『Abstract
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 simulated 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
plots 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.
Keywords: Apatite; Ecosystem sustainability; Labradorite plagioclase;
Plasticity of biological activities; Rhizosphere; Weathering』
1. Introduction
2. Materials and methods
2.1. Ecosystem description
2.2. Studied beech stands
2.3. Test mineral
2.4. Experimental design
2.5. Analyses of the test minerals after incubation
2.6. Measurements of root density
2.7. Statistical analyses
3. Results
3.1. SEM observations
3.2. Mass balance
3.2.1. Apatite
3.2.2. Labradorite
3.3. Root density
4. Discussion
4.1. Site comparison
4.2. Rhizosphere effect
4.3. Possible regulation of the rhizospheric biological activity
by nutrient availability
4.4. Conclusion
Acknowledgements
References