Anthropogenic impact on amorphous silica pools in temperate soils

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<sub>a</sub>) and easily soluble (PSi<sub>e</sub>) Si pools at four sites along a gradient of anthropogenic disturbance in southern Sweden. Land use clearly affects ASi pools and their distribution. Total PSi</sub>a</sub> and PSi<sub>e</sub> for a continuous forested site at Siggaboda Nature Reserve (66 900 ± 22 800 kg SiO<sub>2</sub> ha<sup>−1</sup> and 952 ± 16 kg SiO<sub>2</sub> ha<sup>−1</sup>) are significantly higher than disturbed land use types from the Råshult Culture Reserve including arable land (28 800 ± 7200 kg SiO<sub>2</sub> ha<sup>−1</sup> and 239 ± 91 kg SiO<sub>2</sub> ha<sup>−1</sup>), pasture sites (27 300 ± 5980 kg SiO<sub>2</sub> ha<sup>−1</sup> and 370 ± 129 kg SiO<sub>2</sub> ha<sup>−1</sup>) and grazed forest (23 600 ± 6370 kg SiO<sub>2</sub> ha<sup>−1</sup> and 346 ± 123 kg SiO<sub>2</sub> ha<sup>−1</sup>). Vertical PSi<sub>a</sub> and PSi<sub>e</sub> profiles show significant (<i>p</i> < 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<sub>a</sub> 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<sup>−1</sup> 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

Amorphous silica analysis in terrestrial runoff samples

The correct analysis of amorphous silica concentration (CASi) in natural waters is crucial if one wants to correctly quantify terrestrial and/or riverine ASi fluxes. Soil ASi measurements are conducted with a constant solid to solution ratio (). As the suspended particulate matter concentration (CSPM), and therefore, cannot be exactly known a priori in river samples. It is important to understand how variations in effect analysed CASi. The objectives of this paper are (i) to investigate whether and how variations in values affect measured CASi in river runoff samples and (ii) to investigate whether or not it is possible to define a range of within which CASi in runoff and/or soil samples can be accurately measured. For the laboratory experiment 30 runoff samples with a wide range of CSPM, typical for the Belgian Loam Belt, were prepared and analysed using the alkaline digestion method (0.1M Na2CO3). Our study confirmed that the alkaline digestion method proposed by DeMaster can be used for runoff samples provided that is within certain limits: at very low (<0.1kgm-3), subsample heterogeneity results in high variability of measured CASi while at higher values (>0.8kgm-3) incomplete dissolution of ASi as well as the reduction of mineral dissolution rates results in underestimated CASi. As both errors compensate one another, the range of applicable -values can be extended above the theoretically correct limit (1.6kgm-3). The finding that reliable measurements can be made within a relatively wide range of values (0.1≤≤1.6kgm-3) is important. It is now possible to propose a method for the measurement of ASi in runoff samples. We make recommendations for ASi analysis distinguishing samples with a low and high CSPM. For samples with a low CSPM (≤1.6kgm-3) the standard procedure is proposed while for samples with a high CSPM (>1.6kgm-3) an adapted procedure is proposed, analogue to that for soil samples. However, one should be aware that the range and limits for proposed here may depend on the type of sediment to be analysed: it is therefore recommended to evaluate the performance of the method again before it is used in other environments. © 2011

The continental Si cycle and its impact on the ocean Si isotope budget

AbstractThe silicon isotope composition of biogenic silica (δ30SiBSi) in the ocean is a function of the δ30Si of the available dissolved Si (DSi; H2SiO4), the degree of utilisation of the available DSi, and, for some organisms, the concentration of DSi. This makes δ30SiBSi in sediment archives a promising proxy for past DSi concentrations and utilisation. At steady-state, mean δ30SiBSi must equal a weighted average of the inputs, the majority of which are of continental origin. Variation in the functioning of the continental Si cycle on timescales similar to the residence time of DSi in the ocean (~10ka) may therefore contribute to downcore variability in δ30SiBSi on millennial or longer timescales. The direction and magnitude of change in published δ30SiBSi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers. They document glacial values that are typically 0.5 to 1.0‰ lower than interglacial values and together hint at coherent and predictable glacial–interglacial variability in whole-ocean δ30Si driven by a change in mean δ30Si of the inputs. In this contribution, we review the modern inputs of DSi to the ocean and the controls on their isotopic composition, and assess the evidence for their variability on millennial-plus timescales.Today, 9.55×1012molyr−1 DSi enters the ocean, of which roughly 64% and 25% are direct riverine inputs of DSi, and DSi from dissolution of aeolian and riverborne sediment, respectively. The remainder derives from alteration or weathering of the ocean crust. Each input has a characteristic δ30Si, with our current best estimate for a weighted mean being 0.74‰, although much work remains to be done to characterise the individual fluxes. Many aspects of the continental Si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean δ30Si of DSi entering the ocean. These changes relate to i) a cooler, drier glacial climate, ii) lowered sea level and the exposure of continental shelves, iii) the presence of large continental ice-sheets, and iv) altered vegetation zonation.Using a simple box-model with a Monte-Carlo approach to parameterisation, we find that a transition from a hypothesised glacial continental Si cycle to the modern Si cycle can drive an increase in whole ocean δ30Si of comparable rate and magnitude to that recorded in δ30SiBSi. This implies that we may need to revisit our understanding of aspects of the Si cycle in the glacial ocean. Although we focus on the transition from the last glacial, our synthesis suggests that the continental Si cycle should be seen as a potential contributory factor to any variability observed in ocean δ30SiBSi on millennial or longer timescales

Catchment management strongly decreases the sediment transport in rivers: a comprehensive study in May Zeg-zeg (North Ethiopian Highlands)

An overall approach to assess the effectiveness of soil conservation measures at catchment scale is the comparison of sediment budgets before and after implementation of a catchment management programme. In the May Zeg-zeg catchment (187 ha) in Tigray, north Ethiopia, integrated catchment management has been implemented since 2004: stone bunds were built in the whole catchment, vegetation was allowed to regrow on steep slopes and other marginal land, stubble grazing partly abandoned, and check dams built in gullies. Land use and management were mapped and analysed for 2000 and 2006, whereby particular attention was given to the quantification of changes in soil loss due to the abandonment of stubble grazing. Sediment yield was also measured at the catchment‘s outlet. A combination of decreased soil loss (from 14.3 t ha-1 y-1 in 2000 to 9.0 t ha-1 y-1 in 2006) and increased sediment deposition (from 5.8 to 7.1 t ha-1 y-1) has led to strongly decreased sediment yield (from 8.5 to 1.9 t ha-1 y-1) and sediment delivery ratio (from 0.6 to 0.21). This diachronic comparison of sediment budgets revealed that integrated catchment management is most effective and efficient and is the advisable and desirable way to combat land degradation in Tigray and other tropical mountains

Differential effects of water erosion and tillage erosion on carbon dynamics on Arable Land

Global agricultural soil erosion has been proved to be a carbon sink. Water erosion and tillage erosion are the two dominant forms of soil redistribution processes in agricultural catchments. However, there is still no research trying to evaluate whether they play different roles in perturbing the carbon dynamics of agricultural land. By calibrating a spatially distributed soil erosion model on a small agricultural catchment, the proportion of the deposits by water erosion and tillage erosion at different positions of the soil bank formed at the field border can be estimated: grain size analysis confirms that the relative contribution of tillage and water vary with landscape position. The results derived from the water erosion model are further processed with a model elucidating soil organic matter dynamics through the soil profile that is calibrated with detailed measured soil carbon profiles both on the slope and at the soil bank. Different decomposition rates for the carbon deposited by different erosion processes were derived from model simulations by matching observed and simulated profiles of total carbon as well as of delta 13C. Incubation of soil cores at both water erosion and tillage erosion dominated deposits was also conducted so that an attempt can be made to discriminate between the effects of burial and carbon quality on carbon decomposition rates. Different effects of water erosion and tillage erosion on the carbon dynamics in the catchment were assessed and possible mechanisms behind them are discussed using evidence from total carbon content, delta 13C and grain size profiles

Silicate weathering in the Ganges alluvial plain

The Ganges is one of the world's largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny, weathering and global climate change. Three regions can be recognised in the Ganges basin, with the Himalayan orogeny to the north and the plateaus of peninsular India to the south together delimiting the Ganges alluvial plain. Despite constituting approximately 80% of the basin, weathering processes in the peninsula and alluvial plain have received little attention. Here we present an analysis of 51 water samples along a transect of the alluvial plain, including all major tributaries. We focus on the geochemistry of silicon and its isotopes. Area normalised dissolved Si yields are approximately twice as high in rivers of Himalaya origin than the plain and peninsular tributaries (82, 51 and 32 kmol SiO2 km(-2) yr(-1), respectively). Such dissolved Si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the DSi mobilised during the initial weathering process is retained in secondary clay minerals. However, the silicon isotopic composition of dissolved Si (expressed as delta Si-30) varies from +0.8 parts per thousand in the Ganges mainstem at the Himalaya front to +3.0 parts per thousand in alluvial plain streams and appears to be controlled by weathering congruency, i.e. by the degree of incorporation of Si into secondary phases. The higher delta Si-30 values therefore reflect decreasing weathering congruency in the lowland river catchments. This is exploited to quantify the degree of removal using a Rayleigh isotope mass balance model, and consequently derive initial silica mobilisation rates of 200, 150 and 107 kmol SiO2 km(-2) yr(-1), for the Himalaya, peninsular India and the alluvial plain, respectively. Because the non-Himalayan regions dominate the catchment area, the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment (41% and 34%, respectively). (C) 2015 The Authors. Published by Elsevier B.V.Co7hs Times Cited:5 Cited References Count:63</p

The fate of buried organic carbon in colluvial soils: a long-term perspective

Colluvial soils are enriched in soil organic car- bon (SOC) in comparison to the soils of upslope areas due to the deposition and progressive burial of SOC. This burial of SOC has important implications for the global carbon cycle, but the long-term dynamics of buried SOC remain poorly constrained. We addressed this issue by determining the SOC burial efficiency (i.e. the fraction of originally de- posited SOC that is preserved in colluvial deposits) of buried SOC as well as the SOC stability in colluvial soils. We quan- tified the turnover rate of deposited SOC by establishing sed- iment and SOC burial chronologies. The SOC stability was derived from soil incubation experiments and the δ 13 C val- ues of SOC. The C burial efficiency was found to decrease with time, reaching a constant ratio of approximately 17 % by about 1000–1500 yr post-burial. This decrease is attributed to the increasing recalcitrance of the remaining buried SOC with time and a less favourable environment for SOC decom- position with increasing depth. Buried SOC in colluvial pro- files was found to be more stable and degraded in compari- son to SOC sampled at the same depth at a stable reference location. This is due to the preferential mineralisation of the labile fraction of the deposited SOC. Our study shows that SOC responds to burial over a centennial timescale; how- ever, more insight into the factors controlling this response is required to fully understand how this timescale may vary, depending on specific conditions such as climate and depo- sitional environment

Estimated storage of amorphous silica in soils of the circum-Arctic tundra region

We investigated the vertical distribution, storage, landscape partitioning, and spatial variability of soil amorphous silica (ASi) at four different sites underlain by continuous permafrost and representative of mountainous and lowland tundra, in the circum-Arctic region. Based on a larger set of data, we present the first estimate of the ASi soil reservoir (0-1 m depth) in circum-Arctic tundra terrain. At all sites, the vertical distribution of ASi concentrations followed the pattern of either (1) declining concentrations with depth (most common) or (2) increasing/maximum concentrations with depth. Our results suggest that a set of processes, including biological control, solifluction and other slope processes, cryoturbation, and formation of inorganic precipitates influence vertical distributions of ASi in permafrost terrain, with the capacity to retain stored ASi on millennial timescales. At the four study sites, areal ASi storage (0-1 m) is generally higher in graminoid tundra compared to wetlands. Our circum-Arctic upscaling estimates, based on both vegetation and soil classification separately, suggest a storage amounting to 219 ± 28 and 274 ± 33 Tmol Si, respectively, of which at least 30% is stored in permafrost. This estimate would account for about 3% of the global soil ASi storage while occupying an equal portion of the global land area. This result does not support the hypothesis that the circum-Arctic tundra soil ASi reservoir contains relatively higher amounts of ASi than other biomes globally as demonstrated for carbon. Nevertheless, climate warming has the potential to significantly alter ASi storage and terrestrial Si cycling in the Arctic