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Project
Transport of dissolved Silicon in upslope catchments with different land uses.
Dissolved Si (DSi) is an important nutrient in aquatic systems. River discharge delivers DSi to the oceans. Riverine DSi fluxes are affected by land use changes as DSi export from forests are higher than from croplands. Hypothesis were suggested to explain the differences in DSi export but the causes were poorly analyzed.
Previous studies mentioned the higher amount of biogenic Si (BSi) in forest soils as the main reason for higher DSi fluxes from forests, since BSi is an important source of DSi in soils. This hypothesis was confirmed with a leaching test. Our experiment proved BSi in soils reacted kinetically in contrast to other soil minerals. In intensive agriculture, no kinetic reaction occurred: the small amount of BSi (2-3 mg Si g-1 soil) became inert.
We made a conceptual model based on a literature review which suggested that other parameters would also influence DSi export. The leaching test showed Si release was high at pH <4 and varied with water flux rates. Dissolution processes were enhanced in the acidforest soils compared to neutral cropland soils. An increase in water flux with a factor 2.5 resulted in an increase in Si flux with a factor close to 2.5 in cropland, where reactions were (near) equilibrium. The impact of water flux variation was less important in the forest (factor 1.36 - 2.10). Chemical analysis of soil water sampled in situ proved that the biogeochemical Si cycle is controlled by different processes along a land use gradient. In forests, dissolved Si can complex with Al and clays (illite, smectite) dissolution/precipitation likely affects with the Si cycle. In arable land, hydrological conditions clearlycontrol Si concentrations as they are inversely correlated with soil water content.
To quantify DSi fluxes through soils, wesimulated the kinetic reaction of BSi in soils and evaluate the effect of recent deforestation on DSi fluxes. Therefore, we set up a numerical model which coupled transport and geochemical calculations in the software HP1. Due to the increase in water flux after deforestation, the amounts of reacted BSi decreased but the total Si flux exported from the soilwas higher. Our model also proved that clay stability was influenced bythe chemistry of the infiltrating rain: cation-rich throughfall solution enhanced clay stability compared to rain water.
We also showed that Si reprecipitation could affect significantly Si export from a catchment. In one of the studied catchments, we noticed a clear decrease in DSi concentrations from groundwater to the river. Precipitation ofopal-CT was also observed in a coring. Our calculations showed that currently in this catchment DSi flux was 25% lower than expected, due to the Si reprecipitation.
Finally we compared DSi fluxes through the soil from all land uses. Deforestation could increase the DSi fluxof 66% compared to forest. The development of agriculture decreased DSiexport of 33% in the grassland and 44% in the cropland compared to the forest.
Our study proved that the export of DSi is controlled by different processes according to the land use type and the regional geochemical conditions (precipitation of opal-CT). In forests, DSi export from soil is the high as important BSi amounts are present andthe soils are acid. After deforestation, the increase in water flux increases the DSi flux exported from the soil. When intensive agriculture develops, soils become neutral, BSi amounts diminish and become less reactive, water infiltration diminishes. All these processes together cause a decrease in DSi export along the land use gradient.
Previous studies mentioned the higher amount of biogenic Si (BSi) in forest soils as the main reason for higher DSi fluxes from forests, since BSi is an important source of DSi in soils. This hypothesis was confirmed with a leaching test. Our experiment proved BSi in soils reacted kinetically in contrast to other soil minerals. In intensive agriculture, no kinetic reaction occurred: the small amount of BSi (2-3 mg Si g-1 soil) became inert.
We made a conceptual model based on a literature review which suggested that other parameters would also influence DSi export. The leaching test showed Si release was high at pH <4 and varied with water flux rates. Dissolution processes were enhanced in the acidforest soils compared to neutral cropland soils. An increase in water flux with a factor 2.5 resulted in an increase in Si flux with a factor close to 2.5 in cropland, where reactions were (near) equilibrium. The impact of water flux variation was less important in the forest (factor 1.36 - 2.10). Chemical analysis of soil water sampled in situ proved that the biogeochemical Si cycle is controlled by different processes along a land use gradient. In forests, dissolved Si can complex with Al and clays (illite, smectite) dissolution/precipitation likely affects with the Si cycle. In arable land, hydrological conditions clearlycontrol Si concentrations as they are inversely correlated with soil water content.
To quantify DSi fluxes through soils, wesimulated the kinetic reaction of BSi in soils and evaluate the effect of recent deforestation on DSi fluxes. Therefore, we set up a numerical model which coupled transport and geochemical calculations in the software HP1. Due to the increase in water flux after deforestation, the amounts of reacted BSi decreased but the total Si flux exported from the soilwas higher. Our model also proved that clay stability was influenced bythe chemistry of the infiltrating rain: cation-rich throughfall solution enhanced clay stability compared to rain water.
We also showed that Si reprecipitation could affect significantly Si export from a catchment. In one of the studied catchments, we noticed a clear decrease in DSi concentrations from groundwater to the river. Precipitation ofopal-CT was also observed in a coring. Our calculations showed that currently in this catchment DSi flux was 25% lower than expected, due to the Si reprecipitation.
Finally we compared DSi fluxes through the soil from all land uses. Deforestation could increase the DSi fluxof 66% compared to forest. The development of agriculture decreased DSiexport of 33% in the grassland and 44% in the cropland compared to the forest.
Our study proved that the export of DSi is controlled by different processes according to the land use type and the regional geochemical conditions (precipitation of opal-CT). In forests, DSi export from soil is the high as important BSi amounts are present andthe soils are acid. After deforestation, the increase in water flux increases the DSi flux exported from the soil. When intensive agriculture develops, soils become neutral, BSi amounts diminish and become less reactive, water infiltration diminishes. All these processes together cause a decrease in DSi export along the land use gradient.
Date:1 Oct 2009 → 10 Dec 2014
Keywords:Dissolved silicon, Biogenic, Mineral, Transport, Land use, Upslope cathment
Disciplines:Geology
Project type:PhD project