Monitoring Biomacromolecular degradation of Calluna Vulgaris in a 23 year field experiment using solid state 13C-NMR and pyrolysis-GC/MS

Abstract

The qualitative and quantitative changes in lignin, polysaccharide, aliphatic, cutin and protein biomacromolecules in Calluna vulgaris shoots collected at three intervals (initial 0.5 y, 7 y and 23 y) in a field decomposition experiment have been investigated using quantitative solid state 13C nuclear magnetic resonance (NMR) spectroscopy and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Protein showed rapid decomposition with virtually none remaining after 7 y. The absolute weight losses of the other macromolecules after 23 y in the field show the sequence: polysaccharides>aliphatics/cutin>lignin. However, relative decomposition rates varied significantly during each interval, with polysaccharides and aliphatic/cutin being the highest in the first and second intervals, respectively, and polysaccharides being the lowest in the final interval. Py-GC/MS provided evidence that part of the remaining polysaccharides in the 23 y decomposed litter can be attributed to microbial production. Degradation of lignin is shown as progressive oxidation of alcoholic functions into the corresponding ketones or aldehydes and preferential degradation of syringyl over guaiacyl moieties. An unusual preferential removal of p-coumaric acid units over syringyl and guaiacyl units was observed, probably due to selective hydrolysis of its ester bond with lignin moieties either enzymatically by specific microorganisms or chemically. The specific soil environment with its low temperature, high rainfall and high acidity appears to limit microbial activities and leads to accumulation of carbon below ground.