Cell wall compositional changes during incubation of plant roots measured by mid-infrared diffuse reflectance spectroscopy and fiber analysis

Authors: White, Kathryn; REEVES, III, JAMES - Former ARS Employee; COALE, FRANK - University Of Maryland

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2015
Publication Date: 2/15/2016
Citation: White, K.E., Reeves, III, J., Coale, F. 2016. Cell wall compositional changes during incubation of plant roots measured by mid-infrared diffuse reflectance spectroscopy and fiber analysis. Geoderma. 264:205-213.

Interpretive Summary: Decomposing plant roots are an important source of material for the formation of soil organic matter, but information on root cells’ chemical composition or how that composition changes as they decompose is limited. This study utilized chemical and spectroscopic analytical techniques to characterize changes in roots of important agricultural crops and forages - alfalfa, corn, soybeans, fescue, orchardgrass and switchgrass, during the initial month of decomposition. Roots were incubated in inoculated sand microcosms in the laboratory and sampled weekly. The amount of root tissue from each species declined rapidly after one week, as sugars, starches and easily decomposed proteins were degraded by microorganisms. After the first week, only alfalfa continued to lose significant amounts of root material to microbial degradation, with ultimately only 22% of the initial amount of root remaining after four weeks. Between 61% and 81% of the roots of the remaining five species still remained after four weeks. Following the first week, the remaining root tissue was primarily composed of cell wall structural compounds such as cellulose, hemicellulose, and lignin. Chemical and spectroscopic analysis revealed differences in the relative decomposability of these compounds in alfalfa roots, but not for the other species. Resistance of alfalfa cell wall compounds to degradation followed the trend, lignin>hemicellulose>cellulose. Knowledge of how roots decompose aids in understanding soil organic matter formation and how it responds to differing land management decisions, such as switching between annual row crops and perennial forages or following soil disturbance. This information is useful to scientists as well as farmers and other land managers. Scientists will use this information to better understand the processes governing soil organic matter cycling and its potential effect on soil health, productivity and the potential for atmospheric carbon sequestration. Land managers can use this information to make informed decisions regarding management, and tailor crop choices to improve soil health by providing readily degradable soil organic matter sources or to increase soil carbon storage.

Technical Abstract: Plant roots, particularly the constituents of root cell walls (hemicellulose, cellulose and lignin), are important contributors to soil organic matter. Little is known about the cell wall composition of many important crop species or compositional changes as roots decay. The objectives of this study were to quantify changes in root cell wall composition during a four week laboratory incubation by forage fiber analysis, and characterize those changes using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The roots of six important crops, forage and native grass species, were incubated at 25° C and sampled weekly. Alfalfa lost 78% of initial mass over four weeks, while the remaining species lost between 19% and 38%. For all species, the majority of this loss occurred during Week 1, and only alfalfa mass loss was significant (P<0.05) each week. The trends observed for whole root decomposition were parallelled by the decomposability of root cell walls. Significant changes in hemicellulose, cellulose and lignin concentrations over time were only observed in alfalfa roots. Significant changes in decomposability of these constituents was likewise only observed in alfalfa, with cellulose the most decomposable fraction, followed by hemicellulose and lignin. Analysis by DRIFTS supported the fiber analysis results and revealed important changes in root cell wall composition. The disappearance of peaks due to starch in the perennial alfalfa and switchgrass roots following Week 1 helped to explain the greater initial mass loss in both of these species relative to the roots of the annuals. The spectral data also illustrated the resistance of alfalfa lignin to decomposition, the preservation of carbonyl compounds and the degradation of readily decomposed proteins. Finally, changes potentially indicative of wax compound preservation were found in the DRIFTS spectra of alfalfa even though the amount of wax was too small to quantify by fiber analysis. This research study reveals differences in the rate at which crop roots decompose, and important changes that can occur in readily decomposable roots over relatively short time scales. These results provide valuable information contributing to the understanding and prediction of short term soil organic matter dynamics, which will help to predict possible impact of management changes or soil disturbance on soil health and productivity, as well as long term organic C stabilization and the potential for C sequestration.