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Five new scenarios, or Shared Socioeconomic Pathways (SSPs), have been developed, spanning a range of challenges to mitigation and challenges to adaptation. The Shared Socioeconomic Pathway 4 (SSP4), “Inequality” or “A Road Divided,” is one of these scenarios, characterized by low challenges to mitigation and high challenges to adaptation. We describe, in quantitative terms, the SSP4 as implemented by the Global Change Assessment Model (GCAM), the marker model for this scenario. We use demographic and economic assumptions, in combination with technology and non-climate policy assumptions to develop a quantitative representation of energy, land-use and land-cover, and emissions consistent with the SSP4 narrative. The scenario is one with stark differences within and across regions. High-income regions prosper, continuing to increase their demand for energy and food. Electrification increases in these regions, with the increased generation being met by nuclear and renewables. Low-income regions, however, stagnate due to limited economic growth. Growth in total consumption is dominated by increases in population, not increases in per capita consumption. Due to failures in energy access policies, these regions continue to depend on traditional biofuels, leading to high pollutant emissions. Declining dependence on fossil fuels in all regions means that total radiative forcing absent the inclusion of mitigation or impacts only reaches 6.4 W m −2 in 2100, making this a world with relatively low challenges to mitigation. We explore the effects of mitigation effort on the SSP4 world, finding that the imposition of a carbon price has a varied effect across regions. In particular, the SSP4 mitigation scenarios are characterized by afforestation in the high-income regions and deforestation in the low-income regions. Furthermore, we find that the SSP4 is a world with low challenges to mitigation, but only to a point due to incomplete mitigation of land-related emissions. [ABSTRACT FROM AUTHOR]

Abstract: Partitioning the soil surface CO2 flux (R S) flux is an important step in understanding ecosystem-level carbon cycling, given that R S is poorly constrained and its source components may have different sensitivities to climate change. Trenched plots are an inexpensive but labor-intensive method of separating the R S flux into its root (autotrophic) and soil (heterotrophic) components. This study tested if various methods of plant suppression in trenched plots affected R S fluxes, quantified the R S response to soil temperature and moisture changes, and estimated the heterotrophic contribution to R S. It was performed in a boreal black spruce (Picea mariana) plantation, using a randomized complete block design, during the 2007 and 2008 growing seasons. Trenched plots had significantly lower R S than control plots, with differences appearing ∼100 days after trenching; spatial variability doubled immediately after trenching but then declined throughout the experiment. Most trenching treatments had significantly lower (by ∼0.5 μmol CO2 m−2 s−1) R S than the controls, and there was no significant difference in R S among the various trenching treatments. Soil temperature at 2 cm explained more R S variability than did 10-cm temperature or soil moisture. Temperature sensitivity (Q 10) declined in the control plots from ∼2.6 (at 5 °C) to ∼1.6 (at 15 °C); trenched plots values were higher, from 3.1 at 5 °C to 1.9 at 15 °C. We estimated R S for the study period to be 241 ± 40 g C m−2, with live roots contributing 64% of R S after accounting for fine root decay, and 293 g C m−2 for the entire year. These findings suggest that laborious hand weeding of trenched plot vegetation may be replaced by other methods, facilitating future studies of this large and poorly-understood carbon flux. [Copyright &y& Elsevier]

Diffusion-based moisture functions could provide insight into soil physical processes and potentially represent a more rigorous approach to model soil moisture-respiration relationships. However, large prediction bias remains when these functions are evaluated with field observations. Here we revisit the concept of diffusion limited substrate bioaccessibility by coupling Michaelis-Menten microbial uptake kinetics with linear mass transfer. The analytical solution of this microbial-substrate system demonstrates that moisture interacts with heterogeneous substrate distribution to control the dynamic transitions between diffusion- and reaction-limited regimes, leading to deviations of mean-field rates from spatially averaged rates. This result highlights the inadequacy of current simplified diffusion-based moisture functions in capturing the nonlinearity between substrate distribution and microbial utilization, and calls for continued development of effective upscaling approaches and experimental validation. • Diffusion-based moisture functions overlook soil spatial heterogeneity. • Transitions between diffusion- and reaction-limited regimes are dynamic. • Macroscopic rates deviate from spatially averaged rates. • Microscale nonlinearity is not captured by diffusion-based moisture functions. [ABSTRACT FROM AUTHOR]

Field observations of the soil-to-atmosphere CO 2 flux—soil respiration, R S —are a prime example of 'long tail' data that historically have had neither centralized databases nor an agreed-upon reporting format. This has hindered scientific transparency, analytical reproducibility, and syntheses with respect to this globally-important component of the carbon cycle. Here we propose a new data and metadata reporting format for R S data, based on engagement with a wide range of researchers in the earth and ecological sciences as well as expert advisory panels. Our goal was a reporting format that would be relevant and useful for synthesis activities, optimizing data discoverability and usability while not placing an undue burden on data contributors. We describe previous R S data collection efforts, lessons learned from related databases and data-oriented networks (e.g., FLUXNET) in earth and ecological sciences, and the process of community consultation. The proposed reporting format focuses on chamber-level data and metadata, specifying measurement conditions and, for a given measurement period defined by beginning and ending timestamps, a mean R S flux (or CO 2 concentration) and associated ancillary measurements. With input from the research community, we have also developed research data and metadata templates to support data collection adhering to the reporting format. Fundamentally, this format aims to enable findable, accessible, interoperable, and reusable data, while providing 'future-proofing' capabilities to support reanalyses using as yet unknown algorithms or approaches. This proposed R S reporting format is openly available online and is intended to be a dynamic document, subject to further community feedback and/or change. • We propose a new data and metadata reporting format for soil respiration (R S) data. • The proposed reporting format focuses on chamber-level data and metadata. • Considerable input from the research community shaped this format. • This proposed format is openly available online. [ABSTRACT FROM AUTHOR]

Soils both produce and consume methane (CH 4), a potent greenhouse gas that contributes to climate change. In coastal forests, upland soils are shifting from being CH 4 sinks to sources as sea levels rise, increasingly flooding soils with little prior inundation history. Ecosystem CH 4 budgets are highly uncertain due in part to the difficulty in separating fluxes measured at the soil surface into individual production and consumption processes which are likely to have different responses to future environmental conditions. We measured growing season CH 4 fluxes from soil monoliths transplanted four years prior along an inundation and salinity gradient to determine how changes in abiotic conditions control CH 4 flux rates. To parse net fluxes measured at the soil surface into their component gross rates, we paired field measurements with a stable isotope pool dilution incubation of surface soils. Throughout the growing season, net soil surface CH 4 flux was positively correlated with soil moisture (p < 0.01), with lowland-located soils tending towards CH 4 sources (mean 0.349 ± 1.11 mg CH 4 –C m−2 h−1, error is standard deviation) and upland-located soils tending towards CH 4 sinks (−0.003 ± 0.003 mg CH 4 –C m−2 h−1). Transplanted soils' fluxes were statistically identical to their native neighbors once microtopography-driven differences in soil moisture were controlled for. The pool dilution experiment revealed that production and consumption rates were similar in upland and lowland surface soils (2.82 ± 3.29 μmol CH 4 g−1 dry soil d−1 production, 3.47 ± 2.03 μmol CH 4 g−1 dry soil d−1 consumption), indicating the majority of production likely occurs at depth in lowland soils. Both gross and net fluxes from transplanted soils showed no effect of soil origin after four years, suggesting low resistance of CH 4 cycling to global change drivers. Our results indicate the strength of the coastal forest CH 4 sink is likely to decrease in proportion to sea-level rise. • Field & lab measurements probed controls of net & gross coastal-forest soil methane flux. • In situ lowland soils were methane sources while ex situ surface soils were sinks. • Results indicate soil methane production occurs at depth in low-lying areas. [ABSTRACT FROM AUTHOR]

Core-scale soil carbon fluxes are ultimately regulated by pore-scale dynamics of substrate availability and microbial access. These are constrained by physicochemical and biochemical phenomena (e.g. spatial access and hydrologic connectivity, physical occlusion, adsorption-desorption with mineral surfaces, nutrient and resource limitations). We conducted an experiment to determine how spatial access and resource limitations influence core-scale water-soluble SOM mineralization, and how these are regulated by antecedent moisture conditions. Intact soil cores were incubated at field-moist vs. drought conditions, after which they were saturated from above (to simulate precipitation) or below (to simulate groundwater recharge). Soluble C (acetate) and N (nitrate) forms were added to some cores during the rewetting process to alleviate potential nutrient limitations. Soil respiration was measured during the incubation, after which pore water was extracted from the saturated soils and analyzed for water soluble organic carbon concentrations and characterization. Our results showed that C amendments increased the cumulative CO 2 evolved from the soil cores, suggesting that the soils were C-limited. Drought and rewetting increased soil respiration, and there was a greater abundance of complex aromatic molecules in pore waters sampled from these soils. This newly available substrate appeared to alleviate nutrient limitations on respiration, because there were no further respiration increases with subsequent C and N amendments. We had hypothesized that respiration would be influenced by wetting direction, as simulated precipitation would mobilize C from the surface. However, as a main effect, this response was seen only in the C-amended soils, indicating that surface-C may not have been bioavailable. At the pore scale (pore water samples), drought and the C, N amendments caused a net loss of identified molecules when the soils were rewet from below, whereas wetting from above caused a net increase in identified molecules, suggesting that fresh inputs stimulated the C-and N-limited microbial populations present deeper in the soil profile. Our experiment highlights the complex and interactive role of antecedent moisture conditions, wetting direction, and resource limitations in driving core-scale C fluxes. Physicochemical and biochemical controls on soil carbon mineralization. [Display omitted] • Respiration in soils is C-limited. • Fresh C inputs from drought and C amendments alleviate some of these C limitations. • C amendments help mobilize C in surface soils that is otherwise protected from mineralization. • Wetting from below mobilizes C in fine pores, and wetting from above mobilizes C in coarse pores. [ABSTRACT FROM AUTHOR]

Soil moisture influences soil carbon dynamics, including microbial growth and respiration. The response of such 'soil respiration' to moisture changes is generally assumed to be linear and reversible, i.e. to depend only on the current moisture state. Current models thus do not account for antecedent soil moisture conditions when determining soil respiration or the available substrate pool. We conducted a laboratory incubation to determine how the antecedent conditions of drought and flood influenced soil organic matter (SOM) chemistry, bioavailability, and respiration. We sampled soils from an upland coastal forest, Beaver Creek, WA USA, and subjected them to drying and rewetting treatments. For the drying treatment, field moist soils were saturated and then dried to 75, 50, 35, and 5% saturation. In the rewetting treatment, field moist soils were air-dried and then rewet to 35, 50, 75, and 100% saturation. We measured respiration and water extractable organic carbon (WEOC) concentrations and used 1H-NMR and FT-ICR-MS to characterize the WEOC pool across the treatments. The drying vs. wetting treatment strongly influenced SOM bioavailability, as rewet soils (with antecedent drought) had greater WEOC concentrations and respiration fluxes compared to the drying soils (with antecedent flood). In addition, air-dry soils had the highest WEOC concentrations, and the NMR-resolved peaks showed a strong contribution of protein groups in these soils. Both NMR and FT-ICR-MS analyses indicated increased contribution of complex aromatic groups/molecules in the rewet soils, compared to the drying soils. We suggest that drying introduced organic matter into the WEOC pool via desorption of aromatic molecules and/or by microbial cell lysis, and this stimulated microbial mineralization rates. Our work indicates that even short-term shifts in antecedent moisture conditions can strongly influence soil C dynamics at the core scale. The predictive uncertainties in current soil models may be reduced by a more accurate representation of soil water and C persistence that includes a mechanistic and quantitative understanding of the impact of antecedent moisture conditions. • Antecedent moisture conditions influenced SOM response to moisture changes. • Air-dry soils showed increased contribution of protein-like groups in the extractable SOM pool. • Rewetting soils showed greater contribution of aromatic and complex groups in the extractable SOM pool. • Rewetting soils showed greater respiration fluxes, compared to drying soils. [ABSTRACT FROM AUTHOR]

Coastal landscapes are increasingly exposed to seawater due to sea level rise and extreme weather events. The biogeochemical responses of these vulnerable ecosystems are poorly understood, limiting our ability to predict how their role in local and global biogeochemical cycles will shift under future conditions. Here we evaluate how antecedent conditions influence the biogeochemical response of soil to seawater inundation events based on a 42-day laboratory incubation experiment with soils collected from a natural salinity gradient across a coastal floodplain. We quantified influences of seawater inundation on intact soil cores through high-frequency carbon dioxide (CO 2) and methane (CH 4) gas fluxes measurements as well as ultrahigh resolution characterization of organic matter chemistry and metabolites. Mean CO 2 and CH 4 fluxes were higher after inundation compared to control cores for soils that had low in situ electrical conductivity (EC). Soils with low in situ EC also exhibited significant shifts in organic matter profiles after inundation, with surficial soils (0–7.5 cm) becoming more enriched in phenolic compounds, compared to deeper soils (7.5–15 cm). The number of biochemical transformations inferred from mass spectrometry increased significantly after inundation for soils with low in situ EC. Our results suggest that seawater inundation of low-salinity terrestrial environments can lead to increased microbial activity and increasing likelihood of soil carbon release, with sites experiencing infrequent or new seawater exposure likely to be more sensitive to saltwater exposure relative to sites with more frequent exposure. We conclude that the biogeochemical impacts of future seawater exposure will be modulated by antecedent conditions associated with landscape position within coastal watersheds. Image 1 • Impact of tidal inundations on coastal soils is inadequately addressed. • Biogeochemical response of coastal soils to seawater inundation was investigated. • CH 4 and CO 2 fluxes, and soil organic matter molecular features were evaluated. • Low in situ electrical conductivity soils were impacted by inundation episodes. • Antecedent conditions drive biogeochemical response of soils to seawater inundation. [ABSTRACT FROM AUTHOR]

The ongoing increase of atmospheric temperature may induce soil organic carbon (SOC) loss and exacerbate the greenhouse effect. As a result, there is a great effort to understand the relationship between temperature and the heterotrophic soil respiration rate (R SOIL) as it has significant implications for anticipated change of the Earth system. Soil respiration depends on the size of respiring microbial biomass (MBC) and when R SOIL is measured without concurrent measurement of MBC, the apparent temperature sensitivity of R SOIL could be misinterpreted since MBC can change with temperature within days or weeks of warming. The effect of temperature driven changes in MBC on the apparent temperature sensitivity of R SOIL was evaluated using a meta-analysis of 27 laboratory and field experiments conducted at different temporal scales (1–730 d) and under a wide range of temperatures (2–50 °C) and soil conditions. Across all studies, the apparent temperature sensitivity decreased when MBC decreased with increasing temperature and vice versa. We observed a steep decrease of MBC above optimal temperature (27.1 ± 1.0 °C), which attenuated the apparent temperature sensitivity of R SOIL , an aspect previously explained by the existence of reaction rate temperature optima. The temperature response of the MBC specific respiration rate was, however, highly non-linear and soil specific. Including MBC in soil biogeochemical models requires careful consideration of the variability of temperature-associated physiological changes of soil microorganisms. Without it, microbially explicit models cannot predict temperature induced SOC loss better than older, empirical models based on first order reaction kinetics. Image 1 • Temperature sensitivity of heterotrophic soil respiration is affected by temperature driven changes in microbial biomass. • Attenuation of the temperature sensitivity above ∼25 °C is caused by increased death rate of microbial biomass. • The relationship between temperature and microbial biomass specific respiration is highly non-linear and soil specific. [ABSTRACT FROM AUTHOR]

This paper presents a unified multi-scale model (UMSM) that we developed to simulate hydrological processes in an ecosystem containing both surface water and groundwater. The UMSM approach modifies the Navier–Stokes equation by adding a Darcy force term to formulate a single set of equations to describe fluid momentum and uses a generalized equation to describe fluid mass balance. The advantage of the approach is that the single set of the equations can describe hydrological processes in both surface water and groundwater where different models are traditionally required to simulate fluid flow. This feature of the UMSM significantly facilitates modelling of hydrological processes in ecosystems, especially at locations where soil/sediment may be frequently inundated and drained in response to precipitation, regional hydrological and climate changes. In this paper, the UMSM was benchmarked using WASH123D, a model commonly used for simulating coupled surface water and groundwater flow. Disney Wilderness Preserve (DWP) site at the Kissimmee, Florida, where active field monitoring and measurements are ongoing to understand hydrological and biogeochemical processes, was then used as an example to illustrate the UMSM modelling approach. The simulations results demonstrated that the DWP site is subject to the frequent changes in soil saturation, the geometry and volume of surface water bodies, and groundwater and surface water exchange. All the hydrological phenomena in surface water and groundwater components including inundation and draining, river bank flow, groundwater table change, soil saturation, hydrological interactions between groundwater and surface water, and the migration of surface water and groundwater interfaces can be simultaneously simulated using the UMSM. Overall, the UMSM offers a cross-scale approach that is particularly suitable to simulate coupled surface and ground water flow in ecosystems with strong surface water and groundwater interactions. [ABSTRACT FROM AUTHOR]

• Disturbance effects on forest canopy structural complexity are poorly understood. • We synthesized the results of three forest disturbance manipulations. • Disturbance had variable effects on the temporal dynamics of structural complexity. • Moderate severity disturbance sometimes increased structural complexity. • Disturbance severity, source, and scale constrain temporal changes in complexity. The temporal dynamics of forest canopy structure are influenced by disturbances that alter vegetation quantity and distribution. While canopy structural indicators such as leaf area index (LAI), canopy cover, and canopy height have been widely studied in the context of disturbance, the post-disturbance temporal dynamics of structural complexity, which summarizes the heterogeneity of vegetation arrangement, are poorly understood. With the goal of advancing conceptual and empirical understanding of the temporal dynamics of structural complexity following disturbance, we synthesized results from three large-scale disturbance manipulation experiments at the University of Michigan Biological Station (UMBS): the 4-year Forest Resilience Threshold Experiment (FoRTE) manipulating levels of disturbance severity; the decade-long Forest Accelerated Succession Experiment (FASET), in which all early successional tree species were stem-girdled within 39 ha in the same landscape; and forest chronosequences established following clear-cut harvesting. We found that the temporal dynamics of canopy structure following disturbance were dependent upon three factors: (1) the source and severity of disturbance; (2) the spatial and temporal scales of analysis; and (3) the measure of structure assessed. Unlike vegetation area index and canopy cover, which initially decreased in response to disturbance, structural complexity measures such as canopy and top rugosity did not consistently respond to moderate levels of disturbance severity. Over multi-decadal timescales, structural complexity increased to a maximum, regardless of whether fire occurred at the time of stand establishment, but intervening low-to-moderate severity disturbance in regrown century-old forests altered trajectories of canopy rugosity. We conclude that structural complexity indicators display a more nuanced temporal and directional response to disturbance than conventional leaf area and cover indexes. Predicting what disturbance conditions modify trajectories of structural complexity remains critical to disturbance characterization and the inference of ecosystem functioning. [ABSTRACT FROM AUTHOR]

Highlights: [•] We continuously measured soil CO2 efflux (R S) in a temperate forest. [•] We quantified effect of snowpack and freeze–thaw cycles (FTCs) on the R S. [•] The enhancement of R S due to snowpack mainly resulted from snow insulation. [•] The FTC-induced enhancement of R S was mainly biologically originated. [•] Seasonally explicit empirical models performed well in estimation of R S. [Copyright &y& Elsevier]