Compaction changes the arrangement of particles or groups of particles in soils, which is reflected in changes in size, shape and connectivity or continuity of pores. The exposure, and hence the reactivity of clay surfaces, also changes. We can use the concept of soil architecture, which describes how pore spaces are enclosed by surfaces, to discuss soil changes on compaction. The clay particles provide most of the surfaces or the wall elements in soil architecture. Most studies of the effects of soil compaction detail changes in pores; few studies have measured changes in surfaces, so these changes must be inferred from other studies. This paper will examine how possible changes in clay surfaces due to soil compaction could alter soil functions in ecosystems. The main functions that could be affected are storage and release of water and plant nutrients; decomposition of organics and recycling of carbon and nutrients; physicochemical and biological buffering; weathering of inorganic materials; and stability of habitat for biota. Clays in soils interact as individual clay units or as groups of clay particles. These may be domains in the 1 mum size range, clusters of domains in the 50 mum range, or micropeds in the 500 mum range. Inter-clay bonding and bonding with organic molecules changes with scale as interparticle position and amount of exposed surface changes. Compaction involves rearrangement or relative movement of groups of clay particles. The size of structure units affected depends on the nature of the applied stress. This shearing breaks inorganic and organic bonds, which decreases soil strength and allows for further collapse of soil structure. The shearing can remove organic coatings and expose more clay surfaces, which can then become reactive. These fresh surfaces would be expected to be sites for further bonding, thus stabilizing the compacted structure. Compaction brings clay particles closer together, resulting in more parallel relative orientation. This can have several effects. The influence of domains in the soil architecture is decreased, and the soil takes on a massive structure with interparticle bonding dominant. Oriented clays allow for greater interparticle repulsion, and greater swelling pressure, which would decrease aggregate stability and change pore size distribution. This closer proximity cuts off some of the surfaces to reactions with molecules because of steric hindrance, which would decrease organic-inorganic bonding. Proximity also changes the habitat for biota, generally leading to anaerobic conditions. Bacteria depend upon the surfaces to retain nutrients and energy sources, as well as to buffer against toxic chemical concentrations. Ion layers around the clay particle become compressed, with higher surface acidity and enhanced hydrolysis. This would affect habitat for roots and bacteria, as well as leading to greater chemical weathering of minerals. Most of these changes in surfaces are second-order effects relative to changes in pores, but surface changes can be significant in recycling of nutrients by bacteria, in chemical weathering, in the buffering function of soils, and in changing the physical and chemical habitat for biota.
