Silicon is an essential nutrient for marine diatoms, which dominate the export of organic carbon to the deep ocean. Despite the dominance of the oceanic Si budget by fluvial inputs and the importance of the land biosphere in controlling Si losses from rocks and soils to rivers, few studies have considered how continental biogeochemical Si fluxes have varied on an orbital time scale. Stable Si isotopes provide valuable tracers for the biogeochemical cycle of Si because the light isotope 28Si is preferentially incorporated into pedogenic clays and biogenic silica (BSi) that precipitate from aqueous solution. Lake sediments commonly contain BSi produced by higher plants (opal phytoliths) as well as siliceous aquatic microfossils such as diatom frustules and sponge spicules. In this talk, we will briefly review the Si-isotope systematics of terrestrial and freshwater ecosystems. Isotopic analysis of BSi has been aided by the development of effective cleaning techniques. In addition, a new stepped-fluorination method permits δ29Si, δ30Si and δ18O to be measured on a single aliquot of cleaned BSi, thereby allowing variations in biotic Si cycling to be compared directly with the palaeoclimatic record. To demonstrate the potential of this approach, we have reconstructed changes in the Si cycle within the catchment-lake ecosystem of subalpine Lake Rutundu, Mt. Kenya (3078 m a.s.l.), over the last ~38 cal. ka BP, using a novel combination of lake-sediment fluxes and multi-isotope (molecular δ13C, bulk δ15N, and diatom δ18O and δ30Si) data. Under the cold, dry climate of the last glacial (38.3-14.3 ka BP), high diatom productivity was maintained by substantial losses of dissolved SiO2 and soil nutrients (P, Fe) in runoff from a sparse, leaky, shrub grassland, and by fixation of atmospheric N2 by cyanobacteria in the lake. At times of lowest lake level, the Si supply was maintained by groundwater drawdown. During the following period of enhanced monsoon rainfall and seasonality (14.3-9.5 ka BP), rapid Si cycling by fire-prone, mesic grassland was associated with substantial aeolian transport of opal phytoliths by smoke plumes, but greatly reduced nutrient losses in runoff. Invasion of tall, subalpine shrubs after 9.5 ka BP further enhanced landscape stability, leading to very low sediment fluxes of both phytoliths and diatoms. This case study offers new insights into processes that may have operated at biome to continental scales during the Quaternary.