Carbon and nitrogen dynamics in acid detergent fibre lignins of beech (Fagus sylvatica L.) during the growth phase

To study the incorporation of carbon and nitrogen in different plant fractions, 3-year-old-beech (Fagus sylvatica L.) seedlings were exposed in microcosms to a dual-labelling experiment employing C-13 and N-15 throughout one season. Leaves, stems, coarse and fine roots were harvested 6,12 and 18 weeks after bud break (June to September) and used to isolate acid-detergent fibre lignins (ADF lignin) for the determination of carbon and nitrogen and their isotope ratios. Lignin concentrations were also determined with the thioglycolic acid method. The highest lignin concentrations were found in fine roots. ADF lignins of all tissues analysed, especially those of leaves, also contained significant concentrations of nitrogen. This suggests that lignin-bound proteins constitute an important cell wall fraction and shows that the ADF method is not suitable to determine genuine lignin. ADF lignin should be re-named as ligno-protein fraction. Whole-leaf biomass was composed of 50 to 70% newly assimilated carbon and about 7% newly assimilated nitrogen; net changes in the isotope ratios were not observed during the experimental period. In the other tissues analysed, the fraction of new carbon and nitrogen was initially low and increased significantly during the time-course of the experiment, whereas the total tissue concentrations of carbon remained almost unaffected and nitrogen declined. At the end of the experiment, the whole-tissue biomass and ADF lignins of fine roots contained about 65 and 50% new carbon and about 50 and 40% new nitrogen, respectively. These results indicate that significant metabolic activity was related to the formation of structural biopolymers after leaf growth, especially below-ground and that this activity also led to a substantial binding of nitrogen to structural compounds.