An invasive wetland grass primes deep soil carbon pools

Habitat-specific responses of soil organic matter decomposition to Spartina alterniflora invasion along China's coast

Plant invasions cause a fundamental change in soil organic matter (SOM) turnover. Disentangling the biogeographic patterns and key drivers of SOM decomposition and its temperature sensitivity (Q10 ) under plant invasion is a prerequisite for making projections of global carbon feedback. We collected soil samples along China's coast across saltmarshes to mangrove ecosystems invaded by the smooth cordgrass (Spartina alterniflora Loisel.). Microcosm experiments were carried out to determine the patterns of SOM decomposition and its thermal response. Soil microbial biomass and communities were also characterized accordingly. SOM decomposition constant dramatically decreased along the mean annual temperature gradient, whereas the cordgrass invasion retarded this change (significantly reduced slope, p < 0.05). The response of Q10 to invasion and the soil microbial quotient peaked at midlatitude saltmarshes, which can be explained by microbial metabolism strategies. Climatic variables showed strong negative controls on the Q10 , whereas dissolved carbon fraction exerted a positive influence on its spatial variance. Higher microbial diversity appeared to weaken the temperature-related response of SOM decomposition, with apparent benefits for carbon sequestration. Inconsistent responses to invasion were exhibited among habitat types, with SOM accumulation in saltmarshes but carbon loss in mangroves, which were explained, at least in part, by the SOM decomposition patterns under invasion. This study elucidates the geographic pattern of SOM decomposition and its temperature sensitivity in coastal ecosystems and underlines the importance of interactions between climate, soil, and microbiota for stabilizing SOM under plant invasion.

Complex eco-evolutionary responses of a foundational coastal marsh plant to global change

Predicting the fate of coastal marshes requires understanding how plants respond to rapid environmental change. Environmental change can elicit shifts in trait variation attributable to phenotypic plasticity and act as selective agents to shift trait means, resulting in rapid evolution. Comparably, less is known about the potential for responses to reflect the evolution of trait plasticity. Here, we assessed the relative magnitude of eco-evolutionary responses to interacting global change factors using a multifactorial experiment. We exposed replicates of 32 Schoenoplectus americanus genotypes 'resurrected' from century-long, soil-stored seed banks to ambient or elevated CO2 , varying levels of inundation, and the presence of a competing marsh grass, across two sites with different salinities. Comparisons of responses to global change factors among age cohorts and across provenances indicated that plasticity has evolved in five of the seven traits measured. Accounting for evolutionary factors (i.e. evolution and sources of heritable variation) in statistical models explained an additional 9-31% of trait variation. Our findings indicate that evolutionary factors mediate ecological responses to environmental change. The magnitude of evolutionary change in plant traits over the last century suggests that evolution could play a role in pacing future ecosystem response to environmental change.

Species shifts induce soil organic matter priming and changes in microbial communities

Invasion of plant species with functional traits that influences the rhizosphere can have significant effects on soil organic matter (SOM) dynamics if the invasive species stimulates soil microbial communities with, for example, an enhanced supply of labile carbon and oxygen. We evaluated these effects along a Phragmites invasion chronosequence spanning over 40 years. Using a δ¹³C and δ¹⁵N enriched substrate, we separated SOM-derived and substrate-derived carbon (C) and nitrogen (N) mineralization in surface (top 15 cm), shallow (30-45 cm), and deep (65-80 cm) soils collected from established, newly invaded, and native plant communities. We found all soils were susceptible to SOM priming, but priming profiles differed between vegetation communities, being highest at the surface in native assemblage soils, whereas highest at depth under invasive plants. Changes in functional microbial community composition at depth in Phragmites soils, evidenced by an increase in relative fungal laccase abundance, explained the SOM priming in these deep invaded soils. Our results show that invasive Phragmites maintains a microbial community at depth able to degrade SOM faster than that under native vegetation, evidencing that plant species shifts can fundamentally change soil biogeochemistry, altering element cycling and decreasing SOM residence time. Furthermore, our experimental design allowed to quantify real-time SOM-C and SOM-N gross mineralization, resulting in a new model relating C and N mineralization in these wetland soils and providing new insights on how SOM decomposition impacts N availability and cycling across wetland N pools.

Soils with more clay and dense vegetation were rich in soil carbon along Wadi Al-Sharaea, Makkah, Saudi Arabia

In arid ecosystems, lack of vegetation and nutrients can negatively impact soil carbon (C) content. In the current study, our goals were to assess soil C stocks to a depth of 50 cm in an arid ecosystem (Wadi Al-Sharaea, Saudi Arabia) and determine their relation to different vegetation cover. To address our research objective, a total of 102 quadrate (randomly selected) were established along the desert wadi. Soil samples were collected to a depth of 50 cm with 5 cm interval, then Soil Bulk Density (SBD, g/cm³), Soil Organic C Content (SOC, g C/kg), and stocks (kg C/m²) were estimated. Both soil mechanical and chemical analyses were conducted for a composite soil sample. Study sites were categorized based on their visual vegetation cover (VC) percentage (%) into three major groups: 1) scarce vegetation cover (VC less than 25%); 2) medium vegetation cover (VC is higher than 25% and less than 75%); and lastly 3) dense vegetation cover (VC is higher than 75%). Soils were characterized by higher sand content (48.2%, both fine and coarse compiled) than silt (36.7 ± 1.64%) or clay (10.1 ± 1.28%). There were significant differences among soil Calcium (Ca) and Potassium (K) content (p < 0.05), while those plant communities with medium vegetation cover showed the highest soil content of Ca and K (1.7 ± 0.24 and 0.2 ± 0.03 meq/l, respectively). Plant communities with dense vegetation cover had the lowest SBD (1.96 ± 0.03 g/cm³) and the highest SOC stocks (14.9 ± 2.1 kg C/m²). Moreover, our data analyses indicated that SBD and SOC content had strong and negative correlation, where soils with dense vegetation cover had the most significant correlation (R² = 0.95). Our results recommend that soil carbon stocks to a depth of 50 cm based on different vegetation cover of arid ecosystems should be implemented on global soil carbon budget to better elucidate factors controlling SOC content at the regional and global scales.

Rapid evolution of a coastal marsh ecosystem engineer in response to global change

There is increasing evidence that global change can alter ecosystems by eliciting rapid evolution of foundational plants capable of shaping vital attributes and processes. Here we describe results of a field-scale exposure experiment and multilocus assays illustrating that elevated CO₂ (eCO₂) and nitrogen (N) enrichment can result in rapid shifts in genetic and genotypic variation in Phragmites australis, an ecologically dominant plant that acts as an ecosystem engineer in coastal marshes worldwide. Compared to control treatments, genotypic diversity declined over three years of exposure, especially to N enrichment. The magnitude of loss also increased over time under conditions of N enrichment. Comparisons of genotype frequencies revealed that proportional abundances shifted with exposure to eCO₂ and N in a manner consistent with expected responses to selection. Comparisons also revealed evidence of tradeoffs that constrained exposure responses, where any particular genotype responded favorably to one factor rather than to different factors or to combinations of factors. These findings challenge the prevailing view that plant-mediated ecosystem outcomes of global change are governed primarily by differences in species responses to shifting environmental pressures and highlight the value of accounting for organismal evolution in predictive models to improve forecasts of ecosystem responses to global change.

Alder encroachment alters subsoil organic carbon pool and chemical structure in a boreal peatland of Northeast China

Boreal peatlands have been experiencing increased abundances of symbiotic dinitrogen-fixing woody plants (mainly alder species). However, how alder encroachment alters soil organic carbon (C) pool and stability is unclear. To examine the effects of alder encroachment on soil organic C, we measured soil organic C pool, phenol oxidase (POX) activity, organic C mineralization rate, and organic C chemical structure (alkyl C, O-alkyl C, aromatic C, and carbonyl C) using solid-state ¹³C nuclear magnetic resonance spectroscopy in the 0-10 cm, 10-20 cm, and 20-40 cm depths in the Alnus sibirica islands and adjacent open peatlands in the north of Da'xingan Mountain, Northeast China. A. sibirica islands had 28 %, 25 %, and 30 % greater POX activity and 36 %, 31 %, and 100 % higher organic C mineralization than open peatlands in the 0-10 cm, 10-20 cm, and 20-40 cm soil depths, respectively. Despite no significant changes in the 0-10 cm and 10-20 cm depths, alder encroachment reduced soil organic C pool in the 20-40 cm depth. Soil organic C pool in the 0-40 cm depth was lower in A. sibirica islands (298 Mg ha^-1) than in the open peatlands (315 Mg ha^-1). Moreover, alder encroachment increased alkyl (7 %) and carbonyl (57 %) C fractions but reduced O-alkyl C fraction (16 %) in the 20-40 cm depth, resulting in increased aliphaticity and recalcitrance indices. These findings suggest that alder encroachment will reduce soil organic C accumulation by accelerating microbial decomposition, and highlight that increased biochemical stabilization would attenuate soil organic C loss after alder expansion in boreal peatlands. Our results will help assess and project future C budgets in boreal peatlands.

Changes in Archaeal Community and Activity by the Invasion of Spartina anglica Along Soil Depth Profiles of a Coastal Wetland

Invasion of Spartina spp. in tidal salt marshes may affect the function and characteristics of the ecosystem. Previous studies reported that the invasion alters biogeochemical and microbial processes in marsh ecosystems, yet our knowledge of changing archaeal community due to the invasion is still limited, whereas archaeal communities play a pivotal role in biogeochemical cycles within highly reduced marsh soils. In this study, we aimed to illustrate the influences of the Spartina anglica invasion on soil archaeal community and the depth profile of the influences. The relative abundance of archaeal phyla demonstrated that the invasion substantially shifted the characteristics of tidal salt marsh from marine to terrestrial soil only in surface layer, while the influences indirectly propagated to the deeper soil layer. In particular, two archaeal phyla, Asgardaeota and Diapherotrites, were strongly influenced by the invasion, indicating a shift from marine to terrestrial archaeal communities. The shifts in soil characteristics spread to the deeper soil layer that results in indirect propagation of the influences of the invasion down to the deeper soil, which was underestimated in previous studies. The changes in the concentration of dissolved organic carbon and salinity were the substantial regulating factors for that. Therefore, changes in biogeochemical and microbial characteristics in the deep soil layer, which is below the root zone of the invasive plant, should be accounted for a more accurate illustration of the consequences of the invasion.

Biota-mediated carbon cycling-A synthesis of biotic-interaction controls on blue carbon

Research into biotic interactions has been a core theme of ecology for over a century. However, despite the obvious role that biota play in the global carbon cycle, the effects of biotic interactions on carbon pools and fluxes are poorly understood. Here we develop a conceptual framework that illustrates the importance of biotic interactions in regulating carbon cycling based on a literature review and a quantitative synthesis by means of meta-analysis. Our study focuses on blue carbon ecosystems-vegetated coastal ecosystems that function as the most effective long-term CO₂ sinks of the biosphere. We demonstrate that a multitude of mutualistic, competitive and consumer-resource interactions between plants, animals and microbiota exert strong effects on carbon cycling across various spatial scales ranging from the rhizosphere to the landscape scale. Climate change-sensitive abiotic factors modulate the strength of biotic-interaction effects on carbon fluxes, suggesting that the importance of biota-mediated carbon cycling will change under future climatic conditions. Strong effects of biotic interactions on carbon cycling imply that biosphere-climate feedbacks may not be sufficiently represented in current Earth system models. Inclusion of new functional groups in these models, and new approaches to simplify species interactions, may thus improve the predictions of biotic effects on the global climate.

Distribution characteristics and controls of soil organic carbon at different spatial scales in China's Loess Plateau

Understanding the variations and controls of soil organic carbon (SOC) at different spatial scales can help in selecting edaphic and environmental covariates that enables us to model SOC more accurately. The present study investigated the distribution characteristics and controls of SOC content at various spatial scales, including a deep soil core (204.5 m) taken from land surface down to bedrock (plot scale), two toposequences with different slope aspects (slope scale), and eighty-six soil profiles along a north-south transect under different land uses (regional scale) in China's Loess Plateau. The results showed that SOC content at different spatial scales decreased exponentially with increasing soil depth, but the rate of reduction differed at various spatial scales and in soil layers at different depths. For the deep soil core, the SOC content and the average rate of reduction with depth in the 0-15.5 m soil layer were significantly higher than the corresponding values of the 15.5-34.5 m and 34.5-204.5 m soil layers (p < 0.05). For the toposequences with varying slope aspects, SOC content in the 0-50 cm soil layer declined rapidly with increasing depth; while SOC content in the 50-200 cm soil layer showed relatively no change. There was no significant difference of average SOC content at depths of 0-200 cm for forestland and grassland considering slope aspects that differed or were the same (p > 0.05) due to the similar climatic conditions. However, SOC content within 0-500 cm soil profile under different land uses along the north-south transect exhibited a significant difference (p < 0.05), following the order of farmland (4.94 ± 1.23 g kg^-1) > forestland (3.01 ± 1.45 g kg^-1) > grassland (2.03 ± 0.68 g kg^-1); moreover, the mean SOC content of the 0-500 cm soil profile generally decreased from south to north following the decreasing rainfall and temperature gradient. The average rates of reduction of SOC content in the 0-50 cm soil layer under different land uses (0.0807-0.1756 g kg^-1 cm^-1) were higher than the values of the 50-200 cm (0.0021-0.0154 g kg^-1 cm^-1) and 200-500 cm soil layers (0.0001-0.0017 g kg^-1 cm^-). The SOC content at the plot scale at different depths positively correlated with total nitrogen content. The SOC content at the slope scale was mainly affected by soil water content and saturated hydraulic conductivity, while that at the regional scale was impacted by climate, topography and soil water/clay content. Pedotransfer functions were applied to adequately simulate and predict SOC content at different spatial scales in the studied area, which could provide a foundation to build SOC prediction models and extrapolate the various spatial scales to other loess regions worldwide. Our findings demonstrate the importance of considering the scale effects for efficiently predicting the spatial patterns of SOC and can help in devising better policy to protect or enhance existing SOC stocks.

Biogenic link to the recent increase in atmospheric methane over India

Methane (CH₄) is a prominent Greenhouse Gas (GHG) and its global atmospheric concentration has increased significantly since the year 2007. Anthropogenic CH₄ emissions are projected to be 9390 million metric tonnes by 2020. Here, we present the long-term changes in atmospheric methane over India and suggest possible alternatives to reduce soil emissions from paddy fields. The increase in atmospheric CH₄ concentrations from 2009 to 2020 in India is significant, about 0.0765 ppm/decade. The Indo-Gangetic Plains, Peninsular India and Central India show about 0.075, 0.076 and 0.074 ppm/decade, respectively, in 2009-2020. Seasonal variations in CH₄ emissions depend mostly on agricultural activities and meteorology, and contribution during the agricultural intensive period of Kharif-Rabi (i.e., June-December) is substantial in this regard. The primary reason for agricultural soil emissions is the application of chemical fertilizers to improve crop yield. However, for rice farming, soil amendments involving stable forms of carbon can reduce GHG emissions and improve soil carbon status. High crop production in pot culture experiment resulted in lower potential yield-scaled GHG emissions in rice with biochar supplement. The human impact of global warming induced by agricultural activities could be reduced by using biochar as a natural solution.

Plant species determine tidal wetland methane response to sea level rise

Blue carbon (C) ecosystems are among the most effective C sinks of the biosphere, but methane (CH₄) emissions can offset their climate cooling effect. Drivers of CH₄ emissions from blue C ecosystems and effects of global change are poorly understood. Here we test for the effects of sea level rise (SLR) and its interactions with elevated atmospheric CO₂, eutrophication, and plant community composition on CH₄ emissions from an estuarine tidal wetland. Changes in CH₄ emissions with SLR are primarily mediated by shifts in plant community composition and associated plant traits that determine both the direction and magnitude of SLR effects on CH₄ emissions. We furthermore show strong stimulation of CH₄ emissions by elevated atmospheric CO₂, whereas effects of eutrophication are not significant. Overall, our findings demonstrate a high sensitivity of CH₄ emissions to global change with important implications for modeling greenhouse-gas dynamics of blue C ecosystems.

Dark septate endophyte improves salt tolerance of native and invasive lineages of Phragmites australis

Fungal endophytes can improve plant tolerance to abiotic stress. However, the role of these plant-fungal interactions in invasive species ecology and their management implications remain unclear. This study characterized the fungal endophyte communities of native and invasive lineages of Phragmites australis and assessed the role of dark septate endophytes (DSE) in salt tolerance of this species. We used Illumina sequencing to characterize root fungal endophytes of contiguous stands of native and invasive P. australis along a salinity gradient. DSE colonization was assessed throughout the growing season in the field, and effects of fungal inoculation on salinity tolerance were investigated using laboratory and greenhouse studies. Native and invasive lineages had distinct fungal endophyte communities that shifted across the salinity gradient. DSE colonization was greater in the invasive lineage and increased with salinity. Laboratory studies showed that DSE inoculation increased P. australis seedling survival under salt stress; and a greenhouse assay revealed that the invasive lineage had higher aboveground biomass under mesohaline conditions when inoculated with a DSE. We observed that P. australis can establish mutualistic associations with DSE when subjected to salt stress. This type of plant-fungal association merits further investigation in integrated management strategies of invasive species and restoration of native Phragmites.

Impacts of the rhizosphere effect and plant species on organic carbon mineralization rates and pathways, and bacterial community composition in a tidal marsh

Despite the growing recognition regarding the carbon cycle in the rhizosphere of upland ecosystems, little is known regarding the rhizosphere effect on soil organic carbon (SOC) mineralization in tidal marsh soils. In the current study, in situ rhizobox experiments (including rhizosphere and inner and outer bulk soil) were conducted in an estuarine tidal marsh. Our results showed that a higher abundance of total bacteria, Geobacter, dsrA and mcrA and lower α-diversity were observed in the rhizosphere relative to the bulk soil. Rhizosphere effects shifted the partition of terminal metabolic pathways from sulfate reduction in the bulk soil to the co-dominance of microbial Fe(III) and sulfate reduction in the rhizosphere. Although the rhizosphere effect promoted the rates of three terminal metabolic pathways, it showed greater preference towards microbial Fe(III) reduction in the tidal marsh soils. Plant species had little impact on the partitioning of terminal metabolic pathways, but did affect the potential of total SOC mineralization together with the abundance and diversity of total bacteria. Both the rhizosphere effect and plant species influenced the bacterial community composition in the tidal marsh soils; however, plant species had a less pronounced impact on the bacterial community compared with that of the rhizosphere effect.

Impact of sea level change on coastal soil organic matter, priming effects and prokaryotic community assembly

Salt marshes are coastal areas storing high amounts of soil organic matter (SOM) while simultaneously being prone to tidal changes. Here, SOM-decomposition and accompanied priming effects (PE), which describe interactions between labile and old SOM, were studied under controlled flooding conditions. Soil samples from two Wadden Sea salt marsh zones, pioneer (Pio), flooded two times/day, and lower salt marsh (Low), flooded ∼eight times/month, were measured for 56 days concerning CO2-efflux and prokaryotic community shifts during three different inundation-treatments: total-drained (Drained), all-time-flooded (Waterlogged) or temporal-flooding (Tidal). Priming was induced by 14C-glucose addition. CO2-efflux from soil followed Low>Pio and Tidal>Drained>Waterlogged, likely due to O2-depletion and moisture maintenance, two key factors governed by tidal inundation with regard to SOM mineralisation. PEs in both zones were positive (Drained) or absent (Waterlogged, Tidal), presumably as a result of prokaryotes switching from production of extracellular enzymes to direct incorporation of labile C. A doubled amount of prokaryotic biomass in Low compared to Pio probably induced higher chances of cometabolic effects and higher PE. 16S-rRNA-gene-amplicon-based analysis revealed differences in bacterial and archaeal community composition between both zones, revealing temporal niche adaptation with flooding treatment. Strongest alterations were found in Drained, likely due to inundation-mediated changes in C-binding capacities.

Microbial mechanism for enhanced methane emission in deep soil layer of Phragmites-introduced tidal marsh

The introduction of Phragmites australis is known to substantially increase methane emission in the tidal salt marsh. Previous studies suggested that enhanced carbon input by the introduction may stimulate methanogenic activity. However, the exact mechanisms and the effects of the introduction of P. australis to methane dynamics in the deep soil layer are still unclear. In this study we collected 1 m deep intact soil cores and gas samples at native Suaeda japonica- and P. australis-vegetated temperate tidal salt marshes in Suncheon Bay, Republic of Korea. Rates of methane emission and vertical distribution of soil biogeochemistry and microbial communities were analyzed to understand the relationship among chemical and microbiological properties. The introduction of P. australis significantly enhanced methane emission in sites, which was caused by increased DOC and reduced competitive inhibition between sulfate reducer and methanogens. In particular, reduced competitive inhibition between sulfate reducers and methanogens in deep soil layer may play a substantial role in the enhanced methane emission by the introduction of P. australis. Potential methane production was also significantly higher in deeper soil layers than the surface soil layer. We suggest that deep soil layer plays a critical role in the methane dynamics of tidal salt marsh which is introduced by deep root plants, such as P. australis.

Zones of influence for soil organic matter dynamics: A conceptual framework for data and models

Soil organic matter (SOM) is an indicator of sustainable land management as stated in the global indicator framework of the United Nations Sustainable Development Goals (SDG Indicator 15.3.1). Improved forecasting of future changes in SOM is needed to support the development of more sustainable land management under a changing climate. Current models fail to reproduce historical trends in SOM both within and during transition between ecosystems. More realistic spatio-temporal SOM dynamics require inclusion of the recent paradigm shift from SOM recalcitrance as an 'intrinsic property' to SOM persistence as an 'ecosystem interaction'. We present a soil profile, or pedon-explicit, ecosystem-scale framework for data and models of SOM distribution and dynamics which can better represent land use transitions. Ecosystem-scale drivers are integrated with pedon-scale processes in two zones of influence. In the upper vegetation zone, SOM is affected primarily by plant inputs (above- and belowground), climate, microbial activity and physical aggregation and is prone to destabilization. In the lower mineral matrix zone, SOM inputs from the vegetation zone are controlled primarily by mineral phase and chemical interactions, resulting in more favourable conditions for SOM persistence. Vegetation zone boundary conditions vary spatially at landscape scales (vegetation cover) and temporally at decadal scales (climate). Mineral matrix zone boundary conditions vary spatially at landscape scales (geology, topography) but change only slowly. The thicknesses of the two zones and their transport connectivity are dynamic and affected by plant cover, land use practices, climate and feedbacks from current SOM stock in each layer. Using this framework, we identify several areas where greater knowledge is needed to advance the emerging paradigm of SOM dynamics-improved representation of plant-derived carbon inputs, contributions of soil biota to SOM storage and effect of dynamic soil structure on SOM storage-and how this can be combined with robust and efficient soil monitoring.

Phragmites australis meets Suaeda salsa on the "red beach": Effects of an ecosystem engineer on salt-marsh litter decomposition

Suaeda salsa is a pioneer species in coastal wetlands of East Asia and recently an ecosystem engineer species, Phragmites australis, has started to enter into S. salsa communities owing to either autogenic or external drivers. The consequences of this phenomenon on the ecosystem functions of coastal wetlands are still unclear, especially for decomposition processes. Here we compared the decomposition rate of S. salsa litter, and associated litter chemistry dynamics, between sites with and without P. australis encroachment. We conducted a litter transplantation experiment to tease apart the effects of litter quality and decomposing environment or decomposer community composition. Our results showed that P. australis encroachment led to higher carbon and phosphorus losses of S. salsa litter, but equal losses of total mass, lignin, hemicellulose and nitrogen. Phragmites australis encroachment might affect decomposition rate indirectly by making S. salsa produce litter with higher lignin concentrations or via increasing the fungal diversity for decomposition. Moreover, P. australis as an ecosystem engineer might also alter the allocation of total phosphorus between the plants and the soils in coastal wetlands. Our findings indicate that P. australis could impact aboveground and belowground carbon and nutrient dynamics in coastal wetlands, and highlight the important consequences that encroaching plant species, especially ecosystem engineers, can have on ecosystem functions and services of coastal wetlands, not only in East Asia but probably also elsewhere in the world.

Exploring the oxygen sensitivity of wetland soil carbon mineralization

Soil oxygen availability may influence blue carbon, which is carbon stored in coastal wetlands, by controlling the decomposition of soil organic matter. We are beginning to quantify soil oxygen availability in wetlands, but we lack a precise understanding of how oxygen controls soil carbon dynamics. In this paper, we synthesize existing data from oxic and anoxic wetland soil incubations to determine how oxygen controls carbon mineralization. We define the oxygen sensitivity of carbon mineralization as the ratio of carbon mineralization rate in oxic soil to this rate in anoxic soil, such that higher values of this ratio indicate greater sensitivity of carbon mineralization to oxygen. The estimates of oxygen sensitivity we derived from existing literature show a wide range of ratios, from 0.8 to 33, across wetlands. We then report oxygen sensitivities from an experimental mesocosm we developed to manipulate soil oxygen status in realistic soils. The variation in oxygen sensitivity we uncover from this systematic review and experiment indicates that Earth system models may misrepresent the oxygen sensitivity of carbon mineralization, and how it varies with context, in wetland soils. We suggest that altered soil oxygen availability could be an important driver of future blue carbon storage in coastal wetlands.

The Case for Digging Deeper: Soil Organic Carbon Storage, Dynamics, and Controls in Our Changing World

Most of our terrestrial carbon (C) storage occurs in soils as organic C derived from living organisms. Therefore, the fate of soil organic C (SOC) in response to changes in climate, land use, and management is of great concern. Here we provide a unified conceptual model for SOC cycling by gathering the available information on SOC sources, dissolved organic C (DOC) dynamics, and soil biogeochemical processes. The evidence suggests that belowground C inputs (from roots and microorganisms) are the dominant source of both SOC and DOC in most ecosystems. Considering our emerging understanding of SOC protection mechanisms and long-term storage, we highlight the present need to sample (often ignored) deeper soil layers. Contrary to long-held biases, deep SOC—which contains most of the global amount and is often hundreds to thousands of years old—is susceptible to decomposition on decadal timescales when the environmental conditions under which it accumulated change. Finally, we discuss the vulnerability of SOC in different soil types and ecosystems globally, as well as identify the need for methodological standardization of SOC quality and quantity analyses. Further study of SOC protection mechanisms and the deep soil biogeochemical environment will provide valuable information about controls on SOC cycling, which in turn may help prioritize C sequestration initiatives and provide key insights into climate-carbon feedbacks.

Phosphorus alleviation of salinity stress: effects of saltwater intrusion on an Everglades freshwater peat marsh

Saltwater intrusion and salinization of coastal wetlands around the world are becoming a pressing issue due to sea level rise. Here, we assessed how a freshwater coastal wetland ecosystem responds to saltwater intrusion. In wetland mesocosms, we continuously exposed Cladium jamaicense Crantz (sawgrass) plants and their peat soil collected from a freshwater marsh to two factors associated with saltwater intrusion in karstic ecosystems: elevated loading of salinity and phosphorus (P) inputs. We took repeated measures using a 2 × 2 factorial experimental design (n = 6) with treatments composed of elevated salinity (~9 ppt), P loading (14.66 μmol P/d), or a combination of both. We measured changes in water physicochemistry, ecosystem productivity, and plant biomass change over two years to assess monthly and two-year responses to saltwater intrusion. In the short-term, plants exhibited positive growth responses with simulated saltwater intrusion (salinity + P), driven by increased P availability. Despite relatively high salinity levels for a freshwater marsh (~9 ppt), gross ecosystem productivity (GEP), net ecosystem productivity (NEP), and aboveground biomass were significantly higher in the elevated salinity + P treated monoliths compared to the freshwater controls. Salinity stress became evident after extended exposure. Although still higher than freshwater controls, GEP and NEP were significantly lower in the elevated salinity + P treatment than the +P treatment after two years. However, elevated salinity decreased live root biomass regardless of whether P was added. Our results suggest that saltwater intrusion into karstic freshwater wetlands may initially act as a subsidy by stimulating aboveground primary productivity of marsh plants. However, chronic exposure to elevated salinity results in plant stress, negatively impacting belowground peat soil structure and stability through a reduction in plant roots.

Plant-Sediment Interactions in Salt Marshes - An Optode Imaging Study of O2, pH, and CO 2 Gradients in the Rhizosphere

In many wetland plants, belowground transport of O2 via aerenchyma tissue and subsequent O2 loss across root surfaces generates small oxic root zones at depth in the rhizosphere with important consequences for carbon and nutrient cycling. This study demonstrates how roots of the intertidal salt-marsh plant Spartina anglica affect not only O2, but also pH and CO2 dynamics, resulting in distinct gradients of O2, pH, and CO2 in the rhizosphere. A novel planar optode system (VisiSens TD®, PreSens GmbH) was used for taking high-resolution 2D-images of the O2, pH, and CO2 distribution around roots during alternating light-dark cycles. Belowground sediment oxygenation was detected in the immediate vicinity of the roots, resulting in oxic root zones with a 1.7 mm radius from the root surface. CO2 accumulated around the roots, reaching a concentration up to threefold higher than the background concentration, and generally affected a larger area within a radius of 12.6 mm from the root surface. This contributed to a lowering of pH by 0.6 units around the roots. The O2, pH, and CO2 distribution was recorded on the same individual roots over diurnal light cycles in order to investigate the interlinkage between sediment oxygenation and CO2 and pH patterns. In the rhizosphere, oxic root zones showed higher oxygen concentrations during illumination of the aboveground biomass. In darkness, intraspecific differences were observed, where some plants maintained oxic root zones in darkness, while others did not. However, the temporal variation in sediment oxygenation was not reflected in the temporal variations of pH and CO2 around the roots, which were unaffected by changing light conditions at all times. This demonstrates that plant-mediated sediment oxygenation fueling microbial decomposition and chemical oxidation has limited impact on the dynamics of pH and CO2 in S. anglica rhizospheres, which may in turn be controlled by other processes such as root respiration and root exudation.

Competitive interactions between native Spartina alterniflora and non-native Phragmites australis depend on nutrient loading and temperature

We explored the nature and impact of competitive interactions between the salt marsh foundational plant Spartina alterniflora and invasive Phragmites australis in New England under varying levels of anthropogenic influence from nutrient loading and temperature warming. Plants were grown with and without competition in mesocosms over a four-month growing season. Mesocosms were split evenly among three levels of nutrient additions and two temperatures varying by an average of ~3° C, manipulated using small greenhouses. We measured aboveground productivity as total biomass, numbers of new stems, and mean stem height. Nutrient enrichment increased all growth parameters, while competition generally reduced aboveground biomass and the production of new stems in both species. Most importantly, smooth cordgrass suffered no negative consequences of competition when no nutrients were added and temperature was elevated. The results of this study suggest that minimizing nutrient loading into coastal marshes could be an important factor in slowing the spread of common reed into the low marsh zone of New England salt marshes as global temperatures continue to warm.

Cosmopolitan Species As Models for Ecophysiological Responses to Global Change: The Common Reed Phragmites australis

Phragmites australis is a cosmopolitan grass and often the dominant species in the ecosystems it inhabits. Due to high intraspecific diversity and phenotypic plasticity, P. australis has an extensive ecological amplitude and a great capacity to acclimate to adverse environmental conditions; it can therefore offer valuable insights into plant responses to global change. Here we review the ecology and ecophysiology of prominent P. australis lineages and their responses to multiple forms of global change. Key findings of our review are that: (1) P. australis lineages are well-adapted to regions of their phylogeographic origin and therefore respond differently to changes in climatic conditions such as temperature or atmospheric CO2; (2) each lineage consists of populations that may occur in geographically different habitats and contain multiple genotypes; (3) the phenotypic plasticity of functional and fitness-related traits of a genotype determine the responses to global change factors; (4) genotypes with high plasticity to environmental drivers may acclimate or even vastly expand their ranges, genotypes of medium plasticity must acclimate or experience range-shifts, and those with low plasticity may face local extinction; (5) responses to ancillary types of global change, like shifting levels of soil salinity, flooding, and drought, are not consistent within lineages and depend on adaptation of individual genotypes. These patterns suggest that the diverse lineages of P. australis will undergo intense selective pressure in the face of global change such that the distributions and interactions of co-occurring lineages, as well as those of genotypes within-lineages, are very likely to be altered. We propose that the strong latitudinal clines within and between P. australis lineages can be a useful tool for predicting plant responses to climate change in general and present a conceptual framework for using P. australis lineages to predict plant responses to global change and its consequences.