Wim Clymans

Agricultural silica harvest: have humans created a new loop in the global silica cycle?

Silica (Si) is of great concern to agronomists because it has a beneficial effect on plant resistance to various stresses, enabling yield optimization in economically important crop species. Yet biogenic silica (BSi) cycling in soils controls a large part of the Si export fluxes to rivers and oceans. Despite the importance of agricultural-harvest-related Si removal, previous studies have not addressed this topic thoroughly. By performing a detailed quantification of agricultural Si export in Western Europes Scheldt River basin, we show that harvest not only disrupts BSi cycling but also introduces an agricultural Si pathway, with major export Si fluxes as compared with BSi production in climax forest communities and grasslands. Harvesting substantially changes terrestrial Si cycling because reconstitution of BSi to soils in litter fall is prevented. The agricultural Si loop clearly constitutes an important flow of BSi out of terrestrial ecosystems – one that is currently unrecognized in global biogeochem...

Soil organic carbon mobilization by interrill erosion: Insights from size fractions

Sediments mobilized by interrill erosion are often highly enriched in soil organic carbon (SOC) in comparison to source soils. This selectivity may lead to the preferential mobilization of SOC with specific properties, e.g., SOC that is especially susceptible to decomposition. This may then have important implications with respect to the role of soil erosion in the global carbon cycle. We addressed this issue by investigating the behavior of different SOC components in field rainfall simulation experiments on arable fields in loess-derived soils. We characterized the mobilization of mineral-bound organic carbon (MOC) and particulate organic carbon (POC) by interrill erosion using size fractionation and we used the C:N ratio as a tracer variable to determine the composition of the SOC in eroded sediments. MOC was found to be preferentially mobilized by interrill erosion in comparison to POC. The enrichment ratio (i.e., the ratio of the concentration of a soil constituent in the eroded sediment to its concentration in the original soil) of MOC decreased with increasing sediment concentration. The enrichment ratio of POC displayed a similar pattern to that of MOC but enrichment was less pronounced. Furthermore, sediments were found to be enriched in fine POC while they were impoverished with respect to coarse POC. The selective MOC mobilization together with the dominance of MOC in the total SOC pool in the soil explained the dominance of MOC in interrill eroded sediment. The fact that it is mainly MOC that is mobilized by interrill erosion implies that the SOC in the interrill eroded sediments is on average at least as recalcitrant than that in the source soils which may have important implications for the fate of the mobilized SOC. In order to understand the role of soil erosion in C cycling, MOC and POC need to be considered separately not only because they are chemically different but also because of their different behaviors with respect to geomorphic processes.

Landscape cultivation alters d30Si signature in terrestrial ecosystems

Despite increasing recognition of the relevance of biological cycling for Si cycling in ecosystems and for Si export from soils to fluvial systems, effects of human cultivation on the Si cycle are still relatively understudied. Here we examined stable Si isotope (δ30Si) signatures in soil water samples across a temperate land use gradient. We show that – independent of geological and climatological variation – there is a depletion in light isotopes in soil water of intensive croplands and managed grasslands relative to native forests. Furthermore, our data suggest a divergence in δ30Si signatures along the land use change gradient, highlighting the imprint of vegetation cover, human cultivation and intensity of disturbance on δ30Si patterns, on top of more conventionally acknowledged drivers (i.e. mineralogy and climate).

Landscape cultivation alters delta Si-30 signature in terrestrial ecosystems

Despite increasing recognition of the relevance of biological cycling for Si cycling in ecosystems and for Si export from soils to fluvial systems, effects of human cultivation on the Si cycle are still relatively understudied. Here we examined stable Si isotope (δ30Si) signatures in soil water samples across a temperate land use gradient. We show that – independent of geological and climatological variation – there is a depletion in light isotopes in soil water of intensive croplands and managed grasslands relative to native forests. Furthermore, our data suggest a divergence in δ30Si signatures along the land use change gradient, highlighting the imprint of vegetation cover, human cultivation and intensity of disturbance on δ30Si patterns, on top of more conventionally acknowledged drivers (i.e. mineralogy and climate).

Transport of Dissolved Si from Soil to River: A Conceptual Mechanistic Model

This paper reviews the processes which determine the concentrations of dissolved silicon (DSi) in soil water and proposes a conceptual mechanistic model for understanding the transport of Si through soils to rivers. The net DSi present in natural waters originates from the dissolution of mineral and amorphous Si sources in the soil, as well as precipitation processes. Important controlling factors are soil composition (mineralogy and saturated porosity) and soil water chemistry (pH, concentrations of organic acids, CO2 and electrolytes). Together with production, polymerization and adsorption equations they constitute a mechanistic framework determining DSi concentrations. We discuss how key controls differ across soil horizons and how this can influence the DSi transport. A typical podzol soil profile in a temperate climate is used as an example, but the proposed model is transferrable to other soil types. Additionally, the impact of external forcing factors such as seasonal climatic variations and land use is evaluated. This blueprint for an integrated model is a first step to mechanistic modelling of Si transport processes in soils. Future implementation with numerical methods should validate the model with field measurements.

Bacterial and fungal colonization and decomposition of submerged plant litter: consequences for biogenic silica dissolution.

We studied bacterial and fungal colonization of submerged plant litter, using a known Si-accumulator (Equisetum arvense), in experimental microcosms during one month. We specifically addressed the microbial decomposer role concerning biogenic silica (bSiO2) dissolution from the degrading litter. To vary the rates and level of microbial colonization, the litter was combined with a range of mineral nitrogen (N) and phosphorous (P) supplements. Overall microbial growth on plant litter increased with higher levels of N and P. There was a tendency for higher relative bacterial than fungal stimulation with higher nutrient levels. Differences in microbial colonization of litter between treatments allowed us to test how Si remineralization from plants was influenced by microbial litter decomposition. Contrary to previous results and expectations, we observed a general reduction in Si release from plant litter colonized by a microbial community, compared with sterile control treatments. This suggested that microbial growth resulted in a reduction of dissolved Si concentrations, and we discuss candidate mechanisms to explain this outcome. Hence, our results imply that the microbial role in plant litter associated Si turnover is different from that commonly assumed based on bSiO2 dissolution studies in aquatic ecosystems.

Biosilicification drives a decline of dissolved si in the oceans through geologic time

Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis. (Less)

Assessing the potential of sponges (Porifera) as indicators of ocean dissolved Si concentrations

We explore the distribution of sponges along dissolved silica (dSi) concentration gradients to test whether sponge assemblages are related to dSi and to assess the validity of fossil sponges as a palaeoecological tool for inferring dSi concentrations of the past oceans. We extracted sponge records from the publically available Global Biodiversity Information Facility (GBIF) database and linked these records with ocean physiochemical data to evaluate if there is any correspondence between dSi concentrations of the waters sponges inhabit and their distribution. Over 320,000 records of Porifera were available, of which 62,360 met strict quality control criteria. Our analyses was limited to the taxonomic levels of family, order and class. Because dSi concentration is correlated with depth in the modern ocean, we also explored sponge taxa distributions as a function of depth. We observe that while some sponge taxa appear to have dSi preferences (e.g., class Hexactinellida occurs mostly at high dSi), the overall distribution of sponge orders and families along dSi gradients is not sufficiently differentiated to unambiguously relate dSi concentrations to sponge taxa assemblages. We also observe that sponge taxa tend to be similarly distributed along a depth gradient. In other words, both dSi and/or another variable that depth is a surrogate for, may play a role in controlling sponge spatial distribution and the challenge is to distinguish between the two. We conclude that inferences about palaeo-dSi concentrations drawn from the abundance of sponges in the stratigraphic records must be treated cautiously as these animals are adapted to a great range of dSi conditions and likely other underlying variables that are related to depth. Our analysis provides a quantification of the dSi ranges of common sponge taxa, expands on previous knowledge related to their bathymetry preferences and suggest that sponge taxa assemblages are not related to particular dSi conditions. (Less)

Estimated storage of amorphous silica in soils of the circum-Arctic tundra region

We investigated the vertical distribution, storage, landscape partitioning, and spatial variability of soil amorphous silica (ASi) at four different sites underlain by continuous permafrost and rep ...

Erratum: CORRIGENDUM: Landscape cultivation alters δ30Si signature in terrestrial ecosystems

Scientific Reports 5, Article number: 7732 10.1038/srep07732 (2015); Published: January132015; Updated: March152015 Jean-Thomas Cornelis was included in the Acknowledgements but omitted from the author list in the original version of this Article. This has been corrected in the PDF and HTML versions of the Article and in the Supplementary Information. Acknowledgements “F.I.V. thanks Special Research Funding of the University of Antwerp (BOF-UA) for PhD fellowship funding and Patrick Frings, Ryan Taylor and Jean-Thomas Cornelis for proof-reading and editing the manuscript. We also acknowledge Flemish Science Foundation (FWO) for funding the project “Tracking the biological control on Si mobilisation in upland ecosystems” (project number G014609N).” Now reads “F.I.V. thanks Special Research Funding of the University of Antwerp (BOF-UA) for PhD fellowship funding and Patrick Frings and Ryan Taylor for editing the manuscript as native speakers. We also acknowledge Flemish Science Foundation (FWO) for funding the project “Tracking the biological control on Si mobilisation in upland ecosystems” (project number G014609N) and BELSPO for funding the project SOGLO.” Author contributions “F.I.V. collected the samples and wrote the first drafts. C.D. and H.H. optimised and developed the isotopic analytical method, analysed the samples, made the data processing, and co-developed the discussion. F.I.V., W.C., E.S., G.G. and B.R. were involved in site selection and/or installation of the land use gradient. B.R. and A.L.B. provided background data on clay analysis and Si fractions in the soil. P.M., E.S., L.A. and G.G. initialised and conceptualised the work on Si biogeochemistry in joint collaborations. All authors contributed to the writing and methodological development of the paper.” Now reads “F.I.V. collected the samples and wrote the first drafts. C.D. and H.H. optimised and developed the isotopic analytical method, analysed the samples, made the data processing, and C.D., H.H. and J-T C. co-developed the discussion. F.I.V., W.C., E.S., G.G. and B.R. were involved in site selection and/or installation of the land use gradient. B.R. and A.L.B. provided background data on clay analysis and Si fractions in the soil. P.M., E.S., L.A. and G.G. initialised and conceptualised the work on Si biogeochemistry in joint collaborations. All authors contributed to the writing and methodological development of the paper.” The original Article contained an error in the calculation of the weathering index Total Reserve in Bases (TRB) in figure 2b. The correct figure 2 appears below as Figure 1. Figure 1 (a) Scatterplot of biogenic silica (BSi) in mg g−1 dry soil in the soil profile, (b) Total Reserve in Bases (TRB = [Na] + [Mg] + [Ca] + [K]) weathering index calculated on dry soil, in cmol charge kg−1. Sites are represented by symbols: ...