Jean-Dominique Meunier

Plant impact on the biogeochemical cycle of silicon and related weathering processes

Abstract The contribution of plants to the biogeochemical cycle of Si and related weathering processes was studied in an equatorial rainforest ecosystem (Congo) where the biologic turnover of Si is high (58 to 76 kg/ha/y). Litterfall leaves, a soil profile and groundwaters were analysed. Phytoliths and organic matter have a similar distribution with depth in the soil profile. The model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92% of the biogenic silica input is rapidly recycled while about 8% of the biogenic silica input supplies a stable pool of phytoliths, with a lower turnover. Reprecipitation of silica was observed at the base of the soil profile, indicating a local geochemical environment that is oversaturated with respect to amorphous silica. A balance in biogeochemical cycle of Si requires that the vegetation absorb dissolved silicon released from weathering of minerals, which otherwise would be available for mineral neoformation or export from the profile towards regional drainages. Plant uptake of Si increases the chemical weathering rate without increasing the denudation rate. This study shows that the uptake, storage, and release of Si by the vegetation have to be taken into account when using dissolved Si for tracing chemical weathering dynamics.

Phytoliths : indicators of grassland dynamics during the late Holocene in intertropical Africa

Abstract The reconstruction of African tropical grassland history during the late Holocene can be carried out using phytolith analysis. Fossil phytolith assemblages from Lake Guiers, in the Sahelian region of Senegal, and from Lake Sinnda, in the Guineo-Congolian region of Congo were investigated. The results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained. Tall or short grass associations are discriminated by their phytolith index Iph(%) = saddle/(cross + dumbel + saddle), while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths. The phytolith data emphasize that, in the Guineo-Congolian region around Lake Sinnda, the driest phase of the late Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its replacement by a short grass savanna. From ca. 1000 yr B.P., wetter climatic conditions developed, as represented by the setting up of a tall grass savanna woodland. The modern shrub and tall grass savanna was developed ca. 700 yr B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after about 2000 yr B.P. and has not been followed by moister conditions. A tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs. A short period of development of swampy vegetation, which can be related to a lake level change, interrupted the semi-arid adaptation of the vegetation, between about 2000 yr B.P. and the present.

Benefits of plant silicon for crops: a review

Since the beginning of the nineteenth century, silicon (Si) has been found in significant concentrations in plants. Despite the abundant literature which demonstrates its benefits in agriculture, Si is generally not considered as an essential element. The integration of Si in agricultural practices is, however, effective in a few countries. Silicon fertilization by natural silicates has the potential to mitigate environmental stresses and soil nutrient depletion and as a consequence is an alternative to the extensive use of phytosanitary and NPK fertilizers for maintaining sustainable agriculture. This review focuses on recent advances on the mechanisms of Si accumulation in plants and its behavior in soil. Seven among the ten most important crops are considered to be Si accumulators, with concentration of Si above 1% dry weight. New approaches using isotopes and genetics have highlighted the mechanisms of uptake and transfer of Si in planta. There is a general agreement on an uptake of dissolved silica as H4SiO4 and precipitation as amorphous silica particles (the so-called phytoliths), but the mechanism, either active or passive, is still a matter of debate. The benefits of Si are well demonstrated when plants are exposed to abiotic and biotic stresses. The defense mechanisms provided by Si are far from being understood, but evidences for ex planta and in planta processes are given indicating multiple combined effects rather than one single effect. Phytoliths that are located mainly in shoots of monocots return to the soil through litterfall if the plants are not harvested and contribute to the biogeochemical cycle of Si. According to recent progress made on the understanding of the biogeochemical cycle of Si and the weathering process of silicate minerals, phytoliths may significantly contribute to the resupply of Si to plants. We suggest that straw of crops, which contains large amounts of phytoliths, should be recycled in order to limit the depletion of soil bioavailable Si.

Phytoliths as paleoenvironmental indicators, West Side Middle Awash Valley, Ethiopia

Phytolith assemblage analysis offers the potential to refine our knowledge of paleoecosystems where grasses and sedges predominate. In this work, Holocene and Pleistocene sediments from an arid tropical region in Ethiopia have been analyzed for their phytolith content, presented as detailed counts and diagrams according to the Twiss classification. The aim is to test the usefulness of phytolith assemblages to indicate paleoenvironments at Middle Awash, where fossil pollen grains are poorly preserved in sediments that yielded abundant archaeological remains. The vegetation in the Middle Awash subdesertic valley is currently a shrub steppe dominated by C4 grasses adapted to arid conditions, with a narrow riparian forest limited to the Awash River. Our results show that modern surface samples, Holocene and Pleistocene sediments contain well-preserved and different phytolith assemblages, and therefore that no translocation processes from modern soil to geological strata seem to occur. Fossil records and modern assemblages are interpreted using phytolith ratios to estimate the density of the tree cover, the aridity and the proportion of C3 versus C4 grasses, as applied to phytolith assemblages from North America and West Africa. The phytolith assemblages from modern soil samples correctly reflect the proportion of trees and shrubs versus grasses, different in the riparian vegetation and the shrub steppe. Modern phytolith assemblages appear to be a mixed signature of local and regional vegetation. Phytolith analysis of the Holocene sample suggests a grassland, where the grass community is constituted by the Chloridoideae subfamily, adapted to warm and dry conditions and where C3-Pooideae cover the highlands. Phytolith analysis of the Pleistocene sample evidences grassland formation with scattered woody elements, where C4-Panicoideae grasses, adapted to warm and humid conditions dominate the grass cover. However, these conclusions need to be confirmed by more complete study on phytolith assemblages from modern vegetation from Ethiopia.

Effect of silicon on reducing cadmium toxicity in durum wheat (Triticum turgidum L. cv. Claudio W.) grown in a soil with aged contamination.

Agricultural soil contamination and subsequently crops still require alternative solutions to reduce associated environmental risks. The effects of silica application on alleviating cadmium (Cd) phytotoxicity in wheat plants were investigated in a 71-day pot experiment conducted with a historically contaminated agricultural soil. We used amorphous silica (ASi) that had been extracted from a diatomite mine for Si distribution at 0, 1, 10 and 15 ton ASi ha(-1). ASi applications increased plant biomass and plant Si concentrations, reduced the available Cd in the soil and the Cd translocation to shoots, while Cd was more efficiently sequestrated in roots. But ASi is limiting for Si uptake by plants. We conclude that significant plant-available Si in soil contributes to decreased Cd concentrations in wheat shoots and could be implemented in a general scheme aiming at controlling Cd concentrations in wheat.

Phytolith assemblages as a promising tool for reconstructing Mediterranean Holocene vegetation

The reliability of phytolith assemblage analysis for characterizing Mediterranean vegetation is investigated in this study. Phytolith assemblages are extracted from modern and buried Holocene soils from the middle Rhone valley (France). The relation between modern phytolith assemblages and the surrounding vegetation, as well as between fossil assemblages and contemporaneous vegetation, already reconstructed through other proxies, is discussed. We demonstrate that the main northwestern Mediterranean biomes are well distinguished by soil phytolith assemblage analysis. In particular, the density of pine and nonconiferous trees (densities expressed relatively to the grass cover) and the overall degree of opening of the vegetation appear well recorded by three phytolith indexes. North Mediterranean vegetation changes during the Holocene period, mainly tree line shifts, pine wood development and deforestation are poorly documented, due to the scarcity of proxy-preserving sites. Phytolith assemblage analysis of soils, buried soils, and sediments appears to be a promising technique to fill this gap.

Non-reversible geosystem destabilisation at 4200 cal. BP: Sedimentological, geochemical and botanical markers of soil erosion recorded in a Mediterranean alpine lake

A 144-cm-long core was obtained in Lake Petit (2200 m a.s.l., Mediterranean French Alps) in order to reconstruct past interactions between humans, the environment and the climate over the last five millennia using a multidisciplinary approach involving sedimentological, geochemical and botanical analyses. We show a complex pattern of environmental transformation. From 4800 to 4200 cal. BP, podzol-type soils progressively developed under forest cover. This stable situation was interrupted by a major detrital pulse at 4200 cal. BP that we consider as a tipping point in the environmental history. At this point, pedogenetic processes drastically regressed, leading to the development of moderately weathered soils. More frequent detrital inputs are recorded since 3000 cal. BP (ad 1050) as the human impact significantly increased in the catchment area. We conclude that destabilisation of the environment was triggered by climate and exacerbated by human activities to a stage beyond resilience.

Terrestrial plant-Si and environmental changes

Abstract The importance of silica in terrestrial land plants has been recognized since the middle of the 19th century with applications in agronomy and palaeovegetation reconstruction. In this presentation, we will review the latest advances in our understanding of phytolith formation and present a few examples of applications in the field of global environmental changes.

Determination of the silicon concentration in plant material using Tiron extraction

• The quantification of silicon (Si) in plants generally requires a digestion procedure before the determination of the dissolved Si concentration by spectrometric analysis. Recent procedures produce rapid and accurate measurements, but are based on either hazardous chemicals or sophisticated instrumentation. • Here, we describe a simpler procedure using Tiron. Tiron [4,5-dihydroxy-1,3-benzene-disulfonic acid disodium salt, (HO)(2)C(6)H(2)(SO(3)Na)(2)] is currently used as a selective extractant for amorphous silica in soils. Because Si in the shoots is mostly composed of amorphous opaline silica particles (i.e. phytoliths), we tested the Tiron extraction procedure for plants. • Our results are critically discussed in relation to two other standard procedures: electrothermal vaporization determination and high-temperature lithium-metaborate digestion. • We demonstrate that Tiron extraction is an alternative method which allows the rapid, safe and accurate quantification of Si in shoots of various plants covering a wide range of Si concentrations.

Interet de l'etude du cycle biogeochimique du silicium pour interpreter la dynamique des sols tropicaux

Recent studies on the biogeochemical cycle of silica put new constraints into models of genesis and evolution of silica in tropical environments. The role of plants in weathering mass balances are illustrated by phytolith studies. In Dimonika (Congo) latosols, dissolved silica originates mainly from the dissolution of phytoliths rather than from the dissolution of non-biogenic silicates. In andosols from La Reunion island (Indian Ocean), a 15 cm thick, biogenic silica accumulation formed within 4000 years from bamboo forests fires. The turn-over of silica by plants must therefore be taken into account in studies of weathering rates. Weathering rates of trachytic ash layers in La Reunion island show that all the primary minerals are destroyed and that 50% of amorphous secondary Al/Si products are transformed into halloysite. Theses rates, which are faster than the ones obtained in the Hawaii islands, may strongly influence the turn-over of carbon in soils. Besides, phytoliths preserved from dissolution may help to decipher the records of environmental changes in soils. In Salitre (Brazil), the phytolith distribution has been calibrated with charcoal and pollens. The age of phytoliths, which increases with depth, allow to trace the savanna/forest changes.