Dissolved organic carbon
Dissolved organic carbon is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called particulate organic carbon.
Dissolved organic matter is a closely related term often used interchangeably with DOC. While DOC refers specifically to the mass of carbon in the dissolved organic material, DOM refers to the total mass of the dissolved organic matter. So DOM also includes the mass of other elements present in the organic material, such as nitrogen, oxygen and hydrogen. DOC is a component of DOM and there is typically about twice as much DOM as DOC. Many statements that can be made about DOC apply equally to DOM, and vice versa.
DOC is abundant in marine and freshwater systems and is one of the greatest cycled reservoirs of organic matter on Earth, accounting for the same amount of carbon as in the atmosphere and up to 20% of all organic carbon. In general, organic carbon compounds are the result of decomposition processes from dead organic matter including plants and animals. DOC can originate from within or outside any given body of water. DOC originating from within the body of water is known as autochthonous DOC and typically comes from aquatic plants or algae, while DOC originating outside the body of water is known as allochthonous DOC and typically comes from soils or terrestrial plants. When water originates from land areas with a high proportion of organic soils, these components can drain into rivers and lakes as DOC.
The marine DOC pool is important for the functioning of marine ecosystems because they are at the interface between the chemical and the biological worlds. DOC fuels marine food webs, and is a major component of the Earth's carbon cycling.
Overview
DOC is a basic nutrient, supporting growth of microorganisms and plays an important role in the global carbon cycle through the microbial loop. In some organisms that do not feed in the traditional sense, dissolved matter may be the only external food source. Moreover, DOC is an indicator of organic loadings in streams, as well as supporting terrestrial processing of organic matter. Dissolved organic carbon has a high proportion of biodegradable dissolved organic carbon in first order streams compared to higher order streams. In the absence of extensive wetlands, bogs, or swamps, baseflow concentrations of DOC in undisturbed watersheds generally range from approximately 1 to 20 mg/L carbon. Carbon concentrations considerably vary across ecosystems. For example, the Everglades may be near the top of the range and the middle of oceans may be near the bottom. Occasionally, high concentrations of organic carbon indicate anthropogenic influences, but most DOC originates naturally.The BDOC fraction consists of organic molecules that heterotrophic bacteria can use as a source of energy and carbon. Some subset of DOC constitutes the precursors of disinfection byproducts for drinking water. BDOC can contribute to undesirable biological regrowth within water distribution systems.
The dissolved fraction of total organic carbon is an operational classification. Many researchers use the term "dissolved" for compounds that pass through a 0.45 μm filter, but 0.22 μm filters have also been used to remove higher colloidal concentrations.
A practical definition of dissolved typically used in marine chemistry is all substances that pass through a GF/F filter, which has a nominal pore size of approximately 0.7 μm. The recommended procedure is the HTCO technique, which calls for filtration through pre-combusted glass fiber filters, typically the GF/F classification.
Labile and recalcitrant
Dissolved organic matter can be classified as labile or as recalcitrant, depending on its reactivity. Recalcitrant DOC is also called refractory DOC, and these terms seem to be used interchangeably in the context of DOC. Depending on the origin and composition of DOC, its behavior and cycling are different; the labile fraction of DOC decomposes rapidly through microbially or photochemically mediated processes, whereas refractory DOC is resistant to degradation and can persist in the ocean for millennia. In the coastal ocean, organic matter from terrestrial plant litter or soils appears to be more refractory and thus often behaves conservatively. In addition, refractory DOC is produced in the ocean by the bacterial transformation of labile DOC, which reshapes its composition.Due to the continuous production and degradation in natural systems, the DOC pool contains a spectrum of reactive compounds each with their own reactivity, that have been divided into fractions from labile to recalcitrant, depending on the turnover times, as shown in the following table...
This wide range in turnover or degradation times has been linked with the chemical composition, structure and molecular size, but degradation also depends on the environmental conditions, prokaryote diversity, redox state, iron availability, mineral-particle associations, temperature, sun-light exposure, biological production of recalcitrant compounds, and the effect of priming or dilution of individual molecules. For example, lignin can be degraded in aerobic soils but is relatively recalcitrant in anoxic marine sediments. This example shows bioavailability varies as a function of the ecosystem's properties. Accordingly, even normally ancient and recalcitrant compounds, such as petroleum, carboxyl-rich alicyclic molecules, can be degraded in the appropriate environmental setting.
Terrestrial ecosystems
Soil
Dissolved organic matter is one of the most active and mobile carbon pools and has an important role in global carbon cycling. In addition, dissolved organic carbon affects the soil negative electrical charges denitrification process, acid-base reactions in the soil solution, retention and translocation of nutrients, and immobilization of heavy metals and xenobiotics. Soil DOM can be derived from different sources, such as atmospheric carbon dissolved in rainfall, litter and crop residues, manure, root exudates, and decomposition of soil organic matter. In the soil, DOM availability depends on its interactions with mineral components modulated by adsorption and desorption processes. It also depends on SOM fractions by mineralization and immobilization processes. In addition, the intensity of these interactions changes according to soil inherent properties, land use, and crop management.During the decomposition of organic material, most carbon is lost as CO2 to the atmosphere by microbial oxidation. Soil type and landscape slope, leaching, and runoff are also important processes associated to DOM losses in the soil. In well-drained soils, leached DOC can reach the water table and release nutrients and pollutants that can contaminate groundwater, whereas runoff transports DOM and xenobiotics to other areas, rivers, and lakes.
Groundwater
Precipitation and surface water leaches dissolved organic carbon from vegetation and plant litter and percolates through the soil column to the saturated zone. The concentration, composition, and bioavailability of DOC are altered during transport through the soil column by various physicochemical and biological processes, including sorption, desorption, biodegradation and biosynthesis. Hydrophobic molecules are preferentially partitioned onto soil minerals and have a longer retention time in soils than hydrophilic molecules. The hydrophobicity and retention time of colloids and dissolved molecules in soils are controlled by their size, polarity, charge, and bioavailability. Bioavailable DOM is subjected to microbial decomposition, resulting in a reduction in size and molecular weight. Novel molecules are synthesized by soil microbes, and some of these metabolites enter the DOC reservoir in groundwater.Freshwater ecosystems
Aquatic carbon occurs in different forms. Firstly, a division is made between organic and inorganic carbon. Organic carbon is a mixture of organic compounds originating from detritus or primary producers. It can be divided into POC and DOC. DOC usually makes up 90% of the total amount of aquatic organic carbon. Its concentration ranges from 0.1 to >300 mg L−1.Likewise, inorganic carbon also consists of a particulate and a dissolved phase. PIC mainly consists of carbonates, DIC consists of carbonate, bicarbonate, CO2 and a negligibly small fraction of carbonic acid. The inorganic carbon compounds exist in equilibrium that depends on the pH of the water. DIC concentrations in freshwater range from about zero in acidic waters to 60 mg C L−1 in areas with carbonate-rich sediments.
POC can be degraded to form DOC; DOC can become POC by flocculation. Inorganic and organic carbon are linked through aquatic organisms. CO2 is used in photosynthesis by for instance macrophytes, produced by respiration, and exchanged with the atmosphere. Organic carbon is produced by organisms and is released during and after their life; e.g., in rivers, 1–20% of the total amount of DOC is produced by macrophytes. Carbon can enter the system from the catchment and is transported to the oceans by rivers and streams. There is also exchange with carbon in the sediments, e.g., burial of organic carbon, which is important for carbon sequestration in aquatic habitats.
Aquatic systems are very important in global carbon sequestration; e.g., when different European ecosystems are compared, inland aquatic systems form the second largest carbon sink ; only forests take up more carbon.