Degradation and Mobility of Pesticides in Norwegian Soils
Authors: Eklo Ole Martin , Almvik Marit , Hole Halvard , Nyborg Åge Arild , Stenrød Marianne , Borgå Katrine , Dirven Hubert , Grung Merete , Lyche Jan Ludvig , Låg Marit , Nilsen Asbjørn Magne , Sverdrup Line Emilie , Källqvist Torsten 
Published: July 2019

In this report the following topic of pesticides and fate in Norway has been outlined covering: 1. Factors influencing degradation of pesticides. 2. Description and update of datasets on soil and climate in agricultural areas. 3. Normalization of field degradation data as input for modelling fate. 4. Use of degradation data from Norway in model scenarios.

Norwegian laboratory degradation studies indicate that increased soil organic carbon content enhances degradation rates of pesticides that show low sorption (e.g. metalaxyl, bentazone) ,due to increased microbial activity. Whereas pesticides that sorb moderately to strongly to soil (e.g. boscalid, propiconazole), display reduced degradation as organic carbon increases as a consequence of sorption and reduced bioavailability.

Recent DegT50 field studies display a large variation in fungicide degradation rates from Klepp in the south to Tromsø in the north. For the mobile herbicide bentazone, no effect of climate was observed, as degradation rates were coherent at all sites, probably due to rapid leaching. The climate (temperature) seems to be more determinate for fungicide degradation rates than the soil type. Fungicide degradation was slow at two northern sites having low soil temperatures, even though microbial biomass was hugely different at the sites. How soil temperature and moisture affects microbial activity and diversity in various soils, climates and crops is important for the understanding of degradation capacity in Norwegian soils and fields. Microbial activity could be related to both soil, climate and crops/cropping regime – as well as to the nature of the soil organic matter.

The fact that DegT50 values are very much shorter than laboratory values at the same reference conditions, may point to some systematic error in the normalization procedure (e.g. the default simplifications in the Walker and Arrhenius equations), or that the parameters affecting degradation in the laboratory are different from the parameters that affect degradation in the field. Consequently, lab-derived and field-derived DegT50matrix values should be compared and interpreted with care.

The large variations in normalized DegT50 values obtained in field studies in Norway as well as in other regions in Norway cannot be explained by differences in the associated parameters characterizing the soil and microbial community. It is therefore not possible to determine if a certain field study is more or less representative for “Norwegian conditions”. As a conservative approach, the highest, normalized DegT50 from the European field studies should be selected for the Norwegian risk assessment independent on geographic vicinity. As an alternative, when a sufficient number of data are available, a high percentile (e.g 80 or 90-percentile) should be used rather than the geomean.

Each agricultural region in Norway is dominated by one specific soil type for each region. Albeluvisol, Cambisol, Umbrisol, Stagnosol and Histosol in respectively Eastern Norway south, Eastern Norway north, Rogaland, Trøndelag and North of Norway. New updates for Norway include especially Umbrisols and Histosols rich in organic matter. Albeluvisols, Cambisols and Stagnosols are representing the main soil types in the agricultural area in Norway. These are also included in the groundwater (Rustad and Heia) and surface water scenarios (Syverud) developed for Norway. Experience from pesticide fate in the organic rich soils on the south west coast and north of Norway is limited.

Compared to the “normal” temperature and precipitation from 1961 to 1990 with a “new normal” from 1991 to 2014, the climate has changed. For the five described agricultural areas in Norway, annual temperature has increased in average 1 degrees for all five regions and seasons for the new normal. The rainfall has increased for all seasons and regions except for the Northern Norway (Holt in Tromsø) and summer season at Kvithamar (Trøndelag) with lower precipitation in June to September. Annually the precipitation has increased approximately 100 mm in average.

The existing Norwegian scenarios in groundwater and surface water seem to be representative in the meaning of covering the main soil types in the central agricultural areas in South Eastern Norway. However there are no scenarios covering areas of South West and North of Norway containing soil with high organic content, slow degradation and heavy rainfall. Vulnerable areas are not included in these scenarios as the idea of the representativity of soil was to include the main soil types covering the most of the agricultural production areas. The vulnerable areas deals with smaller areas and has to be treated separately. Vulnerable areas are areas with high groundwater levels and sandy soil and mobile pesticides. Hilly areas with clay soil represent high risk of surface runoff with strongly sorbed pesticides. We are lacking experience from areas with high content of organic matter causing slow degradation, combined with heavy rainfall.

 A database with representative soils and climates for various crops should be established in Norway and utilized in a targeted risk assessment approach. Then, the degradation of pesticides to be used in for example fruit/berry cropping, could be evaluated in respect to representative and vulnerable soils and climates in fruit/berry regions in Norway.

A correct risk assessment of pesticide degradation in Norwegian agricultural soils should take the varying climatic zones, the diversity in agricultural soils and crops in Norway into consideration before formulated pesticides are approved. Risk assessment should be based on soils and climates most prevalent for the crop to which the pesticide is to be applied, in addition, vulnerable areas with slow degradation and/or high leaching/runoff risk should be recognized.