Formation of aggregates by plant roots in homogenised soils

The influence of root growth and water regime on the formation of aggregates was studied in modified minirhizotrons under controlled conditions. Two soils, a black earth (67% clay) and a red-brown earth (19% clay) were ground and forced through a 0.5 mm sieve. Ryegrass, pea and wheat were grown for fifteen wetting and drying (wd) cycles for 5 months. Another set of minirhizotrons was not planted and served as a control. Measurements of aggregate size distribution (ASD), aggregate tensile strength (ATS), aggregate stability (AS), aggregate bulk density (ABD) and organic carbon (OC) were made on single aggregates of the 2–4 mm fraction. The results showed that aggregates of the black earth which has a high clay content and shrink/swell properties had more smaller aggregates with higher ATS, AS and ABD than those from the red-brown earth. It was also found that for both soils: (1) w/d cycles and higher root length density (RLD) increased the proportions of smaller aggregates and aggregate strength; (2) differences in the ability of the plant species to influence aggregation was evident and seemed to be related to the RLD. The RLD was in the order ryegrass > wheat > pea. Mechanisms likely to be involved in processes of aggregate formation and stabilization are discussed. They include cracking of soil due to tensile stresses generated during drying of a shrinking soil; changes in pore water pressure within the soil mass caused by water uptake by plant roots generating effective stresses; and biological processes associated with plant roots and root exudates.