Biogeochemical Si Cycle Research Articles

Historical land use change has lowered terrestrial silica mobilization

Continental export of Si to the coastal zone is closely linked to the ocean carbon sink and to the dynamics of phytoplankton blooms in coastal ecosystems. Presently, however, the impact of human cultivation of the landscape on terrestrial Si fluxes remains unquantified and is not incorporated in models for terrestrial Si mobilization. In this paper, we show that land use is the most important controlling factor of Si mobilization in temperate European watersheds, with sustained cultivation (>250 years) of formerly forested areas leading to a twofold to threefold decrease in baseflow delivery of Si. This is a breakthrough in our understanding of the biogeochemical Si cycle: it shows that human cultivation of the landscape should be recognized as an important controlling factor of terrestrial Si fluxes.

The output and bio-cycling of Si in a tropical rain forest developed on young basalt flows (La Reunion Island)

The importance of plant bio-cycling in the biogeochemical cycle of Si has been pointed out by recent studies. We analysed the litter and wood chemical composition of a lowland rainforest (Marelongue Reserve, Réunion Island, Indian ocean) not impacted by human activities, developed on a recent basalt flow dated ci 500 years. The amount of elements stored in the above ground biomass falls within the range of tropical forests developed on moderately fertile older soils with Ca, K, Mg and Si values respectively at 1458 kg ha −1, 1506 kg ha −1, 246 kg ha −1and 17 kg ha −1. The estimated mineral uptake of the old trees gives a flux equivalent to 5 ± 2% of the elements returned to the soil with litterfall. The four species that contribute the most to the litterfall with 70% of the leaf fall represent only 45% of the annual Si bio cycling. The result demonstrates that species that are not highly productive in a rain forest could be significant for the bio-cycling of parent-rock elements such as Si. A mass balance calculation shows that the dissolved silicon (DSi) issued from the dissolution of basalt is 1.8 times the DSi from litterfall. Here, the biogeochemical cycle of Si is characterized by a limited contribution of the bio-cycling compartment compared to other cases reported for rainforests.

Contrasting silicon uptakes by coniferous trees: a hydroponic experiment on young seedlings

Silicon uptake by terrestrial plants impacts the Si land-ocean fluxes, therefore inducing significant modifications for biogeochemical cycle of Si. Understanding the mechanisms that control Si uptakes by forest vegetation is of great interest for the study of the global Si cycle as the world’s total forest area corresponds to about 30% of the land area. Our study compares Si uptake in controlled conditions by two coniferous species (Pseudotsuga menziensii and Pinus nigra) exhibiting contrasting Si uptake in the field. For this purpose, seedlings were grown for 11 weeks under controlled conditions in hydroponics with different Si concentrations (0.2 to 1.6 mM) in nutrient solutions. The Si concentrations were greater in Douglas fir leaves as compared with Black pine leaves and increased, depending on the Si concentration in the nutrient solution. According to mass balance, Si absorption seems to have been driven by passive Si transport at 0.2 mM Si (realistic concentration for forest soil solutions) and was rejective at higher Si concentrations in nutrient solution for both species. For this reason, we attributed the higher Si concentration in Douglas fir leaves to the greater cumulative transpiration of these seedlings. We suggest that contrasting transpiration rates may also play a key role in controlling Si accumulation in leaves at field scale.

Silica content in soil solution and its relation with phytolith weathering and silica biogeochemical cycle in Typical Argiudolls of the Pampean Plain, Argentina—a preliminary study

Little is known on the silica biogeochemical cycle in terrestrial environments. The aim of this work is to assess phytolith’s role on the biogeochemical cycle of Si in Typical Argiudolls under different vegetation of the Pampean Plain, Argentina. The work has been developed in three plots with different vegetal cover: grasses and shelter-belt plantations of Acacia melanoxylon–Celtis tala and Eucalyptus globulus–C. tala. The heavy liquid separation in the soil samples was realized with sodium polytungstate. The silica concentration of the soil solution and groundwaters was determined by UV-VIS spectrophotometry. Acacia and eucalyptus do not produce phytoliths; instead, Dactylis glomerata (grass) is a silica-accumulating species, and their phytolith assemblage is composed basically by oblong and crenate, smooth elongate, rectangular and prickles within isolated phytoliths, and smooth long cells articulated. Phytolith content in soils decreases with depth. Total stock of phytoliths represents 5.9 to 12.9 wt.% and is higher in the arboreal plots. In the A horizons, phytolith fraction represents about 59.6 × 103 to 103.5 × 103 kg/ha. In these horizons, 90.7–94.4% of the phytolith content constitutes the labile pool and 9.3–5.6% the stable pool. In the arboreal plots, SiO2 content in soil solution is higher (406–1,106 μmol/L) and decreases with depth, while in the grass plot, SiO2 content is lesser (421–777 μmol/L) and increases with depth; probably because of differences in the nutritional requirements and root design between vegetal species, therefore, in the different depth uptake from the soil solution. In groundwaters, silica content is very high (932 μmol/L). Phytoliths are very representative in Typical Argiudolls and show a great degree of weathering so they could be into account in the biogeochemical studies since they could contribute with silica content in the soil solution, affecting the terrestrial silica biogeochemical cycle.

Temporal isotopic variations of dissolved silicon in a pristine boreal river

It has previously been concluded that the stable Si isotopes are fractionated during geochemical and biogeochemical elemental transfers, such as weathering and precipitation of clays and biogenic Si, which has opened up the possibility of using Si as a tracer in natural terrestrial ecosystems. Furthermore, quantification of the biogenic impact on the biogeochemical Si cycle has attracted significant scientific interest since biological control has been suggested. Previous observations of seasonal variations in the dissolved Si isotopic pattern further imply that high-frequency riverine sampling during main hydrological events might provide important information about natural processes governing the fluvial biogeochemical Si cycle. Therefore, temporal variations in the isotopic composition of riverine dissolved Si were investigated for the Kalix River, Northern Sweden, the largest pristine river system in Europe, based on high-frequency sampling during a period of 25 weeks from early April to early October 2006. Temporal variations spanning 0.4‰ for δ 29Si and 0.8‰ for δ 30Si of dissolved Si in the Kalix River were observed during the period, suggesting that the riverine dissolved Si input to the oceans cannot be considered to have a constant Si isotopic composition on a short time scale. The results implicate biogeochemical Si-cycling via formation and dissolution of biogenic silica as processes significantly affecting the dissolved Si transport in boreal systems during April to early October. The Si budget in the river system appeared to be controlled by relative Si enrichment during high discharge events and relative Si depletions in the subarctic mountainous and lake dominated areas. The Si enrichments and depletions were accompanied by decreasing and increasing riverine δ 29Si and δ 30Si, respectively. These isotope variations can be explained by release of plant derived silica, depleted in heavier Si isotopes, during the spring snowmelt. Further, increased volumetric contribution from the headwater and losses of dissolved Si due to biogenic silica formation by diatoms in the subarctic lakes at a later period are expected to be responsible for the preferential losses of lighter isotopes, as further verified by land cover analysis.

Anthropogenic perturbations of the silicon cycle at the global scale: Key role of the land‐ocean transition

Silicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO2). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO2. The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO2 in the coastal zone are to be expected over the course of this century.

Factors that Control the Range and Variability of Amorphous Silica in Soils in the Hubbard Brook Experimental Forest

In terrestrial ecosystems, the largest pool of amorphous silica (ASi) is stored in soils and is an important reservoir of biologically active Si for the global biogeochemical cycling of Si. Only limited data are available that quantify the size of this reservoir and often these estimates are made from the physical separation of silt‐sized phytoliths, which can underestimate the ASi pool. Soil samples from five watersheds in a temperate‐zone continental ecosystem at the Hubbard Brook Experimental Forest, New Hampshire, were analyzed for ASi using alkaline digestion. Soils from two of the watersheds were analyzed after experimental forest removal. In undisturbed watersheds, ASi was concentrated at the surface of the soil profile, similar to organic matter, and then progressively decreased with depth. This investigation supports our hypothesis that forest disturbance leads to redistribution of Si in the soil. In fact, although deforestation led to significant decreases in ASi in the upper soil horizons, total profile ASi (∼17,400 kg SiO2 ha−1) remained essentially unchanged, implying translocation downward. Significant increases in the transport of dissolved silica (DSi) by rivers have been observed with deforestation, however, in which the ASi pool in soils may play an important role. Additional studies should target the potential role of ASi as a buffer for DSi losses from deforested watersheds.

Silicon isotope fractionation in rice plants, an experimental study on rice growth under hydroponic conditions

Silicon (Si) isotope composition and Si distribution among different rice plant organs and different parts of rice leaf at maturity were studied, which may provide new insights into the mechanism of Si accumulation in plants and biogeochemical Si cycle. An isotope ratio mass spectrometer (IRMS) was used to examine Si isotope fractionation by rice plant grown in a hydroponic system. The observed 30Si-depletion (about 0.3‰) of whole plant relative to external nutrient solutions suggested biologically mediated Si isotope fractionation occurred during uptake. However, it was not possible to judge the Si uptake mechanism with the data. For δ30Si variation within plant, there was a consistent increasing trend from lower to upper tissues (stem < leaf < husk < grain; leaf sheath < leaf blade base <leaf blade middle < leaf blade top). The phenomenon, reflecting kinetic isotope effects, could be explained that isotope fractionation during Si deposition in rice plant was a Rayleigh-like behavior. The range (−2.7‰ to 2.3‰) of δ30Si variation among rice plant tissues in present experiment exceeded that (−1.7‰ to 2.5‰) of phytoliths observed previously in continents, which would enhance understanding the role of phytoliths on globe Si isotope balance.

Past, present and future state of the biogeochemical Si cycle in the Baltic Sea

The Baltic Sea is one of many aquatic ecosystems that show long-term declines in dissolved silicate (DSi) concentrations due to anthropogenic alteration of the biogeochemical Si cycle. Reductions in DSi in aquatic ecosystems have been coupled to hydrological regulation reducing inputs, but also with eutrophication, although the relative significance of both processes remains unknown for the observed reductions in DSi concentrations. Here we combine present and historical data on water column DSi concentrations, together with estimates of present river DSi loads to the Baltic, the load prior to damming together with estimates of the long-term accumulation of BSi in sediments. In addition, a model has been used to evaluate the past, present and future state of the biogeochemical Si cycle in the Baltic Sea. The present day DSi load to the Baltic Sea is 855 ktons y − 1 . Hydrological regulation and eutrophication of inland waters can account for a reduction of 420 ktons y − 1 less riverine DSi entering the Baltic Sea today. Using published data on basin-wide accumulation rates we estimate that 1074 ktons y − 1 of biogenic silica (BSi) is accumulating in the sediments, which is 36% higher than earlier estimates from the literature (791 ktons y − 1 ). The difference is largely due to the high reported sedimentation rates in the Bothnian Sea and the Bothnian Bay. Using river DSi loads and estimated BSi accumulation, our model was not able to estimate water column DSi concentrations as burial estimates exceeded DSi inputs. The model was then used to estimate the BSi burial from measured DSi concentrations and DSi load. The model estimate for the total burial of BSi in all three basins was 620 ktons y − 1 , 74% less than estimated from sedimentation rates and sediment BSi concentrations. The model predicted 20% less BSi accumulation in the Baltic Proper and 10% less in the Bothnian Bay than estimated, but with significantly less BSi accumulation in the Bothnian Sea by a factor of 3. The model suggests there is an overestimation of basin-wide sedimentation rates in the Bothnian Bay and the Bothnian Sea. In the Baltic Proper, modelling shows that historical DSi concentrations were 2.6 times higher at the turn of the last century (ca. 1900) than at present. Although the DSi decrease has leveled out and at present there are only restricted areas of the Baltic Sea with limiting DSi concentrations, further declines in DSi concentrations will lead to widespread DSi limitation of diatoms with severe implications for the food web.

Modelling the biogeochemical cycle of silicon in soils: Application to a temperate forest ecosystem

We investigated the biogeochemical cycling of silicon (Si) in an acidic brown soil covered by a coniferous forest (Douglas fir). Based on published and original data, we constructed a conceptual model and used a modified version of the reactive transport code MIN3P for model testing and quantification purposes. The model was first calibrated and further validated with respect to biomass data and Si-concentrations in capillary solutions, which were collected monthly over several years by means of suction-cup lysimeters placed at different soil depths. Following sensitivity tests, the model was calibrated quite accurately (limited to a 10% concentration error) by the adjustment of kinetic constants, longitudinal dispersion, and apparent activation energy for K-feldspar dissolution. Calibrated parameter values were constrained by ranges reported in the literature, when available. Mass balance calculations indicate that an average of 60% of the biogeochemical cycle of Si was controlled by biological processes (i.e. Si-uptake and dissolution of phytoliths). Sensitivity analyses suggest that no more than 55% of the Si-cycle is controlled by weathering of primary silicates. Such a large contribution of biological turnover to Si-cycling may be explained by the combined effects of a relatively large Si-content in the litter fall (i.e. specifically in the needles) and high biomass productivity of the coniferous species considered. In addition to potential implications for the global Si cycle, this investigation raises several fundamental questions concerning the nature of Si-uptake mechanisms and physiological use of Si by trees in natural systems.

Biologic cycling of silica across a grassland bioclimosequence

The dynamics of biologic Si cycling in grassland ecosystems are largely unknown and likely to impact mineral weathering rates regionally and diatom productivity globally; key regulatory processes in the global Si cycle are closely tied to the global carbon cycle. Across a bioclimatic sequence spanning major grassland ecosystems in the Great Plains, soil biogenic silica depth distributions are similar to that of soil organic carbon; however, unlike soil organic carbon, quantities of soil biogenic silica decrease with increasing precipitation, despite an increase in annual biogenic inputs through litterfall across the same gradient. Though comprising only 1–3% of the total Si pool, faster turnover of biogenic Si and annual cycling by grasses should positively impact mineral dissolution. Our results suggest that the largest reservoir of biogenic Si in terrestrial ecosystems resides in soils, and emphasize the potential significance of grasslands in the global biogeochemical cycle of Si.

Biogenic silica in tidal freshwater marsh sediments and vegetation (Schelde estuary, Belgium)

To date, estuarine ecosystem research has mostly neglected silica cycling in freshwater intertidal marshes. However, tidal marshes can store large amounts of biogenic silica (BSi) in vegetation and sediment. BSi content of the typical freshwater marsh plants Phragmites australis, Impatiens glandulifera, Urtica dioica, Epilobium hirsutum and Salix sp. was analysed year-round. All herbaceous species accumulated silica in their tissue during their life cycle. Of the live plants, P. aus- tralis contained the most BSi (accumulating from 6 to 55 mg g -1 ). Dead shoots of P. australis had the highest BSi content (up to 72.2 mg g -1 ). U. dioica (<11.1 mg g -1 ), I. glandulifera (<1.1 mg g -1 ), E. hirsutum (<1.2 mg g -1 ) and Salix sp. (<1.9 mg g -1 ) had a much lower BSi content. Except for P. aus- tralis rhizomes (<15 mg g -1 ) underground biomass contained low amounts of BSi (<6 mg g -1 ). Sediment BSi content decreased from the surface (9 to 10 mg g -1 ) to deeper layers (5 to 7 mg g -1 ). There was seasonal variation in sediment BSi. Dissolved Si in porewater was highest in summer (ca. 600 µM) and lowest in winter (ca. 400 µM). P. australis vegetation (aboveground and roots) con- tained up to 126 g m -2 BSi, while the upper 30 cm of sediment accumulated up to 1500 g m -2 , making sediment the largest BSi reservoir in the marsh. We conclude that P. australis wetlands could be an essential, but unrecognised, sink for BSi in the biogeochemical cycling of Si.

Phylogenetic Variation in the Silicon Composition of Plants

Silicon (Si) in plants provides structural support and improves tolerance to diseases, drought and metal toxicity. Shoot Si concentrations are generally considered to be greater in monocotyledonous than in non-monocot plant species. The phylogenetic variation in the shoot Si concentration of plants reported in the primary literature has been quantified. Studies were identified which reported Si concentrations in leaf or non-woody shoot tissues from at least two plant species growing in the same environment. Each study contained at least one species in common with another study. Meta-analysis of the data revealed that, in general, ferns, gymnosperms and angiosperms accumulated less Si in their shoots than non-vascular plant species and horsetails. Within angiosperms and ferns, differences in shoot Si concentration between species grouped by their higher-level phylogenetic position were identified. Within the angiosperms, species from the commelinoid monocot orders Poales and Arecales accumulated substantially more Si in their shoots than species from other monocot clades. A high shoot Si concentration is not a general feature of monocot species. Information on the phylogenetic variation in shoot Si concentration may provide useful palaeoecological and archaeological information, and inform studies of the biogeochemical cycling of Si and those of the molecular genetics of Si uptake and transport in plants.

Biodeposition by an Invasive Suspension Feeder Impacts the Biogeochemical Cycle of Si in a Coastal Ecosystem (Bay of Brest, France)

Biodeposition is a process that strongly affects physical, chemical and biological properties near the sediment–water interface, and mollusks have been shown to influence the cycling of many biogenic elements. In the Bay of Brest ecosystem, the invasive benthic suspension feeder Crepidula fornicata has been proliferating for 50 years, and its influence on phytoplankton dynamics at a seasonal scale, via its role on a coastal silicate pump, has been suggested by Chauvaud et al. (2000) [Chauvaud L., Jean F., Ragueneau O. and Thouzeau G. 2000. Mar. Ecol. Prog. Ser. 200: 35–48] and tested by Ragueneau et al. (2002a) [Ragueneau O., Chauvaud L., Leynaert A., Thouzeau G., Paulet Y.-M., Bonnet S., Lorrain A., Grall J., Corvaisier R., Le Hir M., Jean F. and Clavier J. (2002a). Limnol. Oceanogr. 47(6): 1849–1854]. In the present manuscript, we explore further, at an annual scale, the role of biodeposition in the Si cycle of the Bay of Brest. We show that annual biodeposition Si can represent up to 84% of river silicic acid inputs. We also show that preservation of biogenic silica in C. fornicata mats is very good (27%), leading to an annual retention of Si of 23% of river inputs. We suggest that this high retention is essentially controlled by the biodeposition mechanism, which is directly under the control of the proliferation of the exotic suspension feeder. We conclude that this mechanism might be very important in controlling the Si retention along the land–ocean continuum and should be tested in the many areas of the coastal ocean submitted to the proliferation of such exotic species that modify sedimentation of biogenic matter.

Freshwater marshes as dissolved silica recyclers in an estuarine environment (Schelde estuary, Belgium)

Compared to knowledge about N and P processing in the aquatic continuum of lakes, wetlands and estuaries, knowledge concerning transport and cycling of Si is only fragmentary. Furthermore, Si research in estuaries has mainly been focused on subtidal benthic sediments and uptake and recycling by diatom communities. The biogeochemical cycling of Si in tidal wetlands, which can contain large amounts of Si, has thus far been neglected. We have conducted several whole ecosystem Si mass-balances on a freshwater marsh located in the Schelde estuary (6 tidal cycles, 2 with BSi included). Our measurements show that the freshwater marsh acts as an important source of dissolved Si to the main river (1–18% more export than import, on average 0.114 g m−2). This export is compensated by import of amorphous silica into the marsh (19–55% more import than export). The marsh was shown to act as silica recycler, resupplying biologically available dissolved Si to the estuarine ecosystem. Extrapolations show that during summer and spring months, when dissolved silica is depleted due to diatom growth, almost half of the total dissolved silica load in the main river channel could result from marsh recycling.

Terrestrial ecosystems and the global biogeochemical silica cycle

Most research on the global Si cycle has focused nearly exclusively on weathering or the oceanic Si cycle and has not explored the complexity of the terrestrial biogeochemical cycle. The global biogeochemical Si cycle is of great interest because of its impact on global CO2 concentrations through the combined processes of weathering of silicate minerals and transfer of CO2 from the atmosphere to the lithosphere. A sizable pool of Si is contained as accumulations of amorphous silica, or biogenic silica (BSi), in living tissues of growing plants, known as phytoliths, and, after decomposition of organic material, as remains in the soil. The annual fixation of phytolith silica ranges from 60–200 Tmol yr−1 and rivals that fixed in the oceanic biogeochemical cycle (240 Tmol yr−1). Internal recycling of the phytolith pool is intense with riverine fluxes of dissolved silicate to the oceans buffered by the terrestrial biogeochemical Si cycle, challenging the ability of weathering models to predict rates of weathering and consequently, changes in global climate. Consideration must be given to the influence of the terrestrial BSi pool on variations in the global biogeochemical Si cycle over geologic time and the influence man has had on modifying both the terrestrial and aquatic biogeochemical cycles.

Distribution and dynamics of gibbsite and kaolinite in an oxisol of Serra do Mar, southeastern Brazil

In the Serra do Mar region, in southeastern Brazil, the soil mantle is mainly characterised by (i) a gibbsitic saprolite, (ii) various kaolinitic horizons within the gibbsitic material, (iii) kaolinito-gibbsitic topsoil horizons. This organisation does not match with the thermodynamic stability of gibbsite and kaolinite accompanying the solution percolation through soil profiles. A study of the micromorphological, mineralogical and chemical properties of the soil mantle reveals that this organisation arises from the in situ development of the soil from the crystalline bedrock. The bauxitic weathering of the bedrock, even if it is rich in quartz, can be explained by a fast renewal of the solutions and/or a high solubility of the kaolinite. Recycling of Si and Al by the forest can maintain a dynamic equilibrium of kaolinite in the topsoil horizons, as observed in Amazonia. The kaolinitic compact horizons evolve upslope at the expense of the gibbsitic material. At the contact between kaolinitic and gibbsitic material, dissolution patterns of quartz and gibbsite are observed, indicating that this evolution is in process. These observations and the organisation of the soil mantle set the problem of the apparent stability of gibbsite and kaolinite in this environment. Various assumptions that could explain this organisation of the soil mantle are discussed. Changes in the activity of water due to the pore size diminution and displacement of the gibbsite–kaolinite equilibrium appear insufficient to explain the stability of kaolinite. However, it could be allotted to the slow down of water flows in the soil mantel. Lastly, the eventual role of the complexing organic matter is presented. More investigations on the biogeochemical cycle of Si and Al and on the physico-chemical processes at the soil solution–mineral interface are necessary to explain the stability and dynamics of gibbsite and kaolinite in this environment.

Naturally Weathered Plagioclase Grains from the Idaho Batholith: Observations Using Scanning Electron Microscopy

Rates of primary silicate mineral weathering are important to understanding proton buffering and biogeochemical cycling of Si, Al, alkali metals, and alkaline earth metals in natural watersheds. The objectives of this study were to characterize the surface morphology and secondary coatings of naturally weathered oligoclase grains sampled from the Silver Creek Experimental Watersheds in the southwestern Idaho batholith. Oligoclase grains (0.5–1.0 mm) were sampled from soil, sediment, and rocks of various weathering classes within the watershed, and were characterized using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDXRA). Oligoclase grains from unweathered bedrock (Class 1 rock) and very weakly weathered rock (Class 2) showed little evidence of weathering features such as etch pits and secondary coatings on the mineral surface. As the weathering intensity increased to Class 5, Class 6, R horizon, and C horizon oligoclase samples, the presence of etch pits and secondary coatings increased. Grains from Class 6 rock and the R horizon were characterized by numerous etch pits and secondary coatings, which covered the majority of the grain surface. The EDXRA analyses showed that the secondary coatings had Al/Si ratios approximately two times and Ca/Si ratios approximately 0.2 times that of Class 1 and Class 2 rock, indicating significant Al enrichment and Ca depletion in the weathering products. X-ray diffraction was used to positively identify a kaolin layer silicate (presumably kaolinite) as a major component of the secondary phases found on the feldsdpar surface. Observations of naturally weathered feldspar grains suggest significant differences in weathering environment or intensity between many laboratory studies and actual watersheds. Environmental conditions that may be responsible for differences in weathering intensities between natural watersheds and laboratory studies include physical factors such as solid/solution ratios, water residence times, wetting and drying cycles, and chemical factors such as the presence of secondary coatings on the mineral surface and the reactive surface area of weathered grains. Contribution no. J-2823 from the Montana Agric. Exp. Stn.