|White, Kathryn - Kate|
Submitted to: Geoderma
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/16/2011
Publication Date: 10/15/2011
Citation: White, K.E., Reeves, J.B., Coale, F.J. 2011. Mid-infrared diffuse reflectance spectroscopy for the rapid analysis of plant root composition. Geoderma. 167:197-203. Interpretive Summary: The need to make swift progress in advancing our understanding of the terrestrial carbon (C) cycle and soil C sequestration requires analytical tools capable of rapidly and accurately assessing the composition of the sources of soil organic matter (SOM). Roots are an important source of recalcitrant macromolecules for SOM formation, yet there is virtually no information available about the composition of many species. Knowledge of the properties of root biomass is crucial to understanding the role of roots in SOM formation and is essential for developing management practices, new crops and crop plants that maximize inputs to stable SOM. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is perfect for biomass and soil samples which may require time consuming, expensive and laborious assays, as the need for further conventional analysis is potentially reduced by 90% or more. However, DRIFTS has not been used to characterize the chemical composition of whole root samples and research is needed to assess its capability to distinguish the root components most likely to contribute to stable SOM and C sequestration. Recalcitrant root-derived macromolecules (i.e., lignin and suberin) are likely to be important contributors to stable SOM. Lignin is a large, poly-phenolic molecule with properties that may make it resistant to microbial degredation and an important contributor to SOM. Lignin provides mechanical strength to plants and prevents water absorption by the cell walls lining vascular tissue allowing efficient water transport within the plant. Suberin is a waxy component of root cell walls and is a large polymeric molecule composed of both aromatic and aliphatic components. Suberin serves as a barrier to water transport in roots aiding in the regulation of water and nutrient uptake while blocking harmful substances. Due to its structure suberin is preserved in soil environments and has been found to be a significant contributor to SOM. In order to accurately assess root contributions to SOM it is necessary to differentiate the lignin and suberin components of root biomass, as well as other important constituents such as cellulose and hemicellulose. The objectives of this research were to compare the DRIFTS spectra of roots of different species using whole root samples and root fiber fractions and to identify spectral features indicative of important root macromolecules in order to evaluate the potential of DRIFTS to determine root composition. Spectroscopy revealed that the roots of the same species are similar despite differences in growing conditions, while roots of different species or those that have been genetically modified differ in composition. Root composition varied by species, particularly in terms of potentially recalcitrant molecules. Spectral analysis was consistent with chemical fiber analysis composition data and revealed features that may be indicative of root suberin content. Further research is necessary to confirm that these features are related to suberin.
Technical Abstract: Roots are an important contributor of recalcitrant organic carbon compounds for soil organic matter (SOM) formation, but little is known about the composition of many species. There is a need for techniques capable of rapidly assessing significant, but often overlooked, carbon sinks such as roots. Diffuse reflectance mid-infrared spectroscopy (DRIFTS) has great potential for the analysis and characterization of of plant root composition. The objectives of this research were to compare the DRIFTS spectra of roots of different species using whole root samples and root fiber fractions and to identify spectral features indicative of important root macromolecules in order to evaluate the potential of DRIFTS to determine root composition. A wide variety of roots from agronomic and horticultural crops, ornamental plants, and native plants were collected and analyzed by DRIFTS. In addition, traditional fiber analysis of a subset of roots and DRIFTS analysis of the resulting fiber fractions were utilized to identify spectral features associated with those fractions. Results indicate that the roots of the same species are similar despite differences in growing conditions, while important differences were noted between roots of different species or those that have been genetically modified. Tree root lignins appeared to be similar to their above ground counterparts. Root lignins for all studied species varied by species. Spectral analysis was consistent with chemical fiber analysis composition data and revealed features that may be indicative of root suberin content. Further research is necessary to confirm that these features are related to suberin.