Particulate organic matter (POM) 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 (53 μm) to 2 millimeters.

Particulate organic carbon (POC) 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.

Particulate organic carbon (POC) 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 (DOC), 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 (PON) can also be an important component of particulate organic matter. PON is the fraction of nitrogen found in particulate organic matter (POM) 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 (the ratio of ¹⁵N to ¹⁴N) 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.