Development of biogeochemical interfaces in an artificial soil incubation experiment; aggregation and formation of organo-mineral associations

Abstract

Soils represent a very complex material where mineral, organic and biological components interact together to form a large ‘biogeochemical interface’. In order to study this complex interface in a simplified system, a so-called ‘artificial soil’ incubation experiment was carried out. This experiment was used here specifically to study the effect of microbial activity and mineral interactions on the aggregation occurring during incubation. Artificial soils with different mineral composition but the same texture, were composed of well-defined model materials. The materials considered were quartz, illite, montmorillonite, ferrihydrite, boehmite and charcoal, manure as an organic matter (OM) source and a microbial inoculum extracted from a natural arable topsoil. The artificial soils were incubated for 3 to 18months in the dark, under constant temperature and water content. The pH, organic carbon (OC) and nitrogen content, extractable Fe, Al and Si of the incubated artificial soils were measured, and the amount of macroaggregates>2mm was determined by sieving. Density fractionation was performed at densities of 1.8 and 2.4gcm−3 to separate particulate OM, organo-mineral associates and the mineral fraction. The specific surface area (SSA) of the organo-mineral associate fraction and the amount of OC associated with organo-mineral associates and minerals was determined. The SSA of the model materials and the artificial soils at the start of incubation was determined by BET-N2. The artificial soils developed quickly and CO2 respiration occurred during the entire 18month incubation time. The actual SSA of the ‘soils’ was significantly lower than the sum of the SSA of the pure model materials indicating that occlusion of mineral surfaces by interaction between OM and minerals occurred almost immediately after incubation started. Macroaggregates and organo-mineral associates were formed within 3months of incubation. Macroaggregation decreased after 12months of incubation probably due to decreasing biological activity. The turnover of macroaggregates and continuing formation of organo-mineral associates was consistent with the aggregate hierarchy model. Less macroaggregation was observed in the soil where no clay mineral was present, indicating that clay minerals were important for the formation of macroaggregates. Ferrihydrite and boehmite did not affect the aggregation or formation of organo-mineral associates in this experiment. The results of density fractionation indicate that OM was mainly associated with the clay minerals, probably due to the neutral pH of the artificial soils, leading to a low or negative surface charge of the oxides. The artificial soil incubation experiment showed that interface development and the building of macroaggregates and organo-mineral associates took place within a relatively short time scale. It offers a valuable model where the formation and interactions of soil properties and processes can be studied in a well-defined system.