Silicon has a crucial role in many biogeochemical processes—for example, as a nutrient for marine and terrestrial biota, in buffering soil acidification and in the regulation of atmospheric carbon dioxide. Traditionally, silica fluxes to soil solutions and stream waters are thought to be controlled by the weathering and subsequent dissolution of silicate minerals1,2. Rates of mineral dissolution can be enhanced by biological processes3. But plants also take up considerable quantities of silica from soil solution, which is recycled into the soil from falling litter in a separate soil–plant silica cycle that can be significant in comparison with weathering input and hydrologic output4,5,6,7,8. Here we analyse soil water in basaltic soils across the Hawaiian islands to assess the relative contributions of weathering and biogenic silica cycling by using the distinct signatures of the two processes in germanium/silicon ratios. Our data imply that most of the silica released to Hawaiian stream water has passed through the biogenic silica pool, whereas direct mineral–water reactions account for a smaller fraction of the stream silica flux. We expect that other systems exhibiting strong Si depletion of the mineral soils and/or high Si uptake rates by biomass will also have strong biological control on silica cycling and export.
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The authors declare that they have no competing financial interests.
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We thank A. Moore and P. T. Atkins for field and laboratory assistance. This research was supported by grants from the A. W. Mellon Foundation to L.A.D. and O.A.C., and from the N.S.F. to L.A.D., O.A.C. and A.C.K.
Stream and soil data. (DOC 23 kb)
Supplementary Table S1 Ge and Si in Hawaiian streams. Supplementary Table S2 Ge and Si from Hawaiian soil solutions. Supplementary Table S3 Silica content and Ge/Si ratios of phytoliths from Hawaiian study sites. (DOC 74 kb)
Ge versus Si for Hawaiian stream data set. (PDF 41 kb)
Ge/Si versus 1/Si Si for Hawaiian stream data set. (PDF 43 kb)
Legends to Supplementary Figures S1 and S2. (DOC 20 kb)
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