29 avril 2019
Salle Jean-Baptiste Carnoy - Place Croix du Sud, 4-5
Impact of biochar on biological silicon feedback loop in soil-plant systems
In the continental cycle of silicon (Si), the primary source of Si for plants is the reserve of soil weatherable lithogenic silicates (LSi). LSi weathering releases dissolved Si (DSi), which may follow four routes: formation of pedogenic silicates (PSi), adsorption on oxide surfaces, leaching to watersheds, uptake by plants that form phytoliths (PhSi). Within plant debris, Si returns to soil where phytoliths dissolve to provide plant available DSi. Si from soil to plant and back to soil for plant uptake defines a biological Si feedback loop. In croplands, harvesting and exportation of plant residues disrupt this loop, and aggravate natural soil desilication. Pyrolysis of phytolithic biomass from monocot residues produces silicon-rich biochar which, when applied to soil, is a promising and environmentally friendly technique challenging desilication.
How to enhance the biological Si feedback loop in croplands, given the many benefits of Si in plants?
In a first step, we set up a soil-plant experimental design controlling PhSi supply through biochar, and LSi supply through dosed additions of weatherable LSi minerals to a soil free of them, hence simulating a soil weathering gradient. The contribution of phytoliths to Si accumulation in rice increased from 26% to 80% with increasing soil weathering stage. LSi dissolution contributed to 7-14% of allophanic PSi synthesis, but to less than 1% of plant PhSi. Allophane rapidly formed from LSi dissolution at a maximum rate of 0.85 g kg-1 day-1. The increase in rice plant accumulation of Si enhanced stomatal conductance, photosynthesis, transpiration and water uptake, plant biomass and grain yield.
In a second step, we study the impact of soil properties and processes upon plant available Si. Phytolithic biochar supply increases Si plant uptake and mineralomass. The plant availability of Si is, however, modulated by soil pH and buffering capacity, soil weathering stage and constitution, and thus soil type. Soil aggregation, a major process in croplands, impacts the mobility of Si. Soil microaggregates protect phytoliths from rapid dissolution. They contribute for above 60% to the pools of soil PhSi and DSi.
PhSi supply from biochar thus alleviates natural soil desilication by enhancing the biological Si feedback loop of the terrestrial Si cycle. This effect, however, largely depends on soil constituents, properties and processes.
These novelties open new routes in our understanding of the terrestrial Si cycle with respect to current environmental and agronomic issues.
Jury members :
- Prof. Marnik Vanclooster (UCLouvain), chairperson
- Prof. Bruno Delvaux (UCLouvain), supervisor
- Prof. Jean-Thomas Cornelis (ULiège, Agro-Bio Tech, Belgique), co-supervisor
- Prof. Pierre Bertin (UCLouvain), secretary
- Prof. Eric Struyf (UAntwerp, Belgique)
- Prof. Jean-Dominique Meunier (CEREGE, France)