Variability and transport of suspended sediment,particulate and dissolved organic carbon in the tidal freshwater Hudson River

Measurements of suspended matter, particulate organic carbon and dissolved organic carbon were made over a three year period at stations spanning 150 km of the tidal freshwater Hudson River. Suspended matter concentrations varied from year-to-year and were not related to freshwater discharge. The increase in suspended matter with depth in vertical profiles suggests that, during medium to low flow conditions, resuspension of bottom sediments was as important a source of sediment as loadings from tributaries. Particulate organic carbon showed significant variability among stations, and both autochthonous primary production and detrital organic matter are contributing to POC standing stocks. Dissolved organic carbon represented over half of the total organic carbon in the water column and showed little variation among stations.Examining downstream changes in transport showed that there was significant production of both suspended matter and POC within the study reach during the ice-free season. Tributary loadings within the study reach do not appear to be the cause of these increases in downstream transport. Dissolved organic carbon behaved conservatively in that there was no evidence for net production or net consumption within the river.The spatial/temporal patterns and analyses of transport suggest that suspended matter and POC, but not DOC, were controlled to a significant extent by processes occurring within the river and were not simply related to loadings from outside.     

Alternative explanations for rising dissolved organic carbon export from organic soils

Since 1988, there has been, on average, a 91% increase in dissolved organic carbon (DOC) concentrations of UK lakes and streams in the Acid Waters Monitoring Network (AWMN). Similar DOC increases have been observed in surface waters across much of Europe and North America. Much of the debate about the causes of rising DOC has, as in other studies relating to the carbon cycle, focused on factors related to climate change. Data from our peat‐core experiments support an influence of climate on DOC, notably an increase in production with temperature under aerobic, and to a lesser extent anaerobic, conditions. However, we argue that climatic factors may not be the dominant drivers of DOC change. DOC solubility is suppressed by high soil water acidity and ionic strength, both of which have decreased as a result of declining sulphur deposition since the 1980s, augmented during the 1990s in the United Kingdom by a cyclical decline in sea‐salt deposition. Our observational and experimental data demonstrate a clear, inverse and quantitatively important link between DOC and sulphate concentrations in soil solution. Statistical analysis of 11 AWMN lakes suggests that rising temperature, declining sulphur deposition and changing sea‐salt loading can account for the majority of the observed DOC trend. This combination of evidence points to the changing chemical composition of atmospheric deposition, particularly the substantial reduction in anthropogenic sulphur emissions during the last 20 years, as a key cause of rising DOC. The implications of rising DOC export for the carbon cycle will be very different if linked primarily to decreasing acid deposition, rather than to changes in climate, suggesting that these systems may be recovering rather than destabilising.     

A long-term perspective of dissolved organic carbon transport in Sycamore Creek, Arizona, USA

Dissolved organic carbon (DOC) dynamics were examined over five years (1989–1993) in Sycamore Creek, a Sonoran Desert stream, specifically focusing on DOC concentration in surface and hyporheic waters, and rates of export. In 1989 and 1990, the years of lowest stream discharge (0.08 and 0.04 m³ s^–1 annual mean of daily discharge, respectively), DOC was high, averaging 7.37 and 6.22 mgC l^–1 (weighted annual means). In contrast, from 1991 through 1993, a period of increased flow (1.1, 1.2 and 4.3 m³ s^–1), concentration was significantly lower (P<0.001) with annual mean concentrations of 3.54, 3.49 and 3.39 mgC l^–1. Concentration exhibited little spatial variation between two sampling stations located 6 km apart along the mainstem or between surface and hyporheic waters. Annual export of DOC from Sycamore Creek varied 100-fold over the five-year period from a mean rate of only 24 kgC d^–1 in 1990 to 2100 kgC d^–1 in 1993. Ninety percent of DOC was exported by flows greater than 2.8 m³ s^–1, and 50% during flows greater than 27 m³ s^–1; flows of 2.8 and 24 m³ s^–1 occurred only 9 and 1% of the time. The export of organic matter in Sycamore Creek appears to be coupled to El Niño-Southern Oscillation phenomena. The years of highest export, 1991–1993, had El Niño conditions while 1989 and 1990 had medial conditions.     

氮素添加和CO2浓度升高对白羊草根际和非根际土壤水溶性有机碳、氮的影响

采用盆栽控制试验对黄土丘陵区白羊草在不同CO₂浓度(400和800 μmol·mol^-1)和施氮水平(0、2.5、5.0 g N·m^-2·a^-1)条件下根际和非根际土壤水溶性有机碳(DOC)和水溶性有机氮(DON)的变化特征进行研究.结果表明: CO₂浓度升高对白羊草根际和非根际土壤DOC、水溶性总氮(DTN)、DON、水溶性铵态氮(NH₄⁺-N)、水溶性硝态氮(NO₃^--N)含量均无显著影响.施氮显著提高了根际和非根际土壤DTN、NO₃^--N含量和根际土壤DON含量,显著降低了根际土壤DOC/DON.在各处理条件下,根际土壤DTN、NO₃^--N和DON含量均显著低于非根际土壤,根际土壤DOC/DON显著高于非根际土壤.短期CO₂浓度升高对黄土丘陵区土壤水溶性有机碳、氮含量无显著影响,而氮沉降的增加在一定程度上改善了土壤中水溶性氮素缺乏的状况,但并不足以满足植被对水溶性氮素的需求.     

A comparative study of dissolved organic carbon transport and stabilization in California forest and grassland soils

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year⁻¹ for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models.     

土壤溶解性有机碳在陆地生态系统碳循环中的作用

土壤溶解性有机碳(DOC)是有机碳库的活跃组分,在陆地生态系统碳循环中发挥重要作用.本文从碳循环重要性着手,综述了土壤DOC在土壤碳固持与温室气体排放中的作用;结合我国的现实情况(如土壤酸化、气候变暖等),探讨了土壤DOC的相关影响因素如土壤性质、环境因素、人为活动对土壤DOC的影响及作用机制,对进一步理解土壤DOC在陆地生态系统碳循环与温室气体减排中的作用具有重要意义.     

Photochemical transformations and bacterial utilization of high-molecular-weight dissolved organic carbon in a southern Louisiana tidal stream (Bayou Trepagnier)

UVirradiation of dissolved organic carbon (DOC) in the laboratory can producesmall, labile organic compounds utilizable by microbes, but few studies haveattempted to document this process in situ. ¹³Cnuclear magnetic resonance (NMR) was used to examine the bulk chemicalcomposition of natural and laboratory-irradiated high-molecular-weight DOC(HMW-DOC) from shaded (150 mol m^–2s^–1 average light in surface water) and open (1500mol m^–2 s^–1) field sitesoverone and a half years. ¹³C NMR revealed only small differences incarbon functional groups between laboratory irradiated and non-irradiatedHMW-DOC. However, bacterial protein productivity per cell (BPP) was enhanced innaturally irradiated samples of HMW-DOC in a field mesocosm experiment (p <0.05), suggesting that bacterial growth was enhanced by photochemicalproductionof labile DOC substrates. Absorbance characteristics such as spectral slope,absorbance at 350 nm, and the absorbance ratio 250nm/365 nm revealed that HMW-DOC was photoreactive,yetno differences in these values were found between samples irradiated with andwithout UV-B. In experiments conducted with simulated solar radiation in thelaboratory and with natural light in the field mesocosm experiment, UV-A(320–400 nm) and photosynthetically active radiation (PAR;400–700 nm) were more effective than UV-B (280–320nm) in HMW-DOC photolysis.     

The effect of land use on dissolved organic carbon and nitrogen uptake in streams

1. Agricultural and urban land use may increase dissolved inorganic nitrogen (DIN) concentrations in streams and saturate biotic nutrient demand, but less is known about their impacts on the cycling of organic nutrients. To assess these impacts we compared the uptake of DIN (as ammonium, NH₄⁺), dissolved organic carbon (DOC, as acetate), and dissolved organic nitrogen (DON, as glycine) in 18 low‐gradient headwater streams in southwest Michigan draining forested, agricultural, or urban land‐use types. Over 3 years, we quantified uptake in two streams in each of the three land‐use types during three seasons (spring, summer and autumn). 2. We found significantly higher NH₄⁺ demand (expressed as uptake velocity, V_f) in urban compared to forested streams and NH₄⁺V_f was greater in spring compared to summer and autumn. Acetate V_f was significantly higher than NH₄⁺ and glycine V_f, but neither acetate nor glycine V_f were influenced by land‐use type or season. 3. We examined the interaction between NH₄⁺ and acetate demand by comparing simultaneous short‐term releases of both solutes to releases of each solute individually. Acetate V_f did not change during the simultaneous release with NH₄⁺, but NH₄⁺V_f was significantly higher with increased acetate. Thus, labile DOC V_f was not limited by the availability of NH₄⁺, but NH₄⁺V_f was limited by the availability of labile DOC. In contrast, neither glycine nor NH₄⁺V_f changed when released simultaneously indicating either that overall N‐uptake was saturated or that glycine and NH₄⁺ uptake were controlled by different factors. 4. Our results suggest that labile DOC and DON uptake can be equivalent to, or even higher than NH₄⁺ uptake, a solute known to be highly bioreactive, but unlike NH₄⁺ uptake, may not differ among land‐use types and seasons. Moreover, downstream export of nitrogen may be exacerbated by limitation of NH₄⁺ uptake by the availability of labile DOC in headwater streams from the agricultural Midwestern United States. Further research is needed to identify the factors that influence cycling of DOC and DON in streams.     

Complementary pathways of dissolved organic carbon removal pathways in clear-water Amazonian ecosystems: photochemical degradation and bacterial uptake

Dissolved organic carbon (DOC) photochemical reactions establish important links between DOC and planktonic bacteria. We hypothesize that seasonal changes in DOC quality, related to the flood pulse, drive the effects of light-DOC interactions on uptake by planktonic bacteria uptake in clear-water Amazonian ecosystems. Water samples from two ecosystems (one lake and one stream) were incubated in sunlight during different hydrological periods and were then exposed to bacterial degradation. Photochemical and bacterial degradation were driven by seasonal DOC inputs. Bacterial mineralization was the main degradation pathway of autochthonous DOC in the lake, while allochthonous DOC was more available for photochemical oxidation. We suggest that sunlight enhances the bacterial uptake of refractory DOC but does not alter uptake of labile forms. We also observed a positive relationship between sunlight and bacterial degradation of DOC, instead of competition. We conclude that photochemical reactions and bacteria complementarily degrade the different sources of DOC during the flood pulse in Amazonian clear-water aquatic ecosystems.     

Distribution of dissolved organic carbon and dissolved fulvic acid in mesotrophic Lake Biwa, Japan

The dissolved organic carbon (DOC) concentrations in mesotrophic Lake Biwa were determined by a total organic carbon (TOC) analyzer, and DOC molecular size distributions were determined by size exclusion chromatography (SEC) using a fluorescence detector at excitation/emission (Ex/Em) levels of 300/425 nm with the eluent at pH 9.7. The fluorescence wavelengths for detection were chosen from the result of excitation–emission matrix spectrometry (EEM) analysis for dissolved fulvic acid (DFA) extracted from Ado River (peak A, Ex/Em = 260–270/430–440 nm; peak B, Ex/Em = 300–310/420–430 nm). Ado River DFA was eluted with a retention time (RT) of 7.4–8.9 min and the apparent molecular weight was estimated at 22–87 kDa based on the elution curve for the spherical protein molecular weight standard. A DFA peak eluted at the same retention time as Ado River DFA also appeared in all the samples of Lake Biwa water. From the linear relationship between the peak areas with an RT of 7.4–8.9 min by SEC analysis and DOC values of DFA by TOC analysis of a series of DFA samples (r² = 0.9995), the concentrations of DFA in the lake water were roughly calculated. DFA was distributed within the range 0.25–0.43 mg C l⁻¹ and accounted for 15%–41% of DOC, with the highest ratios observed at a depth of 70 m in August and the lowest at 2.5 m in May.     

Seasonal patterns of viruses, bacteria and dissolved organic carbon in a riverine wetland

SUMMARY 1. Viral and bacterial abundances were studied in relation to environmental attributes over an annual period, for both planktonic and attached (sediment, aquatic macrophyte and submerged wood) habitats, in a riverine wetland.
2. Annual mean abundance of planktonic viruses ranged from 2.3 × 10⁵−3.8 × 10⁵ particles mL⁻¹ and varied according to sampling site. Significant seasonal patterns in viral abundance were evident and appeared to be linked to variations in bacterial abundance, dissolved organic carbon and inorganic nutrients.
3. Annual mean abundance of viruses associated with surfaces ranged from 1.3 × 10⁶ particles cm⁻² on aquatic macrophytes to 1.1 × 10⁷ particles cm⁻² on wood and also showed seasonal patterns. The difference in viral dynamics among the different sites emphasizes the importance of considering habitat diversity within wetlands when examining microbial communities.     

Environmental variability in the reactivity of freshwater dissolved organic carbon to UV-B

Reductions in the global stratospheric ozone layer arethought to be increasing the amount of ultraviolet B (UV-B) radiationreaching the planets surface and may be affecting the chemistry ofdissolved organic carbon (DOC) in surface waters. We studied theabundance of chromophores in DOC collected in four different aquaticenvironments in southwestern Nova Scotia using ¹³C nuclearmagnetic resonance (¹³C NMR) spectroscopy. We showed a clearseasonal pattern in the distribution of structural carbon related tolight-sensitive chromophores. There seemed to be little variation in theUV-B related chemical structure of DOC between lakes and streams, thoughwater from a bog pool showed large differences from the other samples. Thesepatterns of potential UV-B reactivity tend to be dampened however, byvariations in DOC concentrations which also occur seasonally.     

Microbial control of dissolved organic carbon in lakes: research for the future

The dissolved organic carbon (DOC) of lakes dominates any budget of organic carbon in these systems. Limnologists are still limited by techniques and particularly by the lack of measurements of rates of microbial transformation and use of this DOC. There are now four different approaches to the study of the microbial control of DOC in lakes. The first is through measurements of the total DOC. Recent advances in measurement with high temperature combustion will likely lead to higher and more consistent measurements in freshwaters than previously. It is possible that a biologically active fraction may be identified. The second approach is through measurements of microbial incorporation and respiration of ¹⁴C-labeled organic matter. The kinetics of this process are well known but advances in measurement of the size of the substrate pool are still being made. The third approach is to use bacterial growth in batch or continuous flow experiments in order to understand how much of the total DOC can be decomposed by microbes. The assay in this approach may be microbial growth (thymidine incorporation, biomass changes) or change in the DOC (total concentrations, specific compounds, or fractions of the DOC by molecular weight). These methods are promising but are not developed enough for routine use. For example, growth measurements in the laboratory are all subject to experimental artifacts caused by changes in the DOC and in the bacterial populations. Finally, the fourth approach is through the use of isotopes of the natural DOC. In the sea this approach has given the age of the bulk DOC (¹⁴C data). In freshwaters it has great potential for differentiating between bacterial use of terrestrial DOC vs. use of algal-derived DOC (13C data). Stable isotopes are also useful for experimentally labeling DOC produced by algae and following the use of this material by bacteria.     

Climate-induced changes in the dissolved organic carbon budgets of boreal lakes

During 20 years of climatic warming, drought and increased forest firesbetween 1970 and 1990, DOC concentrations declined by 15--25%in lakesof the Experimental Lakes Area, northwestern Ontario, allowing increasedpenetration of both UV and photosynthetically-active radiation (PAR), andcausing deeper euphotic zones and thermoclines. Decreased input to thelakes of DOC from terrestrial catchments and upstream lakes was theprimary reason for the decline, although in-lake removal also increasedslightly. Decreased streamflow caused by drought was more important thanforest fires in affecting DOC exports from catchments. Experimentalacidification of lakes caused even greater losses in DOC, by enhancing ratesof in-lake removal. DOC in Lake 302S, acidified to pH 4.5 during the1980s, declined to less than 10% of preacidificationvalues.     

可溶性有机碳在土壤剖面淋溶过程中的分馏

选取中亚热带两种不同类型土壤为研究对象,采集0～10、10～20、20 ～ 40、40～60、60～80、80～ 100 cm土层土壤样品,提取米槠新近凋落物的可溶性有机碳(DOC)溶液为初始DOC,研究其在土样中淋溶时的分馏现象和截留特征.结果表明:初始DOC在土柱中淋溶时,其浓度随深度增加呈逐层下降的趋势,而且化学结构更为简单,被土壤截留的DOC中主要是疏水性组分,但随着土层深度增加,亲水性DOC的截留量呈上升趋势;红外光谱显示含有芳环的疏水性物质最易被吸附,而烷烃和简单的碳水化合物则最有可能随土壤溶液进入下层土壤;由于到达下层土壤的DOC中易被吸附组分含量的减少,限制了其吸附能力的发挥,因此主要的吸附过程发生在40 cm以上土层,表明DOC本身的化学性质比土壤性质更能影响其吸附行为;不同类型土壤的截留量具有显著差异,这与土壤黏粒和铁铝氧化物的含量呈显著正相关.     

Nitrogen loading into an urban estuary: Lake Pontchartrain (Louisiana,U.S.A.)

We constructed a nitrogen loading budget for the Lake Pontchartrain watershed located north of New Orleans, Louisiana (U.S.A.). Water quality measurements, discharge estimates, and literature values were used to establish the annual and seasonal variations in loading rates for total nitrogen and nitrate. The relatively stable annual loadings (million kg N) are about 10× that of the pre-settlement nitrogen loading, and come from atmosphere (1.3), the watershed (7.8), pumped urban runoff from New Orleans (1.0), and leakage through the Bonnet Carré flood control structure (0.5–0.9). Relatively minor additional amounts come from nitrogen fixation in the Lake. Occasional openings of the Bonnet Carré Spillway (for flood protection) could triple the annual average loading within 1–2 months. Proposed smaller diversions (for wetland restoration) could raise present N loadings by 50%. The results of water quality management, flood protection and wetland restoration may thus have conflicting effects on the Lake's phytoplankton community, which is primarily nitrogen limited. Lowering the total nitrogen loading, however, seems quite possible, especially given that the present loadings are almost all reducible through existing technology, especially sewerage treatment. The analysis demonstrates that the consequences of ecosystem restoration efforts, continued population growth and flood protection to estuarine nitrogen budgets are intertwined with each other, have a seasonal component, and are changing as policies evolve.     

Dynamics of dissolved organic nitrogen in subalpine Castle Lake,California

Dissolved organic nitrogen (DON) concentrations were measured in meso-oligotrophic Castle Lake, California during the ice-free season in 1982, 1983 and 1984. No consistent relationships were found between DON and particulate-N, primary productivity rates or chlorophyll concentrations. However, increases in DON concentrations were observed in the early growing season of 1982 and 1984 when water temperature was rising and a diatom bloom was senescing. DON increased at a high rate (0.31 mg atom N m^–3 day^–1), and then rapidly disappeared. Sediment released appared to be the most important source of DON. Dissolved free amino acids were always less than 7.5% of the DON pool, and did not vary in the composition of specific amino acids during the growing season.     

土地利用变化对挠力河流域可溶性有机碳输出的影响

采用野外采样、室内分析、GIS及统计分析相结合的方法,研究了挠力河流域河水可溶性有机碳（DOC）浓度的季节性动态,以及年均尺度上全流域、100 m河岸带土地利用变化对河水DOC输出的影响.结果表明： 基流状态下,河水DOC浓度在春季、夏季显著高于秋季;有湿地存在的子流域DOC浓度的季节性动态与无湿地存在的子流域存在显著差异,且有湿地存在的子流域中DOC浓度的季节性变异与整个流域的趋势一致;年均尺度上,DOC浓度与全流域湿地以及100 m河岸带范围内的水田面积百分比呈显著正相关,而与全流域尺度的林地百分比呈显著负相关（P<0.05）.表明湿地的存在是影响挠力河流域河水DOC季节性变异的重要因素;全流域的湿地以及100 m河岸带范围内的水田对其具有显著的促进作用,而林地对其有显著的减缓效应,流域过去几十年的土地利用变化改变了河水DOC的平衡状况.     

淹水培养条件下土壤微生物生物量碳、氮和可溶性有机碳、氦的动态

以洞庭湖区2个典型水稻土（红黄泥和紫潮泥）为对象,研究了25℃、淹水培养条件下稻草-硫铵配施和单施硫铵处理土壤微生物生物量碳、氮（SMBC、SMBN）和可溶性有机碳、氦（SDOC、SDON）的动态变化．结果表明,SMBC、SMBN和SDOC、SDON在培养前期达到峰值,之后降低,并趋于稳定．添加底物后,2种土壤不同处理土壤微生物生物量碳与有机碳（SMBC／TC）和土壤微生物生物量氮与全氮（SMBN／TN）的平均值都在2％-3％之间变化;可溶性碳与全碳（SDOC／TC）的平均值为1％左右,可溶性氮与全氮（SDON／TN）平均值为5％-6％．2种土壤中SMBC峰值单施硫铵处理最大,但与稻草-硫铵配施处理差异均不显著;SMBN、SDOC和SDON峰值稻草-硫铵配施最大．稻草．硫铵配施与单施硫铵处理中,低肥力红黄泥的SMBN、SDOC和SDON峰值差异显著;而高肥力紫潮泥SMBN和SDOC峰值差异不显著．前7d,SMBC／SMBN〈10;14d后,同一时刻单施硫铵处理SMBC／SMBN〉稻草．硫铵配施．不同处理的SDOC!SDON3d时最大．28d时最小．     

Sources and sinks of dissolved organic carbon in a forested swamp catchment

Concentrations of dissolved organic carbon (DOC) were measured in precipitation, throughfall, stemflow, and soil, peat and stream water in a 50 ha catchment with a central 5 ha swamp at Mont St. Hilaire, Quebec. DOC concentrations in precipitation were low (2.0 mg L^–1), but increased in passage through the tree canopies as throughfall (9.1–14.6 mg L^–1) and stemflow (23.1–30.1 mg L^–1). For the period July 1–November 15, 1987, 0.5 g DOC m^–2 was imported as precipitation, and forest canopies contributed a further 1.4–1.7 g m^–2 2 to the soil surface. DOC concentrations were higher (46.0 and 67.6 mg L^–1) in upland soil organic horizons, but decreased with depth because subsoil mineral horizons acted as a major sink of DOC. A laboratory experiment using leaf leachate revealed that subsoil horizons were able to adsorb DOC, with equilibrium DOC concentrations ranging from 3 to 19 mg L^–1. Soil organic carbon appeared to be an important determinant of equilibrium DOC concentrations. The swamp was a major source of DOC, with an overall average DOC concentration of 58.6 mg L^–1 and showed strong spatial and temporal variations related to hydrologic and thermal regimes. During base flow periods, stream DOC concentrations were small (< 3 mg L^–1), dominated by water fed from springs draining upland soils. During high flows, stream DOC concentrations increased through the contribution of DOC-rich water originating in the swamp. Sources, sinks and transport of DOC are thus a function of a complex set of inter-related biotic and abiotic process.