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.

Leaf and root litter decomposition is discontinued at high altitude tropical montane rainforests contributing to carbon sequestration

“…Berg (2014) described that in the later phases of decomposition the rate of mass and C loss slows down and is dominated by the degradation of the remaining recalcitrant litter compounds such as lignin. Klotzbücher et al (2011), however, showed that to a large extent lignin is also degraded early during decomposition. The decomposition rate can even cease in later stages, reaching a limit value and leaving behind recalcitrant litter compounds which are little decomposed (Berg, 2014;Berg & McClaugherty, 2008).…”

Who is who in litter decomposition? Metaproteomics reveals major microbial players and their biogeochemical functions

“…Different models for C-acquisition propose a sequential decomposition of polysaccharides, starting with hemicellulose and cellulose degradation followed by the removal of lignin (Berg and Mcclaugherty, 2008; Snajdr et al , 2011). Recently, Klotzbücher et al (2011) suggested that lignin is degraded preferentially in early phases of litter decomposition. In our approach, the dominance of xylanases and in particular cellulases indicates a decomposition phase in which hemicellulose and mainly cellulose can be considered as the major C-source.…”

Are oxygen limitations under recognized regulators of organic carbon turnover in upland soils?

“…Soil organic matter (OM) quantities are regulated by the balance between plant inputs and losses through microbial OM mineralization (i.e., complete oxidation of organic compounds to CO 2 ) or export of dissolved OM in a given soil. Fundamental drivers of OM mineralization are principally climatic factors, such as temperature and precipitation, combined with OM chemistry (Cotrufo et al 2013), the availability of nutrients to the decomposer community (Torn et al 2005;Klotzbücher et al 2011), the formation of protective associations between SOM and soil minerals such as phyllosilicate clays, but in particularly high surface area, hydrated metal oxides (Oades 1988;Torn et al 1997), and physical protection, which constrains the accessibility of substrates to decomposer organisms (Veen and Kuikman 1990;Killham et al 1993). More recent theories highlight the importance of the whole plant-soil-microbe system (Schmidt et al 2011) in regulating OM mineralization rate.…”