Carbon Sequestration in a Large Hydroelectric Reservoir: An Integrative Seismic Approach

Artificial reservoirs likely accumulate more carbon than natural lakes due to their unusually high sedimentation rates. Nevertheless, the actual magnitude of carbon accumulating in reservoirs is poorly known due to a lack of whole-system studies of carbon burial. We determined the organic carbon (OC) burial rate and the total OC stock in the sediments of a tropical hydroelectric reservoir by combining a seismic survey with sediment core sampling. Our data suggest that no sediment accumulation occurs along the margins of the reservoir and that irregular bottom morphology leads to irregular sediment deposition. Such heterogeneous sedimentation resulted in high spatial variation in OC burial—from 0 to 209 g C m^?2 y^?1. Based on a regression between sediment accumulation and OC burial rates (R ² = 0.94), and on the mean reservoir sediment accumulation rate (0.51 cm y^?1, from the seismic survey), the whole-reservoir OC burial rate was estimated at 42.2 g C m^?2 y^?1. This rate was equivalent to 70% of the reported carbon emissions from the reservoir surface to the atmosphere and corresponded to a total sediment OC accumulation of 0.62 Tg C since the reservoir was created. The approach we propose here allows an inexpensive and integrative assessment of OC burial in reservoirs by taking into account the high degree of spatial variability and based on a single assessment. Because burial can be assessed shortly after the survey, the approach combining a seismic survey and coring could, if applied on a larger scale, contribute to a more complete estimate of carbon stocks in freshwater systems in a relatively short period of time.     

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO₂ supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH₄) flux, direct CO₂ and CH₄ fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO₂ concentrations ranged from 6,491 to 14,976 ??atm and directly-measured stream CO₂ outgassing flux was 5,994 ± 677 g C m^?2 y^?1 of stream surface. Stream pCH₄ concentrations ranged from 291 to 438 ??atm and measured stream CH₄ outgassing flux was 987 ± 221 g C m^?2 y^?1. Despite high flux rates from the stream surface, the small area of stream itself (970 m², or 0.007% of watershed area) led to small directly-measured annual fluxes of CO₂ (0.44 ± 0.05 g C m² y^?1) and CH₄ (0.07 ± 0.02 g C m² y^?1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m^?2 y^?1), DOC (0.16 ± 0.03 g C m^?2 y^?1) and coarse plus fine particulate C (0.001 ± 0.001 g C m^?2 y^?1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m^?2 y^?1 as CO₂ outgassing, 11.29 g C m^?2 y^?1 as fluvial DIC and 0.64 g C m^?2 y^?1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40?C50 g C m^?2 y^?1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m^?2 y^?1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m^?2 (±1.5 Mg C ha^?1) that is exchanged annually across Amazon tropical forest canopies.     

Organic carbon burial in lake sediments in the middle and lower reaches of the Yangtze River Basin, China

Lakes are important in the global and regional carbon cycle, and lake sediments potentially store substantial quantities of organic carbon. The middle and lower reaches of the Yangtze River basin (MLYB) are some of the largest agricultural areas in China with an extremely high density of lakes and rivers. The lakes in the region have undergone dramatic changes over the past several decades. In this study, six cores from five lakes (the macrophyte-dominated: Shijiuhu Lake and Honghu Lake; the algae-dominated: Chaohu Lake, Taihu Lake, and Nanyihu Lake) in the MLYB were collected from 2002 A.D. to 2008 A.D. Mass accumulation rates (MARs) of sediment derived from ²¹⁰Pb and ¹³⁷Cs along with total organic carbon content (TOC) were used to determine organic carbon accumulation rates (OC ARs) over the last 100 years. The TOC in the five lakes exhibited a significant increase since the mid or late 20th century, which was consistent with the increase in the lake water trophic status due to nutrient input. The average organic carbon accumulation rates for the Taihu Lake, Nanyihu Lake, Chaohu Lake, Shijiuhu Lake, and Honghu Lake were calculated to be 16.6, 28.9, 9.8, 25.4, and 113.2 g C m^?2 year^?1, respectively, over the past 100 years. Based on the average OC AR of 32.1 g C m^?2 year^?1 from the five lakes, carbon burial in lake sediments may be as much as 6.8 × 10¹³ g C in the MLYB over the past 100 years.     

Potential nitrogen and carbon processing in a landscape rich in milldam legacy sediments

Recent identification of the widespread distribution of legacy sediments deposited in historic mill ponds has increased concern regarding their role in controlling land–water nutrient transfers in the mid-Atlantic region of the US. At Big Spring Run in Lancaster, Pennsylvania, legacy sediments now overlay a buried relict hydric soil (a former wetland soil). We compared C and N processing in legacy sediment to upland soils to identify soil zones that may be sources or sinks for N transported toward streams. We hypothesized that legacy sediments would have high nitrification rates (due to recent agricultural N inputs), while relict hydric soils buried beneath the legacy sediments would be N sinks revealed via negative net nitrification and/or positive denitrification (because the buried former wetland soils are C rich but low in O₂). Potential net nitrification ranged from 9.2 to 77.9 g m^?2 year^?1 and potential C mineralization ranged from 223 to 1,737 g m^?2 year^?1, with the highest rates in surface soils for both legacy sediments and uplands. Potential denitrification ranged from 0.37 to 21.72 g m^?2 year^?1, with the buried relict hydric soils denitrifying an average of 6.2 g m^?2 year^?1. Contrary to our hypothesis, relict hydric layers did not have negative potential nitrification or high positive potential denitrification rates, in part because microbial activity was low relative to surface soils, as indicated by low nitrifier population activity, low substrate induced respiration, and low exoenzyme activity. Despite high soil C concentrations, buried relict hydric soils do not provide the ecological services expected from a wetland soil. Thus, legacy sediments may dampen N removal pathways in buried relict hydric soils, while also acting as substantial sources of NO₃ ^? to waterways.     

Crab Burrowing Limits Surface Litter Accumulation in a Temperate Salt Marsh: Implications for Ecosystem Functioning and Connectivity

Burial of aboveground plant litter by animals reduces the amount available for surface transport and places it into a different environment, affecting decomposition rates and fluxes of organic matter to adjacent ecosystems. Here we show that in a Southwestern Atlantic salt marsh the burrowing crab Neohelice granulata buries aboveground plant litter at rates (0.5–8 g m^?2 day^?1) comparable to those of litter production (3 g m^?2 day^?1). Buried litter has a low probability (0.6%) of returning to the marsh surface. The formation of burrow excavation mounds on the marsh surface is responsible for most litter burial, whereas litter trapped in burrows was an order of magnitude lower than rates of burial under excavation mounds. Crab exclusion markedly increased surface litter accumulation (3.5-fold in just 21 days). Tides with the potential to transport significant amounts of surface litter are infrequent; hence, most litter is buried before it can be transported elsewhere or decomposes on the surface. Crab litter burial can account for the observed low levels of surface litter accumulation in this ecosystem and likely drives organic matter transformation and export. The impacts of ecosystem engineering by this crab species are therefore substantial and comparable in magnitude to the large effects found for tropical crabs and other litter-burying organisms, such as anecic earthworms.     

The effect of nitrogen addition on soil organic matter dynamics: a model analysis of the Harvard Forest Chronic Nitrogen Amendment Study and soil carbon response to anthropogenic N deposition

Recent observations indicate that long-term N additions can slow decomposition, leading to C accumulation in soils, but this process has received limited consideration by models. To address this, we developed a model of soil organic matter (SOM) dynamics to be used with the PnET model and applied it to simulate N addition effects on soil organic carbon (SOC) stocks. We developed the model’s SOC turnover times and responses to experimental N additions using measurements from the Harvard Forest, Massachusetts. We compared model outcomes to SOC stocks measured during the 20th year of the Harvard Forest Chronic Nitrogen Amendment Study, which includes control, low (5 g N m^?2 yr^?1) and high (15 g N m^?2 yr^?1) N addition to hardwood and red pine stands. For unfertilized stands, simulated SOC stocks were within 10 % of measurements. Simulations that used measured changes in decomposition rates in response to N accurately captured SOC stocks in the hardwood low N and pine high N treatment, but greatly underestimated SOC stocks in the hardwood high N and the pine low N treatments. Simulated total SOC response to experimental N addition resulted in accumulation of 5.3–7.9 kg C per kg N following N addition at 5 g N m^?2 yr^?1 and 4.1–5.3 kg C per kg N following N addition at 15 g N m^?2 yr^?1. Model simulations suggested that ambient atmospheric N deposition at the Harvard Forest (currently 0.8 g N m^?2 yr^?1) has led to an increase in cumulative O, A, and B horizons C stocks of 211 g C m^?2 (3.9 kg C per kg N) and 114 g C m^?2 (2.1 kg C per kg N) for hardwood and pine stands, respectively. Simulated SOC accumulation is primarily driven by the modeled decrease in SOM decomposition in the Oa horizon.     

Dust mediated transfer of phosphorus to alpine lake ecosystems of the Wind River Range,Wyoming, USA

Alpine lakes receive a large fraction of their nutrients from atmospheric sources and are consequently sensitive to variations in both the amount and chemistry of atmospheric deposition. In this study we explored the spatial changes in lake water chemistry and biology along a gradient of dust deposition in the Wind River Range, Wyoming. Regional differences were explored using the variation in bulk deposition, lake water, sediment, and bedrock geochemistry and catchment characteristics. Dust deposition rates in the Southwestern region averaged 3.34 g m^?2 year^?1, approximately three times higher than deposition rates in the Northwestern region (average 1.06 g m^?2 year^?1). Dust-P deposition rates ranged from 87 µg P m² day^?1 in the Northwestern region to 276 µg P m² day^?1 in the Southwestern region. Subalpine and alpine lakes in the Southwestern region had greater total phosphorus (TP) concentrations (5–13 µg L^?1) and greater sediment phosphorus (SP) concentrations (2–5 mg g^?1) than similar lakes elsewhere in the region (1–8 µg L^?1 TP, 0.5–2 mg g^?1 SP). Lake phosphorus concentrations were related to dissolved organic carbon (DOC) across vegetation gradients, but related to the percent of bare rock, catchment area to lake area, and catchment steepness across dust deposition gradients. Modern phytoplankton and zooplankton biomasses were two orders of magnitude greater in the Southwest than in the Northwest, and alpine lakes in the Southwest had a unique diatom species assemblage with relatively higher concentrations of Asterionella formosa, Pseudostaurosira pseudoconstruens, and Pseudostaurosira brevistriata. These results suggests that catchment controls on P export to lakes (i.e. DOC) are overridden in dominantly bare rock basins where poor soils cannot effectively retain dust deposited P.     

Fertilization enhancing carbon sequestration as carbonate in arid cropland: assessments of long-term experiments in northern China

Aims

Soil inorganic carbon (SIC), primarily calcium carbonate, is a major reservoir of carbon in arid lands. This study was designed to test the hypothesis that carbonate might be enhanced in arid cropland, in association with soil fertility improvement via organic amendments.

Methods

We obtained two sets (65 each) of archived soil samples collected in the early and late 2000’s from three long-term experiment sites under wheat-corn cropping with various fertilization treatments in northern China. Soil organic (SOC), SIC and their Stable ¹³C compositions were determined over the range 0–100 cm.

Results

All sites showed an overall increase of SIC content in soil profiles over time. Particularly, fertilizations led to large SIC accumulation with a range of 101–202 g C m^?2 y^?1 in the 0–100 cm. Accumulation of pedogenic carbonate under fertilization varied from 60 to 179 g C m^?2 y^?1 in the 0–100 cm. Organic amendments significantly enhanced carbonate accumulation, in particular in the subsoil.

Conclusions

More carbon was sequestrated in the form of carbonate than as SOC in the arid cropland in northern China. Increasing SOC stock through long-term straw incorporation and manure application in the arid and semi-arid regions also enhanced carbonate accumulation in soil profiles.     

Cumulative ecological effects of a Neotropical reservoir cascade across multiple assemblages

Kettle holes are often abundant within agriculturally used moraine landscapes. They are highly enriched with nutrients and considered hotspots of carbon turnover. However, data on their primary productivity remain rare. We analysed two kettle holes typical to Germany with common aquatic plant communities during one year. We hypothesised that gross primary production (GPP) rates would be high compared to other temperate freshwater ecosystems, leading to high sediment deposition. Summer GPP rates (4.5–5.1 g C m^?2 day^?1) were higher than those of most temperate freshwater systems, but GPP rates were reduced by 90% in winter. Macrophytes dominated GPP from May to September with emergent macrophytes accounting for half of the GPP. Periphyton contributed to most of the system GPP throughout the rest of the year. Sediment deposition rates were high and correlated with GPP in one kettle hole. In contrast, due to prolonged periods of anoxia, aerobic sediment mineralisation was low while sediment phosphorus release was significant. Our results suggest that kettle holes have a high potential for carbon burial, provided they do not fully dry up during warm years. Due to their unique features, they should not be automatically grouped with ponds and shallow lakes in global carbon budget estimates.     

Sediment microstructure and resuspension behavior depend on each other

The critical shear stress of resuspension and rates of erosion for cohesive and loosely structured sediments must be obtained by direct measurements since there is no theoretical calculation. An in situ experiment on sediment resuspension was performed in a shallow lake (Langer See, NE Germany; area = 1.27 km², z_max = 3.8 m) in summer 2006 using a hydrodynamically calibrated erosion chamber (Ø 20 cm). Shear velocity (u*) was incrementally increased in 11 steps (0–2.19 cm s^?1) to initiate resuspension events. Entrainment rates (E) of suspended particulate matter (E_SPM), total P (E_TP), chlorophyll a (E_Chl a), and soluble reactive P (E_SRP) were determined by mass balance. Two subsequent critical u* (0.53 cm s^?1 and 1.48 cm s^?1) support the ‘two-layered bed’ model of a fluffy surface aggregate layer (freshly deposited phytodetritus prone to resuspension) and an underlying more consolidated biostabilised layer. Patterns in E_SPM (2–106 g m^?2 h^?1), E_TP (11–532 mg m^?2 h^?1), and E_Chl a (3–24 μg m^?2 h^?1) revealed a sediment surface maximum of TP and Chl a and their theoretical vertical logarithmic decrease within 4 mm sediment depth, the maximum thickness of sediment layer entrained. The advective E_SRP flux (17 mg m^?2 h^?1) was 43 times higher than the diffusive SRP flux (0.4 mg m^?2 h^?1). The TP and Chl a micro-profiles suggest that cohesive sediment bed formation is a function of both settling (fluff) and consolidation (biostabilisation). Thus, sediment microstructure and resuspension behavior depend on each other.     

Carbon sequestration in freshwater wetlands in Costa Rica and Botswana

Tropical wetlands are typically productive ecosystems that can introduce large amounts of carbon into the soil. However, high temperatures and seasonal water availability can hinder the ability of wetland soils to sequester carbon efficiently. We determined the carbon sequestration rate of 12 wetland communities in four different tropical wetlands—an isolated depressional wetland in a rainforest, and a slow flowing rainforest swamp, a riverine flow-through wetland with a marked wet and dry season, a seasonal floodplain of an inland delta—with the intention of finding conditions that favor soil carbon accumulation in tropical wetlands. Triplicate soil cores were extracted in these communities and analyzed for total carbon content to determine the wetland soil carbon pool. We found that the humid tropic wetlands had greater carbon content (P ≤ 0.05) than the tropical dry ones (96.5 and 34.8 g C kg^?1, respectively). While the dry tropic wetlands had similar sequestration rates (63 ± 10 g Cm^?2 y^?1 on average), the humid tropic ones differed significantly (P < 0.001), with high rates in a slow-flowing slough (306 ± 77 g Cm^?2 y^?1) and low rates in a tropical rain forest depressional wetland (84 ± 23 g Cm^?2 y^?1). The carbon accumulating in all of these wetlands was mostly organic (92–100%). These results suggest the importance of differentiating between types of wetland communities and their hydrology when estimating overall rates at which tropical wetlands sequester carbon, and the need to include tropical wetland carbon sequestration in global carbon budgets.     

Influence of Potamogeton pectinatus and microphytobenthos on benthic metabolism,nutrient fluxes and denitrification in a freshwater littoral sediment in an agricultural landscape: N assimilation versus N removal

The influence of Potamogeton pectinatus colonisation on benthic nitrogen dynamics was studied in the littoral zone of a lowland pit lake with high nitrate concentration (~200 μM). Our hypothesis was that in aquatic environments where nitrogen availability is not limiting, colonisation by rooted macrophytes changes the dynamics of the benthic nitrogen cycle, stimulating N assimilation and denitrification and increasing the system capacity to take up external nitrogen loads. To test this hypothesis, we quantified and compared seasonal variations of light and dark benthic metabolism, dissolved inorganic nitrogen (DIN) fluxes, denitrification and N assimilation rates in an area colonised by P. pectinatus and a reference site colonised by microphytobenthos. In both areas, the benthic system was net autotrophic and a sink for DIN (2,241–2,644 mmol m^?2 y^?1). Plant colonisation increased nitrogen losses via denitrification by 30% compared to the unvegetated area. In contrast to what is generally observed in coastal marine systems, where the presence of rooted macrophytes limits denitrification rates, under the very high nitrate concentrations in the studied lake, both denitrification (1,237–1,570 mmol m^?2 y^?1) and N assimilation (1,039–1,095 mmol m^?2 y^?1) played important and comparable roles in the removal of DIN from the water column.     

Peatland carbon stocks and accumulation rates in the Ecuadorian páramo

The páramo is a high altitude tropical Andean ecosystem that contains peatlands with thick horizons of carbon (C) dense soils. Soil C data are sparse for most of the páramo, especially in peatlands, which limits our ability to provide accurate regional and country wide estimates of C storage. Therefore, the objective of our research was to quantify belowground C stocks and accumulation rates in páramo peatland soils in two regions of northeastern Ecuador. Peatland soil cores were collected from Antisana Ecological Reserve and Cayambe-Coca National Park. We measured soil C densities and ¹⁴C dates to estimate soil accumulation rates. The mean peatland soil depth across both regions was 3.8 m and contained an estimated mean C storage of 1282 Mg ha^?1. Peatlands older than 3000 cal. year BP had a mean long-term C accumulation rate of 26 g m^?2 year^?1, with peatlands younger than 500 cal. year BP displaying mean recent rates of C accumulation of 134 g m^?2 year^?1. These peatlands also receive large inputs of mineral material, predominantly from volcanic deposition, that has created many interbedded non-peat mineral soil horizons that contained 48 % of the soil C. Because of large C stocks in Ecuadorian mountain peatlands and the potential disturbance from land use and climate change, additional studies are need to provide essential baseline assessments and estimates of C storage in the Andes.     

Sedimentary Organic Carbon and Nitrogen Sequestration Across a Vertical Gradient on a Temperate Wetland Seascape Including Salt Marshes,Seagrass Meadows and Rhizophytic Macroalgae Beds

Coastal wetlands are key in regulating coastal carbon and nitrogen dynamics and contribute significantly to climate change mitigation and anthropogenic nutrient reduction. We investigated organic carbon (OC) and total nitrogen (TN) stocks and burial rates at four adjacent vegetated coastal habitats across the seascape elevation gradient of Cádiz Bay (South Spain), including one species of salt marsh, two of seagrasses, and a macroalgae. OC and TN stocks in the upper 1 m sediment layer were higher at the subtidal seagrass Cymodocea nodosa (72.3 Mg OC ha⁻¹, 8.6 Mg TN ha⁻¹) followed by the upper intertidal salt marsh Sporobolus maritimus (66.5 Mg OC ha⁻¹, 5.9 Mg TN ha⁻¹), the subtidal rhizophytic macroalgae Caulerpa prolifera (62.2 Mg OC ha⁻¹, 7.2 Mg TN ha⁻¹), and the lower intertidal seagrass Zostera noltei (52.8 Mg OC ha⁻¹, 5.2 Mg TN ha⁻¹). The sedimentation rates increased from lower to higher elevation, from the intertidal salt marsh (0.24 g cm⁻² y⁻¹) to the subtidal macroalgae (0.12 g cm⁻² y⁻¹). The organic carbon burial rate was highest at the intertidal salt marsh (91 ± 31 g OC m⁻² y⁻¹), followed by the intertidal seagrass, (44 ± 15 g OC m⁻² y⁻¹), the subtidal seagrass (39 ± 6 g OC m⁻² y⁻¹), and the subtidal macroalgae (28 ± 4 g OC m⁻² y⁻¹). Total nitrogen burial rates were similar among the three lower vegetation types, ranging from 5 ± 2 to 3 ± 1 g TN m⁻² y⁻¹, and peaked at S. maritimus salt marsh with 7 ± 1 g TN m⁻² y⁻¹. The contribution of allochthonous sources to the sedimentary organic matter decreased with elevation, from 72% in C. prolifera to 33% at S. maritimus. Our results highlight the need of using habitat-specific OC and TN stocks and burial rates to improve our ability to predict OC and TN sequestration capacity of vegetated coastal habitats at the seascape level. We also demonstrated that the stocks and burial rates in C. prolifera habitats were within the range of well-accepted blue carbon ecosystems such as seagrass meadows and salt marshes.

     

Lake eutrophication and its implications for organic carbon sequestration in Europe

The eutrophication of lowland lakes in Europe by excess nitrogen (N) and phosphorus (P) is severe because of the long history of land‐cover change and agricultural intensification. The ecological and socio‐economic effects of eutrophication are well understood but its effect on organic carbon (OC) sequestration by lakes and its change overtime has not been determined. Here, we compile data from ~90 culturally impacted European lakes [~60% are eutrophic, Total P (TP) >30 μg P l^?1] and determine the extent to which OC burial rates have increased over the past 100–150 years. The average focussing corrected, OC accumulation rate (C AR_FC) for the period 1950–1990 was ~60 g C m^?2 yr^?1, and for lakes with >100 μg TP l^?1 the average was ~100 g C m^?2 yr^?1. The ratio of post‐1950 to 1900–1950 C AR is low (~1.5) indicating that C accumulation rates have been high throughout the 20th century. Compared to background estimates of OC burial (~5–10 g C m^?2 yr^?1), contemporary rates have increased by at least four to fivefold. The statistical relationship between C AR_FC and TP derived from this study (r² = 0.5) can be used to estimate OC burial at sites lacking estimates of sediment C‐burial. The implications of eutrophication, diagenesis, lake morphometry and sediment focussing as controls of OC burial rates are considered. A conservative interpretation of the results of the this study suggests that lowland European meso‐ to eutrophic lakes with >30 μg TP l^?1 had OC burial rates in excess of 50 g C m^?2 yr^?1 over the past century, indicating that previous estimates of regional lake OC burial have seriously underestimated their contribution to European carbon sequestration. Enhanced OC burial by lakes is one positive side‐effect of the otherwise negative impact of the anthropogenic disruption of nutrient cycles.     

The role of hydrodynamic sorting on the accumulation and distribution of organic carbon in an impoundment: Englebright Lake,California, USA

The global proliferation of dams is one of the most significant anthropogenic impacts on the environment, resulting in the trapping of massive loads of sediment and nutrients in impoundments. Few studies, however, have examined these impounded sediments to understand patterns of organic carbon (OC) accumulation and the effects of watershed processes on carbon delivery. This study measured total organic carbon (TOC) and stable isotopes of carbon and nitrogen (δ¹³C and δ¹⁵N) in Englebright Lake, CA to relate changes in OC sources and TOC accumulation to natural and anthropogenic events in the watershed and to depositional processes in the lake. Englebright Lake is a representative system for impoundments in small, mountainous rivers, and anthropogenic disturbances in the watershed caused high sediment accumulation rates in the lake. Throughout its 60-year history, 0.35 Tg OC has been trapped behind Englebright Dam and δ¹³C signatures indicate that more than 50% of the OC in Englebright Lake was derived from terrigenous sources. TOC content ranged from 0.03 to 30.24% of dry weight, and differed across depositional regimes; TOC content in topset deposits (0.35 ± 0.58%) was less than in foreset (2.64 ± 5.95%) and bottomset (1.51 ± 1.41%) deposits (p < 0.001) and TOC accumulation associated with flood events was higher (up to 231 kg_OC m^?2 year^?1) than during non-event periods (0.2 to 39 kg_OC m^?2 year^?1). TOC accumulation rates in Englebright Lake were up to an order of magnitude higher than previous estimates of OC burial in California impoundments. As the number and size of dams continues to expand worldwide, the storage of TOC in impoundments will likely add to the growing number of anthropogenic modifications to the global carbon cycle.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Productivity and biochemical composition of <Emphasis Type="Italic">Tetradesmus obliquus</Emphasis> and <Emphasis Type="Italic">Phaeodactylum tricornutum</Emphasis>: effects of different cultivation approaches

The present work evaluated biomass productivity, carbon dioxide fixation rate, and biochemical composition of two microalgal species, Phaeodactylum tricornutum (Bacillariophyta) and Tetradesmus obliquus (Chlorophyta), cultivated indoors in high-technology photobioreactors (HT-PBR) and outdoors both in pilot ponds and low-technology photobioreactors in a greenhouse in southern Italy. Microalgae were grown in standard media, under nitrogen starvation, and in two liquid digestates obtained from anaerobic digestion of agro-zootechnical and vegetable biomass. P. tricornutum, cultivated in semi-continuous mode in indoor HT-PBRs with standard medium, showed a biomass productivity of 21.0?±?2.3 g m^?2 d^?1. Applying nitrogen starvation, the lipid productivity increased from 2.3 up to 4.5?±?0.5 g m^?2 d^?1, with a 24 % decrease of biomass productivity. For T. obliquus, a biomass productivity of 9.1?±?0.9 g m^?2 d^?1 in indoor HT-PBR was obtained using standard medium. Applying liquid digestates as fertilizers in open ponds, T. obliquus gave a biomass productivity (10.8?±?2.0 g m^?2 d^?1) not statistically different from complete medium such as P. tricornutum (6.5?±?2.2 g m^?2 d^?1). The biochemical data showed that the fatty acid composition of the microalgal biomass was affected by the different cultivation conditions for both microalgae. In conclusion, it was found that the microalgal productivity in standard medium was about doubled in HT-PBR compared to open ponds for P. tricornutum and was about 20 % higher for T. obliquus.     

Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ( \( {\text{ER}}_{{{\text{CO}}_{2} }} \) ) or methane ( \( {\text{ER}}_{{{\text{CH}}_{4} }} \) ), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m^?2 year^?1) and ²¹⁰Pb radiodating (336 g C m^?2 year^?1), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO_2-eq m^?2 year^?1). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m^?2 year^?1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m^?2 year^?1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm^?1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m^?2 year^?1), making this site a significant GHG source (>2,000 g CO_2-eq m^?2 year^?1). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m^?2 year^?1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.     

Evolution of carbon fluxes during initial soil formation along the forefield of Damma glacier, Switzerland

Soil carbon (C) fluxes, soil respiration and dissolved organic carbon (DOC) leaching were explored along the young Damma glacier forefield chronosequence (7–128 years) over a three-year period. To gain insight into the sources of soil CO₂ effluxes, radiocarbon signatures of respired CO₂ were measured and a vegetation-clipping experiment was performed. Our results showed a clear increase in soil CO₂ effluxes with increasing site age from 9 ± 1 to 160 ± 67 g CO₂–C m^?2 year^?1, which was linked to soil C accumulation and development of vegetation cover. Seasonal variations of soil respiration were mainly driven by temperature; between 62 and 70 % of annual CO₂ effluxes were respired during the 4-month long summer season. Sources of soil CO₂ effluxes changed along the glacier forefield. For most recently deglaciated sites, radiocarbon-based age estimates indicated ancient C to be the dominant source of soil-respired CO₂. At intermediate site age (58–78 years), the contribution of new plant-fixed C via rhizosphere respiration amounted up to 90 %, while with further soil formation, heterotrophically respired C probably from accumulated ‘older’ soil organic carbon (SOC) became increasingly important. In comparison with soil respiration, DOC leaching at 10 cm depth was small, but increased similarly from 0.4 ± 0.02 to 7.4 ± 1.6 g DOC m^?2 year^?1 over the chronosequence. A strong rise of the ratio of SOC to secondary iron and aluminium oxides strongly suggests that increasing DOC leaching with site age results from a faster increase of the DOC source, SOC, than of the DOC sink, reactive mineral surfaces. Overall, C losses from soil by soil respiration and DOC leaching increased from 9 ± 1 to 70 ± 17 and further to 168 ± 68 g C m^?2 year^?1 at the <10, 58–78, and 110–128 year old sites. By comparison, total ecosystem C stocks increased from 0.2 to 1.1 and to 3.1 kg C m^?2 from the young to intermediate and old sites. Therefore, the ecosystem evolved from a dominance of C accumulation in the initial phase to a high throughput system. We suggest that the relatively strong increase in soil C stocks compared to C fluxes is a characteristic feature of initial soil formation on freshly exposed rocks.