Macro-aggregation of soils by biological bonding and binding mechanisms and the factors affecting these: a review

This review presents an analysis of the labile bonding (organic compounds) and binding (hyphae and roots) mechanisms involved in stabilising soil into macro-aggregates in soils (aggregates >0 · 25 mm diameter). The main emphasis is on the approaches used to study these mechanisms and the factors that affect the aggregating mechanisms at the micro-scale (<100µm) level in soils. Much of the understanding of the stabilisation of soil into macro-aggregates in field soils by labile organic C is derived primarily from the interpretations of studies where soils were incubated under artificial conditions. In addition, many recent studies have assessed the importance of labile organic C in macro-aggregation based only on whole soils analyses of hyphal length, extractable carbohydrate C, microbial biomass C, or aliphatic C. These approaches have contributed little to understanding fundamental factors that can influence the biological mechanisms of macro-aggregation under field conditions. It is proposed that greater consideration of bonding and binding aggregation mechanisms and the factors affecting the mechanisms at the micro-scale level will facilitate development of field management practices that benefit macro-aggregation. Factors influencing the location and persistence of bonding and binding mechanisms in the soil matrix are discussed.