Struyf, E.

Open Marine Archive

Historical land use change has lowered terrestrial silica mobilization 
(to the North Sea and the Scheldt Estuary) 
Struyf Eric 
Dept Biology, University of Antwerp, Universiteitsplein 1C, B-2610 Wilrijk, Belgium 
E-mail: eric.struyf@ua.ac.be  
It has recently been shown that intense biogeochemical cycling of silica occurs in terrestrial soils. 
The annual ecosystem biogeochemical cycling of Si exceeds the annual export from the continents 
to the coastal zone by two orders of magnitude and effectively buffers weathering Si fluxes. Given 
the importance of the ecosystem biological Si filter, human impacts on vegetation cover through 
land use changes will significantly alter the biogeochemical silica cycle, although the impacts on the 
magnitude and fluxes of silica from land to coastal zones are presently unknown. Addressing this 
knowledge gap is essential as the terrestrial silica cycle is closely connected to i) important 
terrestrial and ocean carbon sinks and the ii) occurrence of harmful algal blooms in adjacent coastal 
ecosystems.  
We showed that land use is the most important controlling factor on baseflow Si mobilisation in the 
river basin of the Scheldt Estuary, with historical soil disturbance and sustained cultivation (> 250 
years) of formerly forested areas leading to two- to threefold decrease in baseflow delivery of Si 
from the land surface to the estuary. We propose a new conceptual model where initial 
deforestation leads to increased Si fluxes, with sustained soil disturbance depleting soil pools of 
amorphous silica, eventually leading to reduced Si fluxes.  The Scheldt basin has been one of the 
most densely populated areas in Europe already since the 13th century. As early as 1250, only 10% 
woodland cover remained; in a pristine state the Scheldt basin was almost fully forested (>90 %). In 
this conceptual model, the present Scheldt basin therefore represents a new equilibrium state.  
Despite limiting our study to base-flow conditions (60-90% of total yearly water fluxes) and the 
uncertainty associated with the timeframes in the new conceptual model, our data clearly indicate 
that long-term soil disturbance and a millennium of agricultural development has strongly changed 
biogeochemical Si dynamics in the Scheldt River basin (and likely other temperate river basins). 
Modern agricultural practices also result in increased input of N and P to the aquatic continuum, 
resulting in Si limitation in the Scheldt Estuary, with negative effects on ecosystem quality including 
the development of harmful algal blooms. Our new hypothesis suggests this DSi limitation in 
aquatic systems could be counteracted initially by increased Si export from cultivation of former 
forest soils. As the soil biogenic Si pool gradually declines, this counter-effect diminishes and Si 
mobilization becomes lower compared to pristine conditions. This implies that Si depletion events 
in more recently cultivated areas could currently still be masked by increased DSi export fluxes 
from recyclable biogenic Si pools. In the Scheldt watershed, lowered Si fluxes were already attained 
long before intense fertilization started, and no such masking was observed.  
Our observations emphasize the need to increase our understanding of land use impacts on 
biogeochemical Si cycling. A millennium of soil disturbance after deforestation lead to 2-fold to 
even 3-fold decreases in TSi flux from a watershed where the adjacent coastal zone has experienced 
significant eutrophication problems due to changed N:P:Si river deliveries in the last three decades. 
Our results are the first to show that locally factors controlling terrestrial Si mobilization can be 
separated from factors important at continental and global scales, where controlling factors largely 
include lithology, precipitation and slope. We clearly show that land use should be included in 
watershed scale models for baseline Si mobilization. Our results shed new light on how historical 
cultivation has affected the terrestrial silica cycle, and indicate yet another anthropogenic reduction 
of silica fluxes through the aquatic continuum, adding to globally important reductions in riverine Si 
transport by deposition in reservoirs and in eutrophicated rivers and estuarine sediments. 
The observation that long-term soil disturbance has lowered baseflow mobilization of Si from 
landscapes is novel and part of a complex puzzle linking Si, N, and P continental biogeochemical 
cycles to the ecological quality of coastal waters and C burial in the ocean. Transport of nutrients to 
the coastal zone and the ocean will be strongly dependent on processing within estuaries. The 
yearly average filtering capacity (the ratio of the net process rate to the total inputs) of the Scheldt 
Estuary is 0.12 for DSi, with strong seasonal fluctuations. In addition, tidal marshes will play an 
important role in estuarine Si fluxes, especially in summer. Ratios of processed nutrients in the 
estuary may also vary with discharge. Increasing discharge lowers diatom uptake of Si within the 
estuary, while increasing the net amount of TSi transported through the estuary. Yet, the average 
filtering capacity demonstrates that riverine inputs of Si are an important determining factor for 
eventual fluxes to the coastal zone, and a good understanding of the effect of land use in Si 
dynamics is therefore essential. 
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Historical land use change has lowered terrestrial silica mobilization (to the North Sea and the Scheldt Estuary)

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