Flux of organic carbon in a riverine mangrove wetland in the Florida Coastal Everglades

Short-term (daily) and seasonal variations in concentration and flux of dissolved organic carbon (DOC) were examined over 15 tidal cycles in a riverine mangrove wetland along Shark River, Florida in 2003. Due to the influence of seasonal rainfall and wind patterns on Shark River’s hydrology, samplings were made to include wet, dry and transitional (Norte) seasons. We used a flume extending from a tidal creek to a basin forest to measure vertical (vegetated soil/water column) and horizontal (mangrove forest/tidal creek) flux of DOC. We found significant (p < 0.05) variations in surface water temperature, salinity, conductivity, pH and mean concentration of DOC with season. Water temperature and salinity followed seasonal patterns of air temperature and rainfall, while mean DOC concentration was highest during the dry season (May), followed by the wet (October) and ‘Norte’ (December) seasons. This pattern of DOC concentration may be due to a combination of litter production and inundation pattern of the wetland. In contrast to daily (between tides) variation in DOC flux between the mangrove forest and tidal creek, daily variations of mean water quality were not significant. However, within-tide variation of DOC flux, dissolved oxygen content and salinity was observed. This indicated that the length of inundation and water source (freshwater vs. saltwater) variation across tidal cycles influenced water quality and DOC flux in the water column. Net DOC export was measured in October and December, suggesting the mangrove forest was a source of DOC to the adjacent tidal creek during these periods. Net annual export of DOC from the fringe mangrove to both the tidal creek and basin mangrove forest was 56 g C m⁻² year⁻¹. The seasonal pattern in our flux results indicates that DOC flux from this mangrove forest may be governed by both freshwater discharge and tidal range.     

Controls on fluxes and export of dissolved organic carbon in grasslands with contrasting soil types

Export of dissolved organic carbon (DOC) from grassland ecosystems can be an important C flux which directly affects ecosystem C balance since DOC is leached from the soil to the groundwater. DOC fluxes and their controlling factors were investigated on two grassland sites with similar climatic conditions but different soil types (Vertisol vs. Arenosol) for a 2.5-year period. Parts of both grasslands were disturbed by deep ploughing during afforestation. Contrary to what was expected, ploughing did not increase DOC export but surface soil DOC concentrations decreased by 28% (Vertisol) and 14% (Arenosol). DOC flux from the soil profile was negatively influences by the clay content of the soil with seven times larger DOC export in the clay-poor Arenosol (55 kg C ha^?1 a^?1) than in the clay-rich Vertisol (8 kg C ha^?1 a^?1). At the Arenosol site, highest DOC concentrations were measured in late summer, whereas in the Vertisol there was a time lag of several months between surface and subsoil DOC with highest subsoil DOC concentrations during winter season. DOC export was not correlated with soil organic carbon stocks. Large differences in ¹⁴C concentrations of 22–40 pMC between soil organic carbon and DOC in the subsoil indicated that both C pools are largely decoupled. We conclude that DOC export at both sites is not controlled by the vegetation but by physicochemical parameters such as the adsorption capacity of soil minerals and the water balance of the ecosystem. Only in the acidic sandy Arenosol DOC export was a significant C flux of about 8% of net ecosystem production.     

Composition and variations in the occurrence of dissolved free simple organic compounds of an unproductive lake ecosystem in northern Sweden

Low molecular weight organic carbon compounds are potentially important carbon and energy substrates to heterotrophic production in the aquatic environment. We studied the occurrence of dissolved free amino acids (AA), monosaccharides (CHO), and carboxylic acids (CA) in the subarctic Lake Diktar-Erik. The lake is unproductive with slightly humic water, and receives water via one major inlet stream draining a birch forest environment. The concentration of dissolved organic carbon (DOC) in the inlet stream was strongly correlated with the discharge. This relationship changed from season to season, indicating changes in the sources of the DOC entering the stream. AA and CHO each accounted for an average of less than 0.5% of the DOC. After high discharge events during the ice-free period, AA and CHO occurred in especially high concentrations. CA occurred in higher concentrations during the ice-free period, when it generally accounted for 20–30% of the DOC pool. The CA content relative to the total DOC pool was strongly inversely correlated with overall DOC concentration, and at low DOC levels the relative content of CA was high and vice versa. This followed a seasonal trend, with CA accounting for a smaller proportion of the DOC in winter and a larger part in spring/early summer. A conservative estimate suggested that the studied simple organic carbon compounds potentially could cover 30% of the bacterial gross production in the lake and therefore potentially also was an important source of CO₂ that occur in supersaturated concentrations in the lake.     

Examination of the potential relationship between droughts, sulphate and dissolved organic carbon at a wetland-draining stream

Rising dissolved organic carbon (DOC) concentrations observed at a number of sites in the northern hemisphere over recent decades are the subject of much debate, and recent reports suggest a link between DOC patterns in surface waters and changes in sulphate (SO₄) related to droughts or deposition. In order to investigate the potential influence of changes in SO₄ concentration on DOC patterns in south‐central Ontario, we used long‐term (1980–2001) stream monitoring data from a wetland‐dominated catchment (Plastic Lake‐1 subcatchment, PC1) that has been the focus of intensive investigations of both SO₄ and DOC dynamics. Annual average volume‐weighted DOC concentration increased significantly between 1980 and 2001, whereas SO₄ concentration declined, but the decrease was not significant due to large increases in SO₄ that occurred during drought years. There was no relationship between SO₄ and DOC in annual data series; however, seasonal analyses indicated significant negative correlations between SO₄ and DOC concentrations in spring (March–April–May), summer (June–July–August) and fall (September–October–November). In spring, DOC concentration was negatively correlated with flow whereas SO₄ concentrations increased with flow, and their opposing relationships with discharge explain the negative correlation between SO₄ and DOC in this season. In summer and fall, low SO₄ concentrations occur during periods of low flow as a result of microbial SO₄ reduction, whereas correspondingly high DOC concentrations in the summer and fall can be attributed to optimal conditions (i.e. stagnant flow, warm temperatures) for DOC production in the wetland. Increases in SO₄ (and acidity) following droughts were not associated with declines in DOC; instead the primary impact of droughts on DOC was to limit DOC export due to diminished stream flow. Rather than an acidification effect, we suggest that negative relationships between SO₄ and DOC were either directly (spring) or indirectly (summer/fall) caused by underlying relationships with hydrology.     

Solute dynamics in a North Brazilian mangrove: the influence of sediment permeability and freshwater input

Although water in mangrove sediments influences nutrient cycling in both, mangrove forest and estuary, little information exists on seasonal and vertical distribution of dissolved organic and inorganic compounds in the sediment column. We studied the influence of sediment texture and chemistry, permeability (K), tides, and rainfall on dissolved organic carbon (DOC) and nitrogen (DON), dissolved inorganic phosphate (DIP) and salinity in creek and sediment waters of a mangrove in Pará, Brazil. Water samples were taken from boreholes and piezometers in the mangrove forest and from an adjacent tidal creek at neap and spring tides, during the dry and rainy season. Forest sediment was analysed for carbon (C), nitrogen (N), salinity and permeability. Clay, C and N decreased with depth. Sediment permeability (K) was lowest (<0.1 m day⁻¹) in the upper, clay-rich and crab-burrow-free mud layer. In the deeper, fine sand strata, K ranged from 0.7 to 1.8 m day⁻¹. Tidal range in the creek was 3.5 and 5.5 m for neap and spring tides, respectively. Salinity, DOC, DON and DIP in creek water were inversely related to tidal height. Piezometer data revealed significant water level changes in deeper, sandy sediment layer, which followed, time-lagged, the tidal fluctuations. In contrast, tide did not affect the water level in the upper sediment due to low permeability. Compared with creek water, sediment water was enriched in DOC, DON and DIP because of organic matter input and mineralization. In deeper layers, solute concentration was most likely affected by sorption processes (DOC and DIP) and reduction reactions (DIP). During the rainy season, DOC and DON in creek and sediment water were higher than in the dry season. DIP appeared invariant to seasonal changes. In the rainy season, salt flushing from surface sediments resulted in higher salinities at intermediate sediment depths, while in the deeper layers salinity was lower due to exchange with water from the tidal creek.     

Influence of hydrological pathways on dissolved organic carbon fluxes in tropical streams

Water flow pathways and water balance are fundamental components for understanding the dynamics of C in the soil/water interface of small basins. The objective of this study was to describe the seasonal variations and estimate the annual balance of dissolved organic carbon (DOC ) by comparing two tropical microbasins (preserved forest—PF and cacao plantation—CP ). Twenty‐one weekly collections were conducted from September to December 2012 and from April to June 2013. The calculation of the partial balance considered precipitation (P) as inflow and the stream as outflow. The samples were filtered and analyzed using a TOC analyzer. Overall, the DOC was higher CP compared with FP . The behavior of both venues showed that rainy season caused an increase in concentrations in the overland flow (OF ) and in the stream, and a decrease in the precipitation (P) and in the throughfall (T). In the CP , the outflow and the soil were chiefly responsible for the high DOC concentrations in the stream, when compared to the PF , which is the result of constant OM decomposition. Soil composition contributes to the control of DOC consumption in each type of soil. The balances were negative in both microbasins, although losses were higher in the AFS (agroforestry systems) when compared to the PF , especially during rainy seasons (?8.98 and ?3.05 kg ha^?1 year^?1, CP and FP , respectively). Thus, the high annual loss of DOC in the CP of the microbasins during the rainy season indicates changes in ecosystem metabolism due to the vegetation cover and to the interactions with the soil.     

Shifts in the carbon dynamics in a tropical lowland river system (Tana River,Kenya) during flooded and non-flooded conditions

Rivers transport sediment and carbon (C) from the continents to the ocean, whereby the magnitude and timing of these fluxes depend on the hydrological regime. We studied the sediment and carbon dynamics of a tropical river system at two sites along the lower Tana River (Kenya), separated by a 385 km stretch characterized by extensive floodplains, to understand how the river regime affects within-river C processing as well as the C exchange between floodplain and river. Sampling took place during three different wet seasons (2012–2014), with extensive flooding during one of the campaigns. We measured the suspended sediment concentration, the concentration and stable isotope signature of three different carbon species (particulate and dissolved organic carbon, POC and DOC, and dissolved inorganic carbon, DIC) and other auxiliary parameters. During non-flooded conditions, the total C flux was dominated by POC (57–72%) and there was a downstream decrease of the total C flux. DIC was dominating during the flooded season (56–67%) and the flux of DIC and DOC coming from the inundated floodplains resulted in a downstream increase of the total carbon flux. Our data allowed us to construct a conceptual framework for the C dynamics in river systems, whereby nine major fluxes were identified. The application of this framework highlighted the dominance of POC during non-flooded conditions and the significant CO₂ emissions during the flooded season. Furthermore, it identified the exchange of POC with the floodplain as an important factor to close the C budget of the river.     

Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma,Colorado

A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants.In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.     

Decomposition pathways of 13C-depleted leaf litter in forest soils of the Swiss Jura

Decomposition of leaf litter and its incorporation into the mineral soil are key components of the C cycle in forest soils. In a ¹³C tracer experiment, we quantified the pathways of C from decomposing leaf litter in calcareous soils of a mixed beech forest in the Swiss Jura. Moreover, we assessed how important the cold season is for the decomposition of freshly fallen leaves. The annual C loss from the litter layer of 69–77% resulted mainly from the C mineralization (29–34% of the initial litter C) and from the transfer of litter material to the deeper mineral soil (>4 cm) by soil fauna (30%). Although only 4–5% of the initial litter C was leached as dissolved organic carbon (DOC), this pathway could be important for the C sequestration in soils in the long term: The DOC leached from the litter layer was mostly retained (95%) in the first 5 cm of the mineral soil by both physico-chemical sorption and biodegradation and, thus, it might have contributed significantly to the litter-derived C recovered in the heavy fraction (>1.6 g cm^?3) at 0–4 cm depth (4% of the initial litter C). About 80% of the annual DOC leaching from the litter layer occurred during the cold season (Nov–April) due to an initial DOC flush of water-soluble substances. In contrast, the litter mineralization in winter accounted for only 25% of the annual C losses through CO₂ release from the labelled litter. Nevertheless, the highest contributions (45–60%) of litter decay to the heterotrophic soil respiration were observed on warm winter days when the mineral soil was still cold and the labile litter pool only partly mineralized. Our ¹³C tracing also revealed that: (1) the fresh litter C only marginally primed the mineralization of older SOM (>1 year); and (2) non-litter C, such as throughfall DOC, contributed significantly to the C fluxes from the litter layer since the microbial biomass and the DOC leached from the litter layer contained 20–30% and up to 60% of unlabelled C, respectively. In summary, our study shows that significant amounts of recent leaf litter C (<1 year) are incorporated into mineral soils and that the cold season is clearly less important for the litter turnover than the warm season in this beech forest ecosystem.     

气候变化背景下湖泊有机碳组分对温度和降水格局改变的响应

湖泊湿地是地球上缓解全球变暖的巨大碳汇系统。在气候变化背景下,随着温度、降水等格局的改变,其碳汇功能存在高度的响应过程。基于135篇文献已发表的557项观测数据,运用meta分析的方法,分析全球尺度上气候及地理因素的相互作用对湖泊有机碳组分的影响,包括湖泊沉积物有机碳(Sediment Organic Carbon)、湖水中总有机碳(Total Organic Carbon)、湖水溶解有机碳(Dissolved Organic Carbon)和颗粒有机碳(Particulate Organic Carbon)。结果显示:1)在纬度3°49''N-76°24''N和海拔0-4500m范围内,SOC、TOC、POC和DOC的变化范围分别为0.25-15.6 g C/Kg、1.9-25.11 mg C/L、0.026-24 mg C/L、1-115.4 mg C/L;2)地理因素中,海拔与TOC呈显著的负相关(P < 0.05),每升高1m,TOC 增加0.0077 mg/L;纬度与POC呈显著负相关,与DOC呈显著正相关(P < 0.05),每增加1°,POC减少0.136 mg/L,DOC增加1.18 mg/L;3)气候因素中,年均温与SOC和POC呈显著的正相关,与DOC呈显著负相关(P < 0.05),每升高1℃,SOC和POC增加0.079 g/kg、0.36 mg/L,DOC下降1.52 mg/L;年均降水量与POC呈显著正相关,与DOC呈显著负相关(P < 0.05),每增加100mm,POC增加0.87 mg/L,DOC减少3 mg/L;4)气候和地理因素对SOC、TOC、POC、DOC综合影响的贡献度分别达到16.1%、14%、90%、61.6%;5)最热季度的降水对SOC的影响成正相关,其他季节性气候参数对湖水中TOC和SOC均无显著性影响;最冷季的降水因素是影响POC的主要因素,最潮湿月份、最潮湿季节的降水量和最冷季度的平均温度与POC呈显著正相关关系;最干燥月份、最干旱地区、最热季度和最冷季度的降水量对DOC的影响最显著。     

Microbial activity and organic matter dynamics during 4 years of irrigation with treated wastewater

The global changes in rainfall frequency and quantity have subjected arid and semi-arid regions to long periods of drought. As this phenomenon corresponds to increasing trend of water shortage, the use of treated wastewater (TWW) has been suggested as an alternative for irrigation of agricultural crops in these areas. The aim of the study was to investigate the short- and middle-term effects of TWW irrigation on the soil microbial activities and organic carbon content. The microbial community activity was measured every 1–3 months for 4 years in a persimmon (Diospyros kaki) orchard. These activities were used here as an indicator for the soil health. The hydrolysis activity (detected by fluorescein diacetate hydrolysis (FDA) assay) increased during the irrigation season and was significantly higher in soils irrigated with TWW compared to those irrigated with freshwater (FW). This activity was also negatively correlated with dissolved organic carbon (DOC) concentrations during the irrigation season, suggesting that the community degraded the DOC in the soils regardless of its origin. The irrigation season was also characterized by an increase in nitrification potential in both TWW- and FW-irrigated soils, which coincided with high concentrations of nitrate (50 mg kg⁻¹ soil). Overall, there was an increase in all measured activities during the irrigation season, and they were higher in the TWW soils. However, it appears that after each irrigation season, the potential activity of the community returned to levels similar to or even slightly lower than those of FW-irrigated soil during the wet season, suggesting that the periodic irrigation did not significantly change the soil microbial activity.     

Effects of reclamation of natural wetlands to a rice paddy on dissolved carbon dynamics in the Sanjiang Plain,Northeastern China

Natural wetlands play an important role in the global carbon cycle, and loss of dissolved carbon through water has been indicated as one of the most important carbon sources for riverine ecosystems. During the last century, a large natural wetland area was reported to be converted to other land use types such as rice paddy land around the world. In this study, we explored the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in two natural freshwater wetlands and a rice paddy field, which was reclaimed from the natural wetlands in the Sanjiang Plain, Northeastern China, during the growing season (May–October) of 2009. The DOC and DIC concentrations in the two ecosystems were significantly different (P < 0.05). The mean DOC concentrations during the growing season in the surface water of the Deyeuxia angustifolia and Carex lasiocarpa wetlands were 49.88 ± 5.44 and 27.97 ± 1.69 mg/L, respectively, while it was only 8.63 ± 2.54 mg/L in the rice paddy field. Specific ultra-violet light absorption at 254 nm (SUVA₂₅₄) of DOC increased by an average of 19.54% in the surface water from the natural wetlands to rice paddy, suggesting that DOC mobilized in the natural wetlands was more aromatic than that in the rice paddy field. The mean DIC concentration in surface water of the rice paddy was 5.25 and 5.04 times higher than that in the natural D. angustifolia and C. lasiocarpa wetlands, respectively. The average ratio of DIC to dissolved total carbon (DTC) for the water sampled from the artificial drainage ditch in the rice paddy field was 61.82%, while it was 14.75% from the nearby channel of natural wetlands. The significant differences in dissolved carbon concentration in surface water and channels originating from different land use types suggested that reclamation of natural wetlands to rice paddy field would reduce DOC runoff and increase the DIC concentration to adjacent watersheds. Our study results for the changed pattern in dissolved carbon after the natural wetland was transformed to paddy field could have important implications for studying the impacts of the large-scale land use change to carbon cycle and management.     

Distribution and Characteristics of Dissolved Organic Matter in Mangrove Sediment Pore Waters along the Coastline of French Guiana

Mangroves represent a major environment of tropical coasts. They are highly productive, and act both as a source and a sink of organic carbon. Concentrations and characteristics (fluorescence and hydrophobic–hydrophilic fractions) of dissolved organic matter (DOM) were investigated in relation to the organic content of sediments and to the chemistry of pore waters along the coastline of French Guiana. The pore waters studied were extracted (centrifugation, soil moisture sampler) from sediments cored beneath A. germinans mangrove stands representative of development stages: pioneer, mature and senescent. In order to asses the effects of seasonal changes, two cores were performed in each location, just after dry and wet seasons, respectively. Dissolved organic carbon (DOC) concentrations in pore waters of the upper sediment were found to increase, from 0.7 mmol l⁻¹ under the pioneers to 9 under senescent mangroves. The evolution of sedimentary organic carbon (SedOC) in the same sediment paralleled that of DOC, increasing from 0.7 to 28%. On the contrary, in the lower parts of sediment cores SedOC and DOC displayed contrasting vertical trends: SedOC decreased sharply with depth while DOC increased, reaching concentrations up to 30 mmol l⁻¹ at 50 cm in the older, senescent mangroves. In addition, the Fluorescence/DOC ratios and the hydrophobic contents of DOC were higher at greater depths in most cores, expressing changes in the DOC composition. These results suggest that the DOC of the upper layers originated directly from the SedOC of the enclosing sediment, while the hydrophobic and fluorescent DOC accumulated in the anoxic bottom layer. The mechanisms responsible for this accumulation at depth requires additional research to be fully understood. However, the anoxic conditions and high pH values prevailing in the lower sediment, by lessening DOM sorption and enhancing SedOC dissolution, may be partly responsible for the high DOC concentrations and fluorescences at depth. In addition, seasonal variation may be involved. During the rainy season, water sources were mixed resulting in lower DOC concentrations in the upper sediment, whereas during the dry season, increased evapotranspiration concentrate salts and DOC, which are transported vertically with percolating water.     

Dissolved organic matter and nutrient dynamics of a coastal freshwater forested wetland in Winyah Bay, South Carolina

Seasonally flooded, freshwater cypress-tupelo wetlands, dominated by baldcypress (Taxodium distictum), water tupelo (Nyssa aquatica), and swamp tupelo (Nyssa sylvatica var. biflora) are commonly found in coastal regions of the southeastern United States. These wetlands are threatened due to climate change, sea level rise, and coastal urban development. Understanding the natural biogeochemical cycles of nutrients in these forested wetlands as ecosystems services such as carbon sequestration and nitrogen processing can provide important benchmarks to guide conservation plans and restoration goals. In this study, surface water and soil pore water samples were collected weekly from a cypress-tupelo wetland near Winyah Bay, South Carolina and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), inorganic nitrogen, and phosphate during its flooding period between October 2010 and May 2011. DOC was further characterized by specific ultra-violet absorbance at 254 nm, spectral slope ratio (S_R) (ratio of two spectral slopes between 275–295 nm and 350–400 nm), E2/E3 ratio (ratio between A254 and A365), and fluorescence excitation-emission matrix. In addition, litterfall was collected on a monthly basis for a year while the biomass of the detritus layer (i.e., decomposed duff lying on the wetland floor) was determined before and after the flooding period. Results of the field study showed that concentrations of DOC, DON, NH₄ ⁺–N, and (NO₂ ^? + NO₃ ^?)–N in the surface water were generally higher during the fall, or peak litterfall season (October to December), than in the spring season (March to May). Highest concentrations of 54.8, 1.48, 0.270, and 0.0205 mg L^?1, for DOC, DON, NH₄ ⁺–N, and (NO₂ ^? + NO₃ ^?)–N respectively, in surface waters were recorded during October. Lower SUVA, but higher S_R and E2/E3 ratios of DOC, were observed at the end of the flooding season comparing to the initial flooding, suggesting the wetland system converts high aromatic and large DOC molecules into smaller and hydrophilic fractions possibly through photochemical oxidation. A similar trend was observed in soil pore water, but the pore water generally had greater and relatively stable concentrations of dissolved nutrients than surface water. No obvious temporal trend in phosphate concentration and total nitrogen to total phosphorus ratio (N:P) were found. Results of the laboratory extraction and mass balance calculation suggested fresh litter was a major source of DOC whereas decomposed duff was the source of dissolved nitrogen in surface water. In summary, the biogeochemistry of this isolated cypress-tupelo wetland is not only driven by the vegetation within the wetland system but also by hydrology and weather conditions such as groundwater table position, precipitation, and temperature.     

Permafrost conditions in peatlands regulate magnitude,timing, and chemical composition of catchment dissolved organic carbon export

Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2–3 g C m^?2) dominated by the snow melt period (~70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m^?2), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and ~10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.     

Dissolved Organic Carbon in Alaskan Boreal Forest: Sources, Chemical Characteristics, and Biodegradability

The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV–Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO₂ production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13–15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates.     

Trend analysis of serum progesterone, deoxycorticosterone, deoxycorticosterone sulfate, cortisol, corticosterone, 18-hydroxydeoxycorticosterone and estradiol in early neonates

To elucidate the origin and regulatory mechanism of deoxycorticosterone (DOC) and deoxycorticosterone sulfate during fetal life, the levels of serum DOC, DOC sulfate, progesterone, cortisol, corticosterone and 18-hydroxydeoxycorticosterone (18OH-DOC) were determined in the fraction separated on high performance liquid chromatogram (HPLC) by radioimmunoassay (RIA) using the serum from normal newborn. Elimination curves both of serum DOC and DOC sulfate showed two phases: rapidly decreasing and slowly decreasing ones. Both serum DOC and DOC sulfate correlated with progesterone (r = 0.340, p less than 0.01; r = 0.737, p less than 0.01, respectively). They also correlated with cortisol (DOC, r = 0.467, p less than 0.01; DOC sulfate, r = 0.549, p less than 0.01, respectively). Serum DOC reached normal adult levels by 16 hrs after birth. However serum DOC sulfate concentration was maintained high throughout the entire early neonatal period. On the contrary, the changes in serum cortisol, corticosterone and 18OH-DOC showed a peak surge in the initial phase after delivery. Both serum corticosterone and 18OH-DOC correlated with cortisol (r = 0.518, p less than 0.01; r = 0.410, p less than 0.01, respectively). These findings suggest that, in the fetus, serum DOC and DOC sulfate are mainly produced at extraadrenal sites isolated from normal mineralocorticoids synthesis and after birth they begin to be formed at adrenal glands.     

Soil Fauna Affects Dissolved Carbon and Nitrogen in Foliar Litter in Alpine Forest and Alpine Meadow

Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) are generally considered important active biogeochemical pools of total carbon and nitrogen. Many studies have documented the contributions of soil fauna to litter decomposition, but the effects of the soil fauna on labile substances (i.e., DOC and TDN) in litter during early decomposition are not completely clear. Therefore, a field litterbag experiment was carried out from 13th November 2013 to 23rd October 2014 in an alpine forest and an alpine meadow located on the eastern Tibetan Plateau. Litterbags with different mesh sizes were used to provide access to or prohibit the access of the soil fauna, and the concentrations of DOC and TDN in the foliar litter were measured during the winter (the onset of freezing, deep freezing and thawing stage) and the growing season (early and late). After one year of field incubation, the concentration of DOC in the litter significantly decreased, whereas the TDN concentration in the litter increased. Similar dynamic patterns were detected under the effects of the soil fauna on both DOC and TDN in the litter between the alpine forest and the alpine meadow. The soil fauna showed greater positive effects on decreasing DOC concentration in the litter in the winter than in the growing season. In contrast, the dynamics of TND in the litter were related to seasonal changes in environmental factors, rather than the soil fauna. In addition, the soil fauna promoted a decrease in litter DOC/TDN ratio in both the alpine forest and the alpine meadow throughout the first year of decomposition, except for in the late growing season. These results suggest that the soil fauna can promote decreases in DOC and TDN concentrations in litter, contributing to early litter decomposition in these cold biomes.     

Seasonal variability of dissolved organic carbon ina Mediterranean stream

The seasonal variability of dissolved organic carbon(DOC) flux in a Mediterranean stream subjected todischarges of wide range of intensities and variabledry period was studied as a function of the hydrologicconditions, and the relationship between surface andsubsurface (hyporheic and groundwater) DOCconcentration. DOC concentration in stream water(2.6 mg l^–1 ±1.5 SD) was higher thangroundwater (1.3 mg l^–1 ± 1.2 SD) and lower thanhyporheic water (3.8 mg l^–1 ±1.7 SD),suggesting that, at baseflow, stream DOC concentrationincreases when groundwater discharges through thehyporheic zone. Storms contributed to 39% of annualwater export and to 52% of the total annual DOCexport (220 kg km^–2). A positive relationship wasobserved between Discharge (Q) and stream DOCconcentration. Discharge explained only 40% of theannual variance in stream DOC, but explained up to93% of the variance within floods. The rate of streamDOC changes with discharge change during storms (dDOC/dQ), ranged between 0 and 0.0045 C mgl^–1 s l^–1, with minimum values during Springand Summer, and maxima values in Fall and Winter.These dynamics suggest that storm inputs ofterrigenous DOC vary between seasons. During floods inthe dormant season, DOC recession curves were alwayssteeper than discharge decline, suggesting shortflushing of DOC from the leaching of fresh detritusstored in the riparian zone.     

飞来峡水库蓄水初期营养状态及浮游生物分布特征

于2000年丰水期和枯水期调查分析了新建河流型水库-飞来峡水库在蓄水初期的营养状态及浮游生物特征。结果表明:水库处于营养累积高峰期,已开始回落,但流域上游大量含磷废水使水库中磷含量继续上升,目前属于中营养型。共记录到浮游植物29种(属),以蓝藻门、绿藻门、硅藻门种类较多,其中丰水期以蓝藻和绿藻为主,枯水期是硅藻占优势,除大坝附近外,浮游植物两个季节密度变化不大,为0.39×10⁶cells·L^-1.浮游动物53种(属),其中包括僧帽溞和透明薄皮溞两种嗜寒性种类,丰度为52ind.·L^-1,枯水期高于丰水期,数量以轮虫和无节幼体为多。