Nitrogen-fertilizer addition to an agricultural soil enhances biogenic non-extractable residue formation from...

Glyphosate and nitrogen (N) or (P) phosphorus fertilizers are often applied in combination to agricultural fields. The additional P or N supply to microorganisms might drive glyphosate degradation towards sarcosine/glycine or aminomethylphosphonic acid (AMPA), and consequently determine the speciation of non-extractable residues (NERs): harmless biogenic NERs (bioNERs) or potentially hazardous xenobiotic NERs (xenoNERs). We therefore investigated the effect of P or N-fertilizers on microbial degradation of glyphosate and bioNER formation in an agricultural soil. Four different treatments were incubated at 20 °C for 75 days as follows; I: no fertilizer (2-¹³C,¹⁵N-glyphosate only, control), II: P-fertilizer (superphosphate + 2-¹³C,¹⁵N-glyphosate, effect of P-supply), III: N-fertilizer (ammonium nitrate + 2-¹³C,¹⁵N-glyphosate, effect of N-supply) and IV: ¹⁵N-fertilizer (¹⁵N-ammonium nitrate + 2-¹³C-glyphosate, differentiation between microbial assimilations of ¹⁵N: ¹⁵N-fertilizer versus ¹⁵N-glyphosate). We quantified ¹³C or ¹⁵N in mineralization, extractable residues, NERs and in amino acids (AAs). At the end, mineralization (36-41 % of the ¹³C), extractable 2-¹³C,¹⁵N-glyphosate/2-¹³C-glyphosate (0.42-0.49 %) & ¹⁵N-AMPA (1.2 %), and ¹³C/¹⁵N-NERs (40-43 % of the ¹³C, 40-50 % of the ¹⁵N) were comparable among treatments. Contrastingly, the ¹⁵N-NERs from ¹⁵N-fertlizer amounted to only 6.6 % of the ¹⁵N. Notably, N-fertilizer promoted an incorporation of ¹³C/¹⁵N from 2-¹³C,¹⁵N-glyphosate into AAs and thus the formation of ¹³C/¹⁵N-bioNERs. The ¹³C/¹⁵N-AAs were as follows: 16-21 % (N-fertilizer) > 11-13 % (control) > 7.2-7.3 % (P-fertilizer) of the initially added isotope. 2-¹³C,¹⁵N-glyphosate was degraded via the sarcosine/glycine and AMPA simultaneously in all treatments, regardless of the treatment type. The percentage share of bioNERs within the NERs in the N-fertilized soil was highest (¹³C: 80-82 %, ¹⁵N: 100 %) compared to 53 % (¹³C & ¹⁵N, control) and to only 30 % (¹³C & ¹⁵N, P-fertilizer). We thus concluded simultaneous N & glyphosate addition to soils could be beneficial for the environment due to the enhanced bioNER formation, while P & glyphosate application disadvantageous since it promoted xenoNER formation.