Effect of temperature, pH and total organic carbon variations on microbial turnover of ¹³C₃ ¹⁵N-glyphosate in agricultural soil

Glyphosate is the best-selling and the most-used broad-spectrum herbicide worldwide. Microbial conversion of glyphosate to CO₂ and biogenic non-extractable residues (bioNER) leads to its complete degradation. The degradation of glyphosate may vary in different soils and it depends on environmental conditions and soil properties. To date, the influence of temperature, soil pH and total organic carbon (TOC) on microbial conversion of glyphosate to bioNER has not been investigated yet. The pH or TOC of an agricultural original soil (pH 6.6, TOC 2.1%) was modified using sulfuric acid or farmyard manure (FYM), respectively. Each treatment: original (I), 3% TOC (II), 4% TOC (III), pH 6.0 (IV) and pH 5.5 (V) was amended with ¹³C₃ ¹⁵N-glyphosate and incubated at 10 °C, 20 °C and 30 °C for 39 days. The temperature was the main factor controlling the mineralization and the extractable ¹³C₃ ¹⁵N-glyphosate, whereas higher TOC content and lower pH resulted in enhanced formation of ¹³C-bioNER. After 39 days the cumulative mineralization of ¹³C-glyphosate was in the range of 12–22% (10 °C), 37–47% (20 °C) and 43–54% (30 °C). Extractable residues of ¹³C-glyphosate were in the range of 10–21% (10 °C) and 4–10% (20 °C and 30 °C); whereas those of ¹⁵N-glyphosate were as follows 20–32% (10 °C) and 12–25% (20 °C and 30 °C). The ¹³C-NER comprised about 53–69% of ¹³C-mass balance in soils incubated at 10 °C, but 40–50% in soils incubated at 20 °C and 30 °C. The ¹⁵N-NER were higher than the ¹³C-NER and varied between 62% and 74% at 10 °C, between 53% and 81% at 20 °C and 30 °C. A major formation of ¹³C-bioNER (72–88% of ¹³C-NER) at 20 °C and 30 °C was noted in soil amended with FYM. An increased formation of ¹⁵N-bioNER (14–17% of ¹⁵N-NER) was also observed in FYM-amended soil. The xenobiotic ¹⁵N-NER had a major share within the ¹⁵N-NER and thus need to be considered when assessing the environmental risk of glyphosate-NER.