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Record summer causing long-term damage to soil – herbicide degradation poorer after drought

Heat records and continuing drought like in the summers of 2003 and 2006 make not only plants dry up, but they can also cause long-term damage to soils. This was found by scientists from the GSF Research Centre for Environment and Health, who are engaged in a long-term test investigating the ability of soils to degrade the herbicide isoproturon.

“Since 1997 we have been observing four outdoor soils,” explains Dr. Reiner Schroll (Institute of Soil Ecology), “we were essentially concerned with the question how isoproturon behaves in different soils as a model substance for pesticides.” Until summer 2003 one of these soils degraded isoproturon particularly efficiently: the microorganisms living in the soil mineralised up to 60 per cent of the isoproturon applied within approx. two months.

After the drought in summer 2003, however, the degradation capacity of this soil dropped dramatically. In particular, there was only very little degradation in the top few centimetres. “Drought and heat resulted in very profound changes in the composition of the microbial communities (biocoenosis), which could not even be reversed by extended remoistening of the soil before the investigations were carried out,” Schroll explains the drastic reduction. On the one hand the absolute number of microorganisms dropped, on the other the composition of species changed: apparently the bacteria degrading isoproturon in particular were so badly damaged that they had practically become extinct in the top soil.

The soil has still not recovered completely up to this day: new investigations in April 2006 showed a degradation capacity of only 15 per cent of the isoproturon applied. “Our results show how important long-term experiments are,“ Schroll emphasizes, „it was only when we compared the degradation capacity over several years that we noticed the changes.”

To make a comparison of the degradation capacity beyond vegetation periods possible at all, the various soils have to be analysed in identical weather conditions. In particular the soil water content is of central significance, since it influences the air-oxygen supply to the microorganisms on the one hand, and on the other determines the supply with nutrients and chemicals to be degraded, such as pesticides. Therefore, all soil samples were moistened before the tests in laboratory experiments. It is not the absolute water content that is decisive for the microbial activity, but the water tension, i.e. the force which retains the water in the soil. It had, however, not been known up until now at which water tension the optimum degradation of substances can be expected, so that this water content had to be determined empirically for each soil individually.

While they were conducting these tests, the GSF scientists made yet another very interesting discovery: “Irrespective of the type of soil and the substance to be degraded, the maximum microbial degradation activity always developed at a water tension of -0.015 Megapascal – this value seems to be a natural constant,” Schroll emphasizes. At this water tension, e.g., the soil microorganisms only have to exert a suction of approx. 0.15 millibars, in order to take up water from the soil – microorganisms love a moist environment.

Schroll’s results are particularly relevant, e.g., for the environment in the conurbation of Munich, since one of the soils investigated is a typical agricultural soil, as it is representative for large areas of the Munich gravel plain. When herbicides can no longer be degraded properly there, they can be shifted downwardly more easily and may enter the drinking water. Ploughing could be a countermeasure to this: since the lower soil layers are not affected as badly by the drought, mixing the soil may help, in order to settle the respective microorganisms in the top soil areas again. “For very badly damaged soils the deliberate introduction of suitable microorganisms might also be a possibility,“ explains Schroll, „both measures, however, are more labour-intensive and slightly more expensive than the minimum tillage, which is increasingly applied in agriculture. But if the weather fluctuations with their various effects continue to intensify – and estimates made by colleagues very strongly suggest this – science and agriculture will have to respond with appropriate countermeasures. Unfortunately climate change is a fact, and we have to face it.”

Michael van den Heuvel | alfa
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