INTERACTIONS BETWEEN PEAT AND SODIUM-ACETATE, AMMONIUM-SULFATE, UREA OR WHEAT STRAW DURING INCUBATION STUDIED BY C-13 AND N-15 NMR-SPECTROSCOPY

Transformations of sodium acetate, ammonium sulphate, urea and wheat straw in peat have been studied by determining the distribution of N-15-labelled material, and by C-13 and N-15 nuclear magnetic resonance spectroscopy (NMR) using cross polarization (CP) and magic-angle spinning (MAS). Samples of an oligotrophic blanket peat were incubated for 6 months at 15-degrees-C with N-15 ammonium sulphate, N-15 urea, 13C15N urea, N-15-labelled wheat straw or C-13 sodium acetate. The incubated samples were separated into fractions of > 1 mm, 1-0.5 mm, 0.5-0.25 mm, 0.25-0.15 mm, 0.15-0.05 mm, 0.05-0.005 mm and a water-soluble fraction by wet sieving, and were then freeze-dried. The distribution of N-15 between the fractions was obtained after isotope-ratio analysis by mass spectrometry, and the 0.5-0.25 mm, 0.05-0.005 mm and water-soluble fractions from the incubations were examined by C-13 and N-15 NMR. C-13-labelled acetate increased carbohydrate resonances in the 0.05-0.005 mm and soluble material, but an organic acid derived from the substrate was still present 6 months later. Incorporation of N-15 from ammonium sulphate into the peat was low, and more than 50% of the added N was detected in the soluble fraction still present as (NH4+)-N-15. As carbohydrate and soluble organic matter were detected in the peat, it was concluded that microbial activity and N immobilization were restricted by poor aeration and low pH. Urea, in contrast, interacted with all the fractions examined, with some N-15 being incorporated into a range of compounds that included protein, peptides, amides, amino acids and carbamates or lactam derivatives. A small proportion of labelled N-15 from wheat straw, originally present in the > 1 mm and 1-0.5 mm fractions, had moved into the 0.05-0.005 fraction during incubation and sieving. The C-13 spectra suggested that the presence of the straw may have stimulated decomposition of the peat components.