Incorporation of 15N-TNT transformation products into humifying plant organic matter as revealed by one- and two-dimensional solid state NMR spectroscopy

Solid-state double cross polarization magic angle spinning (DCPMAS) 15N 13C nuclear magnetic resonance (NMR) spectroscopy was applied to study the incorporation of TNT transformation products into humifying plant organic matter. For this approach, 13C-enriched plant material (Lolium perenne) was mixed with quartz sand and aerobically incubated for 11 months after addition of 15N3-2,4,6-trinitrotoluene (TNT). After successive extraction of the incubate with water, methanol and ethyl acetate, approximately 60% of the 15N added as 15N3-TNT (15Nadd) remained in the solid organic residue (SOR-fraction). The acid insoluble fraction (AI) obtained after NaOH and HCl extractions contained approximately 20% of 15Nadd. For both fractions, 15N NMR spectroscopy revealed an almost complete reduction of the TNT after 11 months of aerobic incubation. Most of the reduced nitrogen groups underwent further condensation. The corresponding DCPMAS NMR spectra allowed the identification of amides that are further substituted by alkyl groups that resist even acid hydrolysis. This assigns them to relatively stable compounds rather than to newly synthesized microbial peptides. The results of this study suggest further that the covalent binding of TNT transformation products to plant derived organic matter is mediated by alkylation and acetylation reactions, rather than by 1,4 addition of TNT-derived nitrogenous groups to quinones of the humic material.