Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2016
Publication Date: 3/12/2016
Publication URL: http://handle.nal.usda.gov/10113/5192015
Citation: Mckee, G., Soong, J., Calderon, F.J., Borch, T., Cotrufo, F. 2016. An integrated spectroscopic and wet chemical approach to investigate grass litter decomposition chemistry. Biogeochemistry. 128:107-123. doi:10.1007/s10533-016-0197-5.

Interpretive Summary: In this experiment we used three different techniques to measure chemical changes in big bluestem grass residues as they decompose in the field. Infrared spectroscopy, nuclear magnetic resonance, as well as wet chmistry fractionations show that the fate of plant components such as lignins and cellulose do not follow a straightfoward pattern of decomposition. Wet chemistry shows that the “less tough” plant components like cellulose and hemicellulose tend to be lost early during decomposition. The lignin is the harder to decompose plant material. While lignin does not make up a large portion of the bluestem residue, it becomes slightly enriched during decomposition. Our results using spectroscopic methods, however, show that the chemical resistance to decomposition is not the only factor determining litter chemistry as it gets degraded in the field. For example, chemical indicators of cellulose and hemicellulose, which are considered easy to decompose, are present in the decomposing residue beyond 1 yr. Microbial growth as well as associations between different chemical compounds make litter decomposition a very dynamic and complex processes.

Technical Abstract: Litter decomposition is a key process for soil organic matter formation and terrestrial biogeochemistry. Yet we still lack complete understanding of the chemical transformations which occur in the litter residue as it decomposes. A number of methods such as bulk nutrient concentrations, chemical fractionation and spectroscopy have been used to study changes in litter chemistry during decomposition, often leading to conflicting conclusions about the relative lability of cellulose, hemicellulose and lignin. In this study we sought to monitor the chemical composition of Andropogon gerardii (Big Bluestem grass) litter residue over the course of a thirty six month decomposition experiment within the litter layer of a prairie ecosystem using three techniques: traditional wet chemical fractionation based upon digestibility, nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. We observed a 97% mass loss of the original litter. Non-structural components and the cellulose and hemicellulose fractions not encrusted in lignin were preferentially and rapidly lost within the first 12 months of decomposition. This loss was associated with a parallel increase in microbial components in the litter residue. After 12 months, all fraction mass losses were similar, and the decomposition rates of lignin and the lignin-encrusted cellulose were comparable, likely through hydrolysis. Thus, at the latter stages, the relative litter residue chemical composition remained constant. Each method had its strengths and limitations. Wet chemical fractionation is quantitative and portrays bioavailability, but it does not accurately follow the fate of the initial litter structural compounds, particularly lignin, likely because of chemical transformations and the accumulation of microbial compounds. Spectroscopic methods such as FTIR and NMR are able to determine the bulk structural characteristics, and aid in elucidating transformation mechanisms of the litter as it decomposes but they lack quantitative ability. As a result, we strongly encourage coupling spectroscopic and wet chemical methods to clearly assess chemical transformations in litter residues. Our results indicate that chemical structure cannot fully predict the chemical composition of litter as it degrades, and instead suggests a more dynamic interplay of processes that impact the chemical composition of litter at each stage.