Depth Distribution of Glyphosate and AMPA Under Conventional and No Tillage System in Long-Term Experiments.

Glyphosate (N-(phosphonomethyl glycine) is a post-emergence, non-selective, foliar herbicide. Around 200 million liters of this herbicide are applied every year in Argentina, where the main agricultural practice is no-till (NT), accounting for 78 % of the cultivated land. In this work, we studied the depth distribution of glyphosate in long-term experiments (more than 15 years) at different locations under NT and conventional tillage (CT). Samples were taken from 0-2, 2-5, 5-10, 10-15, and 15-20 cm depth with four replication and two treatments NT and CT at six locations: Balcarce (BA) a loam, Bordenave (BO) sandy loam, Marcos Juarez a silty loam (MJ), Pergamino (PE) silty loam, Parana (PM) silty clay loam and Parana silty clay (PV).

The glyphosate plus AMPA concentration (GA) (expressed as glyphosate concentration) in the first 2 cm of soil was, on the average, 70% greater than in the next 2-5 cm. The mass of glyphosate in CT was higher at 2 to 10 cm depth. The depth concentration of AMPA follows the same trend than glyphosate, although its average concentration at 0-2 cm depth is 28 times higher than the glyphosate concentration at 2-5 cm (glyphosate = 147 mg kg^-1 and AMPA = 4100 mg kg^-1). Beside the AMPA concentration at 0-2 cm depth is greater in NT than in CT, the mass of AMPA is higher in CT only for the Balcarce location.

To our knowledge, this study is the first dealing with the depth distribution of glyphosate concentration in soils under different soil managements. In the present study, it was demonstrated that glyphosate and AMPA are present in soils under agricultural activity with maximum concentration in the first two cm of soil and the AMPA concentration at this depth is greater in NT than in CT.