Investigating the biogeochemistry of plant material decomposition in soil has been restricted by difficulties extracting and identifying organic compounds. In this study the decomposition of 13C- and 15N-labelled Lolium perenne leaves mixed with mineral soil has been investigated over 224 days of incubation under laboratory conditions. Decomposition was followed using short-term rates of CO2 evolution, the amounts of 13C and 15N remaining were determined by mass spectrometry, and 13C and 15N solid-state nuclear magnetic resonance (NMR) spectroscopy was used to characterize chemically the plant material as it decomposed. After 224 days 48% of the added 13C had been lost with a rapid period of CO2 evolution over the first 56 days. The fraction of cross-polarization magic angle spinning (CP MAS) 13C NMR spectra represented by O-alkyl-C signal probably in carbohydrates (chemical shift, 60-90 p.p.m.) declined from 60 to 20% of the spectrum (chemical shift, 0-200 p.p.m.) over 224 days. The rate of decline of the total 13C exceeded that of the 60-90 p.p.m. signal during the first 56 clays and was similar thereafter The fraction of the CP MAS 13C NMR spectra represented by the alkyl- and methyl-C (chemical shift, 10-45 p.p.m.) signal increased from 5 to 14% over the first 14 days and was 19% after 224 days. CP MAS 13C NMR of 13C- and 15N-L. perenne contained in 100-μm aperture mesh bags incubated in the soil for 56 days indicated that the remaining material was mainly carbohydrate but there was an increase in the alkyl- and methyl-C associated with the bag's contents. After 224 days incubation of the labelled 13C- and 15N-L. perenne mixed with the soil, 40% of the added 15N had been lost. Throughout the incubation there was only one signal centred around 100 p.p.m. detectable in the CP MAS 15N NMR spectra. This signal corresponded to amide 15N in peptides and may have been of plant or microbial origin or both. Although there bad been substantial interaction between the added 15N and the soil microorganisms, the associated redistribution of 15N from plant to microbial tissues occurred within the amide region. The feasibility of following some of the component processes of plant material decomposition in soil using NMR has been demonstrated in this study and evidence that microbial synthesis contributes to the increase in alkyl- and methyl-C content of soil during decomposition has been represented.
|Number of pages||9|
|Journal||European Journal of Soil Science|
|Publication status||Print publication - Dec 1997|