Determining the Extent of Delocalization in Mixed-Valence Iron Dimers using X-ray Absorption Spectroscopy

This study examines the extent of charge delocalization in mixed valence compounds. Understanding the structure of charge delocalization is the first step in understanding the local dynamics of charge transfer. This insight has diverse applications such as the ability to mimic biological reactions and to enhance solar technology. Because of its fast time scale, synchrotron radiation was used to probe the iron K-edge for three organometallic systems. In these complexes, two bridged metal atoms share an effective charge of 5+. In a Robin-Day Class II compound, charge is localized and the two iron atoms have effective oxidation states of 2+ and 3+ respectively. For Class III delocalized compounds each metal center has an effective charge of 2.5+. Class II/III compounds exhibit characteristics of both localized and delocalized systems according to various optical spectroscopies. Synchrotron radiation was used to study charge distribution in these poorly-understood Class II/III intermediate systems. In the limit of absolute localization, spectra of the mixed valence species were expected to be a linear combination of the reduced and oxidized species. For the delocalized case, a linear combination was not expected. These two cases were used as calibration limits to determine the extent of delocalization in the unknown Class II/III compound. Results showed that synchrotron radiation classifies the Class II/III compound as localized. However, data also demonstrated that the linear combination model did not hold as expected and a revised model is necessary to better understand this phenomenon

Impacts of Drying and Rewetting on the Radiocarbon Signature of Respired CO2 and Implications for Incubating Archived Soils

The radiocarbon signature of respired CO2 (∆14C-CO2) measured in laboratory soil incubations integrates contributions from soil carbon pools with a wide range of ages, making it a powerful model constraint. Incubating archived soils enriched by “bomb-C” from mid-20th century nuclear weapons testing would be even more powerful as it would enable us to trace this pulse over time. However, air-drying and subsequent rewetting of archived soils, as well as storage duration, may alter the relative contribution to respiration from soil carbon pools with different cycling rates. We designed three experiments to assess air-drying and rewetting effects on ∆14C-CO2 with constant storage duration (Experiment 1), without storage (Experiment 2), and with variable storage duration (Experiment 3). We found that air-drying and rewetting led to small but significant (α < 0.05) shifts in ∆14C-CO2 relative to undried controls in all experiments, with grassland soils responding more strongly than forest soils. Storage duration (4–14 y) did not have a substantial effect. Mean differences (95% CIs) for experiments 1, 2, and 3 were: 23.3‰ (±6.6), 19.6‰ (±10.3), and 29.3‰ (±29.1) for grassland soils, versus −11.6‰ (±4.1), 12.7‰ (±8.5), and −24.2‰ (±13.2) for forest soils. Our results indicate that air-drying and rewetting soils mobilizes a slightly older pool of carbon that would otherwise be inaccessible to microbes, an effect that persists throughout the incubation. However, as the bias in ∆14C-CO2 from air-drying and rewetting is small, measuring ∆14C-CO2 in incubations of archived soils appears to be a promising technique for constraining soil carbon models

From the Top: Surface-derived Carbon Fuels Greenhouse Gas Production at Depth in a Neotropical Peatland

Tropical peatlands play an important role in global carbon (C) cycling but little is known about factors driving carbon dioxide (CO2) and methane (CH4) emissions from these ecosystems, especially production below the surface. This study aimed to identify source material and processes regulating C emissions from deep in a Neotropical peatland on the Caribbean coast of Panama. We hypothesized that: 1) surface derived organic matter transported down the soil profile is the primary C source for respiration products at depth and 2) high lignin content results in hydrogenotrophic methanogenesis as the dominant CH4 production pathway throughout the profile. We used radiocarbon isotopes to determine whether CO2 and CH4 at depth (measured to 2 m) are produced from modern substrates or ancient deep peat, and we used stable C isotopes to identify the dominant CH4 production pathway. Peat organic chemistry was characterized using 13C solid state nuclear magnetic resonance spectroscopy (13C-NMR). We found that deep peat respiration products had radiocarbon signatures that were more similar to surface dissolved organic C (DOC) than deep solid peat. Radiocarbon ages for deep peat ranged from 1200 &ndash; 1800 yrBP at the sites measured. These results indicate that surface derived C was the dominant source for gas production at depth in this peatland, likely because of vertical transport of DOC from the surface to depth. Carbohydrates did not vary with depth across these sites, whereas lignin, which was the most abundant compound (55&ndash;70 % of C), tended to increase with depth. These results suggest that there is no preferential decomposition of carbohydrates, but preferential retention of lignin. Stable isotope signatures of respiration products indicated that hydrogenotrophic rather than acetoclastic methanogenesis was the dominant production pathway of CH4 throughout the peat profile. These results suggest, even C compounds that are typically considered vulnerable to decomposition (i.e., carbohydrates) are preserved deep in these tropical peats, highlighting the importance of anaerobic, waterlogged conditions for preserving tropical peatland C

Soil minerals mediate climatic control of soil C cycling on annual to centennial timescales

Climate and parent material both affect soil C persistence, yet the relative importance of climatic versus mineralogical controls on soil C dynamics remains unclear. To test this, we collected soil samples in 2001, 2009, and 2019 along a combined gradient of parent material (andesite, basalt, granite) and climate (mean annual temperature (MAT): 6.5 &deg;C &ldquo;cold&rdquo;, 8.6 &deg;C &ldquo;cool&rdquo;, 12.0 &deg;C &ldquo;warm&rdquo;). We measured the radiocarbon of heterotrophically respired CO2 (∆14Crespired) and bulk soil C (∆14Cbulk) as proxies for transient and persistent soil C, and characterized mineral assemblages using selective dissolution. Using linear regression, we observed that MAT was not a significant predictor of either ∆14Cbulk or ∆14Crespired, yet climate was highly significant as a categorical variable. Climate explained more variance in ∆14Cbulk and ∆14Crespired over 0&ndash;0.1 m, but parent material explained more from 0.1&ndash;0.3 m. Cool site soil C was more persistent (lower ∆14Cbulk) than cold or warm climate sites, and also more persistent on andesitic soils, followed by basaltic and then granitic soils. Poorly crystalline metal oxides (PCMs) (but not crystalline metal oxides) were significantly (p &lt; 0.1) correlated with ∆14Cbulk, ∆14Crespired, and ∆14Crespired - ∆14Cbulk, indicating their importance for soil C cycling on both short and long timescales. The change in ∆14Crespired observed over the study period was linearly related to MAT for the granite soils with the lowest PCM content, but not in the andesitic and basaltic soils with higher PCM content. This link between PCM abundance and the decoupling of MAT and soil C cycling rates suggests PCMs may attenuate the temperature sensitivity of decomposition.</p

Soil carbon respiration in tropical forest soils along geomorphic and geochemical gradients

Tropical ecosystems and the soils therein have been reported as one of the most important and largest terrestrial carbon (C) pools and are considered important climate regulator. Carbon stabilization mechanisms in these ecosystems are often complex, as these mechanisms crucially rely on the interplay of geology, topography, climate, and biology. Future predictions of the perturbation of the soil carbon pool ultimately depend on our mechanistic understanding of these complex interactions. Using laboratory incubation experiments, we investigated if carbon release from soils through heterotrophic respiration in the African highland forests of the Eastern Congo Basin follows predictable patterns related to topography, soil depth or geochemical soil properties that can be described at the landscape scale and ultimately be used to improve the spatial accuracy of soil C respiration in mechanistic models. In general, soils developed on basalt and granite parent material (mafic and felsic geochemistry of parent material) showed significantly (p <0.05) higher specific respiration than soils developed on sedimentary rocks (mixed geochemistry) with highest rates measured for soils developed on granite. For soils developed on basalt, specific respiration decreased two-fold with soil depth, but not for soils developed on granite or sedimentary rocks. No significant differences in respiration under tropical forest were found in relation to topography for any soil and geochemical background. Using a non-linear, stochastic gradient boosting machine learning approach we show that soil biological, physical and chemical properties can predict the pattern of specific soil respiration (R2=0.41, p<0.05). An assessment of the relative importance of the included predictors for soil respiration resulted in 43 % of the model being driven by geochemistry (pedogenic oxides, nutrient availability), 12 % driven by soil texture and clay mineralogy, 34 % by microbial biomass, C:N, and C:P ratios and 11 % by topographic indices. We conclude that, in order to explain soil C respiration patterns in tropical forests, a complex set of variables need to be considered that differs depending on the local bedrock chemistry. Its effect is likely related to the varying strength of C stabilization with minerals as well as nutrient availability that might drive C input patterns and microbial turnover

Long‐term recovery of the functional community assembly and carbon pools in an African tropical forest succession

On the African continent, the population is expected to expand fourfold in the next century, which will increasingly impact the global carbon cycle and biodiversity conservation. Therefore, it is of vital importance to understand how carbon stocks and community assembly recover after slash-and-burn events in tropical second growth forests. We inventoried a chronosequence of 15 1-ha plots in lowland tropical forest of the central Congo Basin and evaluated changes in aboveground and soil organic carbon stocks and in tree species diversity, functional composition, and community-weighted functional traits with succession. We aimed to track long-term recovery trajectories of species and carbon stocks in secondary forests, comparing 5 to 200 + year old secondary forest with reference primary forest. Along the successional gradient, the functional composition followed a trajectory from resource acquisition to resource conservation, except for nitrogen-related leaf traits. Despite a fast, initial recovery of species diversity and functional composition, there were still important structural and carbon stock differences between old growth secondary and pristine forest, which suggests that a full recovery of secondary forests might take much longer than currently shown. As such, the aboveground carbon stocks of 200 + year old forest were only 57% of those in the pristine reference forest, which suggests a slow recovery of aboveground carbon stocks, although more research is needed to confirm this observation. The results of this study highlight the need for more in-depth studies on forest recovery in Central Africa, to gain insight into the processes that control biodiversity and carbon stock recovery

How temporal patterns in rainfall determine the geomorphology and carbon fluxes of tropical peatlands

Tropical peatlands now emit hundreds of megatons of carbon dioxide per year because of human disruption of the feedbacks that link peat accumulation and groundwater hydrology. However, no quantitative theory has existed for how patterns of carbon storage and release accompanying growth and subsidence of tropical peatlands are affected by climate and disturbance. Using comprehensive data from a pristine peatland in Brunei Darussalam, we show how rainfall and groundwater flow determine a shape parameter (the Laplacian of the peat surface elevation) that specifies, under a given rainfall regime, the ultimate, stable morphology, and hence carbon storage, of a tropical peatland within a network of rivers or canals. We find that peatlands reach their ultimate shape first at the edges of peat domes where they are bounded by rivers, so that the rate of carbon uptake accompanying their growth is proportional to the area of the still-growing dome interior. We use this model to study how tropical peatland carbon storage and fluxes are controlled by changes in climate, sea level, and drainage networks. We find that fluctuations in net precipitation on timescales from hours to years can reduce long-term peat accumulation. Our mathematical and numerical models can be used to predict long-term effects of changes in temporal rainfall patterns and drainage networks on tropical peatland geomorphology and carbon storage

Decomposability of soil organic matter over time: the Soil Incubation Database (SIDb, version 1.0) and guidance for incubation procedures

The magnitude of carbon (C) loss to the atmosphere via microbial decomposition is a function of the amount of C stored in soils, the quality of the organic matter, and physical, chemical, and biological factors that comprise the environment for decomposition. The decomposability of C is commonly assessed by laboratory soil incubation studies that measure greenhouse gases mineralized from soils under controlled conditions. Here, we introduce the Soil Incubation Database (SIDb) version 1.0, a compilation of time series data from incubations, structured into a new, publicly available, open-access database of C flux (carbon dioxide, CO2, or methane, CH4). In addition, the SIDb project also provides a platform for the development of tools for reading and analysis of incubation data as well as documentation for future use and development. In addition to introducing SIDb, we provide reporting guidance for database entry and the required variables that incubation studies need at minimum to be included in SIDb. A key application of this synthesis effort is to better characterize soil C processes in Earth system models, which will in turn reduce our uncertainty in predicting the response of soil C decomposition to a changing climate. We demonstrate a framework to fit curves to a number of incubation studies from diverse ecosystems, depths, and organic matter content using a built-in model development module that integrates SIDb with the existing SoilR package to estimate soil C pools from time series data. The database will help bridge the gap between point location measurements, which are commonly used in incubation studies, and global remote-sensed data or data products derived from models aimed at assessing global-scale rates of decomposition and C turnover. The SIDb version 1.0 is archived and publicly available at https://doi.org/10.5281/zenodo.3871263 (Sierra et al., 2020), and the database is managed under a version-controlled system and centrally stored in GitHub (https://github.com/SoilBGC-Datashare/sidb, last access: 26 June 2020)

Organic matter cycling along geochemical, geomorphic and disturbance gradients in forests and cropland of the African Tropics – Project TropSOC Database Version 1.0

The African Tropics are hotspots of modern-day land-use change and are, at the same time, of great relevance for the cycling of carbon (C) and nutrients between plants, soils and the atmosphere. However, the consequences of land conversion on biogeochemical cycles are still largely unknown as they are not studied in a landscape context that defines the geomorphic, geochemically and pedological framework in which biological processes take place. Thus, the response of tropical soils to disturbance by erosion and land conversion is one of the great uncertainties in assessing the carrying capacity of tropical landscapes to grow food for future generations and in predicting greenhouse gas fluxes (GHG) from soils to the atmosphere and, hence, future earth system dynamics. Here, we describe version 1.0 of an open access database created as part of the project &ldquo;Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to variability in soil geochemistry and land use&rdquo; (TropSOC). TropSOC v1.0 contains spatial and temporal explicit data on soil, vegetation, environmental properties and land management collected from 136 pristine tropical forest and cropland plots between 2017 and 2020 as part of several monitoring and sampling campaigns in the Eastern Congo Basin and the East African Rift Valley System. The results of several laboratory experiments focusing on soil microbial activity, C cycling and C stabilization in soils complement the dataset to deliver one of the first landscape scale datasets to study the linkages and feedbacks between geology, geomorphology and pedogenesis as controls on biogeochemical cycles in a variety of natural and managed systems in the African Tropics. The hierarchical and interdisciplinary structure of the TropSOC database allows for linking a wide range of parameters and observations on soil and vegetation dynamics along with other supporting information that may also be measured at one or more levels of the hierarchy. TropSOC&rsquo;s data marks a significant contribution to improve our understanding of the fate of biogeochemical cycles in dynamic and diverse tropical African (agro-)ecosystems. TropSOC v1.0 can be accessed through the supplementary material provided as part of this manuscript or as a separate download via the websites of the Congo Biogeochemistry observatory and the GFZ data repository where version updates to the database will be provided as the project develops.</p

Reviews and syntheses: The promise of big diverse soil data, moving current practices towards future potential

In the age of big data, soil data are more available and richer than ever, but – outside of a few large soil survey resources – they remain largely unusable for informing soil management and understanding Earth system processes beyond the original study. Data science has promised a fully reusable research pipeline where data from past studies are used to contextualize new findings and reanalyzed for new insight. Yet synthesis projects encounter challenges at all steps of the data reuse pipeline, including unavailable data, labor-intensive transcription of datasets, incomplete metadata, and a lack of communication between collaborators. Here, using insights from a diversity of soil, data, and climate scientists, we summarize current practices in soil data synthesis across all stages of database creation: availability, input, harmonization, curation, and publication. We then suggest new soil-focused semantic tools to improve existing data pipelines, such as ontologies, vocabulary lists, and community practices. Our goal is to provide the soil data community with an overview of current practices in soil data and where we need to go to fully leverage big data to solve soil problems in the next century