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Effect of land use on silica transport
Smis, A. (2009). Effect of land use on silica transport. MSc Thesis. Universiteit Gent. Faculteit Bio-ingenieurswetenschappen: Gent. 179 pp.

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Documenttype: Doctoraat/Thesis/Eindwerk

    Chemical compounds > Silicon compounds > Silica
    Climate change
    Concentration (composition)
    Cycles > Chemical cycles > Geochemical cycle > Biogeochemical cycle
    Dissolved chemicals
    Erosion > Soil erosion
    Land use
    België, Schelde R. [Marine Regions]

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    Silicon (Si) is the second-most abundant element in the earth’s crust and constitutes a base component of quasi each soil type. It is an essential nutrient for the growth of diatoms, but not for other phytoplankton groups. Diatoms are estimated to constitute ~40% of the global oceanic primary production and as such constitute an important part of the global oceanic carbon sequestration and are the main energetic source for estuarine and coastal fish production. The losses of Si to the ocean floor in oceanic diatom communities (~3% of the settling diatom Si flux) need to be replenished by the terrestrial input of Si, which is, in temperate regions, dominated by the riverine Si flux. The relative availability of Si decreased drastically through anthropogenically increased riverine loads of nitrogen (N) and phosphorous (P). The decrease in the relative availability of Si may have resulted in (i) extensive eutrophication problems of lacustrine, estuarine and coastal ecosystems and (ii) a decrease in the CO2 storage in oceanic systems. Weathering of mineral silicates constitutes the basic source of riverine Si and is traditionally assumed to determine the terrestrial Si flux entirely. The absolute availability of Si has generally been expected to have remained constant over the past centuries. It is in this study hypothesized that the absolute availability is not constant, but affected, also in an anthropogenic way, by land use.

    Through influence on the weathering process and through uptake of dissolved Si (DSi) and subsequent accumulation as amorphous Si structures (ASi), plants may strongly regulate the riverine export of DSi from continental to oceanic systems. Recently, a first global estimate of the terrestrial Si uptake showed the possible importance of biological control on the terrestrial DSi flux. Moreover, plant ASi, which is by large the most reactive compound of particulate Si, was hypothesized to constitute an important part of the riverine Si flux. Significant differences in terrestrial Si cycling were observed between different ecosystems. Changes in land use were therefore hypothesized to have influenced and still influence the terrestrial efflux of both DSi and ASi to estuarine and coastal ecosystems. A quantification of such relation or effect was up till now not provided.

    Both at base and peak flow, we observed terrestrial ecosystems as important regulators of the bioreactive Si flux (i.e. ASi + DSi) through the river continuum towards coastal systems. At peak flow, extensive mobilization of ASi was observed in highly cultivated headwater catchments, while DSi transport at base flow varied along a gradient of forest, grassland and cropland dominated catchments.

    The release of DSi at base flow was quantified with respect to land use on a river basin scale in one single region with comparable climatic conditions (Scheldt basin, Northern Belgium). The fifty-two studied subbasins were spread over two distinct soil regions, but within both regions, observed riverine DSi fluxes were higher out of the more forested catchments compared with the highly cultivated catchments. Moreover, the old deciduous forests in the south of Flanders were observed to export more DSi compared with the younger coniferous forests in the north-east of Flanders. We hypothesize that these differences resulted from the presence of an extensive ASi pool in forest soils which was absent under cropland. This hypothesis was substantiated by the observed differences in (i) the seasonal fluctuation of river DSi concentrations at base flow and (ii) the hysteresis pattern in DSi concentration at peak flow between highly cultivated and more forested catchments. Deforestation, which was recently shown to initially enhance the export of DSi, may therefore result in a decreased riverine DSi flux on a long-term.

    The mobilization of ASi was quantified at system scale in eight highly cultivated and erosion-sensitive headwater catchments as a first step towards the understanding of the importance of ASi transport in silica budgets. On a yearly basis, between 11% and 18% of all bio-reactive Si was estimated to be transported as ASi, and 60% to 80% of all ASi was transported during high discharge events, in contrast with 20% to 40% for DSi. The transport of ASi was highly correlated with discharge, transport of suspended particulate matter (SPM), and especially the organic carbon content of the SPM. As the presence of diatoms was assumed negligible in the studied headwater systems, the mobilization of ASi is hypothesized to be strongly linked to soil erosion processes. Phytoliths and mineral ASi phases, present as a result of the high cultivation intensity of the soils, may therefore contribute significantly to the total bio-reactive Si load through the Scheldt river basin. Moreover, the extensive mobilization of ASi in highly cultivated catchments may partly explain the hypothesized decrease in DSi export after large-scale deforestation.

    Our hypothesis suggests that the historical cultivation of vast areas of forests in Flanders initially increased the riverine DSi flux, but has finally resulted in a decreased supply of DSi to the estuarine and coastal system. The increased ASi export from cultivated areas as a result of the enhanced surface erosion may have counteracted the decreased DSi flux. However, only a fraction of the mobilized ASi is expected to reach the river estuary as a result of its sedimentation in the river continuum. Present efforts to reduce the sediment loads from cultivated areas will probably further decrease the downstream ASi load. In contrast, future changes in climate are expected to stimulate the mobilization of ASi.

    The structural decrease in DSi transport towards coastal systems after long-term deforestation probably decreased the diatom production and oceanic carbon sequestration. The severe eutrophication events observed in the Scheldt river and the North Sea were probably partly explained by the lack of a strongly recyclable ASi pool in the cultivated soils of the draining river basins. In contrast, Si limitation in more recently cultivated areas might currently still be masked by enhanced DSi export fluxes from recyclable ASi pools. The effect of land use on estuarine and coastal eutrophication and oceanic carbon sequestration is therefore suggested to be reconsidered. Future ecological models will need to work out the Si cycle in more detail than the current lithology-dependent assumed DSi concentrations. Future global circulation models are suggested to assess our newly hypothesized effect of land use changes on the global climate, and DSi is suggested as a new parameter in the waterquality monitoring network of Flanders. Meanwhile, new studies may confirm and refine our hypotheses and can quantify the change in riverine DSi export, preferably expressed by annual transported Si loads, at different stages after cultivation of formerly Si accumulating natural ecosystems. On the other hand, future research may reveal whether the observed significant ASi mobilization at the headwater catchment scale may contribute to the earlier shown importance of ASi for the total riverine load of bio-reactive Si.

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