Abstract :
[en] Silicon (Si) is widely recognized as an important regulator of the global carbon (C) cycle via its effect on diatom productivity in oceans, and as a beneficial plant nutrient, improving resistance to herbivory and pathogens and mitigating the negative effects of several abiotic stresses. This thesis explores the long-term dynamics of Si in terrestrial ecosystems, and investigates some factors driving soil-plant Si dynamics in agroecosystems. The main study sites are three 2-million-years dune chronosequences located on a climatic gradient in southwestern Australia. Within a chronosequence, plant productivity is limited by nitrogen (N), then by phosphorus (P), as soils age.
We show that soil Si dynamics is primarily driven by geochemical processes in young and middle-aged soils (carbonates dissolution, clay formation, quartz enrichment), but increasingly by biological processes (silica formation in plants followed by its dissolution in soils) in old and highly-weathered soils. A climate-driven increase in biomass production along the climatic gradient seems to enhance this biological Si feedback loop. Besides, the continuous increase in community-level leaf Si concentrations with increasing soil age and P depletion might reflect the importance of silica-based defenses in P-poor environments. This increase is associated with a decrease in leaf total phenol concentrations, suggesting a tradeoff between both defense strategies along N-P gradients. We also propose that the increase in nutrient-acquisition carboxylate-releasing strategies with increasing soil age might explain the increase in leaf Si concentrations, with carboxylates not only mobilizing rhizosphere P, but also Si. Based on the above results and a literature review, we then summarized the biotic and abiotic controls on soil Si dynamics, and wondered whether they could be exploited in agroecosystems. We particularly stress the importance of mycorrhizal associations, silicate-solubilizing bacteria, soil macrofauna, root exudates and large herbivores on soil-plant Si dynamics. These ecological processes might in turn be exploited in cereal-legume intercropping, cover crops implementation, or integrated crop-livestock systems. We finally demonstrate that soil properties and recycling crop residues strongly influence the foliar silicification and its beneficial effects for two crop species (rice and sugarcane) through two case studies in Burkina Faso and Guadeloupe, respectively.
This thesis highlights the major influence of soil age and weathering degree on soil-plant Si dynamics, from both a biogeochemical and ecological perspective, and demonstrate that knowledge from complex natural systems might help to improve the Si-use efficiency and subsequent sustainability of modern agroecosystems. Besides, this thesis stresses the need to develop multidisciplinary approaches to better understand elements mobility in natural ecosystems and agroecosystems.
