Continental export of si to the coastal zone is closely linked to the ocean carbon sink and to the dynamics of phytoplankton blooms in coastal ecosystems. Presently, however, the impact of human cultivation of the landscape on terrestrial si fluxes remains unquantified and is not incorporated in models for terrestrial si mobilization. In this paper, we show that land use is the most important controlling factor of si mobilization in temperate European watersheds, with sustained cultivation ( > 250 years) of formerly forested areas leading to a twofold to threefold decrease in baseflow delivery of si. This is a breakthrough in our understanding of the biogeochemical si cycle: it shows that human cultivation of the landscape should be recognized as an important controlling factor of terrestrial si fluxes.

Silicon is one of the most important elements in the current age of the anthropocene. It has numerous industrial applications, and supports a high-tech multibillion Euro industry. Silicon has a fascinating biological and geological cycle, interacting with other globally important biogeochemical cycles. In this review, we bring together both biological and geological aspects of the silicon cycle to provide a general, comprehensive review of the cycling of silicon in the environment. We hope this review will provide inspiration for researchers to study this fascinating element, as well as providing a background environmental context to those interested in silicon.The Earth's crust consists primarily of silicates (Si oxides, 90% of all minerals); consequently silicon (hereafter referred to as Si) is the second-most abundant element in the earth's crust (28.8%) after oxygen [1]. During silicate weathering dissolved soil CO 2 is used in a reaction where ortho-silicic acid (H 4 SiO 4 ) is dissolved and released from the crystalline structure of silicate minerals. In the environment dissolved silicate (DSi), i.e. ortho-silicic acid (H 4 SiO 4 ), is transported through soil and exported to rivers and eventually the ocean [2] (Fig. 1). The silicate weathering process consumes CO 2 . For example, in the weathering of anorthite (over kaolinite) to gibbsite, DSi is produced and CO 2 is consumed [3]: CaAl 2 Si 2 O 8 þ2:CO 2 þ8:H 2 O!Ca 2þ þ2:Al OH ð Þ 3 þ2:H 4 SiO 4 þ2:HCO À 3Ca 2þ þ 2HCO À 3 ! CaCO 3 þ H 2 CO 3

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.

Question How does the interaction between silicon (Si) and vegetation affect local and global ecological processes, higher levels of ecological organization, and terrestrial‐ and watershed‐scale Si fluxes? Location We selected several ecosystems throughout the world, from river headwaters to estuaries, being examples of (i) terrestrial vegetation, (ii) aquatic and floodplain vegetation, and (iii) tidal wetland vegetation. Methods We provide examples of the importance of linking Si use by terrestrial and aquatic vegetation, to larger‐scale Si flux consequences towards and through rivers. Cross‐disciplinary studies achieve the best understanding of vegetation effects on the global Si cycle, and the role of Si as a plant functional trait. Conclusion Si use by plants has not always received the research attention of other elements. Yet, today the importance of Si for plant functioning is slowly becoming better understood. Silicon is a crucial element for many plant species, being important for decomposition processes, plant competitiveness and stress tolerance. The inclusion by vegetation scientists of Si uptake as a plant functional trait is important to assess links between plant physiology, plant distribution and plant tolerance to environmental changes, but also to understand the role of vegetation on Si fluxes through the watershed. However, lack of knowledge regarding the biological control of the Si cycle hinders accurate quantification. Only a concerted effort bringing scientists together from a broad array of disciplines will provide this new direction for research on vegetation–Si cycling.