Highlights
- •Distribution of minerals in microaggregates revealed by SEM-EDS.
- •Coaction of EPS and mineral size on forming the complexes of microaggregates.
- •EPS is a dispersant in the mineral mixture containing nano-sized goethite.
- •EPS is an agglomerant in the system containing micro-sized goethite.
- •No effect in the goethite size on the complexes forming without EPS.
Abstract
Soil microaggregates composed of mineral–mineral and organo-mineral complexes are crucial for controlling many soil ecosystem functions such as soil stability and persistence of organic matter. However, mechanisms of how microbial-derived organic matter (e.g., extracellular polymeric substances, EPS) and particle size of the mineral shape mineral–mineral and organo-mineral complexes in multiphase system are unclear. Here, batch experiments (i.e., mineral mixtures of quartz, kaolinite, and three different sizes of goethite) were used to determine the effect of goethite size and EPS on the formation of mineral–mineral and organo-mineral complexes. Our results revealed that the multiphase system containing micro-sized goethite had a higher proportion of 50–250 μm aggregates than these containing nano-sized goethite in the presence of EPS. When EPS was absent, the particle distribution was not related to goethite size, and the mineral suspension was homogeneous. The mechanisms of mineral–mineral and organo-mineral complexes formation were further investigated by measuring zeta potential. Negatively charged EPS adsorbed onto positively charged goethite surface through electrostatic interactions, resulting in stable organo-mineral complexes and inhibition of mineral–mineral complexes. EPS-goethite interaction resulted in a marked decrease of phosphorus in the suspension, suggesting preferred adsorption of EPS-P compounds to goethite. This interaction led to EPS fractionation which was affected by goethite size due to the particle size-dependent surface interactions. The information obtained in this study is of fundamental significance for understanding the initial formation of microaggregates in the edaphic microenvironment.
