Soils lose ability to self-clean after drought, study finds
Drought greatly reduces the natural ability of soils to degrade pesticides, a research study has found, suggesting the chemicals are more likely to accumulate as the climate becomes drier.
Scientists took the commonly used herbicide isoproturon as a ‘model substance for pesticides’ and studied its degradation in soils near Munich, Germany, over a period of nine years, looking closely at the effects of weather conditions on the degradation process.
Isoproturon, a herbicide used on cereals, is very widely used in European countries including the UK, France and Germany, and one of the pesticides most widely present in surface freshwaters in the UK. As with other pesticides, its accumulation in soils is of concern because of its high toxic effects on aquatic life and it being classed as a potential carcinogen by the EU.
While the microbial communities living in the soils degraded the herbicide very efficiently in conditions of normal humidity, the droughts of 2003 and 2006 the process had practically stopped in the top soil, the study found.
Under normal conditions, 60% of the pesticide was gone two months after being applied, but during the drought of 2003 the degradation capacity of the soil dropped dramatically.
“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,” said Dr Reiner Schroll who led the study.
The drought made the number of microorganisms in the soil drop and changed their composition – the bacteria that degrade isoproturon had practically disappeared from the top soil.
The effect is likely to spread as droughts become increasingly common in a warming climate, Dr Schroll said.
“For very badly damaged soils the deliberate introduction of suitable microorganisms might also be a possibility. 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,” Dr Schroll said.