The Effect of Lignin on Biodegradability

Tom Richard

Plant cell wall material is composed of three important constituents: cellulose, lignin, and hemicellulose. Lignin is particularly difficult to biodegrade, and reduces the bioavailability of the other cell wall constituents. A bit of knowledge about each of these constituents is helpful in understanding the vastly different rates that different plant materials decompose. This discussion also presents the mathematical models developed to compensate for the effect of lignin on biodegradability in anaerobic systems, and suggests some constraints on applying these models to aerobic composting systems.

Cell Wall Constituents

Cellulose is a long chain of glucose molecules, linked to one another primarily with glycosidic bonds. The simplicity of the cellulosic structure, using repeated identical bonds, means that only a small number of enzymes are required to degrade this material. Although people do not produce the enzymes required for cellulose degradation (and thus do not get much energy from eating paper, straw or other cellulosic material), some microorganisms do. Cows and other ruminants create an environment in their rumen which encourages this microbial degradation, converting cellulose to volatile fatty acids and microbial biomass which the ruminant can then digest and use.

Hemicelluloses are branched polymers of xylose, arabinose, galactose, mannose, and glucose. Hemicelluloses bind bundles of cellulose fibrils to form microfibrils, which enhance the stability of the cell wall. They also cross-link with lignin, creating a complex web of bonds which provide structural strength, but also challenge microbial degradation (Ladisch et al., 1983; Lynch, 1992).

Lignin is a complex polymer of phenylpropane units, which are cross-linked to each other with a variety of different chemical bonds. This complexity has thus far proven as resistant to detailed biochemical characterization as it is to microbial degradation, which greatly impedes our understanding of its effects. Nonetheless, some organisms, particularly fungi, have developed the necessary enzymes to break lignin apart. The initial reactions are mediated by extracellular lignin and manganese peroxidases, primarily produced by white-rot fungi (Kirk and Farrell, 1987. Actinomycetes can also decompose lignin, but typically degrade less than 20 percent of the total lignin present (Crawford, 1986; Basaglia et al., 1992). Lignin degradation is primarily an aerobic process, and in an anaerobic environment lignin can persist for very long periods (Van Soest, 1994).

Because lignin is the most recalcitrant component of the plant cell wall, the higher the proportion of lignin the lower the bioavailability of the substrate. The effect of lignin on the bioavailability of other cell wall components is thought to be largely a physical restriction, with lignin molecules reducing the surface area available to enzymatic penetration and activity (Haug, 1993).

Modeling Lignin's Impacts on Biodegradability in Anaerobic Systems

Chandler et al. (1980) formulated a mathematical correction for bioavailability of an organic substrate based on its lignin content. Using data collected from the anaerobic degradation of a range of lignocellulosic materials (40 day retention time), they developed a linear relationship to describe this effect:

Kayhanian and Tchobanoglous (1992) proposed using this equation to adjust C/N ratios for mixture calculations in a sequenced anaerobic / aerobic process. The effect, for highly lignified materials, can be significant. For example, using their lignin data for newspaper versus office paper:

Thus, while about 82% of the carbon in office paper is biodegradable, only 22% of the carbon in newsprint would be available through anaerobic digestion. Put another way, it would take almost 4 tons of newsprint amendment to provide the same bioavailable carbon as 1 ton of office paper. This clearly has significant implications for mixture ratio calculations.

Further evaluation of Chandler et al.'s (1980) relationship compared the predicted biodegradability with long term (75 day) batch studies in a high-solids anaerobic digestor (Kayhanian, 1995). The predicted biodegradability of this solid waste mixture based on its lignin content (typically 4%) was 68%, which was comparable to the 70% biodegradability measured in the long term batch study.

The linear relationship given by Chandler et al. (1980) is simple, and appears to provide reasonable accuracy for materials of relatively low lignin content. While Chandler et al.'s relationship makes mechanistic sense for relatively small lignin fractions, materials with a high lignin content may be affected differently. With a large amount of lignin present, some of the lignin would be overlapping other lignin molecules rather than cellulose, so the incremental effect will be smaller(Conrad et al., 1984). Recent analysis of extensive databases on the maximum digestibility of lignocellulosic materials in the rumen suggests a log-linear relationship provides a better fit (Van Soest, 1996):

Applying the formula of Van Soest (1996) to the cell wall fraction, we can calculate an overall biodegradable carbon content:

This biodegradable carbon content can then be used to calculate biodegradable C/N ratios using the usual formulas. If we apply this equation to newsprint, wheat straw, maple wood chips and poultry manure, using data from the Table of Lignin and Other Constituents of Selected Organic Materials and other sources, we get the following biodegradable C/N ratios (access a No-Frames version of the Table of Lignin here):

Material	Carbon (%) (Total)	C/N (Total)	Carbon (%) (biodegradable)	C/N (biodegradable)	Lignin (%) (dry basis)	Cell wall (%) (dry basis)	Nitrogen (%) (dry basis)
newsprint	39.3	115.5	18.4	54.2	20.9	97.0	0.34
wheat straw	51.1	88.7	33.6	58.4	23.0	95.0	0.58
manure, poultry	43.3	9.6	41.8	9.3	2.0	38.0	4.51
wood chips, maple	49.7	51.2	43.8	45.1	12.7	32.0	0.97

Note, however, that when correcting carbon/nitrogen ratio calculations for lignin content, it may also be necessary to reduce the carbon/nitrogen goal. The typical recommended C/N ratio of 30:1 must presumably already include some discount for lignin, which is a component of most common carbonaceous materials.

It is also important to remember that these formulas are all based on data from anaerobic systems. Since lignin is degradable (albeit slowly) in aerobic systems, the restriction on biodegradability will be less in aerobic composting.

Lignin degradation under aerobic conditions

There is some debate and perhaps significant variability in the rate of lignin decomposition in aerobic systems. Lynch and Wood (1985) state that "little, if any, lignin degradation occurs during composting", and Iiyama et al. (1995) assume constant lignin as the basis of their calculations of polysaccharide degradation. However, Hammouda and Adams (1989) measured lignin degradation ranging from 17% to 53% in grass, hay and straw during 100 days of composting, and Tomati et al. (1995) measured a 70% reduction in the lignin content of olive waste compost after 23 days under high moisture (65-83%) thermophillic conditions. Interestingly, after this initially high decomposition rate under thermophillic conditions, Tomati et al. found no further reductions in lignin content during the subsequent 67 days under mesophillic conditions. In contrast, in a laboratory incubation study, Horwath et al. (1995) measured 25% lignin degradation during mesophillic composting and 39% during thermophillic composting of grass straw during 45 day experiments.

Adding small quantities of nitrogen to woody materials can increase lignin degradation rates. Over a two week incubation with a white-rot fungi at 39-40°C (the optimum temperature for growth of Phanerochaete chrysosporium, the fungi used in this experiment), adding only 0.12% nitrogen (dry weight basis), lignin degradation in alder pulp increased from 5.2% to 29.8% (Yang et al., 1980). In this same study, the increase in lignin degradation of hemlock pulp with 0.12% supplemental nitrogen was only 2.2% to 3.9%, and additional nitrogen did not provide further benefit. The differences between plant species is likely related to differences in lignin structure, with gymnosperm lignin composed of coniferyl alcohols, angiosperm lignin composed of both coniferyl and sinapyl alcohols, and grass lignin of coniferyl, sinapyl, and p -coumaryl alcohols (Ladisch et al., 1983).

While significant lignin degradation appears possible during aerobic composting, a number of factors are likely to affect the decomposition rate. Conditions which favor the growth of white-rot fungi, including adequate nitrogen, moisture, and temperature, all appear to be important in encouraging lignin decomposition, as does the composition of the lignocellulosic substrate itself.

The impact of lignin degradation on the biodegradability of the remaining carbon has not been extensively researched. In one of the few studies which might provide such insight, Latham (1979) measured a 5 to 11% increase in anaerobic digestability of barley straw after 3 to 4 week aerobic incubations at 30°C with various pure cultures of white-rot fungal species. Increases in biodegradability would likely be even greater with a mixed culture under themophillic conditions, as evidenced by the lignin degradation rates cited above.

Pretreatment to enhance biodegradability

Biodegradability can be enhanced by pretreatment of lignocellulosic materials, including acid (Grethelin, 1985) or alkali treatment (Jackson, 1977; Van Soest, 1994), ammonia and urea (Basaglia et al., 1992; Van Soest, 1994), physical grinding and milling (Ladisch et al., 1983; Fahey et al, 1992), fungal degradation and steam explosion (Sawada et al, 1995), and combined alkali and heat treatment (Gossett et al., 1976). Gharpuray et al. (1983) examined several of these pretreatment options individually and in combination, and found that those treatments which enhanced specific surface area were most effective at increasing enzymatic hydrolysis.

While pretreatment may be uneconomical when considered as a separate process in compost feedstock preparation, in some cases it may be incorporated in other preprocessing operations at little additional cost. However, because many lignocellulosic ingredients in composting serve dual roles as energy sources and porosity enhancers, treatments which reduce porosity and pore size distributions may prove counterproductive to maintaining an aerobic process.

Summary and Conclusions

Researchers have developed quantitative relationships between lignin content and the biodegradability of lignocellulosic materials during anaerobic digestion. However, before applying these formulas to aerobic composting other factors should be considered. Several studies indicate significant biodegradation of lignin can occur during composting, which would increase the availability of other plant cell wall materials. Bioavailability will also be affected by particle size and other factors for which no quantitative correction presently exists. When analyzing aerobic composting systems, the mathematical relationships developed by Chandler et al. (1980) and Van Soest (1996) are best used in a comparative sense, to help understand the differences in bioavailability of different composting substrates.

Acknowledgment

Martin Traxler provided very helpful discussions and comments during the creation of this document.

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