Increase in the fracture toughness and bond energy of clay by a root exudate

Root exudates help drive the formation of the rhizosphere by binding soil particles, but the underlying physical mechanisms have not been quantified. This was addressed by measuring the impact of a major component of root exudates, polygalacturonic acid (PGA), on the interparticle bond energy and fracture toughness of clay. Pure kaolinite was mixed with 0, 1.2, 2.4, 4.9 or 12.2 g PGA kg(-1) to form test specimens. Half of the specimens were washed repeatedly to remove unbound PGA and evaluate the persistence of the effects, similar to weathering in natural soils. Fracture toughness, K(IC), increased exponentially with added PGA, with washing increasing this trend. In unwashed specimens K(IC) ranged from 54.3 +/- 2.5 kPa m(-1/2) for 0 g PGA kg(-1) to 86.9 +/- 4.7 kPa m(-1/2) for 12.2 g PGA kg(-1). Washing increased K(IC) to 61.3 +/- 1.2 kPa m(-1/2) for 0 g PGA kg(-1) and 132.1 +/- 4.9 kPa m(-1/2) for 12.2 g PGA kg(-1). The apparent bond energy, gamma, of the fracture surface increased from 5.9 +/- 0.6 J m(-2) for 0 g kg(-1) to 12.0 +/- 1.1 J m(-2) for 12.2 g kg(-1) PGA in the unwashed specimens. The washed specimens had gamma of 13.0 +/- 1.9 J m(-2) for 0 g kg(-1) and 21.3 +/- 2.6 J m(-2) for 12.2 g PGA kg(-1). Thus PGA, a major component of root exudates, has a large impact on the fracture toughness and bond energy of clay, and is likely to be a major determinant in the formation of the rhizosphere. This quantification of the thermodynamics of fracture will be useful for modelling rhizosphere formation and stability.