Maataloustieteellinen A ikakauskirja Vol. 61: 45—53, 1989 Effect of annual use of pesticides on soil microorganisms and sugar beet yields HELVI HEINONEN-TANSKIPAAVO SIMOJOKI 2 , KYÖSTI PAININKO 3 , NILS NUORMALA 3 and RIITTA SILVO 4 1 University of Kuopio, Department of Environmental Engineering, POB 6, SF-70211 Kuopio, Finland 2 Agricultural Research Centre, Central Finland Research Station, SF-41340 Laukaa, Finland 3 Sugar Beet Research Centre, SF-25170 Kotalato, Finland 4 Water Protection Association of Saimaa Region, Hietakallionkatu 2, SF-53850 Lappeenranta, Finland Abstract. Sugar beet is often cultivated for several years on the same fields, using many pesticides. We have therefore studied the effects of a pesticide programme on soil microorganisms and sugar beet yields in Perniö and Laukaa. The pesticides in use were thiram, hymexazol, dimethoate, phenmedipham and metamitron and, in Laukaa only, alloxidim-Na. Pesticides were used either in the normal doses or at 150 % of the normal dose. The normal doses of pesticide application had a favourable effect on sugar beet yields in both experiments. The sugar yield was higher in plots with the normal pesticide doses than in the control plots. The overdoses increased neither the sugar beet yields nor the sugar yields as compared to the normal plots. The soil microorganisms were affected by pesticides in some but not in all cases. The most sensitive were the ureolytic microorganisms and the dehydrogenase activities. Index words: Alloxidim-Na, dimethoate,hymexazol, metamitron, phenmedipham, thiram, total number, spore-forming aerobes, ureolytics, dehydrogenase, nitrification, cellulolysis Introduction The cultivation of sugar beet nowadays re- quires the use of pesticides. Sugar beet also requires a very good soil structure so that the developing roots get water and nutrients but do not lack air. For a long time in spring and early summer the soil is covered by crops only partly, and is therefore exposed to erosion. The pressure of the heavy machines in use in- 45 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=m_PLoapG9M2FnZJ8.tb2jwvj9dM4ez9QBGrzilQ.hrotPpLRlZL_Syh0NQyvrre2Ba2cEf-u75QQzzEkCYHLghKafCH65GMklwZGAbK9Yvu5rn--X3yrffJ_0c3IG_Vs94sfiwuVf42hi-MJ9Z0tDC9khnAS1fZh2JpF4dbmyVj8YKJ6BZ6RB-VuCcoa1L90bkndC3XPs79Ez_Sn8CVxZb-pX_Xs4BG8l_fm33f-tzx1mXG8Y5mRxyE6XvNldCDwPN9EDWjVsVpNqDhNJaecNzL5zfOfrwPBJtI7W-1UtpBeiTd7-wSapbc creases the risk of soil tightening which has also been found in Finnish sugar beet fields (Erjala & Raininko, 1985; Raininko, 1988). Soil microorganisms which form soil ag- gregates have favourable effects on the soil structure and microorganisms of sugar beet soil are thereforeparticularly important. The microorganisms are also the most important factors for the decomposition and forming of nutrients available for plants and in the degradation of pesticides which could leave residues. The effects of individual pesticides as well as some pesticide programmes on the soil biology of sugar beet fields have been stud- ied (Pestemer & Malkomes, 1983), but the pesticides used in these experiments are not used in Finland. The aim of our study was to determine the possible microbial response to pesticides in Finland. The pesticide doses for the present study were selected to correspond a) to the normal, recommended dose and b) to 150 % of this dose, because slight overdoses (or underdoses) of pesticides may be typical resulting from uneven distribution or broken spraying machines, which is rather common in Finland (Luoma & Lavonen, 1987). Seed dressing compounds thiram and hymexazol are fungicides. This mixture has been found to control effectively the damp- ing-off of sugar beet (Vestberg et al. 1982). Hymexazol is known to affect, in particu- lar, Fusarium, Aphanamyces, Pythium, Cor- ticium (Pesticide Manual, 1979) Mortierella, some Phytophtora (Tsao & Guy, 1977), Al- ternaria, Botrytis, and Phoma betae (Vur- banov et al. 1984). The effects of hymexazol on non-target soil microbes have been de- scribed very sporadically. Hymexazol only in high concentrations inhibits Saccharomyces, Pseudomonas (Kamimura et al. 1976), red clover rhizobia (Heinonen-Tanski et al. 1982) and various phytopathogenic bacteria (Tomi- ta et al. 1975; Oros et al. 1983). The Bengal gram rhizobia studied by Garg et al. (1981) were more sensitive to hymexazol than the above mentioned microbes. Hymexazol is degraded by soil microorganisms to carbon di- oxide, acetoacetamide and oxazolone (Naka- nishi et al. 1974). As a fungicide, thiram is known to reduce the fungal activity of soil (Anderson et al. 1981; Hickisch et al. 1984) and the num- bers of some bacteria, e.g. nitrogen-fixers (reviewed by Torstensson, 1979). Thiram ap- plication increases the number of phosphate mobilizing microbes in soil (Wainwright & Sowden, 1977). The effect of dimethoate on soil microbes is poorly known, but may be short-lived (Congreoado et al. 1979) owing to the rela- tively rapid degradation of dimethoate (Bro- Basmussen et al. 1969). Phenmedipham in normal doses decreases the biological activity of some sugar beet soils for a short time (Bellinck & Mayau- don, 1978; Verstraete et al. 1979) and in- hibits the number of Azotobacterium (Simon- Sylvestre & Beaumont, 1982). On the other hand, it increases cellulose degradation and ammonification (Simon-Sylvestre, 1979). The effects of phenmedipham on nitrifica- tion may be inhibitory or stimulatory, depend- ing on soil type (Simon-Sylvestre, 1979; Verstraete et al. 1979). The oxidation of ammonia to nitrite is less sensitive than the oxidation of nitrite to nitrate (Ratnayake & Audus, 1978), which can result in the enrich- ment of nitrite in soil. Excessive concentra- tions of phenmedipham strongly inhibit soil nitrification (Tena et al. 1984). As a general conclusion, the effects of phenmedipham are less negative on soil microorganisms than those of metamitron or some other herbicides used with sugar beet (Verstraete et al. 1979). Metamitron can stimulate urease activities in soil, but it has only minor effects on phosphatase, on the numbers of various microbial groups (Voets et al. 1977; Gadkari, 1984), and on nitrogen and carbon transformation (Malkomes, 1987). The nitrogen fixation of some cyano- bacteria is inhibited by metamitron (Gad- kari, 1987). Alloxidim-Na degraded microbiologically rapidly in two sandy soils at 25 °C (Ono et al. 46 1984). The half-lives were approximately 5 6 days. Many unidentified and identified metabolites (including C02) were found. In Swedish experiments, however, alloxidim-Na could still be found up to 2—3 months after the spring spraying (Nilsson, 1984). The per- sistence was higher in northern Sweden than in southern Sweden. Alloxidim-Na has a slight inhibitory effect on pea nodulation (Bebb et al. 1985), but no effect on the degradation of herbicide benazolin (Kostowska et al. 1982). Materials and methods The sugarbeet (Salohill) has been cultivated in loam containing 10.7 % organic matter in Laukaa (62° 28' N, 25° 56' E) and in silty clay containing 8.7 % organic matter in Per- niö (60° 17' N 23° 7' E) in 1982—1985. The seeding (105 seeds/ha) was done in Perniö by sown-to-stand by 15 cm seed spacing with simultaneous fertilizing (Erjala & Raininko, 1985; Raininko, 1988). The soils are described in Table 1. The trials were carried out in four parallel plots (9 m x 12 m in Laukaa and 14.4 m X 36 m in Per- niö). The control plots were not dosed with pesticides, the weeds were hand-weeded in the second weeks of June and July. The pesticides were applied at normal recommended dosages or 150 % of the normal dosages. The agricul- tural operations and sampling schedules were Table 1. Properties of the soils at the beginning of the trials. pH K mg/l P mg/l Mg mg/l Ca mg/l Laukaa 6.2 108 14 142 1 708 Perniö 6.7 241 21 614 4 422 Sand °7o Silt % Clay % Laukaa 30 43 27 Perniö 17 27 55 Table 2. Agricultural schedule and sampling times. Time Control plots Normal pesticide 150 %of the normal dose pesticide dose May sampling sampling sampling Late May seeding seeding + seed dressing with seeding + seed dressing l with 6.8 g/ha thiram + 17.5 g/ha 6.8 g/ha thiram + 17.5 g/ha hymexazol2 hymexazol2 Early June 450 g/ha phenmedipham + 675 g/ha phenmedipham + 240 g/ha dimethoate 360 g/ha dimethoate Middle of June sampling sampling sampling Late June 450 g/ha phenmedipham + 675 g/ha phenmedipham + 2800 g/ha metamitron + 4200 g/ha metamitron + 240 g/ha dimethoate 360 g/ha dimethoate Early July 471 g/ha phenmedipham + 707 g/ha phenmedipham + 1500 g/ha alloxidim-Na3 2250 g/ha alloxidim-Na 3 Early Aug. sampling sampling sampling Early Sep. sampling sampling sampling Oct. harvesting harvesting harvesting 1 150 °7o of the normal dosage in Laukaa 2 not in 1982 in Laukaa 1 alloxicim-Na only in Laukaa 47 done as needed; they are presented in Table 2. In some cases dimethoate and a second her- bicide treatment with phenmedipham and metamitron were applied before the second sampling. Alloxidim-Na was used only inLau- kaa. No hymexazol was used in 1982 in Lau- kaa. The sub-samples, to a depth of 3 cm, were collected from five parts of the plots, bulked and mixed. The samples were never taken from the area within 1 m of the border of each plot. The microbiological inoculations were made on the sampling day. The soils were kept at 5°C for the next day for the dehydrogenase determinations. For the determinationof the nitrification activity, the soil samples were air- dried at room temperature for 3—lo days. The number of “total” microorganisms were determined on Taylor’s (1951) agar. The numbers of spore-forming aerobes were determined on agar of Fenchel and Hem- mingsen (1974) after destroying the non-spor- ing microorganisms by heat-treatment at 80°C for 20 min. The ureolytics were determined by the MPN-technique in the Christensen (1946) medium. The incubations were kept at 15°C for 3—4 weeks. The determination of dehydrogenase has been described by Mettälä et ai. (1982) and the nitrification by Heinonen-Tanski et al. (1985). The cellulose decomposition was de- termined by the polyester-bag method, follow- ing the weight loss of filter paper (5 g, Schleicher & Schiill 604) buried to a depth of 2 cm for three to four months. The papers were washed carefully, dried at room temper- ature and weighed. The /-tests and paired /-tests were per- formed by using natural log transformations of the microbial groups at all the sampling times (14) and both of the places (2) and treat- ments (2). Results The soil microbial results are presented in Table 3, giving the means for the microbial numbers and activities during the entire ex- perimental time. The most significant statisti- cal differences in Perniö were found for de- Table 3. Geometrical mean of microbial numbers and the arithmetic means for dehydrogenase, nitrification and cellulolysis activities in 1982—1985 in control plots, plots with the normal pesticide dosage and plots with 150 % of the normal pesticide dosage. Laukaa experiment Control Normal 150 % of the plots pesticide normal Total number x 107 41 36 43 Spore-forming aerobes x 104 83 85 70 Ureolytics x 104 77 57* 52"* Dehydrogenase TPF ug/g 80.6 73.8* 74.1 nitrification NOrN ug/g 73.877.6 71.5 Cellulolysis % 40.541.9 40.2 Perniö experiment Control Normal 150 % of the plots pesticide normal Total number x 10' 22 20 25 Spore-forming aerobes x 104 80 81 76 Ureolytics x 10* 12 11 11 Dehydrogenase TPF ug/g 51.5 46.8* 46.0** Nitrification NOj-N ug/g 119 126 112 Cellulolysis % 36.234.3 32.0 ** P