Rhizosphere impact on the dissolution of test minerals in a forest ecosystem

As soil minerals are the principal input of nutrients in non-fertilized forests, the parameters which influence their dissolution must be determined to predict ecosystem sustainability. Notably, biological activities within the rhizosphere, such as root and micro-organism exudation and respiration, considerably affect mineral dissolution rate. Numerous laboratory studies have even demonstrated that certain biological processes involved in mineral weathering can be stimulated in low-nutrient availability conditions, resulting in an improvement of plant nutrition. The objective of this work was to determine in the field if the mineral dissolution rate linked to root and root-associated micro-organism activity is increased in low-nutrient availability conditions. Here, the impact of the rhizosphere on the dissolution of test minerals containing Ca (fluorapatite and labradorite plagioclase) was assessed in an acid forest soil in two stands of mature beeches (Fagus sylvatica) presenting two levels of Ca availability: a control plot as well as a plot fertilized with Ca. Mineral-test bags were inserted at three different depths (− 2.5, − 10 and − 20 cm) in the control and the Ca-fertilized plots into both a zone with roots as well as a zone where roots had been excluded, thus permitting to assess the effect of the rhizosphere on the mineral dissolution. After four years of incubation in the soil, the minerals were weighed and observed by scanning electron microscope. In the control stand, linear dissolution voids were only observed on the mineral surfaces incubated in the zone with roots, suggesting that local biological activities occurring in the rhizosphere affect mineral weathering. This positive effect of the rhizosphere in the control stand was confirmed by quantification of the mineral dissolution, which revealed an increase of fluorapatite and labradorite weathering, reaching factors 3 to 4 at 20-cm depth. In contrast, the beech rhizosphere did not increase mineral dissolution, hyphae colonisation or linear dissolution marks in the Ca-fertilized stand. These results suggest that the rhizospheric biological activities acting on mineral weathering could be regulated by the nutrient availability in the ecosystem. This plasticity of the rhizospheric biological activities may thus contribute to the maintenance of ecosystem sustainability.