Hydraulic properties of soil aggregates as influenced by compaction

Hydraulic properties of aggregates affect soil processes like gas, water, and nutrient fluxes, the accessibility and storage of nutrients, and define the living conditions for microbes including their decomposing or precipitation capacities. However, these properties depend on the soil and aggregate strength and are therefore affected by soil compaction. In this study the effects of soil compaction on water and ethanol sorptivity and water repellency index of variously sized aggregates of loamy Orthic Luvisol was assessed. The various compaction levels were obtained by three tractor traffic treatments using a 3.4Mg tractor with a contact area stress of 135kPa and a rear axle load of 22.6kN: (NC) zero traffic, (MC) three passes, (SC) five passes. Water and ethanol sorptivity data of the initially air-dry soil aggregate fractions: 8-10mm, 14-16mm, 20-22mm were determined by a steady state flow using an infiltration device. Water repellency was identified by the ethanol/water sorptivity method. All the measurements were done for aggregates from topsoil (0.05-0.15m) and subsoil (0.25-0.35m) layers. Effect of soil compaction on the aggregate hydraulic properties was related to the soil layer and aggregate size fraction. Aggregate water and ethanol sorptivities of all aggregate fractions were highest under NC. In subsoil layer (B horizon) the sorptivities were lowest under SC. The response of repellency index to compaction was greater for aggregates from subsoil than topsoil layer. All subsoil aggregate fractions displayed maximum mean repellency index in SC. The 20-22mm aggregates from the subsoil layer in the moderately and most compacted soil exhibited the highest repellency index (2.32-2.60). Among all 540 individual repellency index values, about one fourth exceeded the value 1.95 which defines soil as subcritical water repellent. The results show that the responses of the aggregate hydraulic properties to compaction markedly depend on interactions between compaction level, aggregate size and depth.