The influence of weathering and soil organic matter on Zn isotopes in soils

Zinc is an essential micronutrient that is ultimately released during mineral weathering. In soils, organic matter plays a key role in influencing Zn partitioning and therefore on Zn biogeochemical cycling. Soil organic matter is partitioned between carbon that is more readily available for decomposition by microorganisms, and more stable carbon transiently preserved from decomposition. The role of the stable pool of soil organic matter on Zn biogeochemical cycling remains poorly understood. The pool of stable carbon is controlled by combination with mineral constituents or is material that is intrinsically resistant to decomposition. The Zn stable isotopes are fractionated by interactions between Zn and soil mineral and organic constituents. This study reports the Zn isotope composition of five Icelandic soil profiles derived from the same parent basalt and characterized by contrasting degrees of weathering and organic matter content (δ66Zn = + 0.10 ± 0.05 to + 0.35 ± 0.02‰), the distribution of reactive mineral constituents available to form associations with soil organic matter, and the amount of stable organic carbon. Throughout these soils, the δ66Zn isotope variations are little influenced by mineral constituents, but rather by soil organic matter content. These data suggest that a combination of organic matter accumulation and Zn loss by leaching is required to explain the observed decrease in Zn concentration in soils and lighter soil δ66Zn with increasing organic carbon content. These results suggest that the presence of stable organic carbon in soils provides a pool of light Zn, attributed to the Zn isotope signature of organic matter partially preserved from decomposition. Crucially, this stable organic carbon pool may also contribute to the formation of the light Zn isotope sink reported in organic-rich marine sediments, a key output required to explain the oceanic mass balance of Zn isotopes

Understanding Root Uptake of Nutrients, Toxic and Polluting Elements in Hydroponic Culture

peer reviewe

Tracing the origin of dissolved silicon transferred from various soil-plant systems towards rivers: a review

Silicon (Si) released as H4SiO4 by weathering of Si-containing solid phases is partly recycled through vegetation before its land-to-rivers transfer. By accumulating in terrestrial plants to a similar extent as some major macronutrients (0.1–10% Si dry weight), Si becomes largely mobile in the soil-plant system. Litter-fall leads to a substantial reactive biogenic silica pool in soil, which contributes to the release of dissolved Si (DSi) in soil solution. Understanding the biogeochemical cycle of silicon in surface environments and the DSi export from soils into rivers is crucial given that the marine primary bio-productivity depends on the availability of H4SiO4 for phytoplankton that requires Si. Continental fluxes of DSi seem to be deeply influenced by climate (temperature and runoff) as well as soil-vegetation systems. Therefore, continental areas can be characterized by various abilities to transfer DSi from soil-plant systems towards rivers. Here we pay special attention to those processes taking place in soil-plant systems and controlling the Si transfer towards rivers. We aim at identifying relevant geochemical tracers of Si pathways within the soil-plant system to obtain a better understanding of the origin of DSi exported towards rivers. In this review, we compare different soil-plant systems (weathering-unlimited and weathering-limited environments) and the variations of the geochemical tracers (Ge/Si ratios and d30Si) in DSi outputs. We recommend the use of biogeochemical tracers in combination with Si mass-balances and detailed physico-chemical characterization of soil-plant systems to allow better insight in the sources and fate of Si in these biogeochemical systems

Ge and Si Isotope Behavior During Intense Tropical Weathering and Ecosystem Cycling

Chemical weathering of volcanic rocks in warm and humid climates contributes disproportionately to global solute fluxes. Geochemical signatures of solutes and solids formed during this process can help quantify and reconstruct weathering intensity in the past. Here, we measured silicon (Si) and germanium (Ge) isotope ratios of the soils, clays, and fluids from a tropical lowland rainforest in Costa Rica. The bulk topsoil is intensely weathered and isotopically light (mean ± 1σ: δ³⁰Si = −2.1 ± 0.3‰, δ⁷⁴Ge = −0.13 ± 0.12‰) compared to the parent rock (δ³⁰Si = −0.11 ± 0.05‰, δ⁷⁴Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ³⁰Si = −2.5 ± 0.2‰, δ⁷⁴Ge = −0.16 ± 0.09‰), demonstrating a whole‐system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ³⁰Si = 0.2 − 0.6‰, δ⁷⁴Ge = 2.2 − 2.6‰) with solute‐rich interbasin groundwater (δ³⁰Si = 1.0 ± 0.2‰, δ⁷⁴Ge = 4.0‰). Using a Ge‐Si isotope mass balance model, we calculate that 91 ± 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays, 9 ± 9% by vegetation, and only 0.2 ± 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering 39 ± 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soil δ⁷⁴Ge closely tracks the depletion of Ge by chemical weathering (τGe), whereas δ³⁰Si and Ge/Si both reflect the loss of Si (τ_{Si}). Because of the different chemical mobilities of Ge and Si, a δ⁷⁴Ge‐δ³⁰Si multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation

Pet soil thickness and carbon storage in the Belgian High Fens: insights from multi-sensor UAV remote sensing

editorial reviewe

Quantifying exchangeable base cations in permafrost: a reserve of nutrients about to thaw

Permafrost ecosystems are limited in nutrients for vegetation development and constrain the biological activity to the active layer. Upon Arctic warming, permafrost thaw exposes large amounts of soil organic carbon (SOC) to decomposition and minerals to weathering but also releases organic and mineral soil material that may directly influence the soil exchange properties (cation exchange capacity, CEC, and base saturation, BS). The soil exchange properties are key for nutrient base cation supply (Ca2+, K+, Mg2+, and Na+) for vegetation growth and development. In this study, we investigate the distributions of soil exchange properties within Arctic tundra permafrost soils at Eight Mile Lake (Interior Alaska, USA) because they will dictate the potential reservoir of newly thawed nutrients and thereby influence soil biological activity and vegetation nutrient sources. Our results highlight much lower CEC density in surface horizons (∼9400 cmolc m−3) than in the mineral horizons of the active layer (∼16 000 cmolc m−3) or in permafrost soil horizons (∼12 000 cmolc m−3). Together, with the overall increase in CEC density with depth and the overall increase in BS (percentage of CEC occupied by exchangeable base cations Ca2+, K+, Mg2+, and Na+) with depth (from ∼19 % in organic surface horizons to 62 % in permafrost soil horizons), the total exchangeable base cation density (Ca2+, K+, Mg2+, and Na+ in g m−3) is up to 5 times higher in the permafrost than in the active layer. More specifically, the exchangeable base cation density in the 20 cm upper part of permafrost about to thaw is ∼850 g m−3 for Caexch, 45 g m−3 for Kexch, 200 g m−3 for Mgexch, and 150 g m−3 for Naexch. This estimate is needed for future ecosystem prediction models to provide constraints on the size of the reservoir in exchangeable nutrients (Ca, K, Mg, and Na) about to thaw. All data described in this paper are stored in Dataverse, the online repository of Université catholique de Louvain, and are accessible through the following DOI: https://doi.org/10.14428/DVN/FQVMEP (Mauclet et al., 2022b).</p

Investigating the role of changing permafrost hydrology on carbon and nutrient fluxes using very-high resolution remote sensing and geochemical techniques (Stordalen, Sweden)

editorial reviewe

Constraining modern day silicon cycling in Lake Baikal

Constraining the continental silicon cycle is a key requirement in attempts to understand both nutrient fluxes to the ocean and linkages between silicon and carbon cycling over different timescales. Silicon isotope data of dissolved silica (δ30SiDSi) are presented here from Lake Baikal and its catchment in central Siberia. As well as being the world's oldest and voluminous lake, Lake Baikal lies within the seventh largest drainage basin in the world and exports significant amounts of freshwater into the Arctic Ocean. Data from river waters accounting for c. 92% of annual river inflow to the lake suggest no seasonal alteration or anthropogenic impact on river δ30SiDSi composition. The absence of a change in δ30SiDSi within the Selenga Delta, through which 62% of riverine flow passes, suggest a net balance between biogenic uptake and dissolution in this system. A key feature of this study is the use of δ30SiDSi to examine seasonal and spatial variations in DSi utilisation and export across the lake. Using an open system model against deep water δ30SiDSi values from the lake, we estimate that 20-24% of DSi entering Lake Baikal is exported into the sediment record. Whilst highlighting the impact that lakes may have upon the sequestration of continental DSi, mixed layer δ30SiDSi values from 2003 and 2013 show significant spatial variability in the magnitude of spring bloom nutrient utilisation with lower rates in the north relative to south basin