14C-Glyphosate Mineralization in Soils Enriched with Glucose and Phosphate

Abstract

14C-Glyphosate (N-(phosphonomethyl) glycine) mineralization was studied in three upper soils, two Entic Haplustoll and one Typic Ustifluvent, with different physical and chemical characteristics but with similar climatic conditions, soil use and management. The evolved 14C-CO2 and CO2 were tracked over 28 d, sampled to 2 d, 7 d, 15 d, and 28d, while soil was incubated under laboratory conditions in three different treatments: glyphosate (GLY), glyphosate + glucose (GLY+C) and glyphosate + glucose + phosphate (GLY+C+P). The incubations were conducted in closed jars at 28±1ºC. At the end of the incubation period, the percentage of 14C-CO2 released ranged from 46.9% to 61.2% (GLY), 44.6% to 60.3% (GLY+C) and 45.1% to 62% (GLY + C+ P). In the S1 soil, 14C-glyphosate mineralization in the GLY+C+P treatment was significantly decreased (p<0.05) with respect to the GLY and GLY+C treatments. The observed changes in glyphosate mineralization may be associated with the presence of exogenous inorganic P which may have been utilized by microorganisms as a preferred alternative P source. S2 soil had the highest 14C-glyphosate mineralization but did not show any significant differences among the three treatments. S3 soil, with the highest concentration of organic C in the >50 µm, also revealed the lowest 14C-glyphosate mineralization, perhaps because the organic C was acting as an easily available carbon source. Addition of glucose (GLY+C) also depressed glyphosate mineralization in S3 soil. Soil organic matter (SOM) accumulation in the coarser soil size fraction (>50 µm) was easily oxidizable by the soil microorganisms, although it is not clear if this process negatively or positively impacted glyphosate mineralization.