Wetting and drying cycles in the maize rhizosphere under controlled conditions. Mechanics of the root-adhering soil

Mechanical properties of the topsoil (sandy Podsol and silty Luvisol, FAO) adhering to maize (Zea mays L.) roots and its bulk soil counterpart were st udied as a function of soil texture and final soil water suction at harvest , with three soil water suction values of approximately 30, 50 and 60 kPa. Two scales of observation were also selected: the whole soil:root system an d the root-adhering soil aggregates. Three methods were used to characteriz e the stability of the soil:root system: mechanical shaking in air, and dis persion by low-power ultrasonication, with or without preliminary immersion of the soil:root system in water. Soil disruption kinetics, which were fit ted with first-order kinetics equations, were analyzed and discussed. For e xample, silty soil ultrasonication kinetics, without preliminary water-imme rsion, could be divided into two parts: the first faster part, which was ch aracterized by a mean rate K value of 6.8-7.2 mJ(-1), is attributed to soil slaking, whereas the second slower part, which was characterized by a mean rate K value of 1.5-1.6 mJ(-1), was attributed to the rupture of the 'firm ly root-adhering soil' from the roots. A clear plant effect was observed fo r both aggregate tensile strength and friability, with higher aggregate str ength for the root-adhering silty soil (450-500 kPa) than for its bulk silt y soil counterpart (410-420 kPa), and lower friability (coefficient of vari ation of the aggregate strength) for the root-adhering silty soil (e.g. 67% at a soil water suction value of 30 kPa) than for its bulk silty soil coun terpart (e.g. 49% at asoil water suction value of 30 kPa). These effects we re attributed to root exudation, which was significantly higher for the dri est silty topsoil than for the wetter ones. In conclusion, the mechanical p roperties of the silty topsoil adhering to the maize roots are attributed t o both physical and biological interactions occurring in the maize rhizosph ere.