DOM is an abbreviation for the dissolved organic matter in the soil . According to the definition, the molecule size must be below 0.45 µm , otherwise it is a so-called particulate organic substance. A distinction is made between dissolved organic carbon ( DOC ), dissolved organic nitrogen ( DON ), dissolved organic phosphorus ( DOP ) and dissolved organic sulfur ( DOS ). The DOM is for the most part localized in the organic edition, especially in the Oh horizon . About 10 to 30% get into the mineral soil or are formed there.
The DOM consists of humic and fulvic acids , as the main components, as well as amino acids , carbohydrates and fatty acids . In addition, organic metal complexes and clay-humus complexes are included, provided they are smaller than 0.45 µm. The living components of the DOM are viruses . Microorganisms are usually too big and therefore do not play a role.
The DOM can basically be obtained in two ways, via extraction processes and lysimeters or suction cups . During an extraction in the laboratory, an equilibrium solution is obtained, and the entire soil solution is obtained with lysimeters / suction cups. Depending on the process, a DOM of different quality and quantity results.
Through (micro) biological activity, the potential DOM is first formed from the solid organic substance . If this is then resolved through hydrological or physicochemical processes, the DOM is created.
There are a variety of factors that influence DOM formation:
- pH value : DOM formation is supported by a high pH value, as the hydrophilicity increases with a higher degree of OH - - dissociation .
- Metal ions: More metal ions lead to increased complexation combined with precipitations and thus less DOM.
- Temperature: DOM formation increases with increasing temperature because solubility and microbial activity increase.
- Water content / flow: With a higher water content and stronger flow, more organic matter (potential DOM) can be dissolved.
- Temperature and humidity dynamics: Both freeze-thaw and dehydration-rewetting cycles increase the availability of organic matter (aggregate destruction, dead microbial biomass ) and thus strengthen DOM formation.
Sources and sinks in the ground
Sources of DOM include humus , litter, root exudates and microorganisms. Litter and the resulting humus are of great importance, root exudates of only minor importance. In the case of microorganisms, there has so far only been agreement on the qualitative relevance. At which point these play a role is under discussion.
Possible depressions in the soil are the washout, i.e. H. the discharge from the soil into deeper layers, ground or surface water , the degradation by microorganisms and the adsorption on soil particles. The degradation is usually supported by adsorption on surfaces. In addition, organic matter can be accumulated in the soil matrix through adsorption.
DOM is important in the nutrient cycle ( carbon , nitrogen , phosphorus , sulfur ) because it represents a mobile fraction. It is a substrate for microorganisms and is possibly also taken up by plants. With the DOM, pollutants are sometimes mobilized , transported and made bioavailable . The DOM also plays a role in podsolization .