Lignin and Other Carbon Sources for Composting

The Details in the Carbon

Wood shavings, dried grasses, leaves, straws, stalks, and paper products are plant dry matter commonly added to compost as “browns” for their carbon richness. Brown materials aid in effectively decomposing nitrogen-rich (a.k.a. “greens”) waste products such as manure, food waste, and animal mortalities. In addition, brown materials are used as bulking agents for aeration and regulating moisture. These brown materials are also characterized as lignocellulosic biomass. Lignocellulosic biomass is comprised of cellulose, hemicellulose, and lignin polymers (repeating chains of molecules) of carbon, hydrogen, and oxygen. This article briefly describes the roles of cellulose, hemicellulose, and lignin. As well, gives some tips on speeding up the decomposition of browns when applying hot composting in batches – a practice commonly used with composters manufactured by Actium Composting.

Simply put, the process of composting reduces the mass and alters the chemical composition of a mixture of organic materials to become a valuable product for soil fertility. Ideally, mixed organic materials become indistinguishable as they are converted to carbon dioxide, water vapour, heat and humus by microorganisms and enzymes. Lignocellulosic biomass provides fuel for the composting process and is the main constituent of the finished compost product.

Bacteria and fungi in an active compost obtain much of their energy (sugars) stored in cellulose and hemicellulose – resulting in these two polymers degrading relatively quickly once accessed by these microorganisms. Lignin is a more complex polymer and is tougher to break down. Fully decomposed lignin forms into humus – the dark earthy matter that gives finished compost properties of moisture and nutrient retention, bulking, and permeability. 

Often layers of lignin cover the easily degradable cellulose and hemicellulose  –  preventing microorganisms from accessing these sugars and slowing down the composting process. In a passive (or cold compost), the slow decay of lignin is not a big deal. Materials that still need time to decompose are left in the bin. Alternatively, in a hot composting batch setup, it is preferred for all materials to be decomposed promptly. In addition to optimizing the C:N ratio of inputs (see article Greens and Browns: Composting Input Management), other practices can be applied when hot composting to improve the degradability of lignocellulosic biomass, such as:

  • Reducing particle size: Shredding or cutting lignocellulosic biomass into smaller pieces increases the surface area. This allows microorganisms more access to cellulose and hemicellulose and improves the breakdown of lignin. Moreover, smaller particles promote a more evenly heated compost and prevent heat losses.
  • Regulate moisture:  Lignin is naturally hydrophobic (water-repelling) and can persist for a very long time in anaerobic environments (high moisture content and low oxygen). Instead, lignin decomposition occurs primarily in an aerobic environment. Therefore, regulating moisture content to be approximately 50% and regularly turning a compost batch will maintain aerobic conditions and hot temperatures for thermophile organisms to thrive and decompose lignin.
  • Encourage a thermophilic environment for natural lignin degraders: Lignin is resistant to degradation by most microbes, as lignin serves to protect plants from microbial attacks. Few bacteria and fungi species have been determined to break down lignin. White rot fungi are the most well-known and effective lignin biodegraders. These fungi have developed specialized enzymes to break lignin apart. For white rot fungi to thrive in a compost system, they require adequate nitrogen (C:N ratio between 25-30:1) and thermophilic conditions – an aerobic environment with compost temperatures ranging between 40-65°C (104-149°F). The rotating and insulated drum is a supportive feature of Actium composters for sustaining thermophilic conditions. In general, the topic of microbial degradation of lignin is understudied. However, there is an emerging interest in identifying more lignin biodegraders – as lignin has received more attention as a potential renewable material to be an alternative for petroleum-based non-biodegradable plastic products. 
  • Select browns with low lignin content: Different types of browns have differing lignin contents. For instance, the lignin content in office paper (0-15%) and cardboard (10-19%) are lower than in newspaper (18-30%).  Choosing brown materials with lower lignin content will result in faster biodegradation and faster batch turnover in a hot composting system. Other brown materials that have relatively low lignin content include wheat and barley straw (6-19%), corn stover (7-19%) and deciduous leaves (1-24%). While hard-wood stems (18-25% ), softwood stems (25-35%), dry grasses (10-30%) and nutshells (30-40%) tend to have a higher lignin content. 

Overall, achieving finished high-quality compost within a short period requires knowledge of inputs added and maintaining an ideal environment for decomposition. It is beneficial to understand the cellulose, hemicellulose, and lignin content of brown materials added to a hot compost to ensure microorganisms have the energy to thrive. As well, foster aerobic and warm conditions for specialized microorganisms to degrade more robust and resistant organic materials efficiently.