Does application of the herbicide glyphosate promote priming effects in soil?

Abstract: Priming effects have gotten revived interest in the last decade due to the concerns of global warming. Priming are defined as an increase in decomposition of soil organic matter due to the addition of organic or mineral substances. The worldwide used herbicide glyphosate is rapidly degraded by microorganisms. Hence, it might give rise to priming effects in soils. This was tested by adding glyphosate in three different concentrations (0.5, 5 and 50 µg/ g soil) as well as one control (no addition) to a grassland soil. The soil was partly amended with cellulose to obtain observations in both C- and N-limited soils. The SOM mineralization rate was found to increase significantly at the concentration of 5 µg glyphosate/ g soil. The relative increase compared to control was 26% in the C-limited soil and 9% and 12% in the two N-limited soils. Priming was only observable during the first 24 hours. Based on these findings, glyphosate seem to be able to cause priming effects in soils. The relative increase in SOM mineralization was not remarkably high and the effects were found to be short term. Nevertheless, the herbicide is believed to have a possible significance on the CO2-flux from soil to the atmosphere. The mineralization rate of N in the soil organic matter was not found to increase by glyphosate addition. In contrast, a trend for decreasing mineralization rate was found in the C-limited soil.