Particulate organic matter
Particulate organic matter is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 millimeters to 2 millimeters.
Particulate organic carbon is a closely related term often used interchangeably with POM. POC refers specifically to the mass of carbon in the particulate organic material, while POM refers to the total mass of the particulate organic matter. In addition to carbon, POM includes the mass of the other elements in the organic matter, such as nitrogen, oxygen and hydrogen. In this sense POC is a component of POM and there is typically about twice as much POM as POC. Many statements that can be made about POM apply equally to POC, and much of what is said in this article about POM could equally have been said of POC.
Particulate organic matter is sometimes called suspended organic matter, macroorganic matter, or coarse fraction organic matter. When land samples are isolated by sieving or filtration, this fraction includes partially decomposed detritus and plant material, pollen, and other materials. When sieving to determine POM content, consistency is crucial because isolated size fractions will depend on the force of agitation.
POM is readily decomposable, serving many soil functions and providing terrestrial material to water bodies. It is a source of food for both soil organisms and aquatic organisms and provides nutrients for plants. In water bodies, POM can contribute substantially to turbidity, limiting photic depth which can suppress primary productivity. POM also enhances soil structure leading to increased water infiltration, aeration and resistance to erosion. Soil management practices, such as tillage and compost/manure application, alter the POM content of soil and water.
Overview
Particulate organic carbon is operationally defined as all combustible, non-carbonate carbon that can be collected on a filter. The oceanographic community has historically used a variety of filters and pore sizes, most commonly 0.7, 0.8, or 1.0 μm glass or quartz fiber filters. The biomass of living zooplankton is intentionally excluded from POC through the use of a pre-filter or specially designed sampling intakes that repel swimming organisms. Sub-micron particles, including most marine prokaryotes, which are 0.2–0.8 μm in diameter, are often not captured but should be considered part of POC rather than dissolved organic carbon, which is usually operationally defined as < 0.2 μm.Typically POC is considered to contain suspended and sinking particles ≥ 0.2 μm in size, which therefore includes biomass from living microbial cells, detrital material including dead cells, fecal pellets, other aggregated material, and terrestrially derived organic matter. Some studies further divide POC operationally based on its sinking rate or size, with ≥ 51 μm particles sometimes equated to the sinking fraction. Both DOC and POC play major roles in the carbon cycle, but POC is the major pathway by which organic carbon produced by phytoplankton is exported – mainly by gravitational settling – from the surface to the deep ocean and eventually to sediments, and is thus a key component of the biological pump.
Particulate organic nitrogen can also be an important component of particulate organic matter. PON is the fraction of nitrogen found in particulate organic matter that exists in solid or suspended form, rather than dissolved in water. PON primarily originates from phytoplankton during photosynthetic growth, but it can also form from zooplankton fecal pellets, detritus, and aggregated organic debris from the breakdown of larger organisms. PON plays an important role in the marine nitrogen cycle and the biological carbon pump. When particles containing organic nitrogen sink from the surface ocean, they transport both nitrogen and carbon to deeper waters. In deep waters, microbial remineralization converts the material back into dissolved forms. This vertical flux helps sustain deep-ocean microbial communities and regulates nutrient availability in surface waters, thus influencing global productivity patterns.
PON is commonly measured by filtering seawater samples to isolate particulate matter, which is then analyzed for total nitrogen and isotopic composition. Isotopic ratios like δ15N provide valuable information about nitrogen sources and biogeochemical transformations. Low δ15N values in PON can indicate a primary contribution from nitrogen fixation, whereas higher values often reflect the assimilation of nitrate regenerated from deeper layers, as the lighter 14N isotope is preferentially used in the early fixation process. Recent studies now use dual-isotope analysis. This process measures both δ15N and δ18O of nitrate. The data allows scientists to better understand nitrification, denitrification, and other nitrogen cycle processes. These combined measurements allow scientists to distinguish between physical mixing and microbial fractionation effects that influence nitrate and particulate nitrogen in the ocean.
By tracking PON concentrations and isotopic compositions, researchers can estimate the proportion of "new" versus "regenerated" production, evaluate the efficiency of the biological pump, and quantify how nitrogen moves through and out of marine ecosystems.
Terrestrial ecosystems
Soil organic matter
is anything in the soil of biological origin. Carbon is its key component comprising about 58% by weight. Simple assessment of total organic matter is obtained by measuring organic carbon in soil. Living organisms contribute about 15% of the total organic matter in soil. These are critical to operation of the soil carbon cycle. What follows refers to the remaining 85% of the soil organic matter - the non-living component.As shown below, non-living organic matter in soils can be grouped into four distinct categories on the basis of size, behaviour and persistence. These categories are arranged in order of decreasing ability to decompose. Each of them contribute to soil health in different ways.
Dissolved organic matter : is the organic matter which dissolves in soil water. It comprises the relatively simple organic compounds which easily decompose. It has a turnover time of less than 12 months. Exudates from plant roots are included here.
Particulate organic matter : is the organic matter that retains evidence of its original cellular structure, and is discussed further in the next section.
Humus: is usually the largest proportion of organic matter in soil, contributing 45 to 75%. Typically it adheres to soil minerals, and plays an important role structuring soil. Humus is the end product of soil organism activity, is chemically complex, and does not have recognisable characteristics of its origin. Humus is of very small unit size and has large surface area in relation to its weight. It holds nutrients, has high water holding capacity and significant cation exchange capacity, buffers pH change and can hold cations. Humus is quite slow to decompose and exists in soil for decades.
Resistant organic matter: has a high carbon content and includes charcoal, charred plant materials, graphite and coal. Turnover times are long and estimated in hundreds of years. It is not biologically active but contributes positively to soil structural properties, including water holding capacity, cation exchange capacity and thermal properties.
Role of POM in soils
Particulate organic matter includes steadily decomposing plant litter and animal faeces, and the detritus from the activity of microorganisms. Most of it continually undergoes decomposition by microorganisms and usually has a turnover time of less than 10 years. Less active parts may take 15 to 100 years to turnover. Where it is still at the soil surface and relatively fresh, particulate organic matter intercepts the energy of raindrops and protects physical soil surfaces from damage. As it is decomposes, particulate organic matter provides much of the energy required by soil organisms as well as providing a steady release of nutrients into the soil environment.The decomposition of POM provides energy and nutrients. Nutrients not taken up by soil organisms may be available for plant uptake. The amount of nutrients released during decomposition depends on the biological and chemical characteristics of the POM, such as the C:N ratio. In addition to nutrient release, decomposers colonizing POM play a role in improving soil structure. Fungal mycelium entangle soil particles and release sticky, cement-like, polysaccharides into the soil; ultimately forming soil aggregates
Soil POM content is affected by organic inputs and the activity of soil decomposers. The addition of organic materials, such as manure or crop residues, typically results in an increase in POM. Alternatively, repeated tillage or soil disturbance increases the rate of decomposition by exposing soil organisms to oxygen and organic substrates; ultimately, depleting POM. Reduction in POM content is observed when native grasslands are converted to agricultural land. Soil temperature and moisture also affect the rate of POM decomposition. Because POM is a readily available source of soil nutrients, is a contributor to soil structure, and is highly sensitive to soil management, it is frequently used as an indicator to measure soil quality.