Effect of temperature, pH and total organic carbon variations on microbial turnover of 13CA315N-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 (C3N)-C-13-N-15-glyphosate and incubated at 10 degrees C, 20 degrees C and 30 degrees C for 39 days. The temperature was the main factor controlling the mineralization and the extractable (C3N)-C-13-N-15-glyphosate, whereas higher TOC content and lower pH resulted in enhanced formation of C-13-bioNER. After 39 days the cumulative mineralization of C-13-glyphosate was in the range of 12-22% (10 degrees C), 37-47% (20 degrees C) and 43-54% (30 degrees C). Extractable residues of C-13-glyphosate were in the range of 10-21% (10 degrees C) and 4-10% (20 degrees C and 30 degrees C); whereas those of N-15-glyphosate were as follows 20-32% (10 degrees C) and 12-25% (20 degrees C and 30 degrees C). The C-13-NER comprised about 53-69% of C-13-mass balance in soils incubated at 10 degrees C, but 40-50% in soils incubated at 20 degrees C and 30 degrees C. The N-15-NER were higher than the C-13-NER and varied between 62% and 74% at 10 degrees C, between 53% and 81% at 20 degrees C and 30 degrees C. A major formation of C-13-bioNER (72-88% of C-13-NER) at 20 degrees C and 30 degrees C was noted in soil amended with FYM. An increased formation of N-15-bioNER (14-17% of N-15-NER) was also observed in FYM-amended soil. The xenobiotic N-15-NER had a major share within the N-15-NER and thus need to be considered when assessing the environmental risk of glyphosate-NER. (C) 2018 Elsevier B.V. All rights reserved.