Shifts in soil microbial community structure, nitrogen cycling and the concomitant declining N availability in ageing primary boreal forest ecosystems

AbstractPlant growth in boreal forests is commonly limited by a low supply of nitrogen, a condition that may be aggravated by high tree below-ground allocation of carbon to ectomycorrhizal (ECM) fungi and associated microorganisms. These in turn immobilise N and reduce its availability to plants as boreal ecosystems develop. Here, we studied a boreal forest ecosystem chronosequence created by new land rising out of the sea due to iso-static rebound along the coast of northern Sweden. We used height over the ocean to estimate ecosystem age and examined its relationship to soil microbial community structure and the gross turnover of N. The youngest soils develop with meadows by the coast, followed by a zone of N2-fixing alder trees, and primary boreal conifer forest on ground up to 560 years old. The young soils in meadows contained little organic matter and microbial biomass per unit area. Nitrogen was turned over at low rates when expressed per area (m−2), but specific rates (per gram soil carbon (C)) were the highest found along the transect. In the zone with alder, the amounts of soil C and microbial biomass were much higher (bacterial biomass had doubled and fungal biomass quadrupled). Rates of gross N mineralisation (expressed on an area basis) were highest, but the retention of added labelled NH4+ was lowest in this soil as compared to other ages. The alder zone also had the largest extractable pools of inorganic N in soil and highest N % in plant foliage. In the older conifer forest ecosystems the amounts of soil C and N, as well as biomass of both bacteria and fungi increased. Data on organic matter 14C suggested that the largest input of recently fixed plant C occurred in the younger coniferous forest ecosystems. With increasing ecosystem age, the ratio of microbial C to total soil C was constant, whereas the ratio of microbial N to total soil N increased and gross N mineralization declined. Simultaneously, plant foliar N % decreased and the natural abundance of 15N in the soil increased. More specifically, the difference in δ15N between plant foliage and soil increased, which is related to relatively greater retention of 15N relative to 14N by ECM fungi as N is taken up from the soil and some N is transferred to the plant host. In the conifer forest, where these changes were greatest, we found increased fungal biomass in the F- and H-horizons of the mor-layer, in which ECM fungi are known to dominate (the uppermost horizon with litter and moss is dominated by saprotrophic fungi). Hence, we propose that the decreasing availability of N to the plants and the subsequent decline in plant production in ageing boreal forests is linked to high tree belowground C allocation to ECM fungi, a strong microbial sink for available soil N

Forest Structure and Fine Root Biomass Influence Soil CO 2 Efflux in Temperate Forests under Drought

Soil respiration is rarely studied at the landscape scale where forest and soil properties can be important drivers. We performed forest and soil inventories in 150 temperate forest sites in three German landscapes and measured in situ soil CO 2 efflux with the soda-lime method in early summer 2018 and 2019. Both years were affected by naturally occurring summer droughts. Our aim was to investigate the impact of forest structural and compositional properties, soil properties and climate on soil CO 2 efflux at the landscape. Forest properties explained a large portion of soil CO 2 efflux variance (i.e., 14% in 2018 and 20% in 2019), which was comparable or larger than the portion explained by soil properties (i.e., 15% in 2018 and 6% in 2019), and much larger than that of climate. Using Structural Equation Modeling, we found that forest structural properties, i.e., tree density and basal area, were negatively linked to soil CO 2 efflux, while forest composition, i.e., conifer share and tree species richness, was not important. Forest structure effects on soil CO 2 efflux were either direct or mediated by fine root biomass under dry summer conditions. Summer soil CO 2 efflux was positively linked to fine root biomass but not related to total soil organic carbon stocks or climate. Forest structural properties influence soil CO 2 efflux under drought events and should be considered when predicting soil respiration at the landscape scale

Thank You to Our 2020 Peer Reviewers

Thank you to the reviewers of AGU Advances. In 2020, we all faced the enormous and unexpected challenges of the Covid‐19 pandemic, with its host of new and competing demands on our time. Thus, we are especially grateful to the 154 people who provided reviews for AGU Advances and helped our fledgling journal complete its first year. Peer‐review is essential to the process of doing and publishing science, and our reviewers have helped define our new journal by indicating papers expected to have broad impact that advance a discipline, have broad impact across disciplines, or have policy relevance. All papers submitted to AGU Advances first go through an editorial consultation. We are committed to respecting reviewers’ time and only send papers for review that the consulting editors agree meet our criteria. Sometimes this means we send papers back to the authors with suggestions how to improve the fit to our journal. Another way we try to streamline the review process is by giving the authors the option to transfer reviews if after review we decide the paper is better suited to another AGU journal. As a relatively new journal, we still have few enough reviewers that we do not want to identify them by name. Nonetheless, you know who you are. Please accept our sincere thanks for generously sharing your expertise and working to improve AGU Advances

Confronting Racism to Advance Our Science

As individuals serving on the AGU Advances editorial board, we condemn racism, affirm that Black Lives Matter, and recognize that inequality is built into the systems that have allowed us to prosper. We aim to persistently foster discussion about racism, inequity, and the need to make our community more diverse and inclusive. This will help AGU Advances do a better job in publishing important science that inclusively reflects the ideas and contributions of all in our community

Isotopic evidence for the contemporary origin of high-molecular weight organic matter in oceanic environments

Previous work has suggested that apparent old C-14 ages for oceanic DOC are the result of mixing of different organic carbon fractions. This report provides direct evidence for a contemporary C-14 age of a high-molecular-weight (HMW) fraction of colloidal organic carbon (greater than or equal to 10 kD). Colloidal organic matter, COM(10) (from 10 kDaltons (kD) to 0.2 mu m), isolated from the upper water column of the Gulf of Mexico and the Middle Atlantic Bight (MAB) region, generally has a contemporary age (i.e., younger than a few decades), while COM(1) (from 1 kD to 0.2 mu m), is apparently old: 380-4500 y BP. Thus, HMW COM(10) (3-5% of DOC) from the upper water column is derived from living particulate organic matter (POM) and cycles rapidly, while a significant fraction of low-molecular-weight (less than or equal to 1 kD) DOM is likely more refractory, and cycles on much longer time scales. The presence of pigment biomarker compounds in COM(1) from the upper water column points to selected phytoplankton species as one of the sources of COM. Terrestrial carbon as another source of COM is suggested from the inverse correlation between Delta(14)C and delta(13)C values, as well as the increasing delta(13)C values with increasing salinity. Th-234-derived turnover times of COM(10) and COM(1) from both the Gulf of Mexico and MAB are consistently short, 1-20 and 3-30 days, respectively. These short residence times support the hypothesis that C-14 ages of colloidal fractions of DOC are the result of COM fractions being a mixture of several endmembers with fast and slow turnover rates

Single-cell analysis: visualizing pharmaceutical and metabolite uptake in cells with label-free 3D mass spectrometry imaging

Detecting metabolites and parent compound within a cell type is now a priority for pharmaceutical development. In this context, three-dimensional secondary ion mass spectrometry (SIMS) imaging was used to investigate the cellular uptake of the antiarrhythmic agent amiodarone, a phospholipidosis-inducing pharmaceutical compound. The high lateral resolution and 3D imaging capabilities of SIMS combined with the multiplex capabilities of ToF mass spectrometric detection allows for the visualization of pharmaceutical compound and metabolites in single cells. The intact, unlabeled drug compound was successfully detected at therapeutic dosages in macrophages (cell line: NR8383). Chemical information from endogenous biomolecules was used to correlate drug distributions with morphological features. From this spatial analysis, amiodarone was detected throughout the cell with the majority of the compound found in the membrane and subsurface regions and absent in the nuclear regions. Similar results were obtained when the macrophages were doped with amiodarone metabolite, desethylamiodarone. The FWHM lateral resolution measured across an intracellular interface in a high lateral resolution ion images was approximately 550 nm. Overall, this approach provides the basis for studying cellular uptake of pharmaceutical compounds and their metabolites on the single cell level

COSORE: A community database for continuous soil respiration and other soil‐atmosphere greenhouse gas flux data

Globally, soils store two to three times as much carbon as currently resides in the atmosphere, and it is critical to understand how soil greenhouse gas (GHG) emissions and uptake will respond to ongoing climate change. In particular, the soil‐to‐atmosphere CO2 flux, commonly though imprecisely termed soil respiration (RS), is one of the largest carbon fluxes in the Earth system. An increasing number of high‐frequency RS measurements (typically, from an automated system with hourly sampling) have been made over the last two decades; an increasing number of methane measurements are being made with such systems as well. Such high frequency data are an invaluable resource for understanding GHG fluxes, but lack a central database or repository. Here we describe the lightweight, open‐source COSORE (COntinuous SOil REspiration) database and software, that focuses on automated, continuous and long‐term GHG flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation. Contributed datasets are mapped to a single, consistent standard, with metadata on contributors, geographic location, measurement conditions and ancillary data. The design emphasizes the importance of reproducibility, scientific transparency and open access to data. While being oriented towards continuously measured RS, the database design accommodates other soil‐atmosphere measurements (e.g. ecosystem respiration, chamber‐measured net ecosystem exchange, methane fluxes) as well as experimental treatments (heterotrophic only, etc.). We give brief examples of the types of analyses possible using this new community resource and describe its accompanying R software package

Negotiating an Irish peace: International negotiation, public opinion and the politics of a two-level game

This project contributes to the theoretical literature on international negotiation and the interaction between public opinion and foreign policy, and the substantive literature on the dynamics of the Anglo-Irish peace process, by exploring the conditions under which public opinion can act as a domestic political constraint in international negotiations. This project makes three primary theoretical contributions. First, it fills a gap in the existing two-level games literature, which largely ignores the impact of public opinion, by specifically considering the role public opinion plays in the negotiation process. Second, it helps to move the two-level games approach from the realm of metaphor to that of theory by specifying the conditions under which one particular domestic actor, the public, can successfully constrain decision-makers. Third, it contributes to the public opinion and foreign policy literature by identifying an important link between public preferences and policy outcomes. ^ Specifically, three main findings emerge from this research: (1) Public opinion can act as a constraint when there is a lack of congruence between public preferences and decision-makers\u27 preferences. (2) Public opinion can act as a constraint when the public has either direct or indirect ratification power over an international agreement. (3) Issue salience forges the critical link between incongruent preferences and domestic constraint when the public\u27s ratification power is indirect. ^ At a substantive level, an examination of negotiations and agreements reached between the Republic of Ireland and Britain between 1980 and 1995 shows that public attitudes in the Republic of Ireland, but not Britain, have constrained decision-makers on a central issue in the peace process, the competing British and Irish claims to jurisdiction over Northern Ireland. The ability of the Irish public to constrain decision-makers results from their direct ratification power over any agreement that would require Ireland to drop or modify its claim to jurisdiction over Northern Ireland. In contrast, because the British public must rely on indirect ratification, issue salience holds the key to understanding their failure to constrain. The issue of Northern Ireland policy is shown not to be salient for the British public, thus rendering indirect ratification ineffective.

A special issue preface: Radiocarbon in the Anthropocene.

The Anthropocene is defined by marked acceleration in human-induced perturbations to the Earth system. Anthropogenic emissions of CO2 and other greenhouse gases to the atmosphere and attendant changes to the global carbon cycle are among the most profound and pervasive of these perturbations. Determining the magnitude, nature and pace of these carbon cycle changes is crucial for understanding the future climate that ecosystems and humanity will experience and need to respond to. This special issue illustrates the value of radiocarbon as a tool to shed important light on the nature, magnitude and pace of carbon cycle change. This article is part of the Theo Murphy meeting issue Radiocarbon in the Anthropocene