Grasses Load Up on Silicon While Legumes Bank Calcium, Revealing a Hidden Chemical Divide in Grasslands

Walk through a European grassland in late spring and you see a tapestry of grasses, legumes and leafy herbs competing for light, water and nutrients. What you cannot see is a chemical divide running through that tapestry. A large field study across 45 semi-natural meadows in Germany has now shown that two overlooked mineral elements, silicon and calcium, split these plant groups apart in striking and contrasting ways, with grasses accumulating vastly more silicon and legumes far more calcium. The findings, published in the journal Plant and Soil, suggest that ecologists may be missing a fundamental dimension of plant strategy by ignoring these elements in trait-based frameworks.

The research team, led by Nahid R. Paeenroudposhti of the Leibniz Centre for Agricultural Landscape Research together with Jörg Schaller and Joana Bergmann, worked within the Biodiversity Exploratories, a long-term research infrastructure spanning three geographically distinct regions of Germany: the Schorfheide-Chorin in the north-east, the Hainich-Dün in the centre, and the Swabian Alb in the south. To keep land-use effects from clouding the picture, the team sampled plots managed at reduced intensity, where fertilisation had stopped and mowing occurred only once per year. In May 2023, they collected fresh leaves and topsoil samples from plots spanning all three regions, targeting twelve herbaceous species, four from each of the three functional groups: grasses, forbs and legumes.

Because not every species grew in every plot, the final dataset comprised 296 composite leaf samples, each pooling leaves from three individual plants of a species within a plot. Back in the laboratory, the leaves were dried, ground to a fine powder and analysed. Silicon concentrations were measured using alkaline extraction in hot sodium carbonate solution, while calcium required microwave-assisted digestion with nitric acid and hydrogen peroxide. Both elements were quantified with inductively coupled plasma optical emission spectroscopy. Soil availability of the two elements was assessed with the Mehlich III extraction method, a standard agroeconomic test that uses a cocktail of ammonium fluoride, EDTA, ammonium nitrate, nitric acid and acetic acid to pull loosely bound nutrients from the soil. The researchers also scanned fresh leaves to calculate specific leaf area, or SLA, the light-capturing surface a leaf builds per unit of dry mass, a cornerstone trait of the global leaf economic spectrum that ranges from fast, acquisitive leaves to slow, conservative ones.

The headline result was a dramatic functional-group split in silicon. Grasses averaged 83.3 milligrams of silicon per gram of leaf tissue, roughly three times the concentration found in forbs at 27.3 milligrams per gram, and an astonishing forty-nine times that of legumes, which averaged just 1.70 milligrams per gram. Calcium told the inverse story. Legumes led with 18.2 milligrams per gram and forbs followed closely at 14.7, while grasses trailed at just 5.40 milligrams per gram, a 63 to 70 percent reduction relative to the other two groups. When the team accounted for these group differences statistically using linear mixed-effects models with species identity and plot as random effects, functional group emerged as the dominant driver of both elements.

The mechanistic explanation for the grass-silicon connection lies in molecular plumbing. Although silicon is not classified as an essential nutrient, most plants take it up passively as monosilicic acid dissolved in the transpiration stream. Grasses, however, possess a specialised cooperative transport system: influx channels of the Lsi1 family and efflux transporters of the Lsi2 family, positioned in root cells to move silicic acid directionally from soil into the xylem. Many grass Lsi1 proteins carry a conserved GSGR selectivity filter, a narrow molecular pore that admits silicic acid while excluding other solutes. This active machinery allows grasses to load silicon far beyond what passive uptake could achieve, underpinning their role as ecosystem-level silicon accumulators. Legumes, which generally lack efficient versions of these transporters, depend on the transpiration stream alone, explaining their minimal silicon content. Forbs proved anything but uniform: the team found that yarrow, Achillea millefolium, accumulated silicon at grass-like levels, and removing it from the analysis strengthened the functional-group effect substantially, echoing previous evidence that some dicot lineages, including members of the daisy family, carry silicon-permeable aquaporins of the NIP-III type.

Calcium follows an entirely different uptake logic. As an essential element, it stabilises cell walls, regulates signalling, supports pathogen resistance and is frequently stored as calcium oxalate crystals that stiffen tissue and deter herbivores. Its entry into roots occurs largely passively through calcium-permeable channels driven by a steep electrochemical gradient, with delivery to leaves riding the transpiration stream. That passive, supply-driven pathway predicted exactly what the field data showed: leaf calcium rose significantly with soil calcium availability across all three functional groups, with a common positive slope. Leaf silicon, by contrast, showed no significant relationship to Mehlich III-extractable soil silicon in any group. Silicon accumulation, it appears, is governed far more by species identity and transporter genetics than by soil supply, at least within the range of availability captured in these grasslands. The authors also caution that short laboratory extractions may imperfectly represent what roots access over a growing season, since plants can mobilise silicon from amorphous soil pools through root exudates and microbial interactions.

One of the most thought-provoking findings concerns the apparent trade-off between the two elements. Across all observations, leaf silicon and leaf calcium were strongly negatively correlated, with a correlation coefficient of minus 0.70, tightening to minus 0.85 when species means were compared. On its face, this looks like plants choosing between two alternative leaf-building materials. But the mixed-effects models revealed something subtler: once functional group was accounted for, the overall negative effect of calcium on silicon disappeared, and within individual species the correlations ranged widely, from moderately negative to clearly positive, with only four of twelve species showing a significant relationship. The apparent trade-off, in other words, is largely an artefact of grasses and legumes sitting at opposite ends of both elemental gradients. It is a community-level pattern, not a universal physiological law operating leaf by leaf.

The study then asked whether these elements connect to the leaf economic spectrum. Silicon showed no significant relationship with specific leaf area at any scale examined, whether across the whole dataset or within groups. That ambiguity makes biological sense: silicon can promote growth, which would link it to acquisitive leaves, yet it also serves as an abrasive anti-herbivore defence, which would associate it with tough, conservative leaves, and previous syntheses have likewise found leaf silicon largely decoupled from leaf economic traits at broad scales. Calcium produced a more tentative but intriguing pattern. Its relationship with SLA differed significantly among functional groups, running marginally negative in legumes, possibly because calcium oxalate crystals add mass to leaf tissue, and marginally positive in forbs, while grasses showed a non-significant positive trend that might reflect transpiration-driven calcium delivery to fast-growing leaves. The team stressed that this calcium-SLA association was sensitive to species composition: removing red clover, which accounted for about a third of legume samples, collapsed the interaction entirely, so they recommend larger species pools before drawing firm conclusions.

Why should this matter beyond the pages of a soil journal? The elemental fingerprints of grasses and legumes ripple through entire ecosystems. Previous work has shown that silicon-rich grass litter accelerates decomposition while suppressing fungal biomass, whereas calcium-rich legume litter promotes fungal decomposition pathways. Shifts in the relative abundance of grasses and legumes, whether driven by land use, nitrogen deposition or climate change, could therefore reshape decomposer communities and nutrient cycling through changes in litter silicon and calcium stoichiometry, effects invisible to trait frameworks that track only carbon, nitrogen and phosphorus.

The authors argue that silicon and calcium carry unique information that cannot be predicted from classical leaf economic traits and should be measured directly and integrated as complementary dimensions in plant functional trait ecology, particularly in field studies. As the first field-based evidence across multiple co-occurring species and functional groups in semi-natural central European grasslands, the work bridges the gap between controlled greenhouse experiments and global syntheses, and points toward future research combining elemental analysis with direct characterisation of biomineral forms, broader species pools and wider land-use gradients, so that the hidden chemistry of leaves can finally take its place alongside the familiar currency of plant ecology.

Subject of Research: Variation of leaf silicon and calcium concentrations among grassland plant functional groups and their relationship to soil availability and specific leaf area

Article Title: Leaf silicon and calcium differ between grassland plant functional groups while only calcium responds to soil background

Article References: Leaf silicon and calcium differ between grassland plant functional groups while only calcium responds to soil background. (n.d.). https://doi.org/10.1007/s11104-026-09063-9

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09063-9

Keywords: silicon, calcium, grassland ecology, plant functional groups, ionome, specific leaf area, leaf economic spectrum, soil chemistry, plant physiology, trait-based ecology, litter decomposition, Biodiversity Exploratories