Amorphous Silica Preservation in an Anthropogenic Soil: An

Chapter 1

Downloaded by 80.82.77.83 on January 14, 2018 | http://pubs.acs.org Publication Date (Web): December 10, 2013 | doi: 10.1021/bk-2013-1154.ch001

Amorphous Silica Preservation in an Anthropogenic Soil: An Explorative Study of “Plaggen” Soils Wim Clymans,*,1,† Toon Verbeeck,1 Sander Tielens,1 Eric Struyf,2 Floor Vandevenne,2 and Gerard Govers1 1Department

of Earth and Environmental Sciences, KU Leuven, Heverlee, B-3001, Belgium 2Department of Biology - Ecosystem Management Research Group, University Antwerp, Wilrijk, B-2610, Belgium *E-mail: [email protected] †Present affiliation: Department of Geology, Lund University, Lund, SE-22362, Sweden

Amorphous Silica (ASi) is present in considerable amounts in most soils and serves as a (micro-)nutrient for many plants. However, our understanding of the response of this important nutrient pool to human or natural disturbances is still very limited. One of the reasons for this is the long time scales involved. This explorative study focuses on the effect of a historical agricultural system, called plaggen management, that was applied on sandy areas in Belgium, the Netherlands and Germany over a period of ca. 1000 yrs on ASi dynamics. The system was designed to maintain high nutrient levels (including C and Si) on arable fields through the addition of mixtures rich in animal manure and vegetation residues. The continuous addition of ASi over such a long time period allows to study if and to what extent ASi is preserved in such a soil system and how Si addition affects the build-up and availability of ASi pools. We quantified ASi pools (Na2CO3 extraction) in a soil profile with plaggen application, and a reference soil without plaggen application. Other measured soil properties were soil organic carbon (SOC) and grain size distribution. There was an important SOC (+20%) and ASi accumulation (+70%) and

© 2013 American Chemical Society Clarson et al.; Progress in Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


preservation at the plaggen site. Si losses from the soil column through leaching and crop harvest might thus be restricted by application of organic residue and Si fertilisation to maintain sustainable nutrient concentrations in the topsoil. Net C and Si accumulation rates were 45 kg C ha-1 y-1 and 77 kg SiO2 ha-1 y-1 respectively, not accounting for the removal of ASi through plant uptake nor for the mineralisation of part of the SOC. The vertical distribution of ASi within the profile, suggests that, contrary to SOC, most of the added ASi has remained stably stored in the soils and that only a smaller, labile pool was removed, most likely through dissolution rather than through plant uptake. Our results indicate that ASi addition leads to a build-up of Si pools in these sandy soils. While this results in increased Si availability, this effect is limited because most ASi remains stored for long time spans. To consolidate our preliminary research results, to answer unresolved questions and to validate proposed hypotheses, future research should: (1) collect additional profiles with higher vertical resolution; (2) include other Si fractions; (3) analyse the relationship between ASi and SOC to better understand the coupling of the ASi and C cycles; (4) develop a modelling approach that would allow one to investigate how ASi pools in soils may respond to future changes.

Introduction Large amounts of Si are stored in terrestrial soils in a more soluble amorphous form than mineral Si. The amorphous Si (ASi) consists mainly of biogenic silica (BSi), primarily in the form of plant siliceous bodies called phytoliths (1), but also as different pedogenic Si forms (2). The uptake of dissolved silica (DSi) in the vegetation and dissolution of ASi in soils has been shown to be an important controlling factor for the DSi export to rivers from catchments dominated by boreal wetlands (3), forests (4) and grasslands (5). This implies that riverine Si fluxes will be affected by biological and pedological processes (6). Quantitative mass-balance studies are necessary to improve our estimates of ASi storage and cycling within terrestrial ecosystems to accurately estimate its effect on Si delivery (7). The few studies performed (1) lack sufficient spatial distribution to integrate in global Si models (8), (2) focus on natural ecosystems, dominantly forests (e.g. (4, 9, 10)) and some grassland (e.g. (5)) or wetland studies (e.g. (11)) and (3) rarely account for the total ASi pool, including both biogenic and pedogenic Si forms (e.g. (12, 13)). Natural ecosystem studies contributed significantly to the contemporary knowledge of terrestrial Si cycling (14). One can however not neglect the effect of human perturbations on Si-mobilisation and Si-storage. Historically land management has altered Si- mobilisation (10, 15, 16), and Si-storage (13, 17). Research interests have therefore shifted to disturbed landscapes (e.g. arable lands 4 Clarson et al.; Progress in Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


and pastures), and processes specific to human land use types (18, 19), like ASi depletion or conversion towards more stable Si forms (20), direct mobilisation of accumulated ASi through soil erosion (21) and the net loss due to harvest into the so-called agricultural loop (22–24). A robust estimate for temperate regions showed that agricultural expansion led to a 10% decrease in the soil ASi pool, and potentially a 20% increase in annual Si delivery towards aquatic systems (13). Acknowledging the uncertainty on this estimate, this finding corroborated the hypothesis that biotic processes rather than abiotic processes control Si dynamics on biological timescales (7). In cultivated areas the harvest of Si accumulating crops (and grasses) leads to a depletion of the plant-available Si stock as large parts of plant Si are not returned (13, 23). Only a part of the harvested crops will re-enter the cropland through direct re-application of biomass or organic fertilisation, i.e. manure (24). As a consequence of reduced Si availability, Si-concentrations in plants may be reduced making them more vulnerable for biotic and abiotic stresses (25), potentially reducing yields (26). In theory, anthropogenic Si addition could lead to a build-up of a bio-available Si pool, as shown for re- established natural vegetation (23). It is however unclear whether original levels can be re-established on relevant timescales, and whether the added Si is available to plants. Although positive effects on crop yields after Si fertilisation have previously been observed (27), studies looking at the effect of Si addition to agricultural systems on ASi pools received limited attention. This study focuses on the effect on soil ASi stocks of a historically applied agricultural system, called plaggen management. In the sandy areas of Western Europe plaggen management was introduced (since 10th century) to improve soil fertility by adding a mixture of manure and organic top layer of neighbouring heathland that was used for grazing. It thus implicitly included Si fertilisation. Nowadays plaggen management has completely been replaced by mineral fertilisation, and arable lands as well as the former heathlands are partly converted to pasture or forest and vice versa. Here, we present a first exploration of the Si dynamics within anthropogenic soils that were formed during periods of plaggen management as they might offer an opportunity to investigate (1) how Si addition built up amorphous silica pools, (2) how present land-use practices could influence these increased amorphous silica pools, and more generally (3) add to our understanding of the controls ASi soil storage, specifically under human disturbed land. Therefore, we quantified the ASi distribution and pools in one anthropogenic soil, i.e. with plaggen application, and one reference soil, without plaggen application. The current land use on both study sites was arable land.

Background The cultural landscape on sandy soils in Western Europe at 1000CE typically consisted of arable fields in the vicinity of the settlements (i.e. infields), and the extensively grazed heathlands at a distance (i.e. outfields) (Fig. 1). These heathlands developed from about 4000 years ago, as a result of forest clearances followed by use of the land for grazing cattle, cutting turf, burning and cutting 5 Clarson et al.; Progress in Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


vegetation for fuel and fodder, activities which all prevented the regeneration of the forest (28). Low intrinsic soil fertility characterised the sandy soils. Persistent agricultural production on arable infields further degraded the nutrient balance of the sandy soils, and therefore their fertility. To sustain sufficient food supply for the growing population, a new agricultural management practice was introduced around 1000 BP: plaggen management.

Figure 1. Representation of the land management system; a) during plaggen management (±1350-1900) and b) present situation (2010). Values indicate measured means (± standard errors) for total amorphous silica pools (PSia, kg SiO2 ha-1) in the soils. Plaggen management for soil improvement was first introduced by Frisian farmers from about 1000 AD (29), and was common practice until the introduction of mineral fertilisers at the beginning of the 20th century (30). Since the Middle Ages, plaggen management replaced shifting cultivation in large parts of Germany, the Netherlands and NE Belgium (31, 32). Except for herding sheep, the heathlands were used to dig up the fertile topsoil: such a topsoil slab was called a plag (plur: plaggen). Afterwards these plaggen were brought into the stables over winter where they became enriched with nutrients as they were mixed 6 Clarson et al.; Progress in Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


with cattle dung (referred to as manure later on). The plaggen material consisted mostly of heather, although grass plaggen, forest litter or peat sods were also used. For centuries these plaggen were spread over croplands and pastures to improve the fertility of the sandy soil in western Europe, causing the development of a very specific soil at the infields: the Plaggic Anthrosol (33). This technique created a flow of nutrients from the heathlands to the arable land thereby making arable production sustainable. A Plaggic Anthrosol has a typical, thick (>0.2m) A-horizon, anthropogenically sustained and enriched in organic carbon (>0.6% SOC). Other typical characteristics are its (loamy) sandy texture, dark grey to black colour and low base saturation (BS

Amorphous Silica Preservation in an Anthropogenic Soil: An

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