Eric Struyf

Grazers: biocatalysts of terrestrial silica cycling

Silica is well known for its role as inducible defence mechanism countering herbivore attack, mainly through precipitation of opaline, biogenic silica (BSi) bodies (phytoliths) in plant epidermal tissues. Even though grazing strongly interacts with other element cycles, its impact on terrestrial silica cycling has never been thoroughly considered. Here, BSi content of ingested grass, hay and faeces of large herbivores was quantified by performing multiple chemical extraction procedures for BSi, allowing the assessment of chemical reactivity. Dissolution experiments with grass and faeces were carried out to measure direct availability of BSi for dissolution. Average BSi and readily soluble silica numbers were higher in faeces as compared with grass or hay, and differences between herbivores could be related to distinct digestive strategies. Reactivity and dissolvability of BSi increases after digestion, mainly due to degradation of organic matrices, resulting in higher silica turnover rates and mobilization potential from terrestrial to aquatic ecosystems in non-grazed versus grazed pasture systems (2 versus 20 kg Si ha−1 y−1). Our results suggest a crucial yet currently unexplored role of herbivores in determining silica export from land to ocean, where its availability is linked to eutrophication events and carbon sequestration through C–Si diatom interactions.

Determination of plant silicon content with near infrared reflectance spectroscopy.

Silicon (Si) is one of the most common elements in the earth bedrock, and its continental cycle is strongly biologically controlled. Yet, research on the biogeochemical cycle of Si in ecosystems is hampered by the time and cost associated with the currently used chemical analysis methods. Here, we assessed the suitability of Near Infrared Reflectance Spectroscopy (NIRS) for measuring Si content in plant tissues. NIR spectra depend on the characteristics of the present bonds between H and N, C and O, which can be calibrated against concentrations of various compounds. Because Si in plants always occurs as hydrated condensates of orthosilicic acid (Si(OH)4), linked to organic biomolecules, we hypothesized that NIRS is suitable for measuring Si content in plants across a range of plant species. We based our testing on 442 samples of 29 plant species belonging to a range of growth forms. We calibrated the NIRS method against a well-established plant Si analysis method by using partial least-squares regression. Si concentrations ranged from detection limit (0.24 ppmSi) to 7.8% Si on dry weight and were well predicted by NIRS. The model fit with validation data was good across all plant species (n = 141, R2 = 0.90, RMSEP = 0.24), but improved when only graminoids were modeled (n = 66, R2 = 0.95, RMSEP = 0.10). A species specific model for the grass Deschampsia cespitosa showed even slightly better results than the model for all graminoids (n = 16, R2 = 0.93, RMSEP = 0.015). We show for the first time that NIRS is applicable for determining plant Si concentration across a range of plant species and growth forms, and represents a time- and cost-effective alternative to the chemical Si analysis methods. As NIRS can be applied concurrently to a range of plant organic constituents, it opens up unprecedented research possibilities for studying interrelations between Si and other plant compounds in vegetation, and for addressing the role of Si in ecosystems across a range of Si research domains.

Biogenic Si analysis in volcanically imprinted lacustrine systems: the case of Lake Rutundu (Mt. Kenya)

Diatoms are important primary producers in lake ecosystems and, as a sink for dissolved (DSi) and biogenic silica (BSi) originating from land, can significantly impact the global Si cycle. After burial in lake sediments, resistant diatom frustules can also be used for reconstructions of past ecosystem change. The BSi content of lake sediments is thus often used as a proxy for past diatom productivity, and measured using a time-step analysis of Si extracted in 0.1xa0M Na2CO3. However, studies in soils and ocean sediments have shown that also certain non-biogenic Si fractions are prone to dissolve in alkaline solutions, contributing to a potential overestimate of sedimentary BSi concentration. In lakes, volcanic and terrestrial Si compounds reactive at high pH are likely to interfere with the analysis. In this study we used a continuous analysis of Si and Al extracted in 0.5xa0M NaOH and a new mathematical model improved from Koning et al. (Aquat Geochem 8:37–67, 2002) to distinguish between biogenic and non-biogenic Si fractions in lake-sediment extracts. We tested this approach in 43 samples of a 19,000-year sediment sequence recovered from Lake Rutundu, a volcanic crater lake on Mt. Kenya. Our results show that a significant fraction of the extractable Si is of non-biogenic origin, especially in the part of the sequence deposited during the glacial period. We conclude that this technique is essential for the characterization of different Si phases, and in particular the diatom-derived BSi, in the sediment of lakes situated in volcanic catchments. It allows calculating a correction, based on the distinct Si:Al ratio of each of those Si phases, that eliminates the contribution from non-BSi fractions.

Dissolved Silicon and Its Origin in Belgian Beers—A Multivariate Analysis

Beer is a quintessential part of Belgian heritage. We performed a detailed analysis of factors controlling Si content in Belgian beers as a case study to coincide with the 2011 IBiS meeting in Antwerp (Belgium). Beer is one of the richest dietary sources of Si. Three decades of research have yielded evidence of a role for Si in human physiology: it plays an essential role in bone mineral density and reduces the biological availability of aluminium. We analysed 119 Belgian beers: highest dissolved Si concentrations were found in high fermentation, traditionally brewed ales. Concentrations ranged between 214 and 2,071xa0μmolxa0Lu2009−u20091. This is probably due to the complexity and length of the brewing procedure: longer, more complicated processing and presence of brewing sediment in the bottle allows more Si to dissolve out of the base products like hop, barley or even rice. As a side effect of fermentation, alcohol content was related to Si content.

Silica Content, Leaf Softness and Digestibility in Tall Fescue (Festuca arundinacea Schreb.)

In tall fescue breeding, a great deal of effort is devoted to develop soft-leaved varieties as it is generally accepted that leaf softness is positively correlated with digestibility and animal preference. In advanced breeding programs, it becomes difficult to discriminate the leaf softness between genotypes. Moreover, there is evidence that the digestibility of the softest varieties is not necessarily higher compared to varieties with coarser leaves. We studied the presence of trichomes (dentation) on the leaf margins and the silica content of plants in relation to the leaf softness and digestibility on a selection of 19 tall fescue clones. On average, soft genotypes had fewer trichomes (2.74 mm−1) on the leaf margins than coarse genotypes (9.03 mm−1), but there was no relation between leaf softness or trichome number and digestibility (R2 = 0.05), nor between silica content and softness or digestibility (R2 = 0.09).

Parameters Influencing Preference by Sheep in Soft Leaved Tall Fescue Genotypes

An important disadvantage of tall fescue (Festuca arundinacea Schreb.) is its low voluntary intake, resulting in suboptimal performances of cattle grazing tall fescue. Ideally selection for this trait is done by animals themselves, but the use of grazing animals in large breeding programmes is laborious. Repeatable, stable and quantifiable parameters that can be linked to animal preference could ease tall fescue breeding. We established a trial to find relations between the grazing preference of sheep and other plant parameters. Twenty clones were selected from a breeding programme and swards of 2 m2 were planted with three replications for each clone. On four different occasions in 2014, sheep were allowed to graze the clones and grazing preference was determined visually. Prior to the grazing, multiple morphological and chemical parameters were measured. Parameters that were correlated with sheep preference were digestibility (r = 0.86), water soluble carbohydrate content (r = 0.74) crude fiber content (r = −0.67), leaf blade width (r = 0.57) and sward height (r = −0.53).