Biol Fertil Soils (2004) 40: 268–276 DOI 10.1007/s00374-004-0776-9 ORIGINAL PAPER Andreas Fließbach . Paul Mäder Short- and long-term effects on soil microorganisms of two potato pesticide spraying sequences with either glufosinate or dinoseb as defoliants Received: 23 December 2003 / Revised: 28 June 2004 / Accepted: 30 June 2004 / Published online: 7 August 2004 # Springer-Verlag 2004 Abstract Pesticides applied to potatoes in Swiss inte- grated farming were evaluated with respect to their cumulative effects on soil microorganisms in a study performed under controlled conditions. Potatoes were treated with a series of herbicides, fungicides and insecticides and were finally defoliated either by hand in the untreated control or with the total herbicides Basta (active ingredient glufosinate) or Super Kabrol (active ingredient dinoseb). Twenty-one and 135 days after the last pesticide application soil samples were collected and analysed for microbial biomass, activity and community level substrate utilisation (CLSU). In the short-term, cumulative pesticide side-effects on microbial biomass and microbial activities averaged 19% for the spraying sequence with glufosinate and 45% for the one with dinoseb. After 135 days these values merely returned to normal. Remarkably, only the CLSU patterns based on Biolog ecoplates showed a lasting effect. We consider this an indication of change in microbial catabolic capabilities that may be due either to induced pesticide degradation capabilities or to a change within the microbial commu- nity. Even though this method has drawbacks in compar- ison to molecular methods of microbial community analysis—in particular because of its culture dependence —it gives a first indication that changes in the microbial community may have occurred. The microbial community structure may then be analysed in more detail with molecular tools. As an additional tool to conventional testing of agrochemicals, microbial community analysis may help in the interpretation of pesticide side-effects and open up new possibilities for their observation. Keywords Potato . Pesticides . Soil microbial biomass . Soil respiration . Community level substrate utilization Introduction Pesticides are developed and applied to destroy or suppress only their target organisms in agricultural crops; however, most often they also have effects on non-target organisms. Among the arable crops, potatoes are frequently treated with pesticide dosages of 5–9.1 kg active ingredient per hectare in Europe (European Com- mission 2002). In many monitoring programs of ground- water quality, pesticides were detectable, the highest concentrations being found for atrazine which often exceeded the EU limit of 0.1 μg/l (BUWAL 2003; Friedrich et al. 2000). Due to the potential risks of pesticides, toxicity and biodegradability have to be evaluated before a new ingredient is allowed to be sold as a plant-protection product (Gerber et al. 1991; OECD 1993). Besides their effects on single species, only pesticide effects on C and N mineralization are proposed as standard tests. Risk assessments are often based on laboratory studies of single substance effects on single species or their performance. In agricultural practice, however, pesticides are often applied as a cocktail or in a spraying sequence in order to reach the aim of disease reduction or prevention and may affect the whole diversity of species in the environment. Effects of pesticides on soil microorganisms are classified as negligible if they are reversible over a certain period or are below a certain threshold, and as persistent or critical, if they remain for >60 days (Domsch et al. 1983). Domsch (1992) found only a minor number of pesticides in this latter critical class and judged that the decision process is efficient enough to eliminate the most hazardous chemicals from agricultural use. Schuster and Schröder (1990) investigated the effects of sequentially and simul- taneously applied pesticides in cereals and concluded their effects to be negligible or transitory. Hart and Brookes (1996) found that 19 years of cumulative pesticide application had no detrimental effect on soil microbial biomass and activity. A. Fließbach (*) . P. Mäder Forschungsinstitut für Biologischen Landbau (FiBL), Ackerstrasse, 5070 Frick, Switzerland e-mail: andreas.fliessbach@fibl.ch Tel.: +41-62-8657225 Fax: +41-62-8657273