Besides well-known effects of climate and parent material on silicate weathering the role of land use change as a driver in the global silicon cycle is not well known. Changes in vegetation cover have altered reservoirs of silicon and carbon in plants and soils. This has potential consequences for plant-Si availability, agricultural yields, and coastal eutrophication, as Si is a beneficial element for many crop plants and an essential nutrient for diatom growth. We here examined the role of sustained and intensive land use and human disturbance on silicon (Si) pool distribution in soils with similar climatological and bulk mineralogical characteristics. We show that land use impacts both biogenic and nonbiogenic Si pools. While biogenic Si strongly decreases along the land use change gradient (from forest to croplands), pedogenic silica fractions (e.g. pedogenic clays) increase in topsoils with a long duration of cultivation and soil disturbance. Our results suggest that nonbiogenic Si pools might compensate for the loss of reactive biogenic silicon in temperate zones.

Diffuse phosphorus (P) export from agricultural land to surface waters is a significant environmental problem. It is critical to determine the natural background P losses from diffuse sources, but their identification and quantification is difficult. In this study, three headwater catchments with differing land use (arable, pasture and forest) were monitored for 3 years to quantify exports of dissolved (<0.45 µm) reactive P and total dissolved P. Mean total P exports from the arable catchment ranged between 0.08 and 0.28 kg ha−1 year−1. Compared with the reference condition (forest), arable land and pasture exported up to 11-fold more dissolved P. The contribution of dissolved (<0.45 µm) unreactive P was low to negligible in every catchment. Agricultural practices can exert large pressures on surface waters that are controlled by hydrological factors. Adapting policy to cope with these factors is needed for lowering these pressures in the future.

Dissolved organic C (DOC) plays an important role in the cycling and distribution of energy and nutrients. However, factors controlling the transport of DOC both within and between ecosystems are not clear. The aim of this work was to identify the contributing pathways for transport of DOC to surface water in catchments contrasting in land use and hydrogeology and during different flow regimes. Stream water was sampled to observe temporal variation of DOC concentrations and quality both seasonally and at the time scale of a rain event. Major cation and silica concentrations in stream water, groundwater, soil pore water, precipitation/throughfall, and riparian zone water samples were combined in an end‐member mixing analysis to determine the contributing end‐members for DOC delivery at the catchment outlet. Results show that the change in DOC concentrations and quality observed in the stream water during a rain event can be explained by a change in contribution of the different end‐members. In the forested catchments with deep groundwater tables, the main pathway for DOC transport from the soil to the surface water during base flow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, DOC in the stream mainly originated from seeps. During rain events, contributions from a surficial transport pathway and riparian zone water gained importance. The importance of contributing pathways changed seasonally and highly depended on the degree of saturation of the vadose zone.

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).

20This paper reviews the processes which determine the concentrations of dissolved silicon (DSi) in