Comparison of Methods to Quantify Lignin in Plant Residues.

The structural components of plant residue are predominantly composed of cellulose, hemicellulose, and lignin. Among these components, lignin is important because it protects the other components from decomposition in soil and so it has been used as an index of soil long term-C storage. There is no ideal procedure for the quantitative measurement of lignin, but two commonly used are acetyl bromide soluble lignin (ABSL) and the alkaline cupric oxide oxidation (CuO oxidation) methods. In this study we compared the two methods to assess some factors that may cause variability in these determinations of lignin concentration. We measured lignin concentrations of aboveground tissues (leaves, stems, and inflorescence) and belowground tissue (roots) of samples collected from both mid-season and end-season sampling of both corn and mixed perennials prairie. The alkaline CuO oxidation method has an advantage because it can be used to determine lignin residues in both plant and soil samples, while the ABSL method is used for plant samples only. The ABSL method is an assessment of the whole lignin concentration, while the CuO oxidation method measures the concentrations of individual lignin monomers. Those concentrations can be summed as an index of “total” lignin. We found that the total lignin concentration of plant residues determined by the ABSL method was considerably higher than the sum of lignin components calculated from the CuO oxidation method. The ABSL method measured higher concentrations of lignin in roots than in aboveground materials. In contrast, the CuO oxidation method produced a higher index of total lignin concentrations in the aboveground samples than in the root samples. For the corn root samples, CuO lignin concentration was significantly higher in the mid-season samples than in the end-season samples. However, for the prairie roots, there were no differences in CuO lignin concentrations between the mid-season and end-season samplings. Across all the data, lignin estimates of the two methods were significantly correlated (r = 0.26, p < 0.05). The correlation was higher (r = 0.51, p<0.05) if only the end-season aboveground samples in 2009 were considered, and highest (r=0.75, p< 0.001) if only the mid-season root samples collected in 2010 were considered. Our results suggest that the effectiveness of the procedures used to quantify lignin was related to the nature of the lignin. The monomeric components of lignin vary with the type of plant, with the morphological parts of a plant, and with the maturity of the plant. The heterogeneity of lignin monomers may contribute to variability in the assessment of “total” lignin by the CuO method.