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Graphical abstract Highlights • Abiotic nitrate incorporation into DON enhances N-retention in temperate rainforest. • About 25% of 15N-nitrate was transformed to DON supporting Ferrous Wheel Hypothesis. • New N-cycle model is proposed where Ferrous Wheel Hypothesis is included. • N 2 O provided further evidence of NO 3 − reduction and CO 2 of DOM oxidation. Abstract We evaluated the abiotic formation of dissolved organic nitrogen (DON) by the fast reaction of iron (Fe) with nitrate (NO 3 −) in the dissolved organic matter (DOM) of volcanic soils in a temperate rainforest (>5000 mm precipitation per year). During five days, the educts and products of abiotic reactions under anoxic conditions were measured in a microcosm experiment depending on the Fe and NO 3 − concentrations. A control zero-Fe was not used because there was no chemical reaction with nitrate addition. Using a novel technique of automated sample preparation for inorganic N (SPIN) attached to a membrane inlet quadrupole mass spectrometry (MIMS), the 15N abundances and inorganic N concentrations were determined directly in aqueous solutions. The results were explained in the context of the Ferrous Wheel Hypothesis which states that Fe(II) is utilized to reduce NO 3 − to nitrite (NO 2 −) that is incorporated into DOM. Fe(II) is regenerated from Fe(III) in anaerobic soil microsites. Here we tested one part of this hypothesis, the processes occurring in DOM (instead of soil organic matter). Using the SPIN-MIMS technique, we could overcome Ferrous Wheel Hypothesis criticism regarding possible Fe interference during NO 3 − analysis. The total recovery of 15N added as NO 3 − fluctuated between 63 and 101%, and the remaining 15N was measured as gaseous N 2 O. The 15N-labelled NO 3 − added decreased immediately after 15 min of incubation. After five days of incubation, approximately 25% of the labelled NO 3 − (e− acceptors) added was transformed to DON in the presence of a high amount of Fe(II) (e− donors). Small amounts of N 2 O and CO 2 provided further evidence of NO 3 − reduction and DOM oxidation, respectively. From these results, we propose a new theoretical model that includes the Ferrous Wheel Hypothesis, where only the transformation of NO 3 − to DON was proven. The present results explain the high retention of NO 3 − in DOM from volcanic soils in ecosystems with high precipitation. [ABSTRACT FROM AUTHOR]

The Chilean Coastal Cordillera features a spectacular climate and vegetation gradient, ranging from arid and unvegetated areas in the north to humid and forested areas in the south. The EarthShape project (“ Earth Surface Shaping by Biota ”) uses this natural gradient to investigate how climate and biological processes shape the Earth's surface. We explored the Critical Zone, the Earth's uppermost layer, in four key sites located in desert, semidesert, Mediterranean, and temperate climate zones of the Coastal Cordillera, with the focus on weathering of granitic rock. Here, we present first results from 16 approximately 2 m-deep regolith profiles to document: (1) architecture of weathering zone; (2) degree and rate of rock weathering, thus the release of mineral-derived nutrients to the terrestrial ecosystems; (3) denudation rates; and (4) microbial abundances of bacteria and archaea in the saprolite. From north to south, denudation rates from cosmogenic nuclides are ~10 t km −2  yr −1 at the arid Pan de Azúcar site, ~20 t km −2  yr −1 at the semi-arid site of Santa Gracia, ~60 t km −2  yr −1 at the Mediterranean climate site of La Campana, and ~30 t km −2  yr −1 at the humid site of Nahuelbuta. A and B horizons increase in thickness and elemental depletion or enrichment increases from north (~26°S) to south (~38°S) in these horizons. Differences in the degree of chemical weathering, quantified by the chemical depletion fraction (CDF), are significant only between the arid and sparsely vegetated site and the other three sites. Differences in the CDF between the sites, and elemental depletion within the sites are sometimes smaller than the variations induced by the bedrock heterogeneity. Microbial abundances (bacteria and archaea) in saprolite substantially increase from the arid to the semi-arid sites. With this study, we provide a comprehensive dataset characterizing the Critical Zone geochemistry in the Chilean Coastal Cordillera. This dataset confirms climatic controls on weathering and denudation rates and provides prerequisites to quantify the role of biota in future studies. [ABSTRACT FROM AUTHOR]

The effects of climate and topography on soil physico-chemical and microbial parameters were studied along an extensive latitudinal climate gradient in the Coastal Cordillera of Chile (26°–38°S). The study sites encompass arid (Pan de Azúcar), semiarid (Santa Gracia), mediterranean (La Campana) and humid (Nahuelbuta) climates and vegetation, ranging from arid desert, dominated by biological soil crusts (biocrusts), semiarid shrubland and mediterranean sclerophyllous forest, where biocrusts are present but do have a seasonal pattern to temperate-mixed forest, where biocrusts only occur as an early pioneering development stage after disturbance. All soils originate from granitic parent materials and show very strong differences in pedogenesis intensity and soil depth. Most of the investigated physical, chemical and microbiological soil properties showed distinct trends along the climate gradient. Further, abrupt changes between the arid northernmost study site and the other semi-arid to humid sites can be shown, which indicate non-linearity and thresholds along the climate gradient. Clay and total organic carbon contents (TOC) as well as Ah horizons and solum depths increased from arid to humid climates, whereas bulk density (BD), pH values and base saturation (BS) decreased. These properties demonstrate the accumulation of organic matter, clay formation and element leaching as key-pedogenic processes with increasing humidity. However, the soils in the northern arid climate do not follow this overall latitudinal trend, because texture and BD are largely controlled by aeolian input of dust and sea salts spray followed by the formation of secondary evaporate minerals. Total soil DNA concentrations and TOC increased from arid to humid sites, while areal coverage by biocrusts exhibited an opposite trend. Relative bacterial and archaeal abundances were lower in the arid site, but for the other sites the local variability exceeds the variability along the climate gradient. Differences in soil properties between topographic positions were most pronounced at the study sites with the mediterranean and humid climate, whereas microbial abundances were independent on topography across all study sites. In general, the regional climate is the strongest controlling factor for pedogenesis and microbial parameters in soils developed from the same parent material. Topographic position along individual slopes of limited length augmented this effect only under humid conditions, where water erosion likely relocated particles and elements downward. The change from alkaline to neutral soil pH between the arid and the semi-arid site coincided with qualitative differences in soil formation as well as microbial habitats. This also reflects non-linear relationships of pedogenic and microbial processes in soils depending on climate with a sharp threshold between arid and semi-arid conditions. Therefore, the soils on the transition between arid and semi-arid conditions are especially sensitive and may be well used as indicators of long and medium-term climate changes. Concluding, the unique latitudinal precipitation gradient in the Coastal Cordillera of Chile is predestined to investigate the effects of the main soil forming factor – climate – on pedogenic processes. [ABSTRACT FROM AUTHOR]

Mechanisms of carbon dioxide (CO 2) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of H 2 O 2 with Fe(II) yielding a hydroxyl radical (OH), in combination with manganese peroxidase (MnP) and lignin peroxidase (LiP). This study aimed to explain the high rate of soil organic matter (SOM) mineralisation and CO 2 release from humid temperate rainforest soils under oxygen-limited conditions. The investigated mechanisms challenge the traditional view that SOM mineralisation in rainforest is slow due to anaerobic (micro)environments under high precipitation and explain intensive CO 2 release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO 2 released from SOM mineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. We used a novel technique that combines labelled H 2 18O 2 and Fe(II) to induce the FR and measured CO18O, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO 2 amount was released when the FR was induced in combination with LiP addition. The CO 2 efflux with LiP was 10-fold that of abiotic FR reactions without enzymes, or in soils amended with MnP. This was consistent with i) the contribution of 18O from peroxide to CO 2 release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the CO18O derived from soil without enzyme addition or with LiP addition. Concluding, abiotic Fenton Reaction coupled with oxidative enzymes, such as LiP, are crucial for SOM oxidation under anaerobic conditions, e.g. in temperate rainforest soils. Unlabelled Image • Fenton reaction and oxidative enzymes explain high CO 2 efflux from anaerobic soils. • Synergism of Fenton reaction and lignin peroxidase activity induce high CO 2 efflux. • H 2 18O 2 labelling allows identifying and differentiating abiotic oxidative processes. [ABSTRACT FROM AUTHOR]