Degradation of glyphosate in a Colombian soil is influenced by temperature, total organic carbon content and pH

Abstract

Glyphosate is one of the most used herbicides in the world. The fate of glyphosate in tropical soils may be different from that in soils from temperate regions. In particular, the amounts and types of non-extractable residues (NER) may differ considerably, resulting in different relative contributions of xenoNER (sorbed and sequestered parent compound) and bioNER (biomass residues of degraders). In addition, environmental conditions and agricultural practices leading to total organic carbon (TOC) or pH variation can alter the degradation of glyphosate. The aim of this study is thus to investigate how the glyphosate degradation and turnover are influenced by varying temperature, pH and TOC of sandy loam soil from Colombia. The pH or TOC of a Colombian soil was modified to yield five treatments: control (pH 7.0, TOC 3%), 4% TOC, 5% TOC, pH 6.5, and pH 5.5. Each treatment received 50 mg kg^-1 of ¹³C₃¹⁵N-glyphosate and was incubated at 10 °C, 20 °C and 30 °C for 40 days. Rising temperature increased the mineralization of ¹³C₃¹⁵N-glyphosate from 13 to 20% (10 °C) to 32-39% (20 °C) and 41-51% (30 °C) and decreased the amounts of extractable ¹³C₃¹⁵N-glyphosate after 40 days of incubation from 13 to 26% (10 °C) to 4.6-12% (20 °C) and 1.2-3.2% (30 °C). Extractable ¹³C₃¹⁵N-glyphosate increased with higher TOC and higher pH. Total ¹³C-NER were similar in all treatments and at all temperatures (47%-60%), indicating that none of the factors studied affected the amount of total ¹³C-NER. However, ¹³C-bioNER dominated within the ¹³C-NER pool in the control and the 4% TOC treatment (76-88% of total ¹³C-NER at 20 °C and 30 °C), whereas in soil with 5% TOC and pH 6.5 or 5.5 ¹³C-bioNER were lower (47-61% at 20 °C and 30 °C). In contrast, the ¹⁵N-bioNER pool was small (between 14 and 39% of the ¹⁵N-NER). Thus, more than 60% of ¹⁵N-NER is potentially hazardous xenobiotic NER which need careful attention in the future.

Keywords: (13)C and (15)N mass balance; Biogenic NER; Degradation pathway; Glyphosate; Tropical soil.