What is the Decomposition Pathway?

The decomposition pathway is the breakdown of biologically consumable organic matter called labile carbon including plant material, root exudates, manure, compost, or any other organic residues, and reduces them into an inert form of organic matter called resistant organic matter. This process is driven by soil microbes, including bacteria and fungi, which consume and transform these materials.

During decomposition, labile carbon cycles within the soil system, with some eventually converting into resistant organic matter. However, the process is inefficient, with only about 8.3% of the original carbon becoming stable organic matter. The remaining carbon is released back into the atmosphere as CO₂ or stays in the soil as labile carbon, continuing to cycle.

How Does It Work?

1. Breaking Down Organic Matter

   Bacteria decompose simple organic compounds like sugars with a low carbon-to-nitrogen (C:N) ratio while Fungi handle more complex materials, such as lignin and cellulose, which have a high C:N ratio. These microbes consume these materials and either 1) incorporate the material into their bodies as microbial biomass, 2) release the carbon via respiration, or 3) release stable by produces as a result. Additionally, microbes can be consumed by other microbes such as nematodes, protozoa, and predictor fungi to continue this cycle.  

2. Building Resistant Organic Matter

   • Over time, microbial activity transforms a small portion of the labile carbon into resistant organic matter, which remains stable in the soil and contributes to long-term carbon storage.

Benefits of the Decomposition Pathway

Despite its inefficiencies at building stable soil organic matter, the decomposition pathway provides several critical benefits:

• Nutrient Cycling: Organic matter releases nutrients into the soil as the microbes consume a release nutrients. This can act as a natural fertilizer for crops. This process supports sustainable farming by reducing dependence on synthetic fertilizers.
• Microbial Stimulation: Decomposition enhances microbial diversity and activity, promoting a healthy soil ecosystem. Predatory organisms, such as nematodes and protozoa, can also suppress harmful pathogens.
• Carbon Fertilization: As organic matter decomposes, it releases CO₂ into the soil, which plants can utilize during photosynthesis.
• Building Soil Carbon: While only a small fraction of carbon converts to stable organic matter, this resistant organic matter is important for improving soil structure and long-term fertility.

Maximizing the Decomposition Pathway

To make the most of this pathway, farmers can focus on two key stages: the growth stage and the decay stage.

1. The Growth Stage: Producing Biomass

The first step in maximizing decomposition is to grow as much biomass as possible, which will feed the system.

• Choose High-Biomass Species: C4 plants, such as tropical grasses, produce more biomass than C3 plants.
• Increase Plant Diversity: Mixing species with different root and canopy structures can fill ecological niches, increasing overall biomass production.
• Optimize Plant Density: Maximize vegetative growth by planting at densities that encourage robust development.
• Enhance Photosynthesis: Provide essential nutrients like nitrogen, phosphorus, magnesium, iron and manganese to increase photosynthetic rates. Well-nourished plants can produce more biomass.
• Utilize Fallow Periods: Grow cover crops or green manure during fallow periods to maintain continuous biomass production throughout the year.

2. The Decay Stage: Enhancing Microbial Activity

Once biomass is terminated, the focus shifts to decomposition, this requires increased microbial activity to rapidly cycle the labile carbon into resistant carbon.

• Minimize Chemical Inputs: Avoid fungicides and salt-based fertilizers, which can harm soil microbes and disrupt the decomposition process.
• Adopt No-Till Practices: Tillage disrupts fungal networks and microbial populations. No-till methods preserve the soil structure and microbial activity.
• Maintain Soil Cover: Protect the soil with plant residue or living cover to prevent temperature fluctuations and retain moisture, creating a stable environment for microbes.
• Promote Diversity: Diverse plant residues stimulate a broader range of microbial communities, enhancing decomposition efficiency.
• Use Inoculants and Biostimulants: Apply microbial inoculants or biostimulants to seeds or soil to boost microbial populations and accelerate decomposition.

Balancing Goals: Slow Decay vs. Rapid Cycling

The C:N ratio of plant material determines how quickly it decomposes:

• High C:N Ratio (Older Plants): Slower decomposition, favoring fungal activity. Ideal for maintaining ground cover.
• Low C:N Ratio (Younger Plants): Faster decomposition, favoring bacterial activity. Better for nutrient cycling.

Farmers can adjust the timing of cover crop termination based on their goals, whether it’s to increase ground cover or speed up nutrient release for the next crop.

A Complementary Tool for Building Soil Carbon

While the decomposition pathway may not be as efficient as the liquid carbon pathway, it offers invaluable benefits for soil health and nutrient cycling. By focusing on maximizing biomass production and microbial activity, farmers can harness this process to improve their soils and create more resilient farming systems.