@article{fdi:010072671,
  title = {{P}rocesses controlling silicon isotopic fractionation in a forested tropical watershed : {M}ule {H}ole {C}ritical {Z}one {O}bservatory ({S}outhern {I}ndia)},
  author = {{R}iotte, {J}ean and {M}eunier, {J}. {D}. and {Z}ambardi, {T}. and {A}udry, {S}. and {B}arboni, {D}. and {A}nupama, {K}. and {P}rasad, {S}. and {C}hmeleff, {J}. and {P}oitrasson, {F}. and {S}ekhar, {M}. and {B}raun, {J}ean-{J}acques},
  editor = {},
  language = {{ENG}},
  abstract = {{A}ssessing the dynamics of the silica cycle in the critical zone remains challenging, particularly within the soil, where multiple processes are involved. {T}o improve our understanding of this cycle in the {T}ropics, and more specifically the role played by vegetation, we combined elemental {S}i mass balance with the delta {S}i-30 signatures of the compartments involved in the waterplant-rock interactions of a tropical forested watershed, {M}ule {H}ole ({S}outhern {I}ndia). {T}o accomplish this, we analysed (1) the d30{S}i values of present-day litter phytoliths from tree leaves and grass, as well as soil amorphous silica ({AS}i); (2) the {S}i 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 delta {S}i-30 signatures in the ephemeral stream. {T}he delta {S}i-30 values of present-day litter phytoliths and soil {AS}i vary within a narrow range of 1.10-1.40 parts per thousand 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 delta {S}i-30 as a proxy for the litter/phytolith source of {S}i. {H}owever, litter-ash dissolution experiments demonstrate that the incipient dissolution of phytoliths fractionates {S}i isotopes, with the preferential dissolution of 28 {S}i over 30 {S}i yielding delta {S}i-30 values as low as -1.41 parts per thousand. {V}alues close to the whole-sample signatures, i.e., above 1 parts per thousand, were recovered in the solution after a few hours of water-ash interaction. {A}t the soil-plant scale, the average delta {S}i-30 value of soil-infiltrating solutions is slightly lighter than the average phytolith signature, which suggests phytoliths as the source of soil dissolved {S}i. {T}he isotopic budget of dissolved {S}i within the soil layer, which was obtained based on previous elemental fluxes, is imbalanced. {E}quilibrating the isotopic budget would imply that up to 4100 mol ha(-1) yr(-1) of silica is taken up by vegetation, which is almost twice as large as that initially estimated from the elemental budget. {T}he additional {S}i flux taken up, and likely stored in woody stems, was estimated assuming that {S}i isotopes followed a steady-state model for the whole {S}i plant uptake and then followed a {R}ayleigh model once in the plants. {T}he delta {S}i-30 value of the additional {S}i flux taken up should be close to 0 parts per thousand, i.e., enriched in light {S}i isotopes compared to the litter. {I}f steady-state conditions apply, the source could correspond to soil {AS}i dissolution or deep (saprolite) root uptake. {A}t the outlet of the watershed, the stream exhibits low delta {S}i-30 values (0.28-0.71 parts per thousand) during peak flows and high delta {S}i-30 values (1.29-1.61 parts per thousand) during the recessions at the end of the rainy season. {H}eavy delta {S}i-30 signatures are consistent with the expected domination of seepage at the end of floods. {T}he light delta {S}i-30 values during peak flow are slightly lower than the overland flow signature and reflect either a sampling bias of overland flow or a minor but significant contribution of another {S}i source within the stream, possibly the partial dissolution of phytoliths from the suspended load, with slight isotopic fractionation. {T}his study confirms that vegetation controls the silicon cycle in this dry tropical forest. {I}t also shows that silicon isotopes yield a better grasp of the mass balance and sources and potential mechanisms involved than the consideration of only silicon concentrations. {H}owever, this proxy still relies on working hypotheses, notably steady-state and/or {R}ayleigh fractionation models, which need to be confirmed in further studies.},
  keywords = {{S}ilicon isotopes ; {S}ilica mass balance ; {P}lant uptake ; {T}ropics ; {S}outh {I}ndia ; {C}ritical zone ; {P}hytoliths ; {INDE} ; {ZONE} {TROPICALE}},
  booktitle = {},
  journal = {{G}eochimica et {C}osmochimica {A}cta},
  volume = {228},
  numero = {},
  pages = {301--319},
  ISSN = {0016-7037},
  year = {2018},
  DOI = {10.1016/j.gca.2018.02.046},
  URL = {https://www.documentation.ird.fr/hor/fdi:010072671},
}