Following the decomposition of ryegrass labelled with 13C and 15N in soil by solid-state nuclear magnetic resonance spectroscopy

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 C0₂ 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 days 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 N 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 had 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.