FOLLOWING THE DECOMPOSITION OF RYEGRASS LABELED WITH C-13 AND N-15 INSOIL BY SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Investigating the biogeochemistry of plant material decomposition in s oil has been restricted by difficulties extracting and identifying org anic compounds. In this study the decomposition of C-13- and N-15-labe lled Lolium perenne leaves mixed with mineral soil has been investigat ed over 224 days of incubation under laboratory conditions. Decomposit ion was followed using short-term rates of CO2 evolution, the amounts of C-13 and N-15 remaining were determined by mass spectrometry, and C -13 and N-15 solid-state nuclear magnetic resonance (NMR) spectroscopy was used to characterize chemically the plant material as it decompos ed. After 224 days 48% of the added C-13 had been lost with a rapid pe riod of CO2 evolution over the first 56 days. The fraction of cross-po larization magic angle spinning (CP MAS) C-13 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 C-13 exceeded that of the 60-90 p.p.m. signal during the first 56 days and was simil ar thereafter. The fraction of the CP MAS C-13 NMR spectra represented by the alkyl- and methyl-C (chemical shift, 10-45 p.p.m.) signal incr eased from 5 to 14% over the first 14 days and was 19% after 224 days. CP MAS C-13 NMR of C-13- and N-15-L. perenne contained in 100-mu m ap erture mesh bags incubated in the soil for 56 days indicated that the remaining material was mainly carbohydrate but there was an increase i n the alkyl-and methyl-C associated with the bag's contents. After 224 days incubation of the labelled C-13- and N-15-L. perenne mixed with the soil, 40% of the added N-15 had been lost. Throughout the incubati on there was only one signal centred around 100 p.p.m, detectable in t he CP MAS N-15 NMR spectra. This signal corresponded to amide N-15 in peptides and may have been of plant or microbial origin or both. Altho ugh there had been substantial interaction between the added N-15 and the soil microorganisms, the associated redistribution of N-15 from pl ant to microbial tissues occurred within the amide region. The feasibi lity of following some of the component processes of plant material de composition 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 represent ed.