New insights into organic-mineral particles: composition, properties and models of molecular structure

Organic-mineral soil particles are formed and altered as a result of pedoge nic processes and anthropogenic impacts on soils. They can be isolated from bulk soil samples by physical disaggregation followed by particle size and /or density fractionation. This review summarizes methodological advances a nd the characteristic features of the chemical composition of organic matte r in these different fractions. It is demonstrated how soil texture and soi l genesis determine the contents of organic C (C-org) in size-separates and their distribution in typical profiles of four important major soil groups . Data from numerous field studies were compiled to show that the proportio ns of C(or)g bound in clay fractions increased under soil uses which reduce the soil organic matter (SOM) content. Coarser and lighter particles are r elatively enriched in native or manured soils. Contents of humic substances , lignin oxidation products, carbohydrates, lipids, and forms of organic N determined by wet-chemical methods indicate that the chemical composition r esults from specific functions of the organic-mineral particles in the turn over of SOM. This is also reflected by C-13-nuclear magnetic resonance and analytical pyrolysis, the applications of which to the study of organic-min eral particles are summarized and compared for the first time. The summariz ed chemical characteristics of organic-mineral particles have direct effect s on other properties such as soil aggregation and the resulting physical b ehaviour of soil. Moreover, the adsorption and distribution of plant nutrie nts, as well as the binding capacity for a wide range of pollutants, clearl y show that the organic matter is a key factor for the physicochemical reac tions at organic-mineral surfaces. Along with evidence for the different mi neralizability of C-org and organic N, and enzymatic properties, the accumu lated knowledge of their composition, structure and properties provides a b asis from which to develop molecular models of organic-mineral particles. M olecular modelling and visualization of humic colloids, SOM and its complex es (organic-mineral particles) with biological substances, metals and xenob iotics are discussed. These model complexes offer a basis for the conceptua l development of SOM macromolecules, mineral soil particles and the compute r-assisted design of organic-mineral particles. Molecular-mechanics calcula tions and geometrical optimization are employed to obtain energy minimized, stable conformations which allow the determination of atomic distances, bo nds, angles, torsions, stretches, etc., at nanochemical level. As a result, exact descriptions of three-dimensional structures can be proposed. The to tal (potential) energy of the organic matter complexes and macromolecules c an be determined simultaneously, and energy derivatives for bond-, angle-, dihedral-, van der Waals', stretch-bend, and electrostatic energies calcula ted. Moreover, quantitative structure-activity relationship software is emp loyed to calculate relevant molecular properties such as surface, volume, a tomic charges, polarizability, refractivity, etc. It is this combination of atomic/molecular structural data and energy derivatives that opens up new insights into binding sites and transport forms between biological and/or x enobiotic substances and SOM. The resulting potential, and limits, for mode lling of organic (humic) macromolecules, organic-mineral particles and mine ral surfaces are delineated.