Summary

Dept. of Plant and Environmental Sciences

2004:39, Marianne Stenrød, IPM
Effects of pedo-climatic conditions on the degradation of glyphosate in soil; Soil microbial activity and glyphosate mineralization at low temperatures and during frost event

Soil supports terrestrial life in several ways including nutrient-cycling and biomass production, and also through detoxification of pollutants and purification of water. To comply with the constant demands to increase the productivity within agriculture we tend to increase the external input of fertilizers and chemical pesticides to the agro-ecosystem. This puts the soil as a unit of detoxification and purification under increasing strain. An increasing use of pesticides is also observed in cold northern countries like Norway, where the agricultural areas are generally small and tightly interacting with water sources.

This work was done within the scope of the strategic institute program ‘Pesticides in the environment’, dealing with fate of pesticides in soil under different climatic conditions and focusing on conditions in Nordic countries. The main objective of this work was to gain increased knowledge on the effect of low temperatures and freeze-thaw activity on microbial degradation of glyphosate in soil. The sub goals included establishing a relation between the climatic origin of soils with specific physico-chemical and biological properties, and the biodegradation of glyphosate under different climatic conditions. We also attempted to evaluate the obtained results in relation to the natural variability of some soil microbial characteristics.

The experimental work was carried out during the period 2001-2004, and involved laboratory incubation studies with soil sampled from the field plots at Sully s/Loire (West France) at N 47°40’ E 2°25’, Grue (South-East Norway) at N 60°28’ E 12°02’, and Målselv (North Norway) at N 69°15’ E 18°33’. These were plots within agricultural fields that had been chosen for the field studies of the strategic institute program ‘Pesticides in the Environment’.

A positive and close connection between general microbial activity and glyphosate mineralization activity was established. This close relationship was shown to be valid at low temperatures and during periods of alternating freezing and thawing of soil. The activity levels in all investigated soils increased with an increase in temperature, but the magnitude of the increase differed and the southernmost sampled soil showed to be more active at all temperatures. This was despite the fact that all soils had similar physico-chemical properties and origin, and was apparently caused by differences in sorption capacities for glyphosate and microbial community structure. Further, the bioavailability of glyphosate was found to affect its mineralization, and the size of the CaCl2-extractable glyphosate pool was positively correlated to the mineralization rate.

Duration of soil frost was found to control the mineralization activity, rather than the frequency of freeze-thaw fluctuations. The positive response to a temperature increase was evident also when fluctuating between -5°C and +5°C, with a rapid increase in activity level after release of soil frost. This shows the potential importance of periods with above zero soil temperatures during winter, to the glyphosate mineralization in soil.

Generally, our results on glyphosate mineralization are mainly applicable to prediction of glyphosate fate under cold climatic conditions in top soil which is ploughed and harrowed annually, before leaching to lower soil layers. Further, they should give a fair indication of the behaviour of pesticides of similar chemical and physical properties in soil; being pesticides that are rapidly degraded in soil when bioavailable, but strongly bound and rapidly inactivated in both mineral and organic soils.

The variability of the investigated general soil microbial characteristics and the glyphosate mineralization process was large both spatially and temporally. Together with the large inherent variability in these soil characteristics, this makes it difficult to establish baseline data and evaluate potential effects of changing climatic conditions.

Keywords: glyphosate mineralization, soil microbial activity, low temperature, freezing and thawing, variability, climate change




Oppdatert: 15.12.04
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