The role of Gracilaria lemaneiformis in eliminating the dissolved inorganic carbon released from calcification and respiration process of Chlamys farreri

Respiration and calcification rate were estimated to quantify the effect of Zhikong scallop Chlamys farreri on marine CO₂ system in Sanggou Bay, China. The C. farreri population in Sanggou Bay sequestered 78.06?±?5.76 g C m^?2 y^?1 for shell formation, while the CO₂ fluxes due to calcification and respiration were 53.95?±?3.98 and 71.69?±?6.51 g C m^?2 y^?1, respectively. In order to eliminate the additional CO₂ released from calcification and respiration process of C. farreri, Gracilaria lemaneiformis were introduced into the integrated system and its role was validated by in situ mesocosm methods. Eight mesocosms (1,000 L) were deployed over 42-h period and consisted of four treatments: seaweed-only, scallop-only (SP), seaweed integrated with scallop (SS), and control (C). The aqueous CO₂ concentration and partial pressure of CO₂ in SP treatments were significantly higher than the other three treatments (p?<?0.01), while there were no difference between SS treatments and C treatments (p?>?0.05). Furthermore, compared with the SP treatments, the presence of the G. lemaneiformis can keep the seawater pH stable. These findings suggest that seaweed and shellfish integrated aquaculture practice cannot only reduce dissolved inorganic carbon but also can alleviate ocean acidification.     

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.     

Spatial and temporal variability of pCO2 and CO2 efflux in seven Amazonian Rivers

Current estimates of CO₂ outgassing from Amazonian rivers and streams have considerable uncertainty since they are based on limited-time surveys of pCO₂ measurements along the Amazon mainstem and mouths of major tributaries, using conservative estimates of gas exchange velocities. In order to refine basin-scale CO₂ efflux estimates from Amazonian rivers, we present a long time (5-year) dataset of direct measurements of CO₂ fluxes, gas transfer velocities and pCO₂ measurements in seven representative rivers of the lowland Amazon basin fluvial network, six non-tidal (Negro, Solimões, Teles Pires, Cristalino, Araguaia and Javaés) and one tidal river (Caxiuanã), with sizes ranging from 4th to 9th order. Surveys were conducted from January 2006 to December 2010, in a total of 389 campaigns covering all stages of their hydrographs. CO₂ fluxes and gas transfer velocities (k) were measured using floating chambers and pCO₂ was measured simultaneously by headspace extraction followed by gas chromatography analysis. Results show high CO₂ flux rate variability among rivers and hydrograph stages, ranging from ?0.8 to 15.3 μmol CO₂ m^?2 s^?1, with unexpected negative fluxes in clear-water rivers during low waters. Non-tidal rivers showed marked seasonal CO₂ flux patterns, with significantly higher exchange during high waters. Seasonality was modulated by pCO₂, which was positive and strongly correlated with discharge. In these rivers k was well correlated with wind speed, which allowed the use of wind data to model k. We estimate a release of 360 ± 60 Tg C year^?1 from Amazonian rivers and streams within a 1.47 million km² quadrant in the central lowland Amazon. Extrapolating these values to the basin upstream of Óbidos, results in an outgassing of 0.8 Pg C to the atmosphere each year. Our results are a step forward in achieving more accurate gas emission values for Amazonian rivers and their role in the annual carbon budget of the Amazon basin.     

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.     

Will CO<Subscript>2</Subscript> Emissions from Drained Tropical Peatlands Decline Over Time? Links Between Soil Organic Matter Quality,Nutrients, and C Mineralization Rates

Conversion, drainage, and cultivation of tropical peatlands can change soil conditions, shifting the C balance of these systems, which is important for the global C cycle. We examined the effect of soil organic matter (SOM) quality and nutrients on CO₂ production from peat decomposition using laboratory incubations of Indonesian peat soils from undrained forest in Kalimantan and drained oil palm plantations in Kalimantan and Sumatra. We found that oil palm soils had higher C/N and lower SOM quality than forest soils. Higher substrate quality and nutrient availability, particularly lower ratios of aromatic/aliphatic carbon and C/N, rather than total SOM or carbon, explained the higher rate of CO₂ production by forest soils (10.80 ± 0.23 µg CO₂–C g C h^?1) compared to oil palm soils (5.34 ± 0.26 µg CO₂–C g C h^?1) from Kalimantan. These factors also explained lower rates in Sumatran oil palm (3.90 ± 0.25 µg CO₂–C g C h^?1). We amended peat with nitrogen (N), phosphorus (P), and glucose to further investigate observed substrate and nutrient constraints across the range of observed peat quality. Available N limited CO₂ production, in unamended and amended soils. P addition raised CO₂ production when substrate quality was high and initial P state was low. Glucose addition raised CO₂ production in the presence of added N and P. Our results suggest that decline in SOM quality and nutrients associated with conversion may decrease substrate-driven rates of CO₂ production from peat decomposition over time.     

Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways

Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO₂ and N₂O immediately following wetting were among the highest ever published (up to 1,588 mg C m^?2 h^?1 and 3,121 μg N m^?2 h^?1). Mean post-wetting CO₂ and N₂O fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m^?2 h^?1; 168 and 187 μg N m^?2 h^?1) than in the sand channels (45 mg C m^?2 h^?1 and 7 μg N m^?2 h^?1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources.     

Multi-decadal Changes in Water Table Levels Alter Peatland Carbon Cycling

Globally, peatlands store a large quantity of soil carbon that can be subsequently modified by hydrologic alterations from land-use change and climate change. However, there are many uncertainties in predicting how carbon cycling and greenhouse gas emissions are altered by long-term changes in hydrology. Therefore, the goal of this study was to quantify how multi-decadal manipulations of water table (WT) levels affected carbon cycling (plant production and net ecosystem exchange from three eddy covariance towers) in a peatland complex modified by levee construction, which created a wetter area up-gradient of the levee (mean WT was 12.1 cm below the surface), a dry area below the levee (36.8 cm), and an adjacent reference site not affected by the levee (21.6 cm). We found that mean total plant production was greatest in the reference site (311.9 g C m^?2 y^?1), followed by the dry site (290.5 g C m^?2 y^?1), and lowest in the wet site (227.1 g C m^?2 y^?1). Net ecosystem exchange during the growing season was negative for all sites (sink), with the wet site having the greatest sink and the dry site having the lowest sink. Ecosystem respiration increased and CH₄ emissions decreased with a decreasing WT level. This research demonstrates that human alteration of peatland WT levels can have long-term (>50 years) consequences on peatland carbon cycling.     

Carbon dioxide and methane emissions from interfluvial wetlands in the upper Negro River basin, Brazil

Extensive interfluvial wetlands occur in the upper Negro River basin (Brazil) and contain a mosaic of vegetation dominated by emergent grasses and sedges with patches of shrubs and palms. To characterize the release of carbon dioxide and methane from these habitats, diffusive and ebullitive emissions and transport through plant aerenchyma were measured monthly during 2005 in permanently and seasonally flooded areas. CO₂ emissions averaged 2193 mg C m^?2 day^?1. Methane was consumed in unflooded environments and emitted in flooded environments with average values of ?4.8 and 60 mg C m^?2 day^?1, respectively. Bubbles were emitted primarily during falling water periods when hydrostatic pressure at the sediment?Cwater interface declined. CO₂ and CH₄ emissions increased when dissolved O₂ decreased and vegetation was more abundant. Total area and seasonally varying flooded areas for two wetlands, located north and south of the Negro River, were determined through analysis of synthetic aperture radar and optical remotely sensed data. The combined areas of these two wetlands (3000 km²) emitted 1147 Gg C year^?1 as CO₂ and 31 Gg C year^?1 as CH₄. If these rates are extrapolated to the area occupied by hydromorphic soils in the upper Negro basin, 63 Tg C year^?1 of CO₂ and 1.7 Tg C year^?1 as CH₄ are estimated as the regional evasion to the atmosphere.     

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.     

Short-term responses of ecosystem carbon fluxes to experimental soil warming at the Swiss alpine treeline

Climatic warming will probably have particularly large impacts on carbon fluxes in high altitude and latitude ecosystems due to their great stocks of labile soil C and high temperature sensitivity. At the alpine treeline, we experimentally warmed undisturbed soils by 4 K for one growing season with heating cables at the soil surface and measured the response of net C uptake by plants, of soil respiration, and of leaching of dissolved organic carbon (DOC). Soil warming increased soil CO₂ effluxes instantaneously and throughout the whole vegetation period (+45%; +120 g C m y^?1). In contrast, DOC leaching showed a negligible response of a 5% increase (NS). Annual C uptake of new shoots was not significantly affected by elevated soil temperatures, with a 17, 12, and 14% increase for larch, pine, and dwarf shrubs, respectively, resulting in an overall increase in net C uptake by plants of 20–40 g C m^?2y^?1. The Q ₁₀ of 3.0 measured for soil respiration did not change compared to a 3-year period before the warming treatment started, suggesting little impact of warming-induced lower soil moisture (?15% relative decrease) or increased soil C losses. The fraction of recent plant-derived C in soil respired CO₂ from warmed soils was smaller than that from control soils (25 vs. 40% of total C respired), which implies that the warming-induced increase in soil CO₂ efflux resulted mainly from mineralization of older SOM rather than from stimulated root respiration. In summary, one season of 4 K soil warming, representative of hot years, led to C losses from the studied alpine treeline ecosystem by increasing SOM decomposition more than C gains through plant growth.     

Role of coarse woody debris in the carbon cycle of Takayama forest,central Japan

Coarse woody debris (CWD) is an important component of the forest carbon cycle, acting as a carbon pool and a source of CO₂ in temperate forest ecosystems. We used a soda-lime closed-chamber method to measure CO₂ efflux from downed CWD (diameter ≥5 cm) and to examine CWD respiration (R _CWD) under field conditions over 1 year in a temperate secondary pioneer forest in Takayama forest. We also investigated tree mortality (input to the CWD pool) from the data obtained from the annual tree census, which commenced in 2000. We developed an exponential function of temperature to predict R _CWD in each decay class (R ² = 0.81–0.97). The sensitivity of R _CWD to changing temperature, expressed as Q ₁₀, ranged from 2.12 to 2.92 and was relatively high in decay class III. Annual C flux from CWD (F _CWD) was extrapolated using continuous air temperature measurements and CWD necromass pools in the three decay classes. F _CWD was 3.0 (class I), 17.8 (class II), and 13.7 g C m^?2 year^?1 (class III) and totaled 34 g C m^?2 year^?1 in 2009. Annual input to CWD averaged 77 g C m^?2 year^?1 from 2000 to 2009. The budget of the CWD pool in the Takayama forest, including tree mortality inputs and respiratory outputs, was 0.43 Mg C ha^?1 year^?1 (net C sink) owing to high tree mortality in the mature pioneer forest. The potential CWD sink is important for the carbon cycle in temperate successional forests.     

Drivers of Sediment Accumulation and Nutrient Burial in Coastal Stormwater Detention Ponds,South Carolina,USA

Stormwater detention ponds are widely utilized as control structures to manage runoff during storm events. These ponds also represent biogeochemical hotspots, where carbon (C) and nutrients can be processed and buried in sediments. This study quantified C and nutrient [nitrogen (N) and phosphorus (P)] sources and burial rates in 14 stormwater detention ponds representative of typical residential development in coastal South Carolina. Bulk sediment accumulation was directly correlated with catchment impervious surface coverage (R² = 0.90) with sediment accumulation rates ranging from 0.06 to 0.50 cm y^?1. These rates of sediment accumulation and consequent pond volume loss were lower than anticipated based on maintenance guidelines provided by the State. N-alkanes were used as biomarkers of sediment source; the derived terrestrial aquatic ratio (TAR_HC) index was strongly correlated with sediment accumulation rate (R² = 0.71) which, in conjunction with high C/N ratios (16–33), suggests that terrestrial biomass drives this sediment accumulation, with relatively minimal contributions from algal derived material. This is counter to expectations that were based on the high algal productivity generally observed in stormwater ponds and previous studies of natural lakes. Sediment C and nutrient concentrations were consistent among ponds, such that differences in burial rates were a simple function of bulk sediment accumulation rate. These burial rates (C: 8.7–161 g m^?2 y^?1, N: 0.65–6.4 g m^?2 y^?1, P: 0.238–4.13 g m^?2 y^?1) were similar to those observed in natural lake systems, but lower than those observed in reservoirs or impoundments. Though individual ponds were small in area (930–41,000 m²), they are regionally abundant and, when mean burial rates are extrapolated to the regional scale (≈ 21,000 ponds), ultimately sequester 2.0 × 10⁹ g C y^?1, 9.5 × 10⁷ g N y^?1, and 3.7 × 10⁷ g P y^?1 in the coastal region of South Carolina alone. Stormwater ponds represent a relatively new but increasingly significant feature of the coastal landscape and, thus, are a key component in understanding how urbanization alters the transport and transformations of C and nutrients between terrestrial uplands and downstream receiving waters.     

Carbon Dioxide and Methane Fluxes From Tree Stems,Coarse Woody Debris,and Soils in an Upland Temperate Forest

Forest soils and canopies are major components of ecosystem CO₂ and CH₄ fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO₂ and CH₄ fluxes. We measured CO₂ and CH₄ fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO₂ sources (4.58 ± 2.46 µmol m^?2 s^?1, mean ± 1 SD) and net sinks of CH₄ (?2.17 ± 1.60 nmol m^?2 s^?1). Coarse woody debris was a CO₂ source (4.23 ± 3.42 µmol m^?2 s^?1) and net CH₄ sink, but with large uncertainty (?0.27 ± 1.04 nmol m^?2 s^?1) and with substantial differences depending on wood decay status. Stems were CO₂ sources (1.93 ± 1.63 µmol m^?2 s^?1), but also net CH₄ sources (up to 0.98 nmol m^?2 s^?1), with a mean of 0.11 ± 0.21 nmol m^?2 s^?1 and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, and L. tulipifera consistently emitted CH₄, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO₂ fluxes, whereas CH₄ emissions from living stems offset net soil and CWD CH₄ uptake by 3.5%. Our results demonstrate the importance of CH₄ emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO₂ and CH₄ dynamics.     

When Water Vanishes: Magnitude and Regulation of Carbon Dioxide Emissions from Dry Temporary Streams

Most fluvial networks worldwide include watercourses that recurrently cease to flow and run dry. The spatial and temporal extent of the dry phase of these temporary watercourses is increasing as a result of global change. Yet, current estimates of carbon emissions from fluvial networks do not consider temporary watercourses when they are dry. We characterized the magnitude and variability of carbon emissions from dry watercourses by measuring the carbon dioxide (CO₂) flux from 10 dry streambeds of a fluvial network during the dry period and comparing it to the CO₂ flux from the same streambeds during the flowing period and to the CO₂ flux from their adjacent upland soils. We also looked for potential drivers regulating the CO₂ emissions by examining the main physical and chemical properties of dry streambed sediments and adjacent upland soils. The CO₂ efflux from dry streambeds (mean ± SD = 781.4 ± 390.2 mmol m^?2 day^?1) doubled the CO₂ efflux from flowing streambeds (305.6 ± 206.1 mmol m^?2 day^?1) and was comparable to the CO₂ efflux from upland soils (896.1 ± 263.2 mmol m^?2 day^?1). However, dry streambed sediments and upland soils were physicochemically distinct and differed in the variables regulating their CO₂ efflux. Overall, our results indicate that dry streambeds constitute a unique and biogeochemically active habitat that can emit significant amounts of CO₂ to the atmosphere. Thus, omitting CO₂ emissions from temporary streams when they are dry may overlook the role of a key component of the carbon balance of fluvial networks.     

Accelerated microbial organic matter mineralization following salt-water intrusion into tidal freshwater marsh soils

The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO₂) and methane (CH₄)], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO₂ and, surprisingly, CH₄ increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following salt-water intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m^?2, respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m^?2, respectively), and to losses in soil organic C content (0 and 44.1 mol C m^?2, respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.     

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.     

Carbon fluxes in forested bog margins along a human impact gradient: could vegetation structure be used as an indicator of peat carbon emissions?

Bog ecosystems are sensitive to anthropogenic disturbance, including drainage and air pollution. Carbon (C) balance measurements to determine the effect of disturbance on bog functioning are laborious; therefore reliable proxies for C fluxes that could facilitate upscaling from single studies to a larger scale would be valuable. We measured peat CO₂ emissions (R _s), CH₄ efflux and vegetation characteristics in four bog areas that formed a gradient from pristine to severely disturbed peatlands, affected by drainage, peat mining, alkaline air pollution and underground oil-shale mining. We expected that sites experiencing higher human impact (i.e., the vegetation was more distinct from that of a natural bog) would have higher R _s and lower CH₄ emissions, but differences in peat C emissions between the most disturbed and pristine sites were not significant. Growing period median R _s ranged from 0.5 to 2.2 g C m^?2 day^?1 for our plots; methane emissions, measured from July to December were an order of magnitude lower, ranging from ?5.9 to 126.7 mg C m^?2 day^?1. R _s and CH₄ emissions were primarily determined by water table depth, as was tree stand productivity. Therefore, stand structural parameters could potentially be good indicators of soil C emissions from poorly drained forested bogs.     

Soil-atmosphere exchange of greenhouse gases in subtropical plantations of indigenous tree species

Indigenous broadleaf plantations are increasingly developing as a prospective silvicultural management approach for substituting in place of large pure conifer plantations in subtropical China. However, little information is known about the effects of tree species conversion on soil-atmosphere greenhouse gas (GHG) exchanges. Four adjacent monospecific plantations were selected in subtropical China to examine the effects of tree species on soil-atmosphere exchanges of N₂O, CH₄ and CO₂. One coniferous plantation was composed of Pinus massoniana (PM), and the three broadleaf plantations were Castanopsis hystrix (CH), Michelia macclurei (MM) and Mytilaria laosensis (ML). We found that mean soil N₂O and CO₂ emissions in the PM plantation were 4.34 μg N m^?2?h^?1 and 43.25 mg C m^?2?h^?1, respectively, lower than those in the broadleaf plantations (>5.25 μg N m^?2?h^?1 and >56.38 mg C m^?2?h^?1). The PM plantation soil had higher mean CH₄ uptake (39.03 μg C m^?2?h^?1) than the broadleaf plantation soils (<32.67 μg C m^?2?h^?1). Variations in soil N₂O emissions among tree species could be primarily explained by the differences in litter C:N ratio and soil total N stock. Differences in soil CH₄ uptake among tree species could be mostly attributed to the differences in mean soil CO₂ flux and water filled pore space (WFPS). Litter C:N ratio could largely account for variations in soil CO₂ emissions among tree species. This study confirms that there is no GHG benefit of converting PM plantation to broadleaf plantations in subtropical China. Therefore, the future strategy of tree species selection for substituting in place of large coniferous plantations in subtropical China needs to consider the potential effects of tree species on soil-atmosphere GHG exchanges.     

The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

Ephemeral streams and wetlands are characterized by complex cycles of submersion and emersion, which influence the greenhouse gas flux rates. In this study we quantify the spatiotemporal variability in CO₂ and CH₄ concentrations and fluxes of an intermittent first-order stream over three consecutive wet and dry cycles spanning 56 days, to assess how hydrologic phase transitions influence greenhouse gas evasion. Water column excess CO₂ ranged from ?11 to 1600 μM, and excess CH₄ from 1 to 15 μM. After accounting for temporal changes in the ratio of wet versus dry streambed hydraulic radius, total CO₂–C fluxes ranged from 12 to 156 mmol m^?2 day^?1, with an integrated daily mean of 61 ± 25 mmol m^?2 day^?1. Soil–air evasion rates were approximately equal to those of water–air evasion. Rainfall increased background water–air CO₂–C fluxes by up to 780% due to an increase in gas transfer velocity in the otherwise still waters. CH₄–C fluxes increased 19-fold over the duration of the initial, longer wet-cycle from 0.1 to 1.9 mmol m^?2 day^?1. Temporal shifts in water depth and site-specific ephemerality were key drivers of carbon dynamics in the upper Jamison Creek watercourse. Based on these findings, we hypothesise that the cyclic periodicity of fluxes of biogenic gases from frequently intermittent streams (wet and dry cycles ranging from days to weeks) and seasonally ephemeral watercourses (dry for months at a time) are likely to differ, and therefore these differences should be considered when integrating transient systems into regional carbon budgets and models of global change.