Agronomic implications of soil organic matter mineralisation

• humification involves a series of reactions leading to the formation of increasingly complex molecules, progressively diverging from the original structure. The result is the synthesis of humic acids, fulvic acids and humin. The transformation rate is influenced not only by the type of organic matter undergoing humification, but also by its interaction with the mineral fraction;
• mineralisation, which is favoured in well-aerated environments with intense biological activity, leads to the decomposition of organic matter. It occurs in the presence of microorganisms, which require energy and Nitrogen sources to carry out their functions and synthesize their compounds. During mineralisation, organic matter is degraded into carbon dioxide and water, while Nitrogen is released and reused in different processes. Carbon and Nitrogen are partly re-immobilised within the soil microbial biomass. The overall result of these processes is a reduction in the Carbon to Nitrogen ratio;
• interaction between organic matter and the mineral fraction occurs through the formation of ionic bonds or ligand exchange between mineral components and organic compounds. This leads to the formation of highly stable complexes, which generally increase soil aggregation and stability while slowing down microbial degradation processes of organic matter.

• temperature
• moisture
• aeration
• pH
• C:N ratio
• residue quality
• microbial activity
• soil exposure to atmospheric agents

Higher temperatures generally accelerate decomposition, as does a favourable water balance, which enhances biological activity; conversely, excess water and poor aeration can limit the process or shift it towards anaerobic pathways. In practical terms, warm, well-aerated soils with intermediate moisture often show faster mineralisation; cold, saturated soils or those poor in easily degradable substrates tend to slow it down.

The pH determines the efficiency and composition of microorganisms (bacteria and fungi) involved in decomposition within the microbiota:

pH 6.5 – 7.5 – the optimal range, where microbial biomass is most active and diverse. Organic matter mineralisation proceeds efficiently, releasing nutrients into the soil;
pH < 5.5 – bacterial activity is strongly inhibited. Decomposition is almost exclusively driven by fungi, which are slower. Organic matter tends to accumulate (peat formation) and mineralisation is greatly reduced;
pH > 8.0 – microbial life decreases due to high salinity or reduced availability of certain micronutrients, limiting the mineralisation process.

Substrate quality also plays a key role: residues that are easily degradable and with a lower C:N ratio tend to mineralise more rapidly, whereas more lignified materials slow down nutrient release. The quality and composition of residues are strongly influenced by the type of vegetation or crops present in the soil.

At the management scale, frequent and deep tillage, limited residue return, and lack of organic inputs tend to promote organic matter loss and therefore faster net mineralisation of soil organic reserves. Conversely, a soil less disturbed by tillage and preferably covered by vegetation (crops and cover crops) for most of the year—thus less exposed to atmospheric agents and better temperature-regulated, especially in warmer months—will exhibit slower mineralisation.

Clearly, fertilisation can make a significant difference in either slowing down or accelerating mineralisation, as well as in restoring soil organic matter when necessary, particularly when carried out using organic or organo-mineral fertilisers.

Additionally, balanced mineralisation releases compounds that can stimulate microbiota vitality, thereby promoting a more functional soil system.

However, when mineralisation is too intense and/or not balanced by new organic inputs, it leads to loss of organic Carbon and progressive depletion of soil organic matter. This may result in reduced structural stability, lower water and nutrient retention capacity, increased susceptibility to erosion, and reduced biological resilience.

From an agronomic perspective, excessively rapid Nitrogen release can also increase leaching losses—with associated environmental risks—especially when crop demand does not coincide with the mineralisation peak.

In light of the above, mineralisation should be considered a dual process: it is essential for both short- and long-term fertility, but if it exceeds the system’s capacity to accumulate new organic matter, it may reduce soil carbon sequestration function.

In other words, the issue is not mineralisation itself, but its misalignment with crop uptake timing and the soil’s ability to replenish the organic fraction.

Soil Carbon storage therefore depends on the balance between inputs of new organic matter, the intensity of microbiota activity, and outputs due to decomposition and mineralisation. When organic inputs and conservative or regenerative agricultural practices prevail, soil can act as a carbon sink; conversely, when intensive tillage and low residue return dominate, the balance tends to become negative.