A new conceptual model for the fate of lignin in decomposing plant litter

Abstract

Lignin is a main component of plant litter. Its degradation is thought to be critical for litter decomposition rates and the build-up of soil organic matter. We studied the relationships between lignin degradation and the production of dissolved organic carbon (DOC) and of CO2 during litter decomposition. Needle or leaf litter of five species (Norway spruce, Scots pine, mountain ash, European beech, sycamore maple) and of different decomposition stage (freshly fallen and up to 27 months of field exposure) was incubated in the laboratory for two years. Lignin degradation was followed with the CuO method. Strong lignin degradation occurred during the first 200 incubation days, as revealed by decreasing yields of lignin-derived phenols. Thereafter lignin degradation leveled off. This pattern was similar for fresh and decomposed litter, and it stands in contrast to the common view of limited lignin degradation in fresh litter. Dissolved organic carbon and CO2 also peaked in the first period of the incubation but were not interrelated. In the later phase of incubation, CO2 production was positively correlated with DOC amounts, suggesting that bioavailable, soluble compounds became a limiting factor for CO2 production. Lignin degradation occurred only when CO2 production was high, and not limited by bioavailable carbon. Thus carbon availability was the most important control on lignin degradation. In turn, lignin degradation could not explain differences in DOC and CO2 production over the study period. Our results challenge the traditional view regarding the fate and role of lignin during litter decomposition. Lignin degradation is controlled by the availability of easily decomposable carbon sources. Consequently, it occurs particularly in the initial phase of litter decomposition and is hampered at later stages if easily decomposable resources decline.

Long-term litter decomposition controlled by manganese redox cycling

“…Litter decomposition regulates the proportion of litter-derived carbon (C) that is either retained in the system as SOM or lost as CO 2 (2), thereby influencing net C storage in soils. Although even small decomposition rate increases may accelerate climate change by virtue of increasing CO 2 emissions from soils (3), uncertainty persists over the ratecontrolling mechanisms (4,5).…”

“…In later stages, decomposition slows when litter has become enriched in lignin and any remaining celluloses and hemicelluloses remain enclosed and protected in ligno-cellulose complexes (8). However, more recent research suggests that lignin readily decomposes in the presence of dissolved organic substrates (5). This effect was attributed to dissolved and readily assimilable substrates providing decomposer organisms with the extra energy required for the cometabolic breakdown of lignin.…”

Foliage and Litter Chemistry, Decomposition, and Nutrient Release in Pinus taeda

“…litter [11]. Decomposition has been suggested in a number of studies [12][13][14][15] to be increased by higher availability of soluble carbon (C) sources rather than by higher nutrient availability. Specifically, the growth of fungal hyphae on litter was suggested to be dependent on the availability of soluble carbohydrates [12].…”

“…Specifically, the growth of fungal hyphae on litter was suggested to be dependent on the availability of soluble carbohydrates [12]. Additions of soluble C have been suggested to produce a positive priming effect on decomposition [13][14][15] where a fraction of this C is used to produce additional enzymes, stimulating decomposition [13]. Soluble C was not higher in fertilized foliage and litter in a number of studies [9,16,17] suggesting fertilization may not increase forest floor decomposition rates by this mechanism.…”

Closing the nutrient loops in (peri-)urban farming systems through composting

“…First, decline of HA (and FA to a smaller extent) is likely indicative for ligninolysis, whereas decline of Hi is indicative for cellulolysis. While it has for a long time been taken for granted that degradation of lignin proceeds at a slower rate than degradation of cellulose, recent data have actually indicated a faster decomposition of lignin than cellulose (Klotzbücher et al, 2011). Second, changes in the various pools are driven both by depletion of the compound during decomposition but also by novel production during degradation and / or changes in solubility of compounds.…”