiben-Getreide-Fruchtfolgen. In DFGForschungsberichf Herbi:ide II. pp. 299-3 IS. VCH Verlagsgesellschaft mbH. Weinheim. Malkomes H. P. (1979) Verhalten einiger bodenbiologischer Parameter nach Vorautlaufapplikation van drei Herbizidcn zu Wintergerste. Nuchrichtenhlarr des Dcumhen P/t~~n~~nschur-tln,~f~~s (Brcrrmschu~c~ig ) 3 I . I 82 - I 8 5. Malkomcs H. P. (1980) Vcrhalten van bodcnbioAktivititen logischcn und Ptlanzcnwuchs bci Nachauflauf-Applikation einer Hcrhizid-Tankmischjung zu Win tcrgetreidc. Zcirschriji Ji’ir ~/ltm~enkrtorkl~~i/~~n ml ~//trrr;cn.sctmr~ 87. 62 I -630. Malkomcs II. P. (IYXS) Einfliissc auf Btrdcnmikroflora und IIotlcnfauna: Einfliisse van Pllanzcnschutzmittcln auf Bodcnmikroorg:lni’imcn und ihre Lcistungcn. Bcrichw ii/w I,cr~tth~~irl.vc~hcrJi, .%m~lcrhc*/i IW. I34 147. M:ilkomcs II. P. and Pestcmcr W. (IYXI) EinnuU van Pll;~t~/cn~chut~mittcln in Wintcrgctrcidc auf biolopischc Aktivitiitcn im Rodcn. Tc~l I: Dchydrogcn;lscaktivillit und Srroha h hau. %c*ir.rchri/i ,Jiir PJl~m:et~krtrnkirc,irc~~r umt /‘jkrrt_c,rt,vctfrtr1. Sont/crtrt~/i IS, 301 -3 I I. Malkurncs tl. P. and Pestcmcr W. (IYH4) Bceinflussung mikrobicller Aktivitlltcn und dcs Dinoseb-Abbaus im Botlcn duroh ausgcwiihltc Umwcltchcmikalien unter Frcilandbedingungen. Zrirschriji Jiiir PJ/~ltrrr~~~nkrcmk/l,eiren umt P/lulr:c~rrsl~tlrrr: 193-20’. Malkomcs H. P. and Wijhler B. (1983) Testing and evaluating some methods to investigate side elTccts of cnvironmental chemicals on soil microorganisms. Ecoroxicolog~ trrrd E,rl.iro,rnre,lruI SuJir v 7, 284. Malkomes H. P. and Wihler B. (1984) Vergleich von Tsstverfahren zur Erfassung der Nebenwirkungen von P11anrenschutzmitteln auf Bodenmikroorganismen am Bcispicl eines Herbizids. Nachrichfenhlurr des Deurschen ~f((r,~:cnschur:d;~,~~sfes (Bruunschweig) 35. 86-92. Marsh J. A. P. and Davies H. A. (1981) Efiects of dichlorprop and mccoprop on respiration and transformation of nitrogen in two soils. Butlerin of Etwirormwnrul Coniamirwivn und Toxrcolog.~ 26, log- I IS.
373
Marsh J. A. P., Davies H. A. and Grossbard E. (1977) The effect of herbicides on respiration and transformation of nitrogen in two soils. I. Metribuzin and glyphosate. Weed Research 17. 77-82. Mudd P. J., Greaves M. P. and Wright S. J. L. (1985) Effects of isoproturon in the rhirosphere of wheat. Weed Research 25. 413-432. Neven H.. Vlassak K. and Heremans K. A. H. (1975) Involved van ipuron. dinosebacetat en triazophos op enkele biologische processen in de bodem. Mededelingen Rijksfucvlreir Lamibouwwetenschappen Gent 40. I22 I 1230. Paeschke R. R. and Heitefuss R. (1978) Der EinfluO van Herbiziden auf metabolisch aktive Bakterien und Pilze im Boden bei erstmaliger und mehrjahriger Anwendung. Zeirschri/ fir PJiun:enkrankheiren und PJiunzenschu!z 85. 471-481. Paeschke R. R.. Ebing W. and Heitefuss R. (1978) Abbau von Herbiziden und deren EinHuB auf die biologische Aktivitat im Boden nach erstmaliger und mehrjihriger Anwendung. Zeirschri/r Ji?r PJlan.-enkrankheiten und Pfltm:enschut: 85. 280-297. Rankov V. (1968) The effect of some herbicides on nitrification in soils. Sorier Soil Science 3. 81-90. Schuster E. (1987) Die Beeinflussung der mikrobiellen Aktivitit des Bodens und der Nebenwirkungen von Pflanzenschutzmitteln durch Zugabe verschiedcner VDLUF.4 -Schrijren -reihc 23, organischer Substrate. KongreOband. 925 -936. Siidcrstriim B.. Baath E. and Lundgrcn B. (1983) Dcxrease in soil microbial activity and biomasses owing to nitrogen amend men ts. Canudian Jourmd of Microbiology 29. 150&l 506. Sole1 Z.. Sandlcr D. and Dinoor A. (1979) Mobility and pcrsistcncc of carbendarim and thiabendazolc applied to soil via drip irrigation. Phyroparhah~gy 69. I273 1277. Sommcr K. (1970) Bccinflussuny dcr Nitrifikation und des N-llaushnltcs in Biiden durch vcrschiedene Pflanzenschutzmittcl. f.cmch~irrsch~~f~l~t~~~Forschung. Sonderhqfr 2.511. 22 30. Stott D. E.. Elliott L. F.. Popendick R. I. and Campbell G. S. (19X6) Low temperature or low water potential elrccts on the microbial decomposition of residue. Soil Biolox) wheat & Biochemistry IS, 577-582. Tag-El-Din A. (1982) Herbicides and respiration flush of soil biomass. Mededelingen Rijksfuculreir LandhouI~Hrrcn.~chuppm Gent 47. I9 I-199. Thompson D. G.. Stephenson G. R.. Salomon K. R. and Skepasts A. (1984) Persistence of (2.4.dichloroand phenoxy)acetic acid 2-(2.4-dichlorophenoxy) propionic acid in agricultural and forest soils of northern and southern Ontario. Journal ofAgriculrural and Food Chemis/ry 32, 578-58 I. Van Schreven D. A., Lindenbergh D. J. and Koridon A. (1970) Etfect of several herbicides on bacterial populations and activity and the persistence of these herbicides in soil. Plunf and Soil 33, 513-532.
0038-0717 90 53.lM + 0.00 Copynghr C, 1990 Pergamon Press plc
SIDE-EFFECTS OF SEQUENTIALLY-APPLIED PESTICIDES ON NON-TARGET SOIL MICROORGANISMS: FIELD EXPERIMENTS Evr SCHUSTER*and D. SCHROEDER Department
of Soil Science, University
of Trier.
(Acceppled 20 Augusr
Postfach 3825. 5500 Trier.
F.R.G.
1989)
plant protection system consisting of 7 pesticide treatments was investigated for its side-effectson the soil microflora. Measurements of microbial activity were based upon microbial biomass,
Summary-A
dehydropenase activity. ammonification and nitritication. Investigations were carried out over 2 yr in a field experiment with four replications. Successive applications of the pesticides caused only slight and short-lived side-etkts on the soil microtlora. These usually disappeared before the next treatment was carried out. The results appeared to be significantly influenced by weather conditions. Thus, side-etkts were less in 1986 despite a more rapid sequence of pesticide treatments compared to the year before.
INTRODCCTION
Pesticides The chemicals applied were typical for a plantprotection system associated with intensive cultivation of ccrcals. It consisted of four herbicides, three fungicides. one insecticide and one growth regulator. The nine chemicals were applied at normal field rates in a sequence of seven treatments (two tank mixtures included). A detailed description is given in Table I. The application was carried out with a motor syringe supplied with teejet nozzles of type 1103 LP yielding a syringe pressure of 0.13 MPa. The chemicals were sprayed at a rate of 400 I. solution ha-‘.
The use of chemicals. including pesticides, has become an integral and economically essential part of modern agriculture. Pesticides arc often applied several times during one growing season and a part of the materials applied always reaches the soil. As they arc designed to be biologically active we arc concerned that continuous inputs of pesticides might affect the soil microflora and so impair soil fertility. Much rcscarch work has been done on testing the side-cffccts of single applications of chemicals. In general, observed effects have been minor and shortlived (&eaves, 1987). However, we do not know much about the more realistic situation where different active substances come together in the soil. There might arise additional stress from the use of mixtures and sequences of pesticides because of the accumulation of toxic agents or the formation of new compounds (from pesticide combinations) which can bc more toxic than the original substances. Our aim was to investigate whether applications of various pesticides in quick succession can adversely affect the activity of the soil microflora.
Weuther condirions Weather conditions for the 2yr experimental period are described in terms of monthly temperature and rainfall and their deviations from long-standing mean values in Fig. I. The data were provided by the weather station Trier-Petri&erg. Further details are probably not appropriate because the measuring station was too far away (ca 25 km) from our field site. The temperatures largely corresponded to the long-standing mean values in both investigation periods. The winter months January and February were colder than average, but there was no soil sampling during that time. The values of monthly rainfall vary around the long-standing mean value. There was a tendency toward more precipitation from March until August 1985. whereas the winter 1985-1986 was drier than normal. March and April 1986 were excessively moist, whereas the following months were drier than the long-standing mean value.
MATERIALS AND METHODS Tar
plots
A complete block design of 2 by IO m plots with four replications was used. The field site was cu 25 km northeast from Trier (western Germany, Mosel valley) near Fiihrcn in the “Wittlicher Senke”. The soil was primarily a Parabraunerde (Glcyic Luvisol) and had been under agricultural cultivation for a long time. The organic matter content averaged 0.85% and the soil pH (CaCI,) averaged 6.5 for all plots. The textural composition was 16.4% sand, 66.7% silt and 16.9% clay. The standing crops were wintcrwheat (1985) and spring barley (1986).
*Present
address: Department
Mcinchen.
of Soil 8050 Freising Weihenstephan,
Science. F.R.G.
Soil sampling
procedure
Thirty to 40 subsamples of topsoil (O-S cm), taken with a special soil-sampling auger, were combined to give one mixed sample of each plot taken for analysis. The soil was thoroughly mixed and, if necessary, allowed to dry to a moisture content of 4@-60% before sieving (< 2 mm). Airdrying of the WHC,,
TU
367
Ew SCHUSTER
368 Table
I. Propcrc~es and
and D. SCHR~DER dosage
of the apphed
pcwcldcs ACI!K
ProductIon Common
Aretlt
Dmoxbacetate
Hoechst
Dm~trobutylphenylacctatc
lsoproturon
Hoechst
.V..V-Dtmethyl-N-[t(
Dtchlorprop
BASF
Dtchlorphcnoxyproplonic
Chlormcquat
BASF
Chlorcholmchloride
Prochloraz+
Schcnng
~V.propyl-N-trichlorophcnony-ethylcarb~moyl~m~dazole
Arelon C-86
DP
Cycocel Sportak
Alpha
name
subrt
Trade-name
company
Carbendazim Baylcton
Captafol+
DF
Chmucal
name
(0.1 49
I-mcthylcthyI)phenyI]carbamidc and
Mcthoxycarbonylamlno-benrimldazoic Bayer
Triadimefon
,V-(1.1.2.2.tetrachlorcthyl-thio)_cyclohe~-J
PInmor
Pinmicarb
Schcring
Dimethyl-2-dimethyl
Roundup
Clyphosate
BASF
N-Phosphonomcthyl-glycine
A nalylical
hlicrobial biomass. The amount of carbon in the physiologically-active microbial biomass was dctcrmined (Anderson et al., 1978). It was found that 400mg glucose IOOg-’ soil was needed for biomass mcasurcmcnts. This quantity of glucose was thoroughly mixed into the soil (two rcplicatcs). Amcndcd soils wcrc then poured into glass cylinders and connected to an “Ultragus U3SB” CO,-analyser (Wiisthoff, Bochum) for automatic analysis of CO? production rates. Incubation was at 22’C for a maximum of 5 h. The maximum initial rates of respiration were dctcrmincd and the values obtained entered into the equation s = 40.04 _Y+ 0.37
ha
50
2.5
60
JO
56
05
8
I5
.J-dicarbonmlde
65
40
6
amino-J-pynmidinyldlmelhylcarbamate
5 36
03 4.0
where x = mg microbial C per unit soil and _V= ml CO: per unit soil h-’ with I mg CO: day-’ = 0.021 ml CO: h-’ (22’C; 101.3 kPa).
Dehychgenase actiriry was determined according to A. Thalmann (Dissertation). A 5 g sample of soil was saturated with 5 ml of a 0.6% solution of 2.3.5-triphenyltetrazolium chloride (TTC) buffered with trishydroxyaminomcthan at a pH of 7.6 and mixed thoroughly with a vortex shaker. The samples were then kept in sealed tubes at 27‘C for 24 h. Triphcnyltctruzolium formazan (TPF), produced by the reduction of XC, was extracted with 25 ml acetone and its concentration dctcrmincd photometrically at 547 nm. Anvnonifiufim W;LSdctcrmincd by the rclcasc of ammonia and nitrate (Beck. 1983). Subsamplcs (IO g) of soil wcrc filled into culture glasses and 3 ml distilled water wcrc carefully dropped onto the surfact (no mixing!). The glasses wcrc closed by Kapscnberg caps and kept for 2 weeks at 27 C. Then the samples wcrc shaken with a solution of I% KCI and tiltcrcd. The contents of ammonia and nitrate (photometrically; Cawsc, 1967) were dctcrmincd before and
Temperature
mm 808040zot
0 J’F’M’A’M’J’J’A
S
0
N
DIJ
I1 F
M
1985
Fig. I. Mean
monthly
temperature
‘)
30
30
Chlor-phenoxy-dimcthyl-triazolbutanonc
exterior of aggregates was avoided. The soil was stored at 4’C until analysis (period not exceeding 3 months). Soils were sampled I day before and 3 days after each pesticide treatment; the application dates for the two years of investigation (1985) and 1986) are given in Table 2.
Dosage (kg
A’H
J
J
A
S
O’N
1986
and rainfall and their deviations in 1985 and 1986.
from the longstanding
mean values
D
Soil microflora response to a sequence of pesticides
369
Table 2. Times of pesticide apphcarmn
(%)
Control Applicauon time Common name
Code
Are111+ Arelon U-t6 DP + CCC Sponak Alpha Corbel Bayleron DF Plnmor Roundup
HI HZ W FI F: F3 II H3
after the incubating. calculated as J = [(NH,-N,
1985 8 23 3 ?J II 28 2
Mar Apr May May Jun Jun AU8
60
I986
70
60
I
I
100 I
90
110
8 Apr 9 May I3 May lb
120 I
-
Hl
1 le Marl
May
23 May 9 Jun I Jul
The rate of ammonification
-
HZ/Wf2311wl
-
Fl
l3Moy)
-
F2
I24 May)
-
F3
(11 Jun )
+
11
(26 Jun)
-
Ii3
I2 Augt
was
+ NO,-NB) - (NH,-N,
The sum of all these (NA) NA = (NH,-N,,
.
+ NO?-N,)]iday
where _V= rate of ammonification as pg N IO g’ soil day- ‘, index 0 = before incubation, index B = after incubation. Measurements were done with two replications. Nitrifcation was determined in a manner analogous to ammonification (Beck, 1979). Instead of water, I ml of I% (NH&SO, solution was added to the soil sample and the conversion of NH, into NO, was measured and referred to as nitrification rate (NU). It has to be considered that: -the amount of added NH, might incrcasc during incubation by an additional rclcasc of NH, through ammonification; -a part of the NO, (and the NH, as well) might be assimilated by the soil micro0ora; and -a part of the NH, might be adsorbed or cvcn fixed to mineral particles. was calculated
(15Augl (26 Auql
(16 Scpl
Fig.
2a.
The
inlluence of’ a sequence of microbial biomass in 198s.
Control 70
80
90
100
pesticides
on
(%I 110
120
130
as
+ NO, - N,)
-(ad&d
NH,-N
+ NO,-N,
+ NH,-N,). -
Negative assimilated The NU by 10 g of exprcsscd calculated
130 1
HP/Wl9Moyl
values of NA indicate that more N is than released by ammonification. is the amount of NH, transformed to NO, soil during the incubation time of I4 days, as a percentage of added NH, and is as
NU (%) = [(NO)-N,
- NO,-N,)
x lOOO]/(added NH, Measurements
+ NA).day.
were done with two replications. Fig.
SloIistictrl uttulysis Data were statistically analysed using the software package SPSS on a Sperry main-frame computer. DifGrences of treatments were examined using the one-factorial analysis of variance, ONEWAY, by means of the Schetl? test. A 95% level of significance is designated by a “+” in all figures.
RESULTS
The microbial biomass increased from 25 to 80 and from 38 to 63 mg C 100 g-’ soil in the course of the years (I985 and 1986). respectively. The time of pesticide application was dependent on both the stage of development of the cereal and the weather
2b. The
influence of a sequence of microbial biomass in 1986.
pesticides
on
conditions. This led to a much shorter investigation period in 1986 with a rapid succession of pesticide applications as compared to the year before (Table 2). The microbial biomass was temporarily adversely affected by the pesticide treatments (Figs Za, b). This could be seen very clearly in 1985. where the application of Aretit + Arelon resulted in a drastic, 50% reduction of microbial biomass. During the following 6 weeks until the next treatment recovery was incomplete. Subsequent application of Dichlorprop + Cycocel caused a further loss of biomass, altogether resulting in a rather long-term depression of microbial biomass for 60days. At the end of May the microflora had recovered completely to the same level
370
EW SCHUSTER and D. Control
SCHR~DER
t-I.1
Control (%) 90
60
I
I
100
110
130
120
I
I
I
\\\\\\\.\
.
* nc
.\a
4
H2/W(23
-
Fl
l3May)
-
F2
i244May)
-
F3
(11 Junl
Fl
(3 May)
-
F2
(24Moyl
-
F3
111 Jun
+
I1
(2EJun)
-
11
(26
-
H3
(24~~1
*
H3
(2 4ugt
.
)
l16Ssp
Fig. ?a. The influcncc of a scqucnce of psticidcs dehydrogenasc activity in 1985.
(264~01
)
(16 kp)
on
Fig. 4n. The
influence
r
I
I
100
a
, 6
120
130
I
I
Ii1
(6
-
H2/W Fl F2 F3
l9May) 113 May) 116 Moyl
-
sequence in
Control(% 110
.
of
ammonification
Control (XI 90
Junl
l154ug)
.
(264ug)
60
4prl
-
(154ugl
70
(aMarI
60 I
70 1
EO 1
90 ,
of
pesticides
on
1985.
I
100
110 I
nprt
120 I Hl
+
130 1 I6 Apr)
-
HZ/WI9t4ay) I13Mayl Fl (16May) F2
(23 May)
-
F3
(23Moyl
11
(9Junl
-
II
(9
Junl
Ii3
I1 Jun)
-H3
II
Jut)
51.5
Fig. 3b. The influence of a sequence of pesticides on dehydrogenase activity in 1986.
Fig. 4b. The
as the control soil. In the following period, losses of biomass were short-lived and the control level was
DHA which was statistically significant only three times in every investigation period. The strong effects of the first treatment on biomass were not demonstrated by DHA though DHA normally proved to be the mom sensitive indicator for side-effects (Maas er al., 1986; B. Auspurg. unpublished Dissertation; Schuster et al., 1987). Further responses to pesticide application were similar to those of microbial biomass: there were only short-lived inhibitions of activity, followed by a quick recovery to the control level. so that side-effects had already disappeared before the next treatment was executed. The ammonification rate, which was determined in unamended soil. was very low (values between 0.4
reached again quickly. In 1986. despite the rapid succession of pesticide treatments, only minor effects were found. Reductions of biomass to a maximum of 10% could be statistically verified only in three cases and even the severe effect of Arctit + Arelon of the year before was not reproduced. In both years, no adverse effect could be seen after the complete pesticide sequence was finished. Dehydropenase activity (DHA) showed a very similar behaviour as microbial biomass (Figs 3a. b). In most casts the pesticides caused an inhibition of
influence
of
ammonification
a
sequence in
of
pesticides
on
1986.
Soil microflora mponx
to a sequence of pesticides
and 0.9 mg N kg-i
soil). Consequently, even small variations between treated and untreated soil led to large differences, if expressed as percentage of the control soil (Figs 4a. b). For this reason statistically significant effects could only be found in two cases each year. although there seemed to be strong sideeffects on the release of ammonia. In 1985 ammonification was stimulated after application of Sportak Alpha and Roundup by factors of SO and 30% respectively. In 1986 these effects were observed only by tendency (not statistically significant). In complete contrast to the year before, there was a strong inhibition of ammonification at the end of the observation period. Nitrilication was almost unaffected (Figs 5a. b). In 1985 the transformation of NH, to NO, was at first depressed by ca 15% but was stimulated shortly afterwards. Later in the year stimulative effects could be seen by tendency. In 1986 there was an initial depression of NO, formation, After mid-May. sideeffects on nitrilication were no longer noted.
371
Control t %) 60 I
70 1
60 I
90 1
100
110 1 -
120 I Hl
130 1 IeMorl
-
W2/Wf23Apprl
+
Fl
I3 May)
-
F2
(24M.a~)
-
F3
IllJUn)
-
It
(26Jun)
+
H3
il .
\\\\\\
l
(2Aug
1
IlSAug
I
l26Aug
1
DISCUSSION
The plant protection system used in our investigation caused only minor cffccts on microbial activitics. The microflora always rccovercd quickly, so that side-effects usually had already disappeared before the next pesticide trcatmcnt was carried out. Only the mixture of Arotit + Arclon induced inhibitions of microbial activity. The adverse effected of Aretit (dinoscbacctatc) wcrc dcscribcd by Malkomcs and collcagucs (Malkomcs and Pcstemer. 198 I. 1984; Malkomcs and Wohlcr. 1984). A. Wiedemann (unpublished Dissertation) and R. Auspurg (koc. cit.). Pacschkc and collcagucs (Paeschke and Hcitefuss, 1978; Passchkc (‘I 01.. 1978) attributed the toxicity of Arctit to its fungicidal ctfccts. When applied repeatcdly in consecutive years, the side-elTects decreased indicating an inductive adaptation of the soil fungi. This corresponds to the diminished effects observed in the second year of our investigation. We cannot really decide whcthcr there is one component of the pesticide mixture which is dominantly responsible for the side-effects. Results of Malkomes (1980) showed that both Arctit and Isoproturon reduced DHA. but the author ascribed the main effect to Aretit. Davis and Marsh (1980) and Tag-El-Din (1982) suggest that lsoproturon is microbiocidal only at higher concentrations (100 )lg g-l). Ncven ef al. (1975) and Mudd CI al. (1985) confirmed negligible effects of isoproturon when used at normal dosages. The increase of ammonification after Aretit + Arelon application might be due to the mineralization of killed microbes. Domsch and Schrijder (1986) showed that increased amounts of N,,, in the soil after the application of biocidal chemicals are at least partly explained by the mineralization of microbial cells, which provide easily available nutrients for the surviving population. Malkomes and Wohler (1983) found different reactions dependant on soil type: a less adsortive soil (loamy sand) showed decreased amounts of N,,, I month after the application of Aretit whereas a sandy loam soil showed significantly higher amounts.
(16Sapl
Fig.
%I. The
influence of a sequcncc of pesticides on nitrification in 198s.
Control TO
80
I
I
90
I b\\-.\
.
(%I
100 5
110
-
+
-
-
d -
Fig.
5b. The
I Hl
120
130
I
I l6Apr)
liZ/W(SMayl Fl (13Moy F2 (16Moy) F3 i23May
I1
l9Jun)
H3
(1
1 I
JutI
influence of a sequence of pesticides on nitritication in 1986.
Nitrification was not disturbed by Aretit. This is in agreement with other results of field experiments (Rankov. 1968; A. Wiedemann, lot. cit.), although laboratory experiments have sometimes shown an inhibition of nitrification which turned out to be concentration dependent (Sommer. 1970; Domsch and Paul, 1974; Johnen ef ol., 1977; Malkomes and Wohler. 1983). Little information is available for the other chemicals which showed only small and short-lived sideeffects in our investigation. Carbendazim, one of the active substances of Sportak Alpha, was degraded within 4 weeks by 60-80% (Sole1 ef al.. 1979). A. Wiedemann (lot. cit.) found no influence by Sportak Alpha on respiration and degradation of cellulose,
3-z
Ew SCHLSTER and D. SCHR~DER
even if applied at IO tnnes higher than normal concentration. Only DHA was inhibited in a laboratory experiment at the high dosage. Dichiorprop is rapidly degraded in soils (Kirkland and Fryer. 19’2; Hance. 1979: Thompson er ~1.. 1984). It seems 10 be non-toxic to microorganisms if applied at normal tieid rates (Sommer, 1970; S. Scholz und H. Kiinig. Dissertation: Marsh and Davies. 198 I; Xlaikomes. 1979). Simiiarily. the active ingredient of Roundup. Giyphosate. seems to be non-toxic. This is confirmed by L6nsj6 er al. (1980). Gomez and Sagardoy (1955) and Grossburd (1985). The toxicity of Buyieton DF appears to be dominantly determined by the active ingredient Captufoi which did not show grave side-effects when used at normal field rates (Atlas er nl.. 1978: A. Wicdemann, lot. cit.) In 1986. side-effects were less than in 1985 despite the more rapid sequence of pesticide treatments. This underlines the inRuence of weather conditions which may take erect in different ways. Ccrtainiy. time and intensity of rainfall determine the input into and distribution of chemicals within the soil. Indirect e&cts result from the efTccts of weather on plant cover and microbial activity. WC also know about the influcncc of soil tcmpcraturc and soil moisture on microbial activltv (Stott (81 [II.. 1986) which leads to a sc:tsonal variation and dynamic of microbi:li popui2linns. Dcpcnding upon lhc composition and slate of :lcti\ lly of the microtIora. chcmicais may thus cause v:lr>ing sid ,-ctrccls. E\cn ttic ditfcrcnt crop cuiturcs might conlributc to the ditycrcnt side-ctl?cts ohscrvcd in this study. B;lricy dovelops more quickly than wheat. FICIICC. the pl;lnt cover U;IS atrcatiy more dcnsc and t:lllcr when the pesticides wcrc appticd. reducing the input ot chcnuc:ils into the soil. These manifold ctl2crs of v;lrling cnvironmcnt;ll conditions ;lrc ohscrvcd in all invcstigtions that inctutlc replications in diffcrcnt years (Marsh and Dlvics. IYX I, Maikomcs, 1971); hl;likomcs and I’cslcmcr, IYX I ). The contradictory results of ammonitication contirm that this test has to be looked upon critically, ;IS mcntioncd above. Deviations from the control soil were rarely statistically significant and the observed ctfects might bc independent of pesticide treatments. This is also supported by the frequent and abrupt change between stimulations and inhibitions. The raw of ammomtication was slightly increased on the treated plots in 1985. Marsh er ~1. (1977). Van Schrrven er trl. (1970) and A. Wicdemann (lot. cir.) have explained such an effect as a stimulation of mineralization of soil nitrogen by pcsticidcs, howcvcr they orered no proof of this theory. It is our opinion that this obscrvarion is mainly attributed lo the problem of referring to a suitable “control plot” in field expcrimenrs. This topic is cxtensivciy discussed by Grossbard (197’)) and Anderson (1978) who assume the higher release of N,,, to be due lo an increased supply of organically-bound nitrogen in the form of uceds killed by herbicides etc. on the treated piors. Summarizing the obtained results we may establish that the sequential application of pesticides in the plant protection system under investigation did not show strong effects on the soil microflora. We did not
find results supporting our hypothesis that there might arise long-term inhibitIons of microbial activity when pesticides are applied in rapid succession. Evaiuatinp the side-effects using the model of Domsch er al. (1983) modified by Maikomes (1985) we may describe ail effects to be tolerable and most as even negligible. Nevertheless. we should bear in mind the shortcomings of present analytical methods as well as the possibility of additive stress arising from the simultaneous occurrence of natural and chemical stress situations. Furthermore. the freqrtencx of anthropogenic stress situations (e.g. number of pesticide treatments) should be taken into account when defining threshold values of tolerance. We must consider that most of the pesticides used in our investigation wcrc non-toxic to microorganisms. at least when used at recommended field rates. Assuming a pesticide treatment system consisting of more compounds as toxic as Arctit one can easily imagine iongcr-lasting side-erects devcioping which would then necessarily be assessed to be critical.
Anderson J. R. (107X) Pusticidc cti’cctson non-tar@ so11 microorganisms. In f’t~~ic~rd~* ~\/rmdM~~,q~~ (I. R. H111and S. J. L. Wright. Us). pp. 313 -353. Academic Press. London.
Domrch K. Ii. and Paul W. (lY74) Simularion and eapcrimcntat anlrlysls of the influence of herbicides on solI nitrification.. Arc,hf~ /ir ,tfikrohio/f~gw 97, 283 -301. Domsch K. H. and Schriidcr M. (1986) EintluB eininer Herbizide auf den mikrobiellen ‘Biomasse-Kohlenst&l und den MineralstickstotTgehalt dcs bodens. In DFGForschun~vhrrrchr Herhkide II. pp. 225-233. VCH Verlagsgesellschaft mbH, Wcinhcim. Domsch K. l-l.. Jagnow G. and Anderson T. H. (1983) An ecological concept for the avxcsmcn~ of side-ctTec& of agrochcmicals on soil microorganisms. Rcridur Rcritwx 86. 66 .lOS. Gomez M. A. and Sagardoy M. A. (IYNS) lnfluencc of glyphosatc herbicide on the microtlors and mesofaurw of a sandy soil in a semiarid rcgwn. Rw Lu~inocm~. Mwrohd. 27. 351 -357. Grcavcs M. P. (1987) Side-etl’cct testing: an alternalive approach. In Pr.rriciL E//kr.r WI Sod .\ficrulk,ra. (L. Somerville and M. P. Grcavcs. Eds). pp. l83-.lYO. Taylor & Francis. London. Grossbard E. (1979) The continuous monitoring of the rye straw. treated evolution of “CO from “C-labelled with herbicides. ~ncubattng undisturbed on the soil surface. In Slrcrn, Decuj, owf if.7 f$!fi,ct 011 nisposal und C’tilixrrirw (E. Grossbard. Ed.). pp. ?9Y-305. Wiley. Chichester.
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