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

Abstract

Solid-state cross polarization magic angle spinning (CPMAS) 15N and double cross polarization (DCP) MAS 15N 13C nuclear magnetic resonance (NMR) spectroscopy was applied to humic fractions and bulk material of 13C-enriched plant residues (13% of total organic carbon) incubated after the addition of K15NO3 (enrichment in 15N: 98%) for up to 9.5 months. The achieved 15N-enrichment was determined by a quadrupole mass spectrometer connected to an elemental analyzer. Those studies were performed to (i) determine the feasibility of DCPMAS 15N 13C 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 15N NMR spectra of humified material. The highest 15N-enrichment (1.4-2.9 mg added 15N 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 15N 13C 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 15N 13C NMR spectra, signals unveiling coupling between 13C and 15N were only detected for amide-C and N-substituted-alkyl-C, supporting the common assignment of the dominant signal in the corresponding 15N NMR spectra to amide-N. 2-D DCPMAS 15N 13C 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.