Soil Biology and Organic Matter
Soil biology and organic matter examine the living organisms of the soil and the dead organic material they process, which together drive decomposition, nutrient cycling, and the build-up of soil organic matter.
Definition
Soil biology is the study of the organisms living in soil and their activities; soil organic matter is the carbon-containing material derived from plant, animal, and microbial residues at various stages of decomposition, including stabilized humus.
Scope
This area covers the nature and stabilization of soil organic matter and humus, the microbial communities that decompose and transform it, the soil fauna that fragment and mix the soil, and the mineralization and immobilization processes that govern nutrient release. It treats the soil as a living system whose biology underlies fertility and carbon storage.
Sub-topics
Core questions
- What is soil organic matter, and why does some of it persist for centuries?
- Which organisms make up the soil community and what do they do?
- How do soil animals fragment residues and mix the soil?
- How do decomposition and microbial activity release or tie up nutrients?
Key concepts
- Soil organic matter and humus
- Decomposition and the soil food web
- Soil microbial communities
- Soil fauna and bioturbation
- Mineralization and immobilization
- Soil organic carbon stabilization
Key theories
- Organic matter persistence as an ecosystem property
- Rather than persisting because of intrinsic molecular recalcitrance, soil organic matter is increasingly understood to persist through physical protection, mineral association, and ecosystem-level controls on decomposition.
- Decomposition and the soil food web
- A food web of microbes, fauna, and their predators decomposes organic residues, cycling carbon and nutrients; the balance between decomposition and stabilization sets organic matter levels and nutrient supply.
- Mineralization-immobilization turnover
- As microbes decompose organic matter they release nutrients (mineralization) or assimilate them into biomass (immobilization), with the carbon-to-nitrogen ratio of residues determining which dominates.
Clinical relevance
Soil biology and organic matter govern soil fertility, structure, water retention, and the storage of carbon that affects the climate; managing residues, tillage, and biological activity to build organic matter is central to sustainable agriculture, soil health, and climate-change mitigation.
History
Early-20th-century soil microbiology, including Waksman's studies of humus and decomposition, established the biological basis of soil fertility. Later work on the soil food web, the dynamics of organic matter, and, more recently, the reframing of organic matter persistence as an ecosystem rather than molecular property, has made soil biology central to carbon and nutrient science.
Key figures
- Eldor A. Paul
- Selman Waksman
- Nyle C. Brady
- Ray R. Weil
Related topics
Seminal works
- schmidt2011
- paul2015
- brady2016
Frequently asked questions
- Why is soil organic matter so important?
- Organic matter supplies and retains nutrients, binds particles into stable structure, holds water, feeds the soil food web, and stores large amounts of carbon; even small changes in its level can markedly affect fertility, soil health, and greenhouse-gas balances.
- How much life is in soil?
- Soil is one of the most biologically diverse habitats on Earth: a single gram of fertile soil can contain billions of bacteria and metres of fungal hyphae, alongside protozoa, nematodes, mites, earthworms, and many other organisms that drive its functions.