The feasibility of using DCPMAS N-15 C-13 NMR spectroscopy for a better characterization of immobilized N-15 during incubation of C-13- and N-15-enriched plant material

摘要

Solid-state cross polarization magic angle spinning (CPMAS) N-15 and double cross polarization (DCP) MAS N-15 C-13 nuclear magnetic resonance (NMR) spectroscopy was applied to humic fractions and bulk material of C-13-enriched plant residues (13% of total organic carbon) incubated after the addition of (KNO3)-N-15 (enrichment in N-15: 98%) for up to 9.5 months. The achieved N-15-enrichment was determined by a quadrupole mass spectrometer connected to an elemental analyzer. Those studies were performed to (i) determine the feasibility of DCPMAS N-15 C-13 NMR spectroscopy as an analytical tool for characterization of humified organic N and (ii) provide more certainty about the assignment of signals commonly observed in solid-state CPMAS N-15 NMR spectra of humified material. The highest N-15-enrichment (1.4-2.9 mg added N-15 per g sample material) was achieved for the acid-insoluble fractions obtained from the supernatant after alkaline extraction of the plant remains. For those samples, the one-dimensional (1-D) DCPMAS N-15 C-13 NMR spectra were obtained after the accumulation of <50,000 scans, indicating that acquisition of corresponding 2-D spectra with acceptable resolution should be feasible within 2 days. In the DCPMAS N-15 C-13 NMR spectra, signals unveiling coupling between C-13 and N-15 were only detected for amide-C and N-substituted-alkyl-C, supporting the common assignment of the dominant signal in the corresponding N-15 NMR spectra to amide-N. 2-D DCPMAS N-15 C-13 NMR spectroscopy demonstrated further that most of the amide-N is also connected to alkyl-C. This finding supports the former assumption that during microbial degradation of plant remains, most of the N that derives from inorganic sources is immobilized into peptide-like structures. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 26]