Anthropogenic impact on amorphous silica pools in temperate soils

Abstract. Human land use changes perturb biogeochemical silica (Si) cycling in terrestrial ecosystems. This directly affects Si mobilisation and Si storage and influences Si export from the continents, although the magnitude of the impact is unknown. A major reason for our lack of understanding is that very little information exists on how land use affects amorphous silica (ASi) storage in soils. We have quantified and compared total alkali-extracted (PSi_a) and easily soluble (PSi_e) Si pools at four sites along a gradient of anthropogenic disturbance in southern Sweden. Land use clearly affects ASi pools and their distribution. Total PSia and PSi_e for a continuous forested site at Siggaboda Nature Reserve (66 900 ± 22 800 kg SiO₂ ha⁻¹ and 952 ± 16 kg SiO₂ ha⁻¹) are significantly higher than disturbed land use types from the Råshult Culture Reserve including arable land (28 800 ± 7200 kg SiO₂ ha⁻¹ and 239 ± 91 kg SiO₂ ha⁻¹), pasture sites (27 300 ± 5980 kg SiO₂ ha⁻¹ and 370 ± 129 kg SiO₂ ha⁻¹) and grazed forest (23 600 ± 6370 kg SiO₂ ha⁻¹ and 346 ± 123 kg SiO₂ ha⁻¹). Vertical PSi_a and PSi_e profiles show significant (p < 0.05) variation among the sites. These differences in size and distribution are interpreted as the long-term effect of reduced ASi replenishment, as well as changes in ecosystem specific pedogenic processes and increased mobilisation of the PSi_a in disturbed soils. We have also made a first, though rough, estimate of the magnitude of change in temperate continental ASi pools due to human disturbance. Assuming that our data are representative, we estimate that total ASi storage in soils has declined by ca. 10 % since the onset of agricultural development (3000 BCE). Recent agricultural expansion (after 1700 CE) may have resulted in an average additional export of 1.1 ± 0.8 Tmol Si yr⁻¹ from the soil reservoir to aquatic ecosystems. This is ca. 20 % to the global land-ocean Si flux carried by rivers. It is necessary to update this estimate in future studies, incorporating differences in pedology, geology and climatology over temperate regions, but data are currently not sufficient. Yet, our results emphasize the importance of human activities for Si cycling in soils and for the land-ocean Si flux.