Lignin lags, leads, or limits the decomposition of litter and soil organic carbon

Abstract

Lignin’s role in litter and soil organic carbon (SOC) decomposition remains contentious. Lignin decomposition was traditionally thought to increase during midstage litter decomposition, when cellulose occlusion by lignin began to limit mass loss. Alternatively, lignin decomposition could be greatest in fresh litter as a consequence of co‐metabolism, and lignin might decompose faster than bulk SOC. To test these competing hypotheses, we incubated 10 forest soils with C4 grass litter (amended with 13C‐labeled or unlabeled lignin) over 2 yr and measured soil respiration and its isotope composition. Early lignin decomposition was greatest in 5 of 10 soils, consistent with the co‐metabolism hypothesis. However, lignin decomposition peaked 6–24 months later in the other five soils, consistent with the substrate‐limitation hypothesis; these soils were highly acidic. Rates of lignin, litter, and SOC decomposition tended to converge over time. Cumulative lignin decomposition was never greater than SOC decomposition; lignin decomposition was significantly lower than SOC decomposition in six soils. Net nitrogen mineralization predicted lignin decomposition ratios relative to litter and SOC. Although the onset of lignin decomposition can indeed be rapid, lignin still presents a likely bottleneck in litter and SOC decomposition, meriting a reconsideration of lignin’s role in modern decomposition paradigms.

Dose–responses for solar radiation exposure reveal high sensitivity of microbial decomposition to changes in plant litter quality that occur during photodegradation

“…This apparent discrepancy is most likely the result of the fact that in many of these field studies abiotic mass loss from photochemical mineralization and biotic decomposition occurred simultaneously. During biotic decomposition, lignin content as a proportion of the remaining material increases relative to other more biotically labile organic compounds, which are rapidly degraded by microbial enzymes (Austin & Ballar e, 2010;Hall et al, 2020). Thus, the relative increase in percentage lignin of decomposing litter over time may outpace the photochemical degradation of lignin, such that the percentage change in decomposed litter does not (a) Linear regression for saccharification as a function of lignin content for litter exposed to all doses of solar radiation during the first phase of the experiment.…”

“…A subsample of the litter was taken for analysis of potential enzyme activities (described in the section of laboratory analyses), and the remaining material was placed in a 60°C drying oven for 48 h and weighed for determination of mass loss. Ash-free dry mass then was determined for all samples to correct for soil contamination from the field (Harmon et al, 1999) for calculations of organic matter (OM) loss associated with biotic decay.…”

Decline in Carbon Decomposition from Litter After Snow Removal is Driven by a Decreased Release of Carbohydrates

“…While much is known about C release from plant litter, how these materials are incorporated into soil organic matter (SOM) is not fully understood (Pries et al 2017;Lavallee et al 2018). In recent years, a growing number of studies have shown that recalcitrant compounds (i.e., lignin) are not selectively preserved in decomposing litter (Hall et al 2020) and that labile fractions are the primary source of stable SOM because that strongly control microbial substrate use e ciency (Cotrufo et al 2015). One piece of strong evidence was that nonstructural compounds (i.e., carbohydrates and polysaccharides) are transferred from plant litter into mineral soils during early decomposition (Villarino et al 2021).…”

Assessing the role of fungal diversity in decomposition: A meta-analysis

“…For example, Baldrian et al (2016) showed that individual wood logs were dominated by one or a few fungal species. Other information about the quality of the plant material, like lignin concentration, can give more insight in the relationship between the plant quality and decomposition (Hall et al, 2020). In the selected studies, only a limited amount of data was available for such plant traits.…”