Processes controlling silicon isotopic fractionation in a forested tropical watershed: Mule Hole Critical Zone Observatory (Southern India)

Abstract

Assessing the dynamics of the silica cycle in the critical zone remains challenging, particularly within the soil, where multiple processes are involved. To improve our understanding of this cycle in the Tropics, and more specifically the role played by vegetation, we combined elemental Si mass balance with the  30 Si signatures of the compartments involved in the water-plant-rock interactions of a tropical forested watershed, Mule Hole (Southern India). To accomplish this, we analysed (1) the  30 Si values of presentday litter phytoliths from tree leaves and grass, as well as soil amorphous silica (ASi); (2) the Si isotope fractionation induced by phytolith dissolution; (3) the silicon mass balance inferred from isotopes at the soil-plant scale; and (4) the consistency between water sources and the  30 Si signatures in the ephemeral stream. The  30 Si values of present-day litter phytoliths and soil ASi vary within a narrow range of 1.10 to 1.40 ‰ for all samples, but two deep vertisol samples which likely trapped phytoliths from different vegetation growing under more humid conditions, as indicated by pollen analysis. A homogeneous signature of litter is a minimum condition for using  30 Si as a

Phytolith‐rich biochar: A potential Si fertilizer in desilicated soils

“…These pools represent various sources of DSi in agricultural soils (Haynes, 2017), because biogenic silica has a much higher dissolution rate than silicate minerals (Fraysse, Pokrovsky, Schott, & Meunier, 2006 1963), which return to soil within plant debris (Smithson, 1956). Soil phytoliths readily dissolve at common pH values of soil solution (Fraysse et al, 2006(Fraysse et al, , 2009, and thus contribute to feed the reservoir of plant-available Si (Alexandre, Meunier, Colin, & Koud, 1997;Bartoli, 1983;Farmer, Delbos, & Miller, 2005;Keller, Guntzer, Barboni, Labreuche, & Meunier, 2012;Li, Song, & Cornelis, 2014;Marxen et al, 2016;Meunier, Guntzer, Kirman, & Keller, 2008;Riotte et al, 2018;Unzué-Belmonte et al, 2016;Yang & Zhang, 2018). As defined in Figure 2, the mineral Si contribution and biological Si feedback loops interact in the Si soil-plant cycle.…”

Agriculture increases the bioavailability of silicon, a beneficial element for crop, in temperate soils

“…DSi concentration in the soil solution can be lowered by plant uptake 19 , uptake by silica-shelled microorganisms 20 and Si adsorption to mineral surfaces 21 . Quantifying the pool of Si that is bioavailable (PAS) is still a challenge 7 .…”

The distribution of Silicon in soil is influenced by termite bioturbation in South Indian forest soils

“…Difference in D:P° ratio between TM and Ctrl soils also shows that TM contained more phytoliths from grasses than control soil samples. Although this study does not allow to reject the assumption that phytoliths from deeper soil layers were selectively incorporated into TM, we consider this hypothesis unlikely since this would have resulted in higher representation of forest indicator phytoliths, as forest is the dominant vegetation at Mule Hole (Riotte et al, 2018). Moreover, phytolith morphotypes vary in size but grass silica short cell phytoliths and the forest indicator spheroids are in the same size range (8-30 microns).…”