Comptes Rendus

Hydrology, environment (Surface geochemistry)
Impact of agriculture on the Si biogeochemical cycle: Input from phytolith studies
Comptes Rendus. Géoscience, Volume 344 (2012) no. 11-12, pp. 739-746.


The cycles of C and Si are closely related to the weathering of silicates. In both cycles, terrestrial plants make significant contributions to the weathering budget and soil formation. The perturbation of the terrestrial Si cycle by human occupation has become a challenging issue because of possible impact on the equilibrium of aquatic ecosystems and agriculture sustainability. The recycling of Si stored in plants as phytoliths may control the Si cycle in the short-term. A review of the recent literature shows that the biogeochemical cycling of Si that is generally not impacted by atmospheric input or fertilisation, is significantly altered by agriculture through the depletion of the phytoliths pool. We present here new evidence that the exportation of straw can lead to the depletion of the soil phytoliths pool in ∼10 years. In order to maintain the current levels of Si in crops, the contribution of other soil silicates such as clay minerals to the phytoavailable silica pool may become a key parameter, when straw is exported.

Les cycles de C et de Si sont intimement liés à l’altération supergène des silicates. Dans les deux cycles, les plantes terrestres apportent des contributions majeures aux bilans d’altération et à la formation des sols. La question de la perturbation du cycle continental de Si par les activités humaines est posée avec des conséquences sur l’équilibre biologique des écosystèmes aquatiques et le maintien d’une agriculture durable. Le recyclage de Si sous forme de phytolithes dans les plantes pourrait contrôler le cycle à des échelles de temps courtes. Une revue de la littérature récente montre que le cycle biogéochimique de Si non impacté par des apports atmosphériques ou des amendements agricoles est modifié de manière significative par une baisse du stock de phytolithes des sols. Nous présentons de nouvelles données qui montrent que l’exportation des pailles peut conduire à une diminution des stocks de phytolithes sur une durée relativement courte, de l’ordre de la dizaine d’années. La contribution d’autres silicates comme les argiles au stock de Si biodisponible pourrait devenir un paramètre clef pour le maintien des teneurs actuelles en Si dans des cultures où les pailles sont exportées.

Published online:
DOI: 10.1016/j.crte.2012.10.004
Keywords: Silicon, Soil, Plant uptake, Phytoliths, Biogeochemical cycle, Weathering
Mot clés : Silicium, Sol, Absorption, Plante, Phytolithes, Cycle biogéochimique, Érosion chimique

Catherine Keller 1; Flore Guntzer 1; Doris Barboni 1; Jérôme Labreuche 2; Jean-Dominique Meunier 1

1 Aix-Marseille université, CNRS, IRD, UMR 7330, centre européen de recherche et d’enseignement en géosciences de l’environnement (CEREGE), Europôle Méditerranéen de l’Arbois, BP 80, 13545 Aix-en-Provence cedex 4, France
2 ARVALIS, Institut du végétal, station expérimentale, 91720 Boigneville, France
     author = {Catherine Keller and Flore Guntzer and Doris Barboni and J\'er\^ome Labreuche and Jean-Dominique Meunier},
     title = {Impact of agriculture on the {Si} biogeochemical cycle: {Input} from phytolith studies},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {739--746},
     publisher = {Elsevier},
     volume = {344},
     number = {11-12},
     year = {2012},
     doi = {10.1016/j.crte.2012.10.004},
     language = {en},
AU  - Catherine Keller
AU  - Flore Guntzer
AU  - Doris Barboni
AU  - Jérôme Labreuche
AU  - Jean-Dominique Meunier
TI  - Impact of agriculture on the Si biogeochemical cycle: Input from phytolith studies
JO  - Comptes Rendus. Géoscience
PY  - 2012
SP  - 739
EP  - 746
VL  - 344
IS  - 11-12
PB  - Elsevier
DO  - 10.1016/j.crte.2012.10.004
LA  - en
ID  - CRGEOS_2012__344_11-12_739_0
ER  - 
%0 Journal Article
%A Catherine Keller
%A Flore Guntzer
%A Doris Barboni
%A Jérôme Labreuche
%A Jean-Dominique Meunier
%T Impact of agriculture on the Si biogeochemical cycle: Input from phytolith studies
%J Comptes Rendus. Géoscience
%D 2012
%P 739-746
%V 344
%N 11-12
%I Elsevier
%R 10.1016/j.crte.2012.10.004
%G en
%F CRGEOS_2012__344_11-12_739_0
Catherine Keller; Flore Guntzer; Doris Barboni; Jérôme Labreuche; Jean-Dominique Meunier. Impact of agriculture on the Si biogeochemical cycle: Input from phytolith studies. Comptes Rendus. Géoscience, Volume 344 (2012) no. 11-12, pp. 739-746. doi : 10.1016/j.crte.2012.10.004. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2012.10.004/

Version originale du texte intégral

1 Introduction

Soils would be in a sustainable state if constituents were removed at the same rate as they are produced. Unfortunately, physical erosion rates of agricultural soils worldwide exceed the natural soil production rate, as well as the natural erosion rates under native vegetation (Montgomery, 2007; Quinton et al., 2010; Wilkinson, 2005). Physical erosion also affects the dynamics of the biogeochemical cycling by removing organic matter, and nutrients such as N, P, and K from the cropped areas (Quinton et al., 2010). However, the human impact on both physical and chemical weathering is not fully assessed, and models taking into account both aspects still need to be developed (Brantley, 2008).

Terrestrial plants play a major role in the evolution of landscapes and in global geochemical cycles (Amundson et al., 2007). Roots and associated microorganisms produce organic acids and chelates, which enhance the chemical weathering of soil minerals and the release of elements in soil solutions (Berner et al., 2005; Lucas, 2001). Enhanced chemical weathering provides pools of elements, which are taken up, stored and recycled at various rates by plants, and are ultimately released in streams at a rate up to 18 times the rate calculated for bare watershed (see review in Berner et al., 2005).

In agriculture, optimised yields are often combined with limited element recycling because of harvesting and subsequent total or partial export from the field. Although at a global scale N and P inputs by fertilizers are larger than exports by plants (Quinton et al., 2010), implying that these two elements do not experience soil depletion, little is known about the other elements taken up by plants that are not brought back to the soil through fertilization or plant recycling. This is particularly the case for Si (Clymans et al., 2011; Struyf and Conley, 2012), the most common element on Earth, which is found in large concentrations in many grass crops (Guntzer et al., 2011). The extent of agriculture impact on the biogeochemical cycle of Si is crucial for maintaining high crop yield (Ma and Takahashi, 2002), as well as for improving the prediction of the global change through modelling (Laruelle et al., 2009). Here we address the question of the perturbation of the phytoliths pool in soil, which has been recognized as a key parameter in the Si biogeochemical cycle in natural ecosystems since the pioneer work of Bartoli (1981, 1983).

2 Si weathering in natural ecosystems

The weathering of silicates, which cover 70% of the continents, is closely linked to the global cycle of carbon (Berner and Kothavala, 2001). In the oceans, dissolved Si mainly originates from the weathering of silicate minerals at the surface of the continents (Struyf et al., 2009). During the Precambrian, oceanic Si was controlled by tectonic, weathering and hydrothermal inputs (Robert and Chaussidon, 2006). Since the Phanerozoic, the oceanic silica levels have been dominated by organic reactions through silica-secreting organisms (Tréguer et al., 1995). Falkowski et al. (2004) showed that the diversification of grasses and diatoms coincided during the Paleogene most likely because Si mobilized by terrestrial plants provided an enhanced nutrient delivery to the oceans that was favorable to radiation of diatoms. On the continents, the role played by plants in the global cycle of Si is less well understood but it is striking that the annual fixation of Si in plants rivals that fixed in the oceanic biogeochemical cycle (Conley, 2002; Loucaides et al., 2010). In addition, Struyf and Conley (2012) recently calculated that the annual fixation of dissolved Si into terrestrial vegetation is 10 to 40 times more than the yearly export of dissolved and biogenic exports to the coastal zone.

Although silicon is not essential to plants, it is considered beneficial because it alleviates both biotic and abiotic stresses (Epstein, 1999; Guntzer et al., 2012). Plants take up Si from the soil solution as silicic acid, which, after translocation to the shoots, is polymerised through water evaporation and precipitated as phytoliths (Alexandre et al., 1997). Phytoliths are composed of opal A (XRD-amorphous phase) particles although some authors have suggested the presence of opal CT (Deelman, 1986). Contrary to many nutrients, Si cannot subsequently be redistributed within the plant, because the phytoliths remain insoluble until plant remains return to the soil and start decomposing. Consequently, Si is mostly located in shoots and concentrations vary between < 0.1 and 10% Si in shoot dry matter (Epstein, 1999).

In natural ecosystems, dissolved silicon (DSi) occurs mainly as Si(OH)4 originating from the dissolution of primary minerals. DSi may follow several paths according to the biogeochemical environmental conditions: adsorption on the surface of oxides, precipitation as amorphous silica or clay minerals, release to the streams by gravity or uptake by plants. Particulate Si can reach the soil through dust fallout or litterfall as phytoliths (see review in Cornelis et al., 2011a; Sommer et al., 2006; Street-Perrott and Barker, 2008). However, the dynamic of soil Si sinks is still not well understood (Struyf and Conley, 2012), and the mechanisms that control the Si cycle are probably time-dependent, with time lapses ranging from seconds (adsorption) to millennia (clay formation), with intermediate periods corresponding to the residence time of phytoliths.

Several studies have evaluated the fluxes of Si between plants, soil solutions and soil minerals mostly in natural forested ecosystems (Table 1a). They show that fluxes vary according to the age and the type of forests, and that plants absorb a significant fraction of dissolved Si, which originates from litterfall decomposition i.e. from phytoliths dissolution. This fraction is usually mostly recycled, while export to streams is limited. The amount of silicon released by weathering of soil minerals other than phytoliths is highly variable and depends on vegetation type.

Table 1

Flux de Si estimés ou calculés dans quelques écosystèmes naturels (a) et agrosystèmes (b).

Plant cycling Output References
kg Si ha−1 yr−1
(a) Natural ecosystems
 Tropical forest (young) 7 7–21 Meunier et al., 2010
 Tropical forest (mature) ca. 50 11–26 Lucas et al., 1993
 Tropical forest (mature) 58–76 16 Alexandre et al., 1997
 Temperate deciduous forest 22 ɛ Bartoli, 1983
 Temperate coniferous forest 5 26 Bartoli, 1983
 Temperate coniferous forest 36 5.0 Gérard et al., 2008
 Temperate coniferous forest 14.27 17.26 Markewitz and Richter, 1998
 Douglas fir 30.6 1.1 Cornelis et al., 2010
 Norway spruce 43.5 0.7 Cornelis et al., 2010
 Black pine 2.3 9.4 Cornelis et al., 2010
 European beech 23.3 6.0 Cornelis et al., 2010
 Oak 18.5 6.7 Cornelis et al., 2010
 Short natural grass (Arik) 22 0.5 Blecker et al., 2006
 Mixed natural glass (Smokey Valley) 56 0.3 Blecker et al., 2006
 Grasslands (Santa Cruz) 12–48 5–19 White et al., 2012
 Tall natural grass (Konza) 67 6.3 Blecker et al., 2006
 Savanna (Ivory Coast) 127 9 Alexandre et al., 2011
(b) Agrosystems
 Non grass crop 3–8 ASi+ DSi? Vandevenne et al., 2011
 Fodder grass 32–53 ASi+ DSi? Vandevenne et al., 2011
 Cereal crop (rice) 270 ASi+ DSi? Desplanques et al., 2006
 Cereal crop (rice) 500 ASi+ DSi? Makabe et al., 2009
 Cereal crop (wheat) 37–113 ASi+ DSi? Vandevenne et al., 2011
 Cereal crop (wheat) 94 ASi+ DSi? Guntzer et al., 2012 (estimates)
 Cereal crop (wheat) 20 ASi+ DSi? Guntzer et al., 2012 (Rothamsted data)

The quantification of the phytoliths pools is still not calibrated and two methods are currently used for the determination of the amorphous Si pools (ASi): (1) a non-destructive technique based on heavy liquid extraction (Alexandre et al., 1997; Kelly, 1990); (2) a destructive technique using alkaline solutions which allows for the quantification of the amorphous Si from silicates (DeMaster, 1981; Saccone et al., 2007). Both techniques are able to extract as well other biogenic silica particles, such as diatom frustules, and pedogenic silica from secondary minerals but only the physical extraction can verify whether the proportion of non-phytolith particles is negligible or not.

Few studies have investigated the Si cycle in natural grasslands (Table 1a): for a range of grasslands, Blecker et al. (2006) calculated that a large pool of Si was recycled through litterfall while output and weathering were limited. The impact of grasslands on Si recycling appears to be of the same order of magnitude as forests because the lower biomass is offset by larger Si concentration and turnover. Indeed, grasses are known to accumulate more Si in their shoots than do most species with concentrations above 1% Si in DM (Hodson et al., 2005; Ma and Takahashi, 2002). However, Blecker et al. (2006) also calculated that grasslands had a twofold increase on mineral dissolution over forests suggesting that Si biocycling may influence mineral weathering to a greater extent in grasslands as compared to forests.

The experimental studies performed by Fraysse et al. (2006, 2009, 2010) show that phytoliths are one of the most important soluble sources of Si as compared to other soil minerals despite their low concentration in soils. Indeed, among all soil minerals, phytoliths have the highest dissolution rate, except at pH < 3–4. The dissolution of 0.1 to 1% of the phytoliths pool may be enough to explain the values of dissolved Si found in surface waters (Fraysse et al., 2010). The high solubility of phytoliths can even control the DSi concentration in rivers as suggested for Hawaiian streams (Derry et al., 2005). However, tracing the proportion of biogenic versus lithogenic origin of riverine DSi in still not well constrained (Struyf and Conley, 2012).

3 Phytoliths in cultivated land

There is little data available on Si mass balance in agrosystems (Table 1b). Most information is related to the amount of Si taken up by plants, but data on both dissolved Si export and chemical weathering are missing. Except for non-grass crops (Table 1b), Si uptake by crops exceeds the uptake by forests and natural grass ecosystems. The five most cultivated crops worldwide, namely sugar cane, maize, rice, wheat, and barley belong to the grass (Poaceae) family (Guntzer et al., 2012). The other main difference between cultivated and natural ecosystems is that through harvesting the plant remains do not necessarily return to the soil, which increases the export part of the Si mass balance (Struyf et al., 2010; Vandevenne et al., 2011). Matichenkov and Bocharnikova (2001) have estimated that the amounts of Si taken up by crops and carried out by rivers are of the same magnitude (ca. 200 Mt Si yr−1). Assuming that phytoliths are the only source of Si for crops, a simple calculation considering an exportation of 50–100 kg Si ha−1 yr−1 (Table 1b) and an initial pool of Si from phytoliths of 1 t Si ha−1 (Bartoli, 1983; Gérard et al., 2008), shows that under temperate climate and initial forest setting, the phytoliths pool would be depleted within a few decades. When comparing data from various ecosystems (Table 2), it appears that the phytolith pools vary according to soil types (Clarke, 2003) and vegetation cover. ASi in agrosystems is of 800–13 458 kg ha−1, which is within the range of values obtained for natural forests (7–31 262 kg ha−1) but largely lower than the range of values obtained for natural grasses (18 000–71 100 kg ha−1). ASi is larger in natural grass ecosystems than in natural forest ecosystems, because grasses are the world largest plant accumulators of Si (Hodson et al., 2005).

Table 2

Stocks de Si phytolithique dans le sol, calculés pour différents écosystèmes naturels et agricoles.

Vegetation cover Soil type Soil phytoliths
kg Si ha−1
Soil depth
Methods References
Natural vegetation
 Tropical forest (young) Leptosol 7 1 Vegetation Meunier et al., 2010
 Tropical forest (mature) Ferrasol 17000 50 Calculation Lucas, 2001
 Tropical forest (mature) Spodosol ?? 50 Calculation Lucas, 2001
 Temperate deciduous forest (Housseras) Dystric Cambisol 450 50 Physical extractiona Bartoli, 1983
 Temperate coniferous forest (Biffontaine) Spodosol 1400 50 Physical extraction Bartoli, 1983
 Temperate coniferous forest Dystric Cambisol 852 120 Calculation Gérard et al., 2008
 Tropical forest (mature) Ferrasol 17100 50 Physical extraction Alexandre et al., 1997
 Mixed continental forest Spodosols 17410 50 Na2CO3b Saccone et al., 2008
 Boreal-nemoral mixed forest Typical Cambisol 31262 85 Na2CO3 Clymans et al., 2011
 Mixed natural grass (Smokey Valley) Mollisol 67000 50 Physical extraction Blecker et al., 2006
 Short grass (Arik) Arid Mollisol 31000 50 Physical extraction Blecker et al., 2006
 Tall grass (Konza) Wet Mollisol 18000 50 Physical extraction Blecker et al., 2006
 Natural grass Mollisol 55000 20 Physical extraction Borrelli et al., 2010
 Acacia stand following natural grass Mollisol 71100 20 Physical extraction Borrelli et al., 2010
 Grasslands (Santa Cruz) Loamy Mollisols 4841–8780 15–25 Physical extraction White et al., 2012
 Savanna Ferrugineous soil 3300 10 Physical extraction Alexandre et al., 2011
 (Sub)arctic wetland Peatland 5607 50 Na2CO3 Struyf et al., 2010b
 Freshwater marsh vegetation Mudflats 15000 30 Na2CO3 Struyf et al., 2006
Agricultural land
 Cereal crop (rice) Fluvisol/Reductisol 800 25 Physical extraction Desplanques et al., 2006
 Cereal crop (wheat) Luvisol 6508 50 Na2CO3 Guntzer et al., 2012
 Cereal crop (wheat) Luvisol 7259 50 Physical extraction Guntzer et al., 2012
 Pasture Typical Cambisol 12757 85 Na2CO3 Clymans et al., 2011
 Arable land (mixed cereals) Typical Cambisol 13458 85 Na2CO3 Clymans et al., 2011

a Physical extraction of phytoliths using densimetric separation (Kelly, 1990).

b Alkaline extraction using Na2CO3 (DeMaster, 1981).

Studying Si soil profiles offers an insight into the dynamics of phytoliths. In natural forests or grass systems (Alexandre et al., 1997, 2011; Blecker et al., 2006; Borrelli et al., 2010; Saccone et al., 2007), ASi is larger at the surface due to input through litterfall. ASi decreases rapidly below the surface but remains constant at depth, which probably reflects different pools of phytoliths, including a more soluble pool at the surface and a more resistant pool at depth (Alexandre et al., 1997). At depth, however, ASi may increase again due to phytoliths translocation (Cornelis et al., 2011b; Fishkis et al., 2010).

Two recent studies, conducted under similar climatic conditions, have reported differences in ASi storage according to land use. The first study (Clymans et al., 2011) used the Na2CO3 extraction method and found that the distribution of ASi followed the same trend in forested and agriculture lands with maximal ASi concentrations in the topsoils. This study also showed larger ASi pools under forest (Table 2). The origin of ASi depletion in agriculture lands was attributed to a reduced ASi replenishment due to a systematic removal of crop residues. However, no information was given on the history of agricultural practices to support this interpretation. In addition, the Na2CO3 extraction technique used to quantify ASi has not been calibrated so far for phytoliths and may either not be adapted to dissolve all types of soil phytoliths (Cabanes et al., 2011) or may give excessive values when inorganic amorphous solids are present (Sauer et al., 2006).

The second study (Guntzer et al., 2012) used a combination of alkaline and physical extractions to test the ASi depletion hypothesis in a 153 y straw export experiment: the Broadbalk continuous winter wheat (Triticum aestivum L.) experiment at Rothamsted Research, UK. This study showed that Si concentration increased with depth, an opposite trend to that found by Clymans et al. (2011). The general trend was a decrease in ASi concentration during the first 30 years of experiment, followed by a low and steady concentration till present. Both alkaline and physical extractions provided similar results, pointing out to a putative ASi depletion in cultivated soils due to straw export (Fig. 1).

Fig. 1

Decrease of ASi (in % Si in DW) over time in the topsoil of the long-term continuous wheat experiment with straw export at Broadbalk (Rothamsted Research, UK); white circles are ASi obtained by Na2CO3 extraction and black triangles are ASi obtained by physical extraction using by ZnBr2; redrawn from Guntzer et al. (2012).

Diminution des concentrations en ASi (en % Si du poids sec de sol), au cours du temps, dans l’horizon de surface de l’expérience de monoculture de blé avec exportation des pailles à Broadbalk (Rothamsted Research, Royaume-Uni). Les cercles représentent les données obtenues par extraction au Na2CO3 et les triangles les données obtenues par extraction physique au ZnBr2. Repris des données originales de Guntzer et al. (2012).

Comparing ASi profiles from these wheat field soils with profiles from similar soils left to return to forest, it appears that 120 years of forest did not significantly modify Si concentration profiles except for the topsoil of the Geescroft Wilderness, which had not been limed and was characterized by a drop of pH down to 4.4 and the presence of unweathered phytoliths. This is in accordance with the limited amount of amorphous Si found in forests under temperate climate and with thermodynamic data indicating a slower phytoliths dissolution rate at low pH (Fraysse et al., 2006), as experienced at the Geescroft Wilderness.

In polder paddy fields of increasing age (Kojima, Japan), Ma and Takahashi (2002) observed a decrease of soluble Si over time in the soil profile, which was correlated to a decrease in grain yield and Si concentrations in rice (Oryza sativa L.) shoots. A pH decrease was also observed in the whole soil profile that might be partly responsible for the decrease in soluble Si. Ma and Takahashi (2002) concluded that both pH decrease and plant uptake were responsible for the decrease in soluble Si concentration in soil. Unfortunately, soluble Si refers to immediately phytoavailable Si and does not include phytoliths and thus cannot be compared to the Broadbalk results. Following results of the Clymans et al. (2011) and Guntzer et al. (2012) experiments, we speculate that changes in soil pH might modulate the ASi pool through increase or decrease of the dissolution rate of phytoliths. While a decrease in pH may reduce the dissolution rate of phytoliths, it may increase that of other soil crystalline silicates (Fraysse et al., 2009). The contribution of soil silicates other than phytoliths to phytoavailable Si may therefore increase with decreasing pH. Indeed, the amount of Si in soil silicates was found to be 30 times larger than the Si phytoliths pool in natural grassland (Blecker et al., 2006), or even 50 times larger than the Si phytoliths pool in the Broadbalk experiment (Guntzer et al., 2012).

4 Preliminary results from a decade export experiment.

A decrease in the soil ASi content, as observed in the Broadbalk experiment, may be related to changes in soil characteristics such as pH through continuous cultivation and not only to straw export. In order to test if the incorporation of straw to soils increases the soil ASi pool, we sampled three plots from a Luvisol at the Arvalis-Institut du végétal station in Boigneville (France), which were cultivated for 36 years with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) from 1970 to 1997, and with sugar beet (Beta vulgaris L.), wheat and pea (Pisum sativum L.) from 1998 to 2006 (Fig. 2a). During 12 years, one of the plots had half of the wheat straw exported after harvest while maize straw was totally exported (CR+E). In the second one, straw was always returned to the soil (CP+R). Both plots were ploughed down to 25 cm and sown classically, while a third one was directly seeded and straw was always returned to the soil (noted as DS+R). In the later, only the first 5 cm of soil were homogenized. Soil samples were taken from the first 20 cm in the conventional ploughing settings (CP+R and CP+E) and the first 5 cm in the direct seeding setting (DS+R).

Fig. 2

Straw restitution experiment at Boigneville, Arvalis experimental station, France. a) Scheme of the crop rotation; b) effect on ASi concentrations using Na2CO3 extraction (white) and ZnBr2 extraction (black) in the topsoil. CP = classical ploughing down to 25 cm; DS = direct seeding; E = straw export and R = straw return.

Expérience de restitution de paille à Boigneville, station expérimentale d’Arvalis, France. a) Déroulé des actions ; b) effet sur les concentrations en ASi (en blanc, extraction alcaline par Na2CO3 et en noir = extraction physique par ZnBr2) dans les horizons de surface. CP = labour classique jusqu’à 25 cm ; SD = semis direct et R = restitution.

All soil samples were air-dried and passed through a 2-mm sieve before Si extraction. We used two techniques for ASi analyses: heavy liquid extraction using zinc bromide (ZnBr2) alkaline extraction by 0.1 M Na2CO3, at 85 °C (Guntzer et al., 2012). The fractions extracted with the non-destructive method were weighed and mounted on slices for optical microscope inspection (Leica DM RXP) in order to characterise their state of conservation and to detect the presence of non-phytolith amorphous silica particles. We calculated the content of Si in phytoliths after checking that the analysed extract was only composed of phytoliths, and assuming that the water content of phytoliths is 10% (Bartoli, 1981), which gave a proportion of 42% of Si in the fraction extracted by zinc bromide. Three or four replicates of each sample were performed with the Na2CO3 procedure while the heavy liquid extraction was conducted only once per sample.

Results indicated an increase in both ASi measurements (Fig. 2b) in the CP+R as compared to the CR+E setting. The effect was still measurable 15 years after the experiment had been terminated. ASi measurements gave also larger values under direct seeding due to concentration of phytoliths in the first five centimeters: although the total phytoliths amount within the whole profile was probably as much as that of the CP+R, this concentration at the surface clearly evidence the importance of straw return or litterfall at the soil surface.

We also observed that the straw restitution influenced the phytoliths types observed in the soils. Indeed, the soil samples that had straw restitution have more (34% and 42%) elongate phytoliths originating from epidermal long cells (observed in wheat stems) and less short cell phytoliths (rondels and trapeziform polylobates) than the soil sample that had straw exportation (20%) (Fig. 3). Also, we did not observe dendriform phytoliths typical of wheat inflorescences (Ball et al., 1993), or cross-shaped and saddle-shaped phytoliths typical of maize (Piperno, 2006).

Fig. 3

Relative abundance (in %) of phytolith morphotypes in topsoil samples that underwent straw restitution at Arvalis experimental station, Boigneville, France. CP, DS, E and R = see Fig. 2.

Abondance relative (en %) des types morphologiques de phytolithes dans les horizons de surface de l’expérience de restitution de paille à Boigneville, station expérimentale d’Arvalis, France. CP, DS, E et R = voir Fig. 2.

Therefore, straw exportation may affect the ASi pools even after a decade. The present results are also in agreement with the experiments showing the progressive increase of soluble Si (available to plants) and Si concentrations in rice crops following incorporation of rice straw into soil (Datnoff et al., 2001).

5 Summary and recommendation

Evidence from the literature and original data suggests that crops modify the silicon biogeochemical cycle through both accelerated Si uptake due to crop plants being more efficient than natural vegetation at taking up nutrients and other elements, and Si export through total or partial shoots removal from the field. The resulting trend is a decrease in long term phytoavailable (less phytoliths) and short term phytoavailable Si (soluble) leading to lower Si concentrations in plants and probable lower Si exports out of the soil. The consequences are: (a) putative changes in the various Si pools in soil and their respective contribution to Si cycle (Carey and Fulweiler, 2011; Struyf et al., 2010a); (b) a possible impact on crop yield and plant health (Savant et al., 1997). The extent of these changes is not enough documented, neither is the impact of Si soil depletion on the long term. More case studies are needed to determine the contribution of other factors that can lead to phytoliths depletion in cultivated land: increasing of soil erosion (Conley et al., 2008) and phytoliths mobility downwards the soil profile (Fishkis et al., 2010).

There is thus a need for characterizing the various Si pools in soil including their dissolution kinetics and their relative contribution to amorphous silicon and phytoavailable silicon. The role of pH needs to be better assessed for constraining the evolution of phytoliths storage in the biogeochemical cycle. It has been recently suggested that the application of nitrogen fertilizers may enhance silicate dissolution because of oxidation of NO3 into NH4+ producing proton (Fortner et al., 2012). Lower pH may increase the delivery of DSi from various minerals into soil solutions to counterbalance the phytoliths loss. Indeed, Makabe et al. (2009) found that available Si (phosphate buffer extraction at pH 6.9) in paddy fields in Japan was related to mineralogy and positively correlated to the amount of the clay fraction. The relative contribution of clays minerals, phytoliths and other amorphous Si forms to the phytoavailable Si pool needs to be clarified. This would allow using Si as a putative tracer of soil element depletion in European soils where silicon losses are not balanced by fertilisers inputs.


This work has been financed by EC2CO, a French national scientific program of CNRS (INSU).


[Alexandre et al., 1997] A. Alexandre; J.D. Meunier; F. Colin; J.M. Koud Plant impact on the biogeochemical cycle of silicon and related weathering processes, Geochim. Cosmochim. Acta, Volume 61 (1997), pp. 677-682

[Alexandre et al., 2011] A. Alexandre; M. Bouvet; L. Abbadie The role of savannas in the terrestrial Si cycle: a case-study from Lamto, Ivory Coast, Global Planet. Changes, Volume 78 (2011), pp. 162-169

[Amundson et al., 2007] R. Amundson; D.D. Richter; G.S. Humphreys; E.G. Jobbagy; J. Gaillardet Coupling between biota and earth materials in the critical zone, Elements, Volume 3 (2007), pp. 327-332

[Ball et al., 1993] T.B. Ball; J.D. Brotherson; J.S. Gardner A typologic and morphometric study of variation in phytoliths from einkorn wheat (Triticum monococcum L.), Canad. J. Bot., Volume 71 (1993), pp. 1182-1192

[Bartoli, 1981] Bartoli, F., 1981. Le cycle biogéochimique du silicium sur roche acide. Application à deux écosystèmes forestiers tempérés (Vosges). PhD Thesis, University of Nancy I.

[Bartoli, 1983] F. Bartoli The biogeochemical cycle of silicon in two temperate foresty ecosystems (R. Hallberg, ed.), Environmental Biogeochemistry, 35, Ecol. Bull., Stockholm, 1983, pp. 469-476

[Berner and Kothavala, 2001] E.K. Berner; Z. Kothavala Geocarb III: a revised model of atmospheric CO2 over Phanerozoic time, Am. J. Sci., Volume 301 (2001), pp. 301-312

[Berner et al., 2005] E.K. Berner; R.A. Berner; K.L. Moulton Plants and mineral weathering: present and past (J.L. Drever, ed.), Surface and groundwater, weathering and soils, Treatise on Geochemistry 5, Elsevier, 2005, pp. 169-188

[Blecker et al., 2006] S.W. Blecker; R.L. McCulley; O.A. Chadwick; E.F. Kelly Biologic cycling of silica across a grassland bioclimosequence, Global Biogeochem. Cycles, Volume 20 (2006) (doi:10.1029/2006GB002690)

[Brantley, 2008] S.L. Brantley Understanding soil time, Science, Volume 321 (2008), pp. 54-56

[Borrelli et al., 2010] N. Borrelli; M.F. Alvarez; M.L. Osterrieth; J.E. Marcovecchio Silica content in the soil solution and its relation with phytolith weathering and silica biogeochemical cycle in Typical Argiudolls of the Pampean Plain, Argentina – a preliminary study, J. Soils Sediments, Volume 10 (2010), pp. 983-994

[Cabanes et al., 2011] D. Cabanes; S. Weiner; R. Shahack-Gross Stability of phytoliths in the archaeological record: a dissolution study of modern and fossil phytoliths, J. Archaeol. Sci., Volume 38 (2011), pp. 2480-2490

[Carey and Fulweiler, 2011] J.C. Carey; R.W. Fulweiler Human activities directly alter watershed dissolved silica fluxes, Biogeochemistry (2011) (doi: 10.1007/s10533-011-9671-2)

[Clarke, 2003] J. Clarke The occurrence and significance of biogenic opal in the regolith, Earth Sci. Rev., Volume 60 (2003), pp. 175-194

[Clymans et al., 2011] W. Clymans; E. Struyf; G. Govers; F. Vandevenne; D.J. Conley Anthropogenic impact on amorphous silica pools in temperate soils, Biogeosciences, Volume 8 (2011), pp. 2281-2293

[Conley, 2002] D.J. Conley Terrestrial ecosystems and the global biogeochemical silica cycle, Global Biogeochem. Cycles, Volume 16 (2002), p. 1121 (doi:10.1029/2002GB001894)

[Conley et al., 2008] D. Conley; G. Likens; D. Buso; L. Saccone; S. Bailey; C. Johnson Deforestation causes increased dissolved silicate losses in the Hubbard Brook experimental forest, Global Change Biol., Volume 14 (2008), pp. 2548-2554

[Cornelis et al., 2011a] J.T. Cornelis; B. Delvaux; R.B. Georg; Y. Lucas; J. Ranger; S. Opfergelt Tracing the origin of dissolved silicon transferred from various soil-plant systems towards rivers/a review, Biogeosciences, Volume 8 (2011), pp. 89-112

[Cornelis et al., 2010] J.-T. Cornelis; J. Ranger; A. Iserentant; B. Delvaux Tree species impact the terrestrial cycle of silicon through various uptakes, Biogeochemistry, Volume 97 (2010), pp. 231-245

[Cornelis et al., 2011b] J.T. Cornelis; H. Titeux; J. Ranger; B. Delvaux Identification and distribution of the readily soluble silicon pool in a temperate forest soil below three distinct tree species, Plant Soil, Volume 342 (2011), pp. 369-378

[Datnoff et al., 2001] Silicon in agriculture. Studies in Plant Science 8 (L.E. Datnoff; G.H. Snyder; G.H. Korndorfer, eds.), Elsevier, 2001

[Deelman, 1986] J.C. Deelman Opal-CT in bamboo, N. Jb. Miner. Mh., Volume H.9 (1986), pp. 407-415

[DeMaster, 1981] D.J. DeMaster The supply and accumulation of silica in the marine environment, Geochim. Cosmochim. Acta, Volume 45 (1981), pp. 1715-1732

[Derry et al., 2005] L.A. Derry; A.C. Kurtz; K. Ziegler; O.A. Chadwick Biological control of terrestrial silica cycling and export fluxes to watersheds, Nature, Volume 433 (2005), pp. 728-731

[Desplanques et al., 2006] V. Desplanques; L. Cary; J.C. Mouret; F. Trolard; G. Bourrié; O. Grauby; J.D. Meunier Silicon transfers in a rice field in Camargue (France), J. Geochem. Explor., Volume 88 (2006), pp. 190-193

[Epstein, 1999] E. Epstein Silicon, Ann. Rev. Plant Physiol. Plant Mol. Biol., Volume 50 (1999), pp. 641-664

[Falkowski et al., 2004] P.G. Falkowski; M.E. Katz; A.H. Knoll; A. Quigg; J.R. Raven; O. Schofield; F.J.R. Taylor The evolution of modern eukaryotic phytoplancton, Science, Volume 305 (2004), pp. 354-360

[Fishkis et al., 2010] O. Fishkis; J. Ingwersen; M. Lamers; D. Denysenko; T. Streck Phytolith transport in soil:a field study using fluorescent labelling, Geoderma, Volume 157 (2010), pp. 27-36

[Fraysse et al., 2010] F. Fraysse; O.S. Pokrovsky; J.D. Meunier Experimental study of terrestrial plant litter interaction with aqueous solutions, Geochim. Cosmochim. Acta, Volume 74 (2010), pp. 70-84

[Fraysse et al., 2006] F. Fraysse; O.S. Pokrovsky; J. Schott; J.D. Meunier Surface properties, solubility and dissolution kinetics of bamboo phytoliths, Geochim. Cosmochim. Acta, Volume 70 (2006), pp. 1939-1951

[Fraysse et al., 2009] F. Fraysse; O.S. Pokrovsky; J. Schott; J.D. Meunier Surface chemistry and reactivity of plant phytoliths in aqueous solutions, Chem. Geol., Volume 258 (2009), pp. 197-206

[Fortner et al., 2012] S.K. Fortner; W.B. Lyons; A.E. Carey; M.J. Shipitalo; S.A. Welch; K.A. Welch Silicate weathering and CO2 consumption within agricultural landscapes, the Ohio-Tennessee River Basin, USA, Biogeosciences, Volume 9 (2012), pp. 941-955

[Gérard et al., 2008] F. Gérard; K.U. Mayer; M.J. Hodson Modelling the biogeochemical cycle of silicon in soils: application to a temperate forest ecosystem, Geochim. Cosmochim. Acta, Volume 72 (2008), p. A304

[Guntzer et al., 2011] F. Guntzer; C. Keller; J.-D. Meunier Benefits of plant silicon for crops. A review, Agron. Sustain. Dev., Volume 32 (2011), pp. 201-213

[Guntzer et al., 2012] F. Guntzer; C. Keller; P.R. Poulton; S.P. McGrath; J.-D. Meunier Long-term removal of wheat straw decreases soil amorphous silica at Broadbalk, Rothamsted, Plant Soil, Volume 352 (2012), pp. 173-184

[Hodson et al., 2005] M.J. Hodson; P.J. White; A. Mead; M.R. Broadley Phylogenetic variation in the silicon composition of plants, Ann. Bot., Volume 96 (2005), pp. 1027-1046

[Kelly, 1990] Kelly, E.F., 1990. Methods for extracting opal phytoliths from soil and plant material. Workshop on biotic indicators of global change, University of Washington (Seattle).

[Laruelle et al., 2009] G.G. Laruelle; V. Roubeix; A. Sferratore; B. Brodherr; D. Ciuffa; D.J. Conley; H.H. Dürr; J. Garnier; C. Lancelot; Q. Le Thi Phuong; J.D. Meunier; M. Meybeck; P. Michalopoulos; B. Moriceau; S. Ní Longphuirt; S. Loucaides; L. Papush; M. Presti; O. Ragueneau; P.A.G. Regnier; L. Saccone; C.P. Slomp; C. Spiteri; P. Van Cappellen The global biogeochemical cycle of silicon: role of the land-ocean transition and sensitivity to anthropogenic perturbations, Global Geochem. Cycles, Volume 23 (2009), p. GB4031 (doi:10.1029/2008GB003267)

[Loucaides et al., 2010] S. Loucaides; T. Behrends; P. Van Capellen Reactivity of biogenic silica: surface versus bulk charge density, Geochim. Cosmochim. Acta, Volume 74 (2010), pp. 517-530

[Lucas, 2001] Y. Lucas The role of plants in controlling rates and products of weathering: importance of biological pumping, Annu. Rev. Earth Planet. Sci., Volume 29 (2001), pp. 135-163

[Lucas et al., 1993] Y. Lucas; F.J. Luizao; A. Chauvel; J. Rouiller; D. Nahon The relation between biological activity of the rainforest and mineral composition of the soils, Science, Volume 260 (1993), pp. 521-523

[Ma and Takahashi, 2002] J.F. Ma; E. Takahashi Soil, fertiliser, and plant silicon research in Japan, Elsevier, 2002

[Makabe et al., 2009] S. Makabe; K. Kakuda; Y. Sasaki; T. Ando; H. Fujii; H. Ando Relationship between mineral composition or soil texture and available silicon in alluvial paddy soils on the Shounai Plain, Japan, Soil Sci. Plant Nutr., Volume 55 (2009), pp. 300-308

[Markewitz and Richter, 1998] D. Markewitz; D.D. Richter The bio in aluminum and silicon geochemistry, Biogeochemistry, Volume 42 (1998), pp. 235-252

[Matichenkov and Bocharnikova, 2001] V.V. Matichenkov; E.A. Bocharnikova The relationship between silicon and soil physical and chemical properties (L.E. Datnoff; G.H. Snyder; G.H. Korndorfer, eds.), Silicon in agriculture. Studies in Plant Science 8, Elsevier, 2001, pp. 209-219

[Meunier et al., 2010] J.-D. Meunier; S. Kirman; D. Strasberg; E. Nicolini; E. Delcher; C. Keller The output and biocycling of Si in a forest developed on young basalt flows (La Réunion Island), Geoderma, Volume 159 (2010), pp. 431-439

[Montgomery, 2007] D.R. Montgomery Is agriculture eroding civilization's foundation?, GSA Today, Volume 17 (2007), pp. 4-9

[Piperno, 2006] D.R. Piperno Phytoliths. A comprehensive guide for archaeologists and paleoecologists, AltaMira Press (Rowman & Littlefield), Lanham, New York, Toronto, Oxford, 2006

[Quinton et al., 2010] J.Q. Quinton; G. Govers; K. Van Oost; R.D. Bardgett The impact of agriculture soil erosion on biogeochemical cycling, Nature Geosci., Volume 3 (2010), pp. 311-314

[Robert and Chaussidon, 2006] F. Robert; M. Chaussidon A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts, Nature, Volume 443 (2006), pp. 969-972

[Saccone et al., 2007] L. Saccone; D.J. Conley; E. Koning; D. Sauer; M. Sommer; D. Kaczorek; S.W. Blecker; E.F. Kelly Assessing the extraction and quantification of amorphous silica in soils of forest and grassland ecosystems, Eur. J. Soil Sci., Volume 58 (2007), pp. 1446-1459

[Saccone et al., 2008] L. Saccone; D.J. Conley; G.E. Likens; S.W. Bailey; D.C. Buso; C.E. Johnson Factors that control the range and variability of amorphous silica in soils in the Hubbard Brook Experimental Forest, Soil Sci. Soc. Am. J., Volume 72 (2008), pp. 1637-1644

[Savant et al., 1997] N.K. Savant; L.E. Datnoff; G.H. Snyder Depletion of plant-available silicon in soils: a possible cause of declining rice yields, Comm. Soil Sci. Plant Anal., Volume 28 (1997), pp. 1245-1252

[Sauer et al., 2006] D. Sauer; L. Saccone; D.J. Conley; L. Herrmann; M. Sommer Review of methodologies for extracting plant-available and amorphous Si from soils and aquatic sediments, Biogeochemistry, Volume 80 (2006), pp. 89-108

[Sommer et al., 2006] M. Sommer; D. Kaczorek; Y. Kuzyakov; J. Breuer Silicon pools and fluxes in soils and landscapes: a review, J. Plant Nutr. Soil Sci., Volume 169 (2006), pp. 310-329

[Street-Perrott and Barker, 2008] F.A. Street-Perrott; P.A. Barker Biogenic silica: a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon, Earth Surf. Proc. Land., Volume 33 (2008), pp. 1436-1457

[Struyf et al., 2006] E. Struyf; A. Dausse; S. Van Damme; K. Bal; B. Gribsholt; H.T.S. Boschker; J.J. Middelburg; P. Meire Tidal marshes and biogenic silica recycling at the land sea interface, Limnol. Oceanogr., Volume 51 (2006), pp. 838-846

[Struyf et al., 2009] E. Struyf; A. Smis; S. Van Damme; P. Meire; D.J. Conley The global biogeochemical silicon cycle, Silicon, Volume 1 (2009), pp. 207-213

[Struyf et al., 2010a] E. Struyf; A. Smis; S. Van Damme; J. Garnier; G. Govers; B. Van Wesemael; D.J. Conley; O. Batelaan; E. Frot; W. Clymans; F. Vandevenne; C. Lancelot; P. Goos; P. Meire Historical land use change has lowered base-line silica mobilization from landscapes, Nat. Commun., Volume 1 (2010), p. 129

[Struyf et al., 2010b] E. Struyf; C.M. Mörth; C. Humborg; D.J. Conley An enormous amorphous silica stock in boreal wetlands, J. Geophys. Res., Volume 115 (2010), p. G04008 (doi:10.1029/2010JG001324)

[Struyf and Conley, 2012] E. Struyf; D.J. Conley Emerging understanding of the ecosystem silica filter, Biogeochemistry, Volume 107 (2012), pp. 9-18

[Tréguer et al., 1995] P. Tréguer; D. Nelson; A. Vanbennekom; D. DeMaster; A. Leynaert; B. Queguiner The silica balance in the world ocean–a reestimate, Science, Volume 268 (1995), pp. 375-379

[Vandevenne et al., 2011] F. Vandevenne; E. Struyf; W. Clymans; P. Meire Agricultural silica harvest: have humans created a new loop in the global silica cycle?, Front. Ecol. Environ. (2011) (doi:10.1890/110046)

[Wilkinson, 2005] B.H. Wilkinson Humans as geologic agents: a deep-time perspective, Geology, Volume 33 (2005), pp. 161-164

[White et al., 2012] A.F. White; D.V. Vivit; M.S. Shulz; T.D. Bullen; R.R. Evett; J. Aagarwal Biogenic and pedogenic controls on Si distributions and cycling in grasslands of the Santa Cruz soil chronosequence, California, Geochim. Cosmochim. Acta, Volume 94 (2012), pp. 72-94

Comments - Policy