Impact of biochar on the biological silicon feedback loop in soil-plant systems
(2019)
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- Abstract
- The continental cycle of silicon (Si) relies on the Si soil-to-plant cycle where 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 silica bodies named phytoliths (PhSi). Within plant debris, Si returns to soil where phytoliths dissolve to provide plant available DSi. DSi from soil to plant and back to soil for plant uptake defines the biological Si feedback loop of the terrestrial cycle of Si. In croplands, harvesting and exportation of plant residues disrupt this loop, and aggravate natural soil desilication. Pyrolysis of phytolithic biomass from monocot residues produces Si-rich biochar which, when applied to soil, is a promising and environmentally friendly technique challenging desilication. Our scientific question is how to enhance the biological Si feedback loop in agroecosystems, 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 supplied phytoliths to Si accumulation in rice increased from 26% to 80% with increasing soil weathering stage, i.e. decreasing total reserve in bases from TRB 405 to 5 cmolc kg-1. 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 photosynthetic activity, water uptake and transpiration, stomatal conductance, plant biomass and grain yield. In a second step, we study the impact of soil properties and processes upon the mobility of Si in the soilplant system. Our results highlight the high potential of phytolithic biochar supply on Si plant uptake and mineralomass. The effective availability of Si for plants is, however, modulated by soil pH and buffering capacity, soil weathering stage and constitution. The effect of Si-rich biochar supply on Si plant uptake and mineralomass will thus depend on soil type. Soil processes may also impact the mobility of Si. One of them is soil aggregation, a major process in agricultural lands. Soil microaggregates protect phytoliths from rapid dissolution. They contribute for above 60% to the pools of soil PhSi and DSi. We conclude that PhSi supply from biochar alleviates natural soil desilication by enhancing the biological Si feedback loop of the terrestrial Si cycle. This effect, however, largely depends on soil constituents and properties, hence on soil type. It depends also on the entrapment of phytoliths within soil microaggregates, which may retard the release of plant available DSi. These novelties open new routes in our understanding of the terrestrial Si cycle with respect to current environmental and agronomic issues.
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Li, Z. (2019). Impact of biochar on the biological silicon feedback loop in soil-plant systems. https://hdl.handle.net/2078.5/56279