Glyphosate effect on soil biochemical properties under conservation tillage

In conservation tillage (CT) the use of herbicides is often required. Glyphosate (C3H8NO5P, N-phosphonomethylglycine) is one of the most used herbicides. Since biochemical parameters are often used as soil quality indexes, the stimulation effects on these indexes induced by CT could be misinterpreted due to the required application of herbicide. The objective of this work was to establish the effect of glyphosate on soil biochemical parameters and how long did these effects last on a sandy clay loam Entisol in SW Spain under different tillages.To test the hypothesis that glyphosate could lead to an erroneous evaluation of biochemical parameters when different tillages are compared, two experiments were carried out in 2011: (i) the first one was conducted under incubation conditions; no-tillage (NT) and reduced tillage (RT) pots with and without herbicide addition were compared, after sunflower (Helianthus annus L.) sowing, during 101 days; (ii) in the second one, samples were directly collected from an experimental farm after wheat (Triticum aestivum L.) sowing; NT and RT soil samples where glyphosate was applied were compared for 60 days with plots under traditional tillage in the absence of glyphosate.Biochemical parameters such as water soluble carbon (WSC), microbial biomass carbon (MBC) and nitrogen (MBN), as well as dehydrogenase (DHA) and β-glucosidase (β-Glu) activities were analyzed in both experiments.Results showed that glyphosate acted as a source of organic carbon in both experiments, however, stimulation effects on MBC, MBN, DHA and β-Glu were more evident and lasted longer for the incubated samples. In this experiment, the maximum values for MBC, MBN and DHA were registered between 18 and 37 days after application, while higher β-Glu values under reduced tillage were still registered after 101 days. Those effects were less noticeable in the on-field experiment.Under our experimental conditions (Entisol, Mediterranean condition, rainfed agriculture, wheat–sunflower–fodder pea rotation), an interval of at least 30 days between glyphosate application and soil sampling should be adopted as a standard to avoid data misinterpretation. Differences in glyphosate metabolism are expected for different conditions and crops and should be evaluated case by case.