Speciation of heavy metals in soils and their immobilization at micro-scale interfaces among diverse soil components.

The molecular composition of soil organic carbon remains contentious. Microbial-, plant- and fire-derived compounds may each contribute, but whether they vary predictably among ecosystems remains unclear. Here we present carbon functional groups and molecules from a diverse spectrum of North American surface mineral soils, collected primarily from the National Ecological Observatory Network and quantified by nuclear magnetic resonance spectroscopy and a molecular mixing model. We find that soils vary widely in relative contributions of carbohydrate, lipid, protein, lignin and char-like carbon, but each compound class has similar overall abundance. Ninety percent of the variance in carbon composition can be explained by three principal component axes representing a trade-off between lignin and protein, a trade-off between carbohydrate and char, and lipids. Reactive aluminium, crystalline iron oxides and pH plus overlying organic horizon thickness—predictors that are all related to climate—best explain variation along each respective axis. Together, our data point to continental-scale trade-offs in soil carbon molecular composition that are linked to environmental and geochemical variables known to predict carbon mass concentrations. Controversies regarding the genesis of soil carbon and its potential responses to global change can be partially reconciled by considering diverse ecosystem properties that drive complementary persistence mechanisms. Environmental factors influence the molecular composition of carbon in soils across continental gradients, according to analyses of North American mineral soils.

Molecular trade-offs in soil organic carbon composition at continental scale

Iron hydroxides serve as an efficient ‘rusty sink’ promoting the stabilization of rhizodeposits into soil organic carbon (SOC). Our work aimed to understand the physicochemical and microbial mechanisms promoting rhizodeposit (rhizo-C) stabilization as influenced by goethite (α-FeOOH) or nitrogen (N), using 13C natural abundance methodologies and DNA sequencing, in the rhizosphere of maize (Zea mays L.). The addition of N fertilizer to soil increased the mineralization of both rhizo-C and SOC, while amendment with α-FeOOH decreased rhizo-C derived CO2 and lowered the rhizosphere priming effect by 0.57 and 0.74-fold, respectively, compared to the control soil. This decrease resulted from the co-precipitation of rhizo-C at the reactive α-FeOOH surfaces as Fe-organic matter complexes (FeOM), which was 10-times greater than the co-precipitation on short-range ordered minerals. The highest portion of rhizo-C (67% of the total accumulated in soil) was protected within macroaggregates (>2 mm). Carbon overlapped with α-FeOOH mainly in >2 mm aggregates, as shown by HRTEM-EDS imaging, suggesting that α-FeOOH associated rhizo-C stimulated aggregate formation. Random forest analysis confirmed that the stabilization of rhizo-C was controlled mainly by physiochemical binding within FeOM complexes and macroaggregates. Rhizo-C mineralization was regulated by the keystone microbiome: Paucimonas (β-Proteobacteria) being an r-strategist with rapid growth under soil without nutrient limitation (N treated) and Steroidobacter (Actinobacteria) with branched filaments that can access C and nutrients under oligotrophic conditions (goethite enriched soil). Two-way orthogonal partial least squares analysis revealed that the rhizosphere priming effect was facilitated mainly by the same genera, most likely due to co-metabolism. The genera belonging to Acidimicrobiaceae (Actinobacteria), Cryptococcus and Cystofilobasidium (Basidiomycota) were positively correlated with the accumulation of rhizo-C in the >2 mm aggregate size, which might due to their high affinity towards α-FeOOH and contribution to the development of aggregation via filamentary structures that interact with microaggregates. We suggest that rhizodeposit stabilization in soil was balanced by microbial mineralization and abiotic associations with the “rusty sink” and organisms with branched filaments contributing to the development of aggregation.

Rusty sink of rhizodeposits and associated keystone microbiomes

Goethite is known to contribute to the co-precipitation of rhizodeposits and thus benefit carbon (C) sequestration, while arbuscular mycorrhizal fungi (AMF) play significant role in soil organic C (SOC), however, the combined effect is less known. To address this paucity in knowledge, we compared the physicochemical stabilization and microbial mineralization of rhizodeposits from maize (Zea mays L.) and the rhizosphere priming effect (RPE) in soils with a combination of goethite addition and AMF inoculation. Here, we showed that compared to the control: i) Co-amendment of AMF and goethite resulted in a 0.6-fold decrease of rhizodeposit derived CO2, and a 2.8-fold larger allocation of rhizodeposits into macro-aggregates, most likely due to precipitation by goethite and macro-aggregate formation stimulated by AMF hyphae. Analyses using μ-FTIR confirmed the spatial distribution of polysaccharides overlapped with Fe–O minerals within macro-aggregates, supporting the concomitant processes of rhizodeposit stabilization and aggregate formation via hyphal-aggregate mineral interactions; ii) Inoculation with AMF accelerated SOC turnover by increasing the RPE (by 6.1 mg C kg−1 day−1, 74% increase) and rhizodeposit stabilization (by 6.2 mg C kg−1 soil day−1, 47% increase). The larger soil priming effect stimulated by AMF was associated with several genera including Solirubrobacter, Pseudomonas and Talaromyces, suggesting these hyper-symbionts were involved in nutrient acquisition (mining hypothesis). Our results enabled the comparison between rhizodeposit stabilization versus rhizodeposit and SOC mineralization, and highlighted the contributions of both goethite (abiotic contribution) and AMF (biotic contribution) to C accrual in a soil-plant system.

Arbuscular mycorrhizal fungi and goethite promote carbon sequestration via hyphal-aggregate mineral interactions

Effects of simulated Cd deposition on soil Cd availability, microbial response, and crop Cd uptake in the passivation-remediation process of Cd-contaminated purple soil.

A strong link exists between iron oxides and soil organic carbon (SOC). However, the role of iron oxides in the preservation of SOC in agricultural soil remains poorly understood. In this study, we comprehensively examined the concentration, molecular composition and biological sources of iron oxide‐bound organic carbon (Fe‐bound OC) in arable soils collected from 12 sites in central and east China. The results indicated that 6.2–31.2% of the SOC was bound to iron oxides in agricultural soil and that the binding mechanisms varied from adsorption in most soils to coprecipitation in those with a large content of organic carbon. The distribution of Fe‐bound OC showed no clear variation in relation to site, but Fe‐bound OC reached a peak in soils with an annual mean temperature of 16.4°C. Correlation analysis demonstrated that TOC might be the main determinant for the amount of Fe‐bound OC and that the binding mechanism is influenced by both TOC and the active Fe ratio. Comprehensive studies of C/N, ¹³C isotope and synchrotron radiation‐based Fourier transform infrared (SR‐FTIR) spectroscopy showed that iron oxides selectively protected plant‐derived aliphatic compounds and polysaccharides in agricultural soil. This study revealed the quantitative characterization, biological sources and molecular composition of Fe‐bound OC in arable soils, which provides useful information for evaluating and managing the global C cycle under the framework of climate change.

Iron oxides selectively stabilize plant‐derived polysaccharides and aliphatic compounds in agricultural soils

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Iron oxides selectively stabilize plant‐derived polysaccharides and aliphatic compounds in agricultural soils