How Fast Can Carbon Be Sequestered When Restoring Degraded Subtropical Thicket?

Is the maximum rate of carbon sequestration reported for the CAM‐C₃ plant Portulacaria afra (spekboom), viz. 15.4 t CO₂ ha^?1 yr^?1, unusual in comparison with other plants with similar physioliogies, or could such rates be expected routinely in restoration with P. afra? Private sector investors in thicket restoration need an answer to this question in order to assess the feasibility of using carbon finance as the main income stream from their investments. A literature review showed that 15.4 t CO₂ ha^?1 yr^?1 is not an unusual rate of carbon sequestration for CAM plants in arid and semi‐arid environments, which suggests that investors in thicket restoration should not consider this an outlier value. The results also suggest that carbon finance could be used to fund restoration using other CAM plants in degraded xeric thickets in countries such as Argentina, Chile, Mexico, and Madagascar.     

Portulacaria afra is constrained under extreme soil conditions in the Fish River Reserve, Eastern Cape, South Africa

Restoration of degraded thicket landscapes can be achieved by planting Portulacaria afra (spekboom) cuttings. The factors determining P. afra abundance in thicket types not dominated by P. afra are of interest to restoration practitioners because they could influence restoration protocols using P. afra cuttings in these landscapes. We consequently investigated the relationship between P. afra cover and soil properties, namely pH, EC, organic C, particle size distribution and total content of 26 elements at 78 sites in the Fish River Reserve, Eastern Cape province, South Africa. P. afra cover showed a consistent pattern of constraint at extreme levels (both high and low) of subsoil EC, organic C, sand content, Ca, Zn and Al. The results suggest that P. afra is most competitive in intermediate edaphic environments in the Fish River Reserve. It remains to be investigated in restored landscapes whether P. afra cutting survivorship and growth are greater in such soils.     

Effects of goat pastoralism on ecosystem carbon storage in semiarid thicket,Eastern Cape,South Africa

Abstract Intensive pastoralism with goats transforms semiarid thicket in the Eastern Cape, South Africa from a dense vegetation of tall shrubs to an open landscape dominated by ephemeral grasses and forbs. Approx. 800 000 ha of thicket (which prior to the introduction of goats had a closed canopy and a Portulacaria afra Jacq. component) have been transformed in this manner. Ecosystem C storage in intact thicket and loss of C due to transformation were quantified. Carbon storage in intact thicket was surprisingly high for a semiarid region, with an average of 76 t C ha^?1 in living biomass and surface litter and 133 t C ha^?1 in soils to a depth of 30 cm. Exceptional C accumulation in thicket may be a result of P. afra dominance. This succulent shrub switches between C₃ and CAM photosynthesis, produces large quantities of leaf litter (approx. 450 g m^?2 year^?1) and shades the soil densely. Transformed thicket had approx. 35% less soil C to a depth of 10 cm and approx. 75% less biomass C than intact thicket. Restoration of transformed thicket landscapes could consequently recoup more than 80 t C ha^?1.     

Spontaneous Return of Biodiversity in Restored Subtropical Thicket: Portulacaria afra as an Ecosystem Engineer

An accepted criterion for measuring the success of ecosystem restoration is the return of biodiversity relative to intact reference ecosystems. The emerging global carbon economy has made landscape‐scale restoration of severely degraded Portulacaria afra (spekboom)‐dominated subtropical thicket, by planting multiple rows of spekboom truncheons, a viable land‐use option. Although large amounts of carbon are sequestered when planting a monoculture of spekboom, it is unknown whether this is associated with the return of other thicket biodiversity components. We used available carbon stock data from degraded, restored, and intact stands at one site, and sampled carbon stocks at restored stands at another site in the same plant community. We also sampled plant community composition at both sites. The total carbon stock of the oldest (50 years) post‐restoration stand (250.8 ± 14 t C ha^?1) approximated that of intact stands (245 t C ha^?1) and we observed a general increase in carbon content with restoration age (71.4 ± 24 t C ha^?1 after 35 and 167.9 ± 20 t C ha^?1 after 50 years). A multiple correspondence analysis separated degraded stands from stands under restoration based on ground cover, floristic composition, and total carbon stock. Older post‐restoration and intact stands were clustered according to woody canopy recruit abundance. Our results suggest that spekboom is an ecosystem engineer that promotes spontaneous return of canopy species and other components of thicket biodiversity. The spekboom canopy creates a cooler micro‐climate and a dense litter layer, both likely to favor the recruitment of other canopy species.     

Comparing carbon sequestration in temperate freshwater wetland communities

High productivity and waterlogged conditions make many freshwater wetlands significant carbon sinks. Most wetland carbon studies focus on boreal peatlands, however, with less attention paid to other climates and to the effects of hydrogeomorphic settings and the importance of wetland vegetation communities on carbon sequestration. This study compares six temperate wetland communities in Ohio that belong to two distinct hydrogeomorphic types: an isolated depressional wetland site connected to the groundwater table, and a riverine flow‐through wetland site that receives water from an agricultural watershed. Three cores were extracted in each community and analyzed for total carbon content to determine the soil carbon pool. Sequestration rates were determined by radiometric dating with ¹³⁷Cs and ²¹⁰Pb on a set of composite cores extracted in each of the six communities. Cores were also extracted in uplands adjacent to the wetlands at each site. Wetland communities had accretion rates ranging from 3.0 to 6.2 mm yr^?1. The depressional wetland sites had higher (P < 0.001) organic content (146 ± 4.2 gC kg^?1) and lower (P < 0.001) bulk density (0.55 ± 0.01 Mg m^?3) than the riverine ones (50.1 ± 6.9 gC kg^?1 and 0.74 ± 0.06 Mg m^?3). The soil carbon was 98–99% organic in the isolated depressional wetland communities and 85–98% organic in the riverine ones. The depressional wetland communities sequestered 317 ± 93 gC m^?2 yr^?1, more (P < 0.01) than the riverine communities that sequestered 140 ± 16 gC m^?2 yr^?1. The highest sequestration rate was found in the Quercus palustris forested wetland community (473 gC m^?2 yr^?1), while the wetland community dominated by water lotus (Nelumbo lutea) was the most efficient of the riverine communities, sequestering 160 gC m^?2 yr^?1. These differences in sequestration suggest the importance of addressing wetland types and communities in more detail when assessing the role of wetlands as carbon sequestering systems in global carbon budgets.     

Changes in carbon storage and fluxes in a chronosequence of ponderosa pine

Forest development following stand‐replacing disturbance influences a variety of ecosystem processes including carbon exchange with the atmosphere. On a series of ponderosa pine (Pinius ponderosa var. Laws.) stands ranging from 9 to> 300 years in central Oregon, USA, we used biological measurements to estimate carbon storage in vegetation and soil pools, net primary productivity (NPP) and net ecosystem productivity (NEP) to examine variation with stand age. Measurements were made on plots representing four age classes with three replications: initiation (I, 9–23 years), young (Y, 56–89 years), mature (M, 95–106 years), and old (O, 190–316 years) stands typical of the forest type in the region. Net ecosystem productivity was lowest in the I stands (?124 g C m^?2 yr^?1), moderate in Y stands (118 g C m^?2 yr^?1), highest in M stands (170 g C m^?2 yr^?1), and low in the O stands (35 g C m^?2 yr^?1). Net primary productivity followed similar trends, but did not decline as much in the O stands. The ratio of fine root to foliage carbon was highest in the I stands, which is likely necessary for establishment in the semiarid environment, where forests are subject to drought during the growing season (300–800 mm precipitation per year). Carbon storage in live mass was the highest in the O stands (mean 17.6 kg C m^?2). Total ecosystem carbon storage and the fraction of ecosystem carbon in aboveground wood mass increased rapidly until 150–200 years, and did not decline in older stands. Forest inventory data on 950 ponderosa pine plots in Oregon show that the greatest proportion of plots exist in stands ～ 100 years old, indicating that a majority of stands are approaching maximum carbon storage and net carbon uptake. Our data suggests that NEP averages ～ 70 g C m^?2 year^?1 for ponderosa pine forests in Oregon. About 85% of the total carbon storage in biomass on the survey plots exists in stands greater than 100 years, which has implications for managing forests for carbon sequestration. To investigate variation in carbon storage and fluxes with disturbance, simulation with process models requires a dynamic parameterization for biomass allocation that depends on stand age, and should include a representation of competition between multiple plant functional types for space, water, and nutrients.     

Spatial heterogeneity of ecosystem carbon fluxes in a broadleaved forest in Northern Germany

Carbon fluxes were investigated in a mature deciduous forest, located in Northern Germany (53°47′N–10°36′E), by means of eddy‐covariance technique, stand survey and models. This forest has been managed following a concept of nature‐oriented forestry since the 1980s. One of the goals of the study was to test whether changed management led to increased carbon sequestration. The forest contains several broadleaved tree species. Depending on wind direction, the fetch‐area of the eddy‐covariance data was dominated by different tree species. Three subplots dominated by Oak, Beech or Alder/Ash could be distinguished from the tower data. In each of these subplots, 30 × 30 m² areas were defined to analyse leaf area index, litterfall and the increase of the wood biomass. Eddy‐covariance analysis showed that the gross primary productivity (GPP′) was higher in the Oak subplot (?1794 g C m^?2 yr^?1) in comparison with the Beech plot and the Alder/Ash plot (?1470 and ?1595 g C m^?2 yr^?1, respectively). The total ecosystem respiration (TER) was the highest in the Alder/Ash‐dominated subplot (1401 g C m^?2 yr^?1) followed by the Oak plot and the Beech plot (1235 and 1174 g C m^?2 yr^?1, respectively). The resulting net ecosystem productivity (NEP) was ?559 g C m^?2 yr^?1 for the Oak‐dominated subplot, ?295 g C m^?2 yr^?1 for the Beech plot and ?193 g C m^?2 yr^?1 for the Alder/Ash plot. From Stand survey and modelling, the net primary productivity was estimated as 1103, 702 and 671 g C m^?2 yr^?1 in the Oak, Beech and Alder/Ash plot, respectively. Also carbon flux with litterfall was the highest in the Oak plot 343 g C m^?2 yr^?1 and lowest in Alder/Ash plot (197 g m^?2 yr^?1) with the Beech plot in between (228 g m^?2 yr^?1). The observations indicate an increase of the proportion of litterfall with increasing GPP′ and a different ability of carbon sequestration of the three stands in medium temporary scale. Only in the Oak stand that comprised the oldest trees and the most structured canopy the carbon sequestration was increased compared with conventionally managed forests.     

The impact of co‐occurring tree and grassland species on carbon sequestration and potential biofuel production

We evaluated how three co‐occurring tree and four grassland species influence potentially harvestable biofuel stocks and above‐ and belowground carbon pools. After 5 years, the tree Pinus strobus had 6.5 times the amount of aboveground harvestable biomass as another tree Quercus ellipsoidalis and 10 times that of the grassland species. P. strobus accrued the largest total plant carbon pool (1375 g C m^?2 or 394 g C m^?2 yr), while Schizachyrium scoparium accrued the largest total plant carbon pool among the grassland species (421 g C m^?2 or 137 g C m^?2 yr). Quercus ellipsoidalis accrued 850 g C m^?2, Q. macrocarpa 370 g C m^?2, Poa pratensis 390 g C m^?2, Solidago canadensis 132 g C m^?2, and Lespedeza capitata 283 g C m^?2. Only P. strobus and Q. ellipsoidalis significantly sequestered carbon during the experiment. Species differed in total ecosystem carbon accumulation from ?21.3 to +169.8 g C m^?2 yr compared with the original soil carbon pool. Plant carbon gains with P. strobus were paralleled by a decrease of 16% in soil carbon and a nonsignificant decline of 9% for Q. ellipsoidalis. However, carbon allocation differed among species, with P. strobus allocating most aboveground in a disturbance prone aboveground pool, whereas Q. ellipsoidalis, allocated most carbon in less disturbance sensitive belowground biomass. These differences have strong implications for terrestrial carbon sequestration and potential biofuel production. For P. strobus, aboveground plant carbon harvest for biofuel would result in no net carbon sequestration as declines in soil carbon offset plant carbon gains. Conversely the harvest of Q. ellipsoidalis aboveground biomass would result in net sequestration of carbon belowground due to its high allocation belowground, but would yield lower amounts of aboveground biomass. Our results demonstrate that plant species can differentially impact ecosystem carbon pools and the distribution of carbon above and belowground.     

Tidal freshwater forest accretion does not keep pace with sea level rise

Soil properties, accretion, and accumulation were measured in tidal freshwater forests (tidal forests) of the Ogeechee, Altamaha, and Satilla rivers of the South Atlantic (Georgia USA) coast to characterize carbon (C) sequestration and nutrient (nitrogen‐N, phosphorus‐P) accumulation in these understudied, uncommon, and ecologically sensitive wetlands. Carbon sequestration and N and P accumulation also were measured in a tidal forest (South Newport River) that experiences saltwater intrusion to evaluate the effects of sea level rise (SLR) and saltwater intrusion on C, N and P accumulation. Finally, soil accretion and accumulation of tidal forests were compared with tidal fresh, brackish and salt marsh vegetation downstream to gauge how tidal forests may respond to SLR. Soil accretion determined using ¹³⁷C and ²¹⁰Pb averaged 1.3 and 2.2 mm yr^?1, respectively, and was substantially lower than the recent rate of SLR along the Georgia coast (3.0 mm yr^?1). Healthy tidal forest soils sequestered C (49–82 g m^?2 yr^?1), accumulated N (3.2–5.3 g m^?2 yr^?1) and P (0.29–0.56 g m^?2 yr^?1) and trapped mineral sediment (340–650 g m^?2 yr^?1). There was no difference in long‐term accretion, C sequestration, and nutrient accumulation between healthy tidal forests and tidal forests of the South Newport River that experience saltwater intrusion. Accelerated SLR is likely to lead to decline of tidal forests and expansion of oligohaline and brackish marshes where soil accretion exceeds the current rate of SLR. Conversion of tidal forest to marshes will lead to an increase in the delivery of some ecosystem services such as C sequestration and sediment trapping, but at the expense of other services (e.g. denitrification, migratory songbird habitat). As sea level rises in response to global warming, tidal forests and their delivery of ecosystem services face a tenuous future unless they can migrate upriver, and that is unlikely in most areas because of topographic constraints and increasing urbanization of the coastal zone.     

Nonlinear root-derived carbon sequestration across a gradient of nitrogen and phosphorous deposition in experimental mesocosms

Enhanced sequestration of plant‐carbon (C) inputs to soil may mitigate rising atmospheric carbon dioxide (CO₂) concentrations and related climate change but how this sequestration will respond to anthropogenic nitrogen (N) and phosphorous (P) deposition is uncertain. We couple isotope, soil C fractionation and mesocosm techniques to assess the sequestration of plant‐C inputs, and their partitioning into C pools with different sink potentials, under an experimental gradient of N and P deposition (0, 10, 30, 60 and 100 kg N ha^?1 yr^?1; and 0, 2, 6, 12 and 20 kg P ha^?1 yr^?1). We hypothesized that N deposition would increase sequestration, with the majority of the C being sequestered in faster cycling soil pools because N deposition has been shown to accelerate the turnover of these pools while decelerating the turnover of slower cycling pools. In contrast to this hypothesis, sequestration into all soil C pools peaked at intermediate levels of N deposition. Given that P amendment has been shown to cause a net loss of soil C, we postulated that P deposition would decrease sequestration. This expectation was not supported by our data, with sequestration generally being greater under P deposition. When soils were amended simultaneously with N and P, neither the shape of the sequestration relationship across the deposition gradient, nor the observed sequestration at the majority of the deposition rates, was statistically predictable from the effects of N and P in isolation. The profound nonlinearities we observed, both for total sequestration responses and the partitioning of C into soil pools with different sink potentials, suggests that the rates of N and P deposition to ecosystems will be the critical determinant of whether they enhance or decrease the long‐term sequestration of fresh plant‐C inputs to soils.     

How strong is the current carbon sequestration of an Atlantic blanket bog?

Although northern peatlands cover only 3% of the land surface, their thick peat deposits contain an estimated one‐third of the world's soil organic carbon (SOC). Under a changing climate the potential of peatlands to continue sequestering carbon is unknown. This paper presents an analysis of 6 years of total carbon balance of an almost intact Atlantic blanket bog in Glencar, County Kerry, Ireland. The three components of the measured carbon balance were: the land‐atmosphere fluxes of carbon dioxide (CO₂) and methane (CH₄) and the flux of dissolved organic carbon (DOC) exported in a stream draining the peatland. The 6 years C balance was computed from 6 years (2003–2008) of measurements of meteorological and eddy‐covariance CO₂ fluxes, periodic chamber measurements of CH₄ fluxes over 3.5 years, and 2 years of continuous DOC flux measurements. Over the 6 years, the mean annual carbon was ?29.7±30.6 (±1 SD) g C m^?2 yr^?1 with its components as follows: carbon in CO₂ was a sink of ?47.8±30.0 g C m^?2 yr^?1; carbon in CH₄ was a source of 4.1±0.5 g C m^?2 yr^?1 and the carbon exported as stream DOC was a source of 14.0±1.6 g C m^?2 yr^?1. For 2 out of the 6 years, the site was a source of carbon with the sum of CH₄ and DOC flux exceeding the carbon sequestered as CO₂. The average C balance for the 6 years corresponds to an average annual growth rate of the peatland surface of 1.3 mm yr^?1.     

Shrub expansion stimulates soil C and N storage along a coastal soil chronosequence

Expansion of woody vegetation in grasslands is a worldwide phenomenon with implications for C and N cycling at local, regional and global scales. Although woody encroachment is often accompanied by increased annual net primary production (ANPP) and increased inputs of litter, mesic ecosystems may become sources for C after woody encroachment because stimulation of soil CO₂ efflux releases stored soil carbon. Our objective was to determine if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen‐fixing shrub Morella cerifera, or if associated stimulation of soil CO₂ efflux mitigated increased litterfall. We monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP in adjacent grasslands. In the final year, we quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM), soil organic carbon (SOC), soil total nitrogen (TN) and soil CO₂ efflux in shrub thickets and adjacent grasslands. Heavy litterfall resulted in a dense litter layer storing an average of 809 g C m^?2 and 36 g N m^?2. Although soil CO₂ efflux was stimulated by shrub encroachment in younger soils, soil CO₂ efflux did not vary between shrub thickets and grasslands in the oldest soils and increases in CO₂ efflux in shrub thickets did not offset contributions of increased litterfall to SOC. SOC was 3.6–9.8 times higher beneath shrub thickets than in grassland soils and soil TN was 2.5–7.7 times higher under shrub thickets. Accumulation rates of soil and litter C were highest in the youngest thicket at 101 g m^?2 yr^?1 and declined with increasing thicket age. Expansion of shrubs on barrier islands, which have low levels of soil carbon and high potential for ANPP, has the potential to significantly increase ecosystem C sequestration.     

Climatic variability,hydrologic anomaly,and methane emission can turn productive freshwater marshes into net carbon sources

Freshwater marshes are well‐known for their ecological functions in carbon sequestration, but complete carbon budgets that include both methane (CH₄) and lateral carbon fluxes for these ecosystems are rarely available. To the best of our knowledge, this is the first full carbon balance for a freshwater marsh where vertical gaseous [carbon dioxide (CO₂) and CH₄] and lateral hydrologic fluxes (dissolved and particulate organic carbon) have been simultaneously measured for multiple years (2011–2013). Carbon accumulation in the sediments suggested that the marsh was a long‐term carbon sink and accumulated ~96.9 ± 10.3 (±95% CI) g C m^?2 yr^?1 during the last ~50 years. However, abnormal climate conditions in the last 3 years turned the marsh to a source of carbon (42.7 ± 23.4 g C m^?2 yr^?1). Gross ecosystem production and ecosystem respiration were the two largest fluxes in the annual carbon budget. Yet, these two fluxes compensated each other to a large extent and led to the marsh being a CO₂ sink in 2011 (?78.8 ± 33.6 g C m^?2 yr^?1), near CO₂‐neutral in 2012 (29.7 ± 37.2 g C m^?2 yr^?1), and a CO₂ source in 2013 (92.9 ± 28.0 g C m^?2 yr^?1). The CH₄ emission was consistently high with a three‐year average of 50.8 ± 1.0 g C m^?2 yr^?1. Considerable hydrologic carbon flowed laterally both into and out of the marsh (108.3 ± 5.4 and 86.2 ± 10.5 g C m^?2 yr^?1, respectively). In total, hydrologic carbon fluxes contributed ~23 ± 13 g C m^?2 yr^?1 to the three‐year carbon budget. Our findings highlight the importance of lateral hydrologic inflows/outflows in wetland carbon budgets, especially in those characterized by a flow‐through hydrologic regime. In addition, different carbon fluxes responded unequally to climate variability/anomalies and, thus, the total carbon budgets may vary drastically among years.     

Carbon bio‐sequestration within the phytoliths of economic bamboo species

The rates of carbon bio‐sequestration within silica phytoliths of the leaf litter of 10 economically important bamboo species indicates that (a) there is considerable variation in the content of carbon occluded within the phytoliths (PhytOC) of the leaves between different bamboo species, (b) this variation does not appear to be directly related to the quantity of silica in the plant but rather the efficiency of carbon encapsulation by the silica. The PhytOC content of the species under the experimental conditions ranged from 1.6% to 4% of the leaf silica weight. The potential phytolith carbon bio‐sequestration rates in the leaf‐litter component for the bamboos ranged up to 0.7 tonnes of carbon dioxide (CO₂) equivalents (t‐e‐CO₂) ha^?1 yr^?1 for these species. Assuming a median phytolith carbon bio‐sequestration yield of 0.36 t‐e‐CO₂ ha^?1 yr^?1, the global potential for bio‐sequestration via phytolith carbon (from bamboo and/or other similar grass crops) is estimated to be ～1.5 billion t‐e‐CO₂ yr^?1, equivalent to 11% of the current increase in atmospheric CO₂. The data indicate that the management of vegetation such as bamboo forests to maximize the production of PhytOC has the potential to result in considerable quantities of securely bio‐sequestered carbon.     

Dissolved carbon leaching from soil is a crucial component of the net ecosystem carbon balance

Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its δ¹³C signature. Leaching of biogenic DIC was 8.3±4.9 g m^?2 yr^?1 for forests, 24.1±7.2 g m^?2 yr^?1 for grasslands, and 14.6±4.8 g m^?2 yr^?1 for croplands. DOC leaching equalled 3.5±1.3 g m^?2 yr^?1 for forests, 5.3±2.0 g m^?2 yr^?1 for grasslands, and 4.1±1.3 g m^?2 yr^?1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4±4.0 g C m^?2 yr^?1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5–98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24–105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems.     

The impact of nitrogen deposition on carbon sequestration in European forests and forest soils

An estimate of net carbon (C) pool changes and long‐term C sequestration in trees and soils was made at more than 100 intensively monitored forest plots (level II plots) and scaled up to Europe based on data for more than 6000 forested plots in a systematic 16 km × 16 km grid (level I plots). C pool changes in trees at the level II plots were based on repeated forest growth surveys At the level I plots, an estimate of the mean annual C pool changes was derived from stand age and available site quality characteristics. C sequestration, being equal to the long‐term C pool changes accounting for CO₂ emissions because of harvest and forest fires, was assumed 33% of the overall C pool changes by growth. C sequestration in the soil were based on calculated nitrogen (N) retention (N deposition minus net N uptake minus N leaching) rates in soils, multiplied by the C/N ratio of the forest soils, using measured data only (level II plots) or a combination of measurements and model calculations (level I plots). Net C sequestration by forests in Europe (both trees and soil) was estimated at 0.117 Gton yr^?1, with the C sequestration in stem wood being approximately four times as high (0.094 Gton yr^?1) as the C sequestration in the soil (0.023 Gton yr^?1). The European average impact of an additional N input on the net C sequestration was estimated at approximately 25 kg C kg^?1 N for both tree wood and soil. The contribution of an average additional N deposition on European forests of 2.8 kg ha^?1 yr^?1 in the period 1960–2000 was estimated at 0.0118 Gton yr^?1, being equal to 10% of the net C sequestration in both trees and soil in that period (0.117 Gton yr^?1). The C sequestration in trees increased from Northern to Central Europe, whereas the C sequestration in soil was high in Central Europe and low in Northern and Southern Europe. The result of this study implies that the impact of forest management on tree growth is most important in explaining the C pool changes in European forests.     

Carbon sequestration and ecosystem respiration for 4 years in a Scots pine forest

The net exchange of CO₂ (NEE) between a Scots pine (Pinus sylvestris L.) forest ecosystem in eastern Finland and the atmosphere was measured continuously by the eddy covariance (EC) technique over 4 years (1999–2002). The annual temperature coefficient (Q₁₀) of ecosystem respiration (R) for these years, respectively, was 2.32, 2.66, 2.73 and 2.69. The light‐saturated rate of photosynthesis (A_max) was highest in July or August, with an annual average A_max of 10.9, 14.6, 15.3 and 17.1 μmol m^?2 s^?1 in the 4 years, respectively. There was obvious seasonality in NEE, R and gross primary production (GPP), exhibiting a similar pattern to photosynthetically active radiation (PAR) and air temperature. The integrated daily NEE ranged from 2.59 to ?4.97 g C m^?2 day^?1 in 1999, from 2.70 to ?4.72 in 2000, from 2.61 to ?4.71 in 2001 and from 5.27 to ?4.88 in 2002. The maximum net C uptake occurred in July, with the exception of 2000, when it was in June. The interannual variation in ecosystem C flux was pronounced. The length of the growing season, based on net C uptake, was 179, 170, 175 and 176 days in 1999–2002, respectively, and annual net C sequestration was 152, 101, 172 and 205 g C m^?2 yr^?1. It is estimated that ecosystem respiration contributed 615, 591, 752 and 879 g C m^?2 yr^?1 to the NEE in these years, leading to an annual GPP of ?768, ?692, ?924 and ?1084 g C m^?2 yr^?1. It is concluded that temperature and PAR were the main determinants of the ecosystem CO₂ flux. Interannual variations in net C sequestration are predominantly controlled by average air temperature and integrated radiation in spring and summer. Four years of EC data indicate that boreal Scots pine forest ecosystem in eastern Finland acts as a relatively powerful carbon sink. Carbon sequestration may benefit from warmer climatic conditions.     

Soil carbon stocks and carbon sequestration rates in seminatural grassland in Aso region,Kumamoto, Southern Japan

Global soil carbon (C) stocks account for approximately three times that found in the atmosphere. In the Aso mountain region of Southern Japan, seminatural grasslands have been maintained by annual harvests and/or burning for more than 1000 years. Quantification of soil C stocks and C sequestration rates in Aso mountain ecosystem is needed to make well‐informed, land‐use decisions to maximize C sinks while minimizing C emissions. Soil cores were collected from six sites within 200 km² (767–937 m asl.) from the surface down to the k‐Ah layer established 7300 years ago by a volcanic eruption. The biological sources of the C stored in the Aso mountain ecosystem were investigated by combining C content at a number of sampling depths with age (using ¹⁴C dating) and δ¹³C isotopic fractionation. Quantification of plant phytoliths at several depths was used to make basic reconstructions of past vegetation and was linked with C‐sequestration rates. The mean total C stock of all six sites was 232 Mg C ha^?1 (28–417 Mg C ha^?1), which equates to a soil C sequestration rate of 32 kg C ha^?1 yr^?1 over 7300 years. Mean soil C sequestration rates over 34, 50 and 100 years were estimated by an equation regressing soil C sequestration rate against soil C accumulation interval, which was modeled to be 618, 483 and 332 kg C ha^?1 yr^?1, respectively. Such data allows for a deeper understanding in how much C could be sequestered in Miscanthus grasslands at different time scales. In Aso, tribe Andropogoneae (especially Miscanthus and Schizoachyrium genera) and tribe Paniceae contributed between 64% and 100% of soil C based on δ¹³C abundance. We conclude that the seminatural, C₄‐dominated grassland system serves as an important C sink, and worthy of future conservation.     

Carbon cost of collective farming collapse in Russia

The collapse of collective farming in Russia after 1990 and the subsequent economic crisis led to the abandonment of more than 45 million ha of arable lands (23% of the agricultural area). This was the most widespread and abrupt land use change in the 20th century in the northern hemisphere. The withdrawal of land area from cultivation led to several benefits including carbon (C) sequestration. Here, we provide a geographically complete and spatially detailed analysis of C sequestered in these abandoned lands. The average C accumulation rate in the upper 20 cm of mineral soil was 0.96 ± 0.08 Mg C ha^?1 yr^?1 for the first 20 years after abandonment and 0.19 ± 0.10 Mg C ha^?1 yr^?1 during the next 30 years of postagrogenic evolution and natural vegetation establishment. The amount of C sequestered over the period 1990–2009 accounts to 42.6 ± 3.8 Tg C per year. This C sequestration rate is equivalent to ca. 10% of the annual C sink in all Russian forests. Furthermore, it compensates all fire and postfire CO₂ emissions in Russia and covers about 4% of the global CO₂ release due to deforestation and other land use changes. Our assessment shows a significant mitigation of increasing atmospheric CO₂ by prolonged C accumulation in Russian soils caused by collective farming collapse.     

Using CO2 to enhance carbon capture and biomass applications of freshwater macroalgae

A major limiting factor in the development of algae as a feedstock for the bioenergy industry is the consistent production and supply of biomass. This study is the first to access the suitability of the freshwater macroalgal genus Oedogonium to supply biomass for bioenergy applications. Specifically, we quantified the effect of CO₂ supplementation on the rate of biomass production, carbon capture, and feedstock quality of Oedogonium when cultured in large‐scale outdoor tanks. Oedogonium cultures maintained at a pH of 7.5 through the addition of CO₂ resulted in biomass productivities of 8.33 (±0.51) g DW m^?2 day^?1, which was 2.5 times higher than controls which had an average productivity of 3.37 (±0.75) g DW m^?2 day^?1. Under these productivities, Oedogonium had a carbon content of 41–45% and a higher heating value of 18.5 MJ kg^?1, making it an ideal biomass energy feedstock. The rate of carbon fixation was 1380 g C m^?2 yr^?1 and 1073.1 g C m^?2 yr^?1 for cultures maintained at a pH of 7.5 and 8.5, and 481 g C m^?2 yr^?1 for cultures not supplemented with CO₂. This study highlights the potential of integrating the large‐scale culture of freshwater macroalgae with existing carbon waste streams, for example coal‐fired power stations, both as a tool for carbon sequestration and as an enhanced and sustainable source of bioenergy.