Forest litter plays important roles in global carbon cycling and nutrient cycling in forest ecosystem. We are still not clear about the long-term decomposition patterns of litter, in part because it is difficult to acquire precise and accurate litter carbon chemistry. Traditional methods to determine litter chemistry, such as wet chemical techniques, measure samples destructively, lack specificity, and in some cases produce erroneous results. One potential approach is solid-state nuclear magnetic resonance (NMR), which can measure samples nondestructively and elucidate chemical structures precisely and is regarded as the best tool to investigate the complex structures of litter and decomposed litter. Several new advanced solid-state NMR techniques for systematically elucidating the structures of complex organic matter have been developed by our team, however, they have not been used to examine changes in litter chemistry during decomposition. This project will employ these new, systematic, advanced NMR to investigate the possible range in variations of litter chemical compositions of different species and sites from the Long-term Intersite Experiment Team (LIDET).
This project is a joint effort between Old Dominion University and Oregon State University. It will train a female graduate student in applying, improving and developing advanced solid-state NMR techniques for ecosystem study. The project will also provide opportunities for recruiting and retaining underrepresented students including African Americans and women which constitute a large body of students at Old Dominion University.
Advanced 13C solid-state NMR techniques were employed to study long-term changes in organic matter constituents of decomposing foliar litter from the Long-term Intersite Experiment Team (LIDET) project across a broad spatial scale. Undecomposed litter samples and ones decomposed for up to 10 years of four species (Acer saccharum (ACSA), Drypetes glauca (DRGL), Pinus resinosa (PIRE), and Thuja plicata (THPL)) at four sites with different climatic conditions (from arctic to tropical forest) were examined for 11 constituents that were grouped into waxes, carbohydrates, lignin/tannins, and proteins/peptides. Decomposition generally led to an enrichment of waxes and a depletion of carbohydrates, however, the changes of general chemical groups in the four litters examined were inconsistent. For example, waxes decreased at the early stage and then increased in PIRE; DRGL showed a significant depletion of lignin/tannins, while the changes of lignin/tannins were relative small and inconsistent for ACSA and THPL. Except for THPL, proteins/peptides were enriched, which could be associated with fungal and bacterial cell walls in the form of chitin and peptidoglycan, respectively. Some chemical differences found in undecomposed litter chemistry were maintained over the course of decomposition leading to multiple compositional endpoints. In many combinations of litter species and sites, the decomposition rate-constants of major groups of materials (i.e., waxes, carbohydrates, lignin/tannins, and proteins/peptides) were very similar and led to small changes in organic matter composition; moreover the extent of compositional changes was not always positively related to cumulative mass losses as some samples had a more "advanced" compositional changes with intermediate than high mass loss stages. Our finding contradicted with both the classical lignin enrichment theory and the newer idea of waxy material enrichment during decomposition. Our study suggested that the extent and path of changes of chemical composition over long-term decomposition could be quite different for various litter species-site combinations. This project trained two female graduate students in applying, improving and developing advanced solid-state NMR techniques for ecosystem study. It helps the understanding of the global carbon cycle and thus global warming.
