Soil Accretion Research Articles

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Evaluating a Steady-State Model of Soil Accretion in Everglades Mangroves (Florida, USA)

To determine whether mangrove soil accretion can keep up with increasing rates of sea level rise, we modeled the theoretical, steady-state (i.e., excluding hurricane impacts) limits to vertical soil accretion in riverine mangrove forests on the southwest coast of Florida, USA. We measured dry bulk density (BD) and loss on ignition (LOI) from mangrove soils collected over a period of 12 years along an estuarine transect of the Shark River. The plotted relationship between BD and LOI was fit to an idealized mixing model equation that provided estimates of organic and inorganic packing densities in the soils. We used these estimates in combination with measures of root production and mineral deposition to calculate their combined contribution to steady-state, vertical soil accretion. On average, the modeled rates of accretion (0.9 to 2.4 mm year−1) were lower than other measured rates of soil accretion at these sites and far less than a recent estimate of sea level rise in south Florida (7.7 mm year−1). To date, however, no evidence of mangrove “drowning” has been observed in this region of the Everglades, indicating that assumptions of the linear accretion model are invalid and/or other contributions to soil accretion (e.g., additional sources of organic matter; feedbacks between physical sedimentation processes and biological responses to short-term environmental change) make up the accretion deficit. This exercise highlights the potential positive impacts of hurricanes on non-steady-state soil accretion that contribute to the persistence of neotropical mangroves in regions of high disturbance frequency such as the Gulf of Mexico and the Caribbean region.

Hydroperiod and Salinity Interactions Control Mangrove Root Dynamics in a Karstic Oceanic Island in the Caribbean Sea (San Andres, Colombia)

Mangroves sustain high soil accretion and carbon sequestration rates, yet it is still unknown if they can keep pace with increasing sea level rise (SLR) across a wider range of coastal geomorphic settings. Because accretion rates are controlled by mineral sediment inputs and organic matter accumulation, it is paramount to assess the relative contribution of root productivity to soil formation. Here, we evaluated root biomass, production, and turnover in three mangrove ecotypes to evaluate the role of soil nutrient limitation, stressors, and hydroperiod in controlling root dynamics in San Andres Island (SAI), a karstic oceanic island in the Caribbean Sea. Root production was modulated by soil stress conditions and not by nutrient availability as it has been reported for other karstic environments. The lowest root biomass allocation, and both production and turnover of fine roots were measured under low flooding duration, and low salinity (<20 PSU) and sulfide concentrations (0.84 ± 0.4 mM). Yet, when soil stress conditions increased during high flooding duration (6207 h y–1) and low oxygen conditions (Eh), root tissues reached the highest biomass and production values, including a relative fast turnover of fine roots (<2 mm; 0.75 y–1). Our results follow the predictions of the plant root longevity cost-benefit hypothesis where plants maintain roots only until the efficiency of resource acquisition is maximized by water and nutrient acquisition. Because of the importance of groundwater in controlling porewater salinity and mangrove root productivity in karstic oceanic islands such as SAI, water use and coastal development should be regulated in the short term to avoid the loss of mangrove area and concomitant ecosystem services.

Processes driving seagrass soils composition along the western Mediterranean: The case of the southeast Iberian Peninsula

Seagrasses are distributed all along the coast of the Mediterranean Sea being Posidonia oceanica and Cymodocea nodosa the most common species. They promote sedimentation, leading to the formation of well-structured soils. Over the last decade, a growing attention has been paid to their role as CO2 sinks in the form of organic carbon (Corg) and to their use as environmental archives. However, most of the knowledge about pedogenetic processes in these soils refer to the rhizosphere. This study aims to understand seagrass soils biogeochemistry in the rhizosphere and below, which in turn can help to understand their long term formation processes. Fifteen cores were strategically sampled along a 350 km stretch of the Southeast Iberian coast, and analyzed for elemental composition (XRF core-scanning), magnetic susceptibility, Corg content and gran size distribution. The cores were dated by 210Pb and 14C-AMS techniques to estimate soil accretion. Principal component analysis was used to explore the main geochemical processes linked to soil formation. The results showed that terrestrial runoff plays a key role in meadow soil composition. Furthermore, Corg accumulation did not follow any general depth trend in our soil records, suggesting that temporal variation in Corg inputs is an important factor in determining carbon depth distribution within the soil. We obtained evidence that the establishment of well-developed, stable C. nodosa meadows in the Mediterranean Sea may be promoted by adverse environmental conditions to P. oceanica settlement. Metal's behavior within the meadow deposit and their interaction with organic matter and carbonates is unclear. The results presented in this paper highlight the importance of the influence of land-based inputs in the characteristics of seagrass meadow deposits, highly determining their Corg content, as well as the need for further studies on metal behavior, to understand their full potential as environmental records.

Evidence does not support the targeting of cryptic invaders at the subspecies level using classical biological control: the example of Phragmites

Classical biocontrol constitutes the importation of natural enemies from a native range to control a non-native pest. This is challenging when the target organism is phylogenetically close to a sympatric non-target form. Recent papers have proposed and recommended that two European moths (Archanara spp.) be introduced to North America to control non-native Phragmites australis australis, claiming they would not adversely affect native P. australis americanus. We assert that these papers overlooked research contradicting their conclusions and that the authors recommended release of the non-native moths despite results of their own studies indicating that attack on native Phragmites is possible after field release. Furthermore, their open-field, host-specificity tests were conducted in non-wetland fields in Switzerland using potted plants, reflecting considerably different conditions than those of North American wetlands. Also, native Phragmites in eastern North America has declined, increasing its potential vulnerability to any new stressors. Because all inadvertently introduced, established, Phragmites-specialist, herbivorous insects have done more harm to native than non-native Phragmites, native Phragmites may experience more intense herbivory than non-native Phragmites from the introduction of Archanara spp. due to demographic mechanisms (e.g., increase in density of the biocontrol agent and spillover onto alternate hosts) or because the herbivores may undergo genetic change. In addition to the risk to native Phragmites, significant biomass reduction of non-native Phragmites may decrease important ecosystem services, including soil accretion in wetlands affected by sea level rise. We strongly caution against the approval of Archanara spp. as biocontrol agents for non-native Phragmites in North America.

Visualizing the dynamics of soil aggregation as affected by arbuscular mycorrhizal fungi.

Stable soils provide valuable ecosystem services and mechanical soil stability is enhanced by the presence of arbuscular mycorrhizal fungi (AMF). Soil aggregation, which is the major driver of mechanical soil stability, is often treated as a static phenomenon, even though aggregate turnover is continually ongoing. In fact, some breakdown of macroaggregates is necessary to allow new aggregate formation and inclusion of new organic matter into microaggregates. We determined how aggregate turnover times were affected by AMF by tracking movement of rare earth elements (REE), applied as their immobile oxides, between aggregate size classes, and using X-ray fluorescence microscopy to spatially localize REEs in a sample of aggregates. Here we show that AMF increased large macroaggregate formation and slowed down disintegration of large and small macroaggregates. Microaggregate turnover was increased in the presence of AMF. Internal aggregate organization suggested that although formation of microaggregates by accretion of soil to particulate organic matter is common, it is not the only mechanism in operation.

Remote sensing of unhelpful resilience to sea level rise caused by mangrove expansion: A case study of islands in Florida Bay, USA

Previous studies have found that vegetated coastal areas can increase their elevation indicating resilience to inundation by sea level rise (SLR), but the potential resilience were ignored or showed controversial results (i.e., soil accretion of vegetated areas vs. SLR). To estimate the resilience influences on 15 islands in Florida Bay (Florida, U.S.), our study used indicators (areas of the 15 islands and their mangrove forests) by analyzing 61-yr high-resolution historical aerial photographs and a 27-yr time-series of Landsat images. In these islands, coastal fringes are dominated by mangroves, and inland parts are dominated by brackish or freshwater species. Our results showed that: (1) despite rising sea levels, these low-lying islands significantly increased in area; (2) all of these islands had significant mangrove expansion, and the landward part of expansion led to the replacement of inland non-mangrove habitats; (3) there was a positive relationship between the increase of island area and mangrove expansion in these islands; (4) without the mangrove expansion, simulations showed that all of the islands had decreased areas by 2014 compared with that in 1953. On the basis of our spatial analyses and previous field studies in our study areas, these islands showed resilience to inundation and the mangrove expansion contributed to processes stabilizing these islands under SLR. Meanwhile, the mangrove expansion were partly at the expense of the habitats previously covered by non-mangrove species, thus potentially leading to a loss of plant diversity. Therefore, the mangrove expansion increased unhelpful resilience to maintain islands in a degraded state losing biodiversity, which should be considered in conservation accounting for future SLR. Moreover, the unhelpful resilience can be monitored by remote sensing based indicators, such as island area.

Long-term assimilation wetlands in coastal Louisiana: Review of monitoring data and management

The term ‘assimilation wetland’ has been applied to natural wetlands in Louisiana into which disinfected, secondarily treated municipal effluent is discharged with the dual purpose of improving regional water quality and enhancing vegetation productivity and soil accretion. Some municipalities began discharging treated effluent into wetlands prior to state regulations, which began in 1992. Here we review data and observations from five assimilation wetlands in the Mississippi River Delta receiving discharge of treated effluent for 26–70 years. In addition, we examine two adjacent forested wetlands, one that receives periodic Mississippi River input and one that does not. Information from these sites provides insight into how long-term nutrient input impacts coastal wetlands. Analysis of tree-ring, leaf litter, accretion, and water quality data shows that input of freshwater containing nutrients and sediments leads to enhanced wetland productivity and soil accretion via increased organic matter burial. In addition, long-term data indicate that assimilation wetlands continue to be nutrient sinks even after decades of effluent discharge, with both nitrogen and phosphorus reduced to background levels. Collectively, these data demonstrate that wetlands benefit from long-term discharge of treated municipal effluent. Properly managed wetland assimilation systems can function for long periods and lead to enhancement of degrading wetland communities in coastal Louisiana.

Impact of Global Change on Nutrient Dynamics in Mangrove Forests

The cycling of essential nutrients is central to mangrove productivity. A mass balance shows that mangroves rely on soil ammonification, nitrification, and dissimilatory reduction to ammonium for available nitrogen. Mangroves are often nutrient limited and show tight coupling between nutrient availability and tree photosynthesis. This relationship and, thus, forest productivity can be disrupted by various disturbances such as deforestation, changes in hydrology due to impoundments, land-use change, increasing frequency and intensity of storms, increasing temperatures, increasing atmospheric CO2 concentrations, and a rising sea-level. Deforestation and hydrological changes are the most devastating to soil nutrient-plant relations and mangrove productivity. Land-use changes can result in positive and negative impacts on mangroves and can also results in increasing frequency of storms and intensity of storms. Increasing temperatures and atmospheric CO2 levels have an initially enhanced effect on mangroves and microbial transformation rates of nitrogen and phosphorus. The effects of rising seas are complex and depend on the local rate of sea-level rise, the soil accretion rate, the subsidence or uplift rate, and the tidal position. If mangroves cannot keep pace with a sea-level rise, seaward mangroves will likely drown but landward mangroves will expand and show enhanced growth and more rapid nutrient cycling if space permits.

Linkages between soil carbon, soil fertility and nitrogen fixation in Acacia senegal plantations of varying age in Sudan.

BackgroundOver the last decades sub-Saharan Africa has experienced severe land degradation and food security challenges linked to loss of soil fertility and soil organic matter (SOM), recurrent drought and increasing population. Although primary production in drylands is strictly limited by water availability, nutrient deficiencies, particularly of nitrogen (N) and phosphorus (P), are also considered limiting factors for plant growth. It is known that SOM (often measured as soil organic carbon (SOC)) is a key indicator of soil fertility, therefore, management practices that increase SOM contents, such as increasing tree cover, can be expected to improve soil fertility. The objectives of this study were to investigate the effect of Acacia senegal (Senegalia senegal) trees on soil nitrogen, phosphorus and potassium (K) in relation to SOC, the potential of A. senegal for N2 fixation, and to identify possible N and P ecosystem limitations.MethodsSoil nutrient (total N, P, K and available P and exchangeable K) concentrations and stocks were determined for the 0–10, 10–20,20–30 and 30–50 cm layers of A. senegal plantations of varying age (ranging from 7 to 24-years-old) and adjacent grasslands (reference) at two sites in semi-arid areas of Sudan. At both sites, three plots were established in each grassland and plantation. The potential of A. senegal for N2 fixation in relation to plantations age was assessed using δ15N isotopic abundances and nutrient limitations assessed using C:N:P stoichiometry.ResultsSoil concentrations of all studied nutrients were relatively low but were significantly and directly correlated to SOC concentrations. SOC and nutrient concentrations were the highest in the topsoil (0–10 cm) and increased with plantations age. Acacia foliage δ15N values were >6‰ and varied little with plantations age. Soil C:N and C:P ratios did not differ between grassland and plantations and only 0–10 cm layer N:P ratios showed significant differences between grassland and plantations.DiscussionThe results indicated that soil fertility in the Sahel region is strongly related to SOM contents and therefore highlighting the importance of trees in the landscape. The higher mineral nutrient concentrations in the topsoil of the plantations may be an indication of ‘nutrient uplift’ by the deeper roots. The high foliar δ15N values indicated that N2 fixation was not an important contributor to soil N contents in the plantations. The accretion of soil N cannot be explained by deposition but may be related to inputs of excreted N brought into the area annually by grazing and browsing animals. The soil C:N:P stoichiometry indicated that the plantations may be limited by P and the grasslands limited by N.

Tidal freshwater forests: Sentinels for climate change

We measured plant community composition and productivity, soil accretion, and C, N, and P burial in a tidal freshwater forest of the Altamaha River, Georgia to gain a better understanding of the ecosystem services they deliver and their ability to keep pace with current and future rates of sea level rise. Ten species were identified in two 0.1 ha plots. Nyssa aquatica (Tupelo Gum) made up 50% of the density and 57% of the total basal area. Nyssa biflora, Liquidambar styraciflua, and Fraxinus pennsylvanica were the next dominant species, collectively accounting for 37% of the density and 26% of the total basal area. Taxodium distichum only accounted for 3% of the density, but 12% of the total basal area. Aboveground productivity, measured as litterfall and stem wood growth, averaged 927 and 1030 g/m2 in 2015 and 2016, respectively, with litterfall accounting for 60% of the total. Tidal forest soils in the streamside and the interior (0–60 cm) contained 3–6% organic C, 0.20–0.40% N, and 270–540 µg/g P. Soil accretion based on 137Cs was 4.0 mm/year on the streamside and 0.2 mm/year in the forest interior. The rate of accretion in the interior is considerably less than the current rate of sea level rise (3.1 mm/year) along the Georgia coast. Because the accretion rate was much higher on the streamside, rates of C sequestration, N and P accumulation, and mineral sediment deposition also were much greater. Low accretion rates in the interior of the forest that accounts for most of the acreage suggests that accelerated sea level rise is likely to lead to foreseeable death of tidal forests from saltwater intrusion and submergence.

Rising tides, rising gates: The complex ecogeomorphic response of coastal wetlands to sea-level rise and human interventions

Abstract Coastal wetlands are vulnerable to submergence due to sea-level rise, as shown by predictions of up to 80% of global wetland loss by the end of the century. Coastal wetlands with mixed mangrove-saltmarsh vegetation are particularly vulnerable because sea-level rise can promote mangrove encroachment on saltmarsh, reducing overall wetland biodiversity. Here we use an ecogeomorphic framework that incorporates hydrodynamic effects, mangrove-saltmarsh dynamics, and soil accretion processes to assess the effects of control structures on wetland evolution. Migration and accretion patterns of mangrove and saltmarsh are heavily dependent on topography and control structures. We find that current management practices that incorporate a fixed gate for the control of mangrove encroachment are useful initially, but soon become ineffective due to sea-level rise. Raising the gate, to counteract the effects of sea level rise and promote suitable hydrodynamic conditions, excludes mangrove and maintains saltmarsh over the entire simulation period of 100 years

Landscape dependent changes in soil properties due to long-term cultivation and subsequent conversion to native grass agriculture

On farmland in undulating landscapes, soil organic carbon (SOC) stocks depend on landscape position. In the North American Prairie Pothole region, we compared a native prairie reference site with a nearby farm undergoing transition to perennial agriculture (“restoration”) after a century of producing annual crops. We quantified legacy effects of farming at four upland landscape positions (to 0.9-m soil depth) and three wetland positions (to 1.0-m soil depth). We also quantified short-term (4years) changes in SOC stocks (to 0.15-m soil depth) during restoration, and how these changes were impacted by historic erosion.Surface (to 0.05-m soil depth) measurements indicated degradation of the cropland soil relative to the prairie at all landscape positions due to less soil organic matter (SOM) and altered soil properties (e.g., water aggregate stability and microbial activity). All upland and wetland positions of the farm lost SOC stocks (Mgha−1) relative to the prairie in the top 1.5 Ggha−1 of soil (~14-cm depth). However, when considering a larger mass of soil, 4.5Ggha−1 (~39-cm depth), loss of SOC stocks was significant (p<0.001) only at the summit (46Mgha−1) and shoulder (63Mgha−1) landscape positions. Differences in SOC stocks between farm and prairie were smaller and not significant at the backslope, footslope, and in wetlands.Contrary to expectations, sites of soil deposition did not accumulate soil carbon after restoration. Accretion of soil C during restoration differed according to the severity of historic erosion (p<0.001), with severely eroded soils gaining soil C at the fastest rate. Historic loss of clay at the shoulder and backslope and subsoil compaction in the wetlands may prevent these landscape positions from full restoration of soil C stocks, net primary productivity, and historic vegetation.

Soil carbon sequestration in freshwater wetlands varies across a gradient of ecological condition and by ecoregion

We evaluated the ability of freshwater riparian wetlands along a gradient of ecological condition to act as sinks for carbon and sediment. We compared rates of carbon accretion and soil accretion across 20 wetlands in the Lake Erie Drift Plain and the Ridge and Valley ecoregions. Soil cores were collected and analyzed using 137Cs dating to quantify long-term (∼50year) rates of sediment and carbon accumulation. Data on hydrology and floristic quality were used to help explain variability in rates. Sites were classified as being in low, moderate, or high ecological condition based on a rapid assessment method, which was verified by their floristic quality. Wetlands of low ecological condition (more human disturbance) had higher mean soil accretion and carbon accretion rates. Soil accretion averaged 0.24±0.17cmyr−1 and 0.14±0.04cmyr−1 in low condition sites and high condition sites, respectively. Carbon accretion averaged 88±50gCm−2yr−1 in low condition and 65±27gCm−2yr−1 in high condition sites. Low condition sites had lower mean soil carbon concentrations in the upper 10cm of the soil profile, suggesting that the higher carbon burial in these sites was related to allochthonous carbon inputs in incoming sediment, rather than autochthonous carbon inputs. There were also striking rate differences between ecoregions. Erie Drift Plain wetlands had significantly higher mean soil accretion rates, compared to Ridge and Valley wetlands. These data indicate that freshwater wetlands play a role in regulating climate by acting as carbon sinks and that anthropogenic disturbance can impact rates of carbon burial.

Science synthesis for management as a way to advance ecosystem restoration: evaluation of restoration scenarios for the Florida Everglades

Restoration of large ecosystems, like the Florida Everglades, is complex because they cover large areas that span many jurisdictional boundaries and encompass a range of stakeholders whose survival and livelihood are intertwined with the ecosystem. The Everglades is well studied, but linking scientific information to restoration action requires that information be coproduced and carefully communicated to make scientific information credible, relevant, and accepted by stakeholders. Guided by decision‐maker key questions, the Synthesis of Everglades Restoration and Ecosystem Services (SERES) project used a team of specialists to synthesize scientific information across disciplines and evaluate four restoration scenarios that differed in water storage and removal of barriers to overland flow. Capital and operation costs were calculated for all scenarios and the results were communicated to policymakers and other stakeholders with the help of a graphic artist. Restoration options were evaluated in areas of hydrology, soil accretion, landscape processes, and small fish and wading bird dynamics. All scenarios provided substantial improvements to the biological system compared to the existing conditions. The option that expanded aboveground surface water storage and reduced barriers to water flow by 69% which allowed an estimated 91% of predrainage flows to reach the Gulf provided the best ecologic benefits for the least resources spent. The SERES project provides a practical process for improving knowledge exchange among stakeholders involved in restoring large‐scale ecosystems.

Low Carbon sink capacity of Red Sea mangroves

Mangroves forests of Avicennia marina occupy about 135 km2 in the Red Sea and represent one of the most important vegetated communities in this otherwise arid and oligotrophic region. We assessed the soil organic carbon (Corg) stocks, soil accretion rates (SAR; mm y−1) and soil Corg sequestration rates (g Corg m−2 yr−1) in 10 mangrove sites within four locations along the Saudi coast of the Central Red Sea. Soil Corg density and stock in Red Sea mangroves were among the lowest reported globally, with an average of 4 ± 0.3 mg Corg cm−3 and 43 ± 5 Mg Corg ha−1 (in 1 m-thick soils), respectively. Sequestration rates of Corg, estimated at 3 ± 1 and 15 ± 1 g Corg m−2 yr−1 for the long (millennia) and short (last century) temporal scales, respectively, were also relatively low compared to mangrove habitats from more humid bioregions. In contrast, the accretion rates of Central Red Sea mangroves soils were within the range reported for global mangrove forests. The relatively low Corg sink capacity of Red Sea mangroves could be due to the extreme environmental conditions such as low rainfall, nutrient limitation and high temperature, reducing the growth rates of the mangroves and increasing soil respiration rates.

Carbon sequestration and nutrient accumulation in floodplain and depressional wetlands

We measured soil organic carbon (C) sequestration and nutrient (nitrogen-N, phosphorus- P) burial in Czech and Midwest U.S. freshwater floodplain and depressional wetlands to evaluate how landscape position and agricultural land use intensity affects C, N, and P retention. Land use in the South Bohemia of the Czech Republic is dominated by forest and pasture, whereas in the Midwest U.S., land use is dominated by row crop agriculture. Cs-137 and 210Pb dating of soil cores revealed comparable rates of soil accretion among wetland types, ranging from 0.5mm/yr in a Czech floodplain wetland to 2.3mm/yr in a U.S. depressional wetland. Carbon sequestration and N & P burial did not differ among floodplain (47+14gC/m2/yr, 3.7+1gN/m2/yr, 0.47+0.16g P/m2/yr) and depressional wetlands (50+19g/m2/yr, 3.6+1.3gN/m2/yr, 0.51+0.14g P/m2/yr). However, sediment deposition in Czech floodplain and depressional wetlands was only 10–50% (150–340g/m2/yr) of rates measured in U.S. wetlands (650–1460g/m2/yr). Our results suggest that, in agricultural landscapes, land use intensity rather than landscape position – floodplain versus depression – drives wetland C sequestration and nutrient retention through increased sediment deposition.

Identifying the Salinity Thresholds that Impact Greenhouse Gas Production in Subtropical Tidal Freshwater Marsh Soils

Increasing salinity due to sea level rise is an important factor influencing biogeochemical processes in estuarine wetlands, with the potential to impact greenhouse gas (GHG) emissions. However, there is little consensus regarding what salinity thresholds will significantly alter the production of GHGs or the physiochemical properties of wetland soils. This study used a fine-scale salinity gradient to determine the impact of seawater concentration on the potential production of CH4, CO2 and N2O and associated soil properties using bottle incubations of tidal freshwater marsh soils from the Min River estuary, SE China. Potential CH4 production was unaffected by salinities from 0 to 7.5‰, but declined significantly at 10‰ and above. Potential CO2 production was stimulated at intermediate salinities (5 to 7.5‰), but inhibited by salinities ≥15‰, while potential N2O production was unaffected by salinity. In contrast, soil dissolved organic carbon and NH4 +-N generally increased with salinity. Overall, this research indicates salinities of ~10–15‰ represent an important tipping point for biogeochemical processes in wetlands. Above this threshold, carbon mineralization is reduced and may promote vertical soil accretion in brackish and salinity wetlands. Meanwhile, low-level saltwater intrusion may leave wetlands vulnerable to submergence due to accelerated soil organic carbon loss.

Regulating effects of climate, net primary productivity, and nitrogen on carbon sequestration rates in temperate wetlands, Northeast China

Temperate wetlands in the Northern Hemisphere have high long-term carbon sequestration rates, and play critical roles in mitigating regional and global atmospheric CO2 increases at the century timescale. We measured soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) from 11 typical freshwater wetlands (Heilongjiang Province) and one saline wetland (Jilin Province) in Northeast China, and estimated carbon sequestration rates using 210Pb and 137Cs dating technology. Effects of climate, net primary productivity, and nutrient availability on carbon sequestration rates (Rcarbon) were also evaluated. Chronological results showed that surface soil within the 0–40cm depth formed during the past 70–205 years. Soil accretion rates ranged from 2.20 to 5.83mm yr−1, with an average of 3.84±1.25mm yr−1 (mean±SD). Rcarbon ranged from 61.60 to 318.5gCm−2 yr−1 and was significantly different among wetland types. Average Rcarbon was 202.7gCm−2 yr−1 in the freshwater wetlands and 61.6gCm−2 yr−1 in the saline marsh. About 1.04×108tons of carbon was estimated to be captured by temperate wetland soils annually in Heilongjiang Province (in the scope of 45.381–51.085°N, 125.132–132.324°E). Correlation analysis showed little impact of net primary productivity (NPP) and soil nutrient contents on Rcarbon, whereas climate, specifically the combined dynamics of temperature and precipitation, was the predominant factor affecting Rcarbon. The negative relationship observed between Rcarbon and annual mean temperature (T) indicates that warming in Northeast China could reduce Rcarbon. Significant positive relationships were observed between annual precipitation (P), the hydrothermal coefficient (defined as P/AT, where AT was accumulative temperature ≥10°C), and Rcarbon, indicating that a cold, humid climate would enhance Rcarbon. Current climate change in Northeast China, characterized by warming and drought, may form positive feedbacks with Rcarbon in temperate wetlands and accelerate carbon loss from wetland soils.

Rise and Fall of an Egyptian Oasis: Artesian Flow, Irrigation Soils, and Historical Agricultural Development in El‐Deir, Kharga Depression, Western Desert of Egypt

The present study examines the geoarchaeological history of an oasis in Kharga Depression in central Egypt. El‐Deir is renowned for its Ptolemaic temple and Roman fortress on the road from former Hibis (Kharga) to the Nile Valley. During the survey, spring mounds and irrigation soils belonging to an ancient agricultural zone were discovered, and further documented by ceramics found on the site. Our methodology combines the geomorphological interpretation of landforms (especially yardangs) with ceramics and 14C‐dated charcoal to distinguish and date former agricultural areas in El‐Deir. The results show that the oasis experienced several phases of soil accretion and destruction through time. Playa sediments were deposited in the humid early Holocene and severely eroded by deflation before the onset of irrigated agriculture between Pharaonic and Persian times. Very fast vertical soil accretion occurred in the Ptolemaic period, but irrigation soils were later destroyed during the Roman period by a combination of wind deflation and flash floods (second to fourth century A.D.), suggesting a period of climate instability. The case of El‐Deir invites reevaluation of constructive agencies for the development of irrigated land and destructive agencies as limiting factors for the sustainability of agricultural practices in late antiquity.