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Mineralisation
Mineralisation
One of the main sorts of decomposition is mineralisation. Mineralisation in soil is a process where organic matter is converted into inorganic minerals. It involves the decomposition of organic material by microorganisms, particularly bacteria, which releases inorganic compounds of nitrogen, phosphorus, and sulphur into the soil. The process of mineralisation is important for plant growth as it provides essential nutrients that are necessary for plant growth and development.
Mineralisation typically occurs in aerobic soils with free-living microbes. Some microbes are capable of mineralisation in anaerobic conditions using nitrogen and phosphate as energy sources.
This process has probably been around for over 500mya, long before soils existed. Both fungi, saprophytic and mycorrhizal, and free-living bacteria would have been working away to release nutrients from organic matter to growing plants.
Mechanism
Mechanism
Soil Organic Matter (SOM) is decomposed to form inorganic compounds, like nitrates and phosphates, important for plant growth. The process involves the conversion of organic matter into inorganic forms, such as carbon dioxide (CO2), water (H2O), ammonia (NH3), and other simple inorganic molecules, like phosphates, nitrates and sulphates. Mineralisation is primarily an aerobic process; it does work anaerobically too, but not as well. Aerobic microorganisms are responsible for breaking down organic compounds into simpler inorganic molecules through oxidative processes, releasing energy in the form of ATP (adenosine triphosphate).
Aerobic
Aerobic
Consider aerobic decomposition to be similar to breathing. The soil inhales oxygen, which is used to change the complex organic matter into other simpler compounds and release carbon dioxide - just like respiration. As, soil went deeper, structures would have to develop to withstand the increasing weight above and creatures living in less aerobic conditions.
Aerobic decomposition is considered more efficient/productive than anaerobic. "Shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralization rate (Keiluweilt 2017)
Mineralisation is split into 3 stages in aerobic conditions
Mineralisation is split into 3 stages in aerobic conditions
Initially, surface bacteria and any interior bacteria typically present initiate mineralization; these organisms include, among others, Bacillus, Clostridium, Proteus and lactic acid bacteria.
Second, there is an intermediate stage when fermentation end-products and components were not utilized by the first decomposers. Pseudomonas, Acinetobacter, Arthrobacter, Enterobacter and s some of the more specialized members of the genus Bacillus are among the organisms involved in this process.
Slow aerobic CO2 release from the most refractory of the organic residues characterizes the last stage. In this case, the one-carbon-compound-oxidizing bacteria and other specialists have a role to play." (Islam 2021)
Anaerobic
Oxygen is not available in anaerobic conditions. Organic molecules degrade due to the activity of non-aerobic living organisms, yielding intermediate compounds such as methane, organic acid, hydrogen sulphide and various substrates. Desulfovibriodesulfuricans, Clostridium botulinum and other organisms are involved in this process (Islam 2021)
Importance of microsites
Importance of microsites
Without anaerobic, aerobic could not cope with the all new growing conditions. “Anaerobic microsites are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration, and selectively protecting otherwise bioavailable, reduced organic compounds such as lipids and waxes from decomposition.“ (Keiluweilt 2017)
Fertilisers
Fertilisers
SOM 'mineralisation' is an important consideration in agricultural practices, such as fertiliser application. Much more attention is now being paid to the relationship between fertiliser use and mineralisation (eg Bolo et al 2024), particularly looking out how organic methods can substitute or aid artificial ones (e.g Tang et al 2022).
N fertiliser suppresses mineralisation
N fertiliser suppresses mineralisation
"We conclude that N fertilizer had a direct suppressive effect on SOM mineralization. These results demonstrate that the direct effect of N fertilizer on microbial activity can exceed the indirect effects of N fertilizer via large changes in net primary productivity that alter organic matter inputs, soil temperature and moisture content. (Mahal et al 2019)
Priming Effect (PE)
Priming Effect (PE)
The priming effect (PE) is the short-term increase or decrease in the rate of soil organic matter mineralisation in response to a stimulus
"Across soils, adding more carbon increases priming, and adding nitrogen suppresses it "(Liu et al 2020) But not that straight forward .............!
N addition increases PE (!)
N addition increases PE (!)
"Nitrogen addition increased PE intensity, which was attributed to the better match between soil resources and microbial demands and enhanced enzyme activities. However, the PE intensity in P-addition soils was lower than that in control soils. This discrepancy may be related to the P-induced decrease of N availability and stronger microbial C/N imbalance" Qui et al 2024
Cycles
Cycles
The decomposition processes complete many soil - or nutrient - cycles – notably carbon, nitrogen, phosphates, and less so sulphur. Mineralisation is key part of the recycling. Nitrogen N, Phosphorus P and sulphur S increase during decomposition, whereas calcium and magnesium change only little. Potassium is not a structural element so is lost to the water.
Carbon (C)
Carbon (C)
Releasing carbon from organic carbon - mineralisation - is essential for life cycles.
"Mineralisation of soil organic C is a two-stage process: non-bioavailable, humified forms are converted by abiotic processes to bioavailable forms which, only then, can undergo mineralisation by the soil microbial biomass". (Luo & Xu 2023).
"Long-term manure application significantly increases Soil Organic Content and total N content and enhanced C and N mineralisation in the three main particle-size fractions." (Cai et al 2016)
Nitrogen (N)
Nitrogen (N)
The nitrogen content of decomposing matter increase during the initial stages of decomposition then declines (Coleman 2018 p192).
Microbes decompose organic N from manure, organic matter and crop residues to ammonium. Because it is a biological process, rates of mineralization vary with soil temperature, moisture and the amount of oxygen in the soil (aeration).
Mineralization readily occurs in warm (68-95°F), well-aerated and moist soils. In decent soils, approximately 60—80 lbs of N per acre is mineralised on average from soil organic matter each year.
Mineralised Nitrogen can be immobilised - ie taken up by soil microbes and become unavailable for plant uptake. The ammonium ions are incorporated into organic matter, resulting in more nitrogen in the soil.
Phosphate (P)
Phosphate (P)
"There is a remarkable variation in net P release between moist and saturated soil conditions, and aerobic condition showed higher P release compared to anaerobic condition" (Islam 2021).
This may have profound effects on soils that are regularly treated with slurry compared to manure. The latter is aerobic, the former encouraging anaerobic conditions. Under anaerobic conditions the bacteria releasing phosphates require nitrogen instead of oxygen to work - so may be utilising N fertiliser instead of it going to plants. It may also mean that there is lot less P release in slurried fields, requiring more P to be added as fertiliser, posing its own problems.
Sulphur (S)
Sulphur (S)
Mineralisation has been going on for several hundred milion years...the other main one is Humification
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