|COALE, FRANK - University Of Maryland|
|REEVES III, JAMES - Retired ARS Employee|
Submitted to: Organic Geochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2017
Publication Date: 1/1/2018
Citation: White, K.E., Coale, F.J., Reeves III, J.B. 2018. Degradation changes in plant root cell wall structural molecules during extended decomposition of important agricultural crop and forage species. Organic Geochemistry. 115:233-245.
Interpretive Summary: Lignin, cellulose and hemicellulose are the primary molecules composing plant root cell walls. These molecules play an important role in the formation of soil organic matter, but little is known about how they change as roots decompose. This study measured changes in root composition and molecular structure for ten important agricultural crops and forages – alfalfa, sorghum x sudangrass, corn, soybean, fescue, orchardgrass, winter rye, wheat, gammagrass and switchgrass – over a nine month period. Roots were incubated in the laboratory in sand microcosms inoculated with a water extract containing indigenous microbial communities from an agricultural soil and sampled at 1, 2, 3, 6 and 9 months. Taken together our results demonstrated that differences in plant root cell wall composition and structure played an important role in the extent of root tissue decomposition over time. This expands knowledge of how roots decompose and ultimately contribute to the formation of soil organic matter which will aid scientists to further understand soil organic matter formation and cycling as well as development of methods to improve soil health, productivity, and the potential for carbon sequestration. This will allow farmers and other land managers with the goal of increasing soil organic matter to make informed crop management and other land use decisions.
Technical Abstract: Little is known about the changes in the cell wall structural molecules lignin, cellulose and hemicellulose as plant roots decompose, despite their importance in the formation of soil organic matter. The objectives of this study were to quantify changes in root composition during 270 d incubations of the roots of ten important grain and forage crop species utilizing forage fiber analysis techniques and to characterize the changes in root cell wall composition and molecular structure using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The roots samples were incubated at 25° C and sampled at 30, 60, 90, 180 and 270 days. Large species dependent variations were observed in the extent of root tissue decomposed over time, ranging from 82.5% of initial mass for alfalfa to 21.5% for switchgrass. Following an initial period of rapid decomposition, fiber analysis revealed that lignin, cellulose and hemicellulose degraded proportionally over time. These results were supported by DRIFTS as similar trends were found in the ratios between the diagnostic peaks for cellulose, the ester and acid carbonyls of hemicellulose and waxes, and for lignin. The DRIFTS spectra revealed spectral features illustrating molecular changes in root composition as roots decomposed. These features were most pronounced in the more extensively decomposed roots. Features potentially indicative of suberin preservation were found in the spectral region between 2800 cm-1 to 3000 cm-1. Examination of the polysaccharide fingerprint region between 1000 cm-1 to 1300 cm-1 revealed changes in decomposing roots possibly indicative of hemicellulose structural changes as more degradable polysaccharides are preferentially degraded. The results demonstrate the effect of differences in cell wall composition and structure on the extent of root tissue decomposition and expand understanding of the role of roots in soil organic matter dynamics. Given the variability in root tissue degradation and changes in cell wall composition observed among species, these results illustrate the necessity for characterization of a broad range of individual species and their decomposition characteristics in order to predict root contributions to soil C cycling and potential C sequestration.