Landscape cultivation alters δ³⁰Si signature in terrestrial ecosystems

Abstract

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 (δ(30)Si) 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 δ(30)Si signatures along the land use change gradient, highlighting the imprint of vegetation cover, human cultivation and intensity of disturbance on δ(30)Si patterns, on top of more conventionally acknowledged drivers (i.e. mineralogy and climate).

Figure 1. Bar charts of soil water DSi δ³⁰Si in different sites.

From top to bottom: (a) deciduous forest (Ronquières), (b) pasture (Blégny), (c) young cropland (Ganspoel) and (d) old cropland (Velm). Data are categorised in 19 size categories of δ³⁰Si, ranging from −0.1 to 1.8‰. The dark part of the bar shows the samples in the unsaturated soil, the grey parts of bars show the samples collected in the saturated soil. See supplementary information for more details on δ³⁰Si values and sampling selection.

Figure 2

(a) Scatterplot of biogenic silica (BSi) in mg g⁻¹ dry soil in the soil profile, (b) Total reserve in bases (TRB) weathering index ( = Na+Ca+K+Mg) calculated from soil. Sites are represented by symbols: Ronquières (circles), Blégny (stars), Ganspoel (triangle) and Velm (squares). Multiple symbols within a site in (b) represent different TRB values calculated from positions and depths along the slope in every site for which soil water δ³⁰Si are available, i.e. 3 in forests, 6 in pasture, 5 in young cropland and 7 in old cropland (See supplementary information for details).

Figure 3. Conceptual outline of land use effects on soil water DSi δ³⁰Si in a temperate weathering regime.

Green arrows represent cycling and/or export of ²⁸Si, dashed arrows represent land use changes: (1) from forest (a) to pasture (b), (2a) from forest to cropland (c) and (2b) from pasture to cropland and (3) from recent (c) to continuous (d) cropland. In (a) light isotopes are intensively recycled (precipitation and dissolution), resulting in an average low δ³⁰Si signature in soil water, with a high variability. In (b) not all biomass (and light Si isotopes) is recycled, and optimal conditions for enhanced weathering and clay dissolution are absent. Soil water δ³⁰Si shifts to higher values. In c, negligible biomass returns, resulting in an increased export of ²⁸Si. In d the pool of ²⁸Si is completely exhausted and continuous harvest of ²⁸Si results in a high and consistent soil water DSi δ³⁰Si signature.