老爷岭多年冻土小流域春季冻融期径流溶解性有机碳变化特征

溶解性有机碳（DOC）的输移过程是流域碳循环中重要的组成部分,对全球碳循环产生重要影响。以大兴安岭多年冻土区的典型森林小流域-老爷岭流域为研究对象,获得2021年4月9日到6月30日冻融期降雨量、气温、土温等气象数据及逐日径流量、径流DOC浓度,计算了冻融循环期（4月9日-28日）和融化期（4月29日-6月30日）流域径流DOC的输出通量,揭示了径流DOC浓度及输出通量的影响因素。结果表明：（1）研究时段内,老爷岭流域径流DOC浓度变化范围为3.88-33.75 mg/L,流域上游的径流DOC浓度变化趋势与下游基本一致,DOC浓度随着温度的升高呈现下降趋势,4月份平均径流DOC浓度明显高于5、6月份。（2）研究时段内流域径流DOC总输出通量为3215.48 kg/km²,其中5月径流DOC输出通量高于4、6月份。径流量与径流DOC输出通量存在显著正相关关系（P<0.05）,是流域DOC输出通量的主导因素。（3）研究时段内流域DOC浓度与平均气温呈极显著负相关（R²=0.5048,P<0.001）;降水样品中的DOC浓度变化范围为1.06-9.42 mg/L,显著低于径流DOC浓度;土壤中DOC含量变化趋势与径流DOC变化趋势一致,0-10 cm、10-20 cm土壤平均DOC浓度范围为77.57-133.99 mg/L。（4）冻融循环期平均日径流DOC浓度（24.02 mg/L）显著（P<0.05）高于融化期（14.64 mg/L）,而融化期平均日DOC输出通量（48.02 kg/km²）是冻融循环期（9.52 kg/km²）的5倍。研究结果揭示了大兴安岭多年冻土小流域春季冻融期径流DOC的输移特征及其影响因素,对理解多年冻土区碳循环有重要意义。     

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.     

Enhanced winter soil frost reduces methane emission during the subsequent growing season in a boreal peatland

Winter climate change may result in reduced snow cover and could, consequently, alter the soil frost regime and biogeochemical processes underlying the exchange of methane (CH₄) in boreal peatlands. In this study, we investigated the short‐term (1–3 years) vs. long‐term (11 years) effects of intensified winter soil frost (induced by experimental snow exclusion) on CH₄ exchange during the following growing season in a boreal peatland. In the first 3 years (2004–2006), lower CH₄ emissions in the treatment plots relative to the control coincided with delayed soil temperature increase in the treatment plots at the beginning of the growing season (May). After 11 treatment years (in 2014), CH₄ emissions were lower in the treatment plots relative to the control over the entire growing season, resulting in a reduction in total growing season CH₄ emission by 27%. From May to July 2014, reduced sedge leaf area coincided with lower CH₄ emissions in the treatment plots compared to the control. From July to August, lower dissolved organic carbon concentrations in the pore water of the treatment plots explained 72% of the differences in CH₄ emission between control and treatment. In addition, greater Sphagnum moss growth in the treatment plots resulted in a larger distance between the moss surface and the water table (i.e., increasing the oxic layer) which may have enhanced the CH₄ oxidation potential in the treatment plots relative to the control in 2014. The differences in vegetation might also explain the lower temperature sensitivity of CH₄ emission observed in the treatment plots relative to the control. Overall, this study suggests that greater soil frost, associated with future winter climate change, might substantially reduce the growing season CH₄ emission in boreal peatlands through altering vegetation dynamics and subsequently causing vegetation‐mediated effects on CH₄ exchange.     

Dynamics of dissolved and particulate organic carbon in a saline and semiarid stream of southeast Spain (Chicamo stream)

Annual variations in the concentration of dissolved (DOC) and particulate organic carbon (CPOC = Coarse; FPOC = Fine; UPOC = Ultrafine) were studied in a 100 m-reach of the Chicamo stream, an intermittent saline stream in southeast Spain. DOC represented the most important fraction of organic carbon flowing in the Chicamo stream (>98%), with concentrations of about 1.7 mgC l^–1 during most of the year, reaching 2.5 mgC l^–1 in summer. One high flow episode during a rain storm in winter was characterized by a considerably increased concentration of DOC (9.4 mgC l^–1). CPOC was the dominant POC fraction. Positive and significant correlations were found for DOC and discharge, which support the idea of allochthonous inputs due to floods. There was no significant correlation between POC and discharge. No significant correlations were found for DOC or POC with the physico-chemical parameters measured, while a negative significant correlation was found between DOC and temperature. The export of total organic carbon from the drainage basin of the Chicamo stream was low (6.2 × 10^–4 gC m^–2 yr^–1) and typical of streams in arid and semi-arid regions. The results of a Principal Component Analysis defined three different phases. The first consisted of short periods, during which floods provide pulses of allochthonous organic carbon and nutrients, the second a dry phase (summer), defined by biotic interactions, during which the stream could acts as a `sink' of organic matter, and the third and final phase which is characterised by hydrological stability.     

A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream

Patterns of dissolved organic carbon (DOC) and nitrogen (DON) delivery were compared between times of stormflow and baseflow in Paine Run, an Appalachian stream draining a 12.4 km² forested catchment in the Shenandoah National Park (SNP), Virginia. The potential in-stream ecological impact of altered concentrations and/or chemical composition of DOM during storms also was examined, using standardized bacterial bioassays. DOC and DON concentrations in Paine Run were consistently low during baseflow and did not show a seasonal pattern. During storms however, mean DOC and DON concentrations approximately doubled, with maximum concentrations occurring on the rising limb of storm hydrographs. The rapid response of DOM concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. Stormflow (4% of the time, 36% of the annual discharge) contributed >50% of DOC, DON and NO₃ ^– flux in Paine Run during 1997. In laboratory bacterial bioassays, growth rate constants were higher on Paine Run stormflow water than on baseflow water, but the fraction of total DOM which was bioavailable was not significantly different. The fraction of the total stream DOC pool taken up by water column bacteria was estimated to increase from 0.03 ± 0.02% h^–1 during baseflow, to 0.15 ± 0.04% h^–1 during storms. This uptake rate would have a minimal effect on bulk DOM concentrations in Paine Run, but storms may still have considerable impact on the bacterial stream communities by mobilizing them into the water column and by supplying a pulse of DOM.     

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.     

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.     

Interactions of the bacterial assemblages of a prairie stream with dissolved organic carbon from riparian vegetation

The zonation of riparian vegetation on the Konza Prairie Research Natural Area of Kansas (tallgrass prairie ecosystem) provided a situation where the influences on uptake rates of the different DOC qualities could be measured in the field. Leachates from grasses disappeared rapidly from stream water in in situ chambers at upstream sites in grassland reaches and at downstream sites in forested reaches. The reverse observation, uptake of leachates from trees, revealed that DOC disappeared rapidly from chambers placed in the forested reaches but bacteria in the grassland reaches did not utilize tree leachate as rapidly. The pattern varies seasonally. The interpretation of these observations was aided by aseptic laboratory culture on the different substrate types and by measurements in poisoned chambers to evaluate abiotic uptake.     

Aquatic export of young dissolved and gaseous carbon from a pristine boreal fen: Implications for peat carbon stock stability

The stability of northern peatland's carbon (C) store under changing climate is of major concern for the global C cycle. The aquatic export of C from boreal peatlands is recognized as both a critical pathway for the remobilization of peat C stocks as well as a major component of the net ecosystem C balance (NECB). Here, we present a full year characterization of radiocarbon content (¹⁴C) of dissolved organic carbon (DOC), carbon dioxide (CO₂), and methane (CH₄) exported from a boreal peatland catchment coupled with ¹⁴C characterization of the catchment's peat profile of the same C species. The age of aquatic C in runoff varied little throughout the year and appeared to be sustained by recently fixed C from the atmosphere (<60 years), despite stream DOC, CO₂, and CH₄ primarily being sourced from deep peat horizons (2–4 m) near the mire's outlet. In fact, the ¹⁴C content of DOC, CO₂, and CH₄ across the entire peat profile was considerably enriched with postbomb C compared with the solid peat material. Overall, our results demonstrate little to no mobilization of ancient C stocks from this boreal peatland and a relatively large resilience of the source of aquatic C export to forecasted hydroclimatic changes.     

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.     

Increasing riverine export of dissolved organic carbon from China

River transport of dissolved organic carbon (DOC) to the ocean is a crucial but poorly quantified regional carbon cycle component. Large uncertainties remaining on the riverine DOC export from China, as well as its trend and drivers of change, have challenged the reconciliation between atmosphere-based and land-based estimates of China's land carbon sink. Here, we harmonized a large database of riverine in-situ measurements and applied a random forest model, to quantify riverine DOC fluxes (F_DOC) and DOC concentrations (C_DOC) in rivers across China. This study proposes the first DOC modeling effort capable of reproducing well the magnitude of riverine C_DOC and F_DOC, as well as its trends, on a monthly scale and with a much wider spatial distribution over China compared to previous studies that mainly focused on annual-scale estimates and large rivers. Results show that over the period 2001–2015, the average C_DOC was 2.25 ± 0.45 mg/L and average F_DOC was 4.04 ± 1.02 Tg/year. Simultaneously, we found a significant increase in F_DOC (+0.044 Tg/year², p = .01), but little change in C_DOC (−0.001 mg/L/year, p > .10). Although the trend in C_DOC is not significant at the country scale, it is significantly increasing in the Yangtze River Basin and Huaihe River Basin (0.005 and 0.013 mg/L/year, p < .05) while significantly decreasing in the Yellow River Basin and Southwest Rivers Basin (−0.043 and −0.014 mg/L/year, p = .01). Changes in hydrology, play a stronger role than direct impacts of anthropogenic activities in determining the spatio-temporal variability of F_DOC and C_DOC across China. However, and in contrast with other basins, the significant increase in C_DOC in the Yangtze River Basin and Huaihe River Basin is attributable to direct anthropogenic activities. Given the dominance of hydrology in driving F_DOC, the increase in F_DOC is likely to continue under the projected increase in river discharge over China resulting from a future wetter climate.     

Effect of dissolved organic carbon quality on microbial decomposition and nitrification rates in stream sediments

1. Microbial decomposition of dissolved organic carbon (DOC) contributes to overall stream metabolism and can influence many processes in the nitrogen cycle, including nitrification. Little is known, however, about the relative decomposition rates of different DOC sources and their subsequent effect on nitrification. 2. In this study, labile fraction and overall microbial decomposition of DOC were measured for leaf leachates from 18 temperate forest tree species. Between 61 and 82% (mean, 75%) of the DOC was metabolized in 24 days. Significant differences among leachates were found for labile fraction rates (P < 0.0001) but not for overall rates (P=0.088). 3. Nitrification rates in stream sediments were determined after addition of 10 mg C L^–1 of each leachate. Nitrification rates ranged from below detection to 0.49 μg N mL sediment^–1 day^–1 and were significantly correlated with two independent measures of leachate DOC quality, overall microbial decomposition rate (r=–0.594, P=0.0093) and specific ultraviolet absorbance (r=0.469, P=0.0497). Both correlations suggest that nitrification rates were lower in the presence of higher quality carbon. 4. Nitrification rates in sediments also were measured after additions of four leachates and glucose at three carbon concentrations (10, 30, and 50 mg C L^–1). For all carbon sources, nitrification rates decreased as carbon concentration increased. Glucose and white pine leachate most strongly depressed nitrification. Glucose likely increased the metabolism of heterotrophic bacteria, which then out‐competed nitrifying bacteria for NH₄⁺. White pine leachate probably increased heterotrophic metabolism and directly inhibited nitrification by allelopathy.     

森林转换对地表径流可溶性有机碳输出浓度和通量的影响

米槠次生林转换成米槠人工幼林和米槠人工促进天然更新幼林(以下简称"人促幼林")后,以这三种森林类型为研究对象,连续监测每次降雨后地表径流量及径流水中可溶性有机碳(DOC)的含量及通量,比较不同森林类型观测结果的差异,并分析降雨对实验结果的影响。结果表明:米槠人工幼林单次产流量是米槠次生林的1.5—19.0倍,观测期间总径流量为5.9倍;米槠人促幼林单次径流量和总径流量均与米槠次生林无显著差异(P0.05)。观测期间米槠次生林、人工幼林、人促幼林径流水DOC浓度值范围为5.9—18.4 mg/L,4.3—13.5 mg/L和3.2—9.9 mg/L,米槠次生林径流水浓度均值(12.6 mg/L)分别是米槠人促幼林(7.6 mg/L)和米槠人工幼林(5.3 mg/L)的1.6和2.4倍。回归分析表明,径流水中DOC浓度与降雨前土壤含水率呈显著相关;降雨前土壤含水率20.8%是一个临界值,含水率低于20.8%时,径流水DOC浓度与降雨前含水率呈显著正相关(P0.05);高于20.8%时,径流水DOC浓度与降雨前土壤含水率呈显著负相关(P0.05)。米槠人工幼林地表径流DOC输出通量是米槠次生林的0.7—5.4倍,观测期间总输出通量为2.1倍;米槠人促林DOC单次通量和观测期间总通量均与米槠次生林差异不显著(P0.05)。三种森林类型DOC输出通量均与降雨量呈显著相关(P0.05)。可见,米槠次生林转变成米槠人工幼林后DOC输出浓度降低,但径流量显著增加,导致DOC输出通量增加;而转变成米槠人促幼林后DOC输出浓度也降低,但径流量并未增加,因而并未增加DOC输出通量。     

Kinetics and mechanisms of dissolved organic carbon retention in a headwater stream

Freeze-dried aqueous extracts of autumn-shed maple leaves, birch leaves, and spruce needles were added to a third-order reach of Bear Brook, New Hampshire at concentrations similar to those predicted to occur during peak leaf fall. Leachate from each species was rapidly removed from solution. With initial concentrations of added leachate of approximately 5 mgl^–1, dissolved organics (DOC) uptake ranged from 73 to 130 mg m^–2 h^–1 for the first five hours of travel downstream from the point of addition. There was no preferential removal of DOC of low molecular weight, or of monomeric carbohydrates relative to phenolics or unidentified DOC.Stream sediments and organic debris rapidly removed DOC from solution in laboratory experiments. No significant flocculation or microbial assimilation of sugar maple leachate occurred in stream water alone. Stream sediments showed small increases in respiration with addition of leaf leachate, but no increase in respiration occurred upon addition of leachate to organic debris. Abiotic adsorption due to the high concentrations of exchangeable iron and aluminium in stream sediments may be responsible for much of the rapid removal of leaf leachate observed in field experiments. Abiotic processes appear to retain DOC within the stream, thereby allowing subsequent metabolism of dissolved organic carbon by stream microflora.     

The significance of storms for the concentration and export of dissolved organic carbon from two Precambrian Shield catchments

Dissolved organic carbon (DOC) concentrations and DOC export arestudied during storms to examine the relationship between DOCconcentration and stream discharge and to assess the importance of stormson DOC export. Storms were monitored in seven subcatchments within twosmall watersheds (Harp 4--21 and Harp 3A) on the Precambrian Shield inCentral Ontario, Canada. Stream DOC concentrations increase during stormsby as much as 100% and 410% in Harp3A and Harp 4--21 respectively. The seasonal regression between DOC andstream discharge is significant in subcatchments without wetlands(r² > 0.7) but is not significant in thetwo subcatchments with small wetland areas (r² <0.06). On average, regressions based on weekly data yield accurate estimatesof DOC export but the variation in regressions among individual storms andthe small number of high DOC samples result in uncertainties of more than30% in DOC export. The period-weighted calculation ofDOC export from weekly data underestimates export by 14%and 22% in Harp 3A and Harp 4--21 respectively. Stormswere responsible for 57% to 68% of theDOC export in the autumn and 29% to 40%of the DOC export in the spring. A single large storm accounted for31% of the autumn DOC export in Harp 3A. The importanceof individual storms for DOC export and the variation in the relationshipbetween DOC and stream discharge among storms make it difficult to predictthe effects of climate change on DOC export and DOC concentrations.     

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.     

Terrestrial export of highly bioavailable carbon from small boreal catchments in spring floods

1. We assessed the terrestrial export of organic carbon, which effectively supported aquatic bacterial production (BP), from small boreal catchments during spring flood. We analysed stream runoff from nine small catchments with different proportions of peat mires and coniferous forests by monitoring the dissolved organic carbon (DOC) flux in combination with conducting bacterial bioassays.
2. Multiple linear regression analysis showed that BP during 7-day-dark bioassays (BP₇; μg C L⁻¹day⁻¹) was explained by both the quantity and quality (low-molecular weight fractions) of the DOC. BP₇ can be used as a measure of export of terrestrial organic carbon that is highly bioavailable.
3. Total export of DOC during spring flood from the different catchments ranged from 20 to 27 kg ha⁻¹ and was negatively correlated to forest cover (%). However, the export of BP₇ carbon was positively correlated to forest cover and varied from about 0.1 kg ha⁻¹ in mire-dominated streams to about 0.2 kg ha⁻¹ in forest-dominated streams.
4. The high bioavailability of forest carbon suggests that forests are the main contributors of BP-supporting carbon in boreal streams although mires have higher area-specific export of DOC.     

Sources and flowpaths of dissolved organic carbon during storms in two forested watersheds of the Precambrian Shield

Dissolved organic carbon (DOC) concentrations and export were studied in two small catchments in central Ontario to examine DOC sources and to assess the hypothesis that organic matter adjacent to the stream is a significant contributor of DOC during storms. Different DOC dynamics and exports were observed according to the depth of the riparian water table. In Harp 4-21, riparian flowpaths were predominantly through A and upper B soil horizons and riparian soils contributed between 73 and 84% of the stream DOC export during an autumn storm. In Harp 3A, riparian flowpaths were predominantly through lower B horizons. Consequently, riparian soils were less important and hillslopes contributed more than 50% of the stream DOC export in subcatchments without wetlands during storms. Wetlands and adjacent soils contributed significantly to DOC export in Harp 3A; 8% of the total catchment area exported 32 to 46% of the storm runoff DOC. DOC export dynamics in wetlands and riparian soils were distinctly different. In wetlands, transport was affected by leaching and flushing of DOC at the wetland surface leading to lower DOC concentrations with successive storms. In riparian soils, groundwater flowpaths were more important and stronger positive relationships between discharge and DOC concentration were observed. Precipitation, throughfall and stemflow were minor sources of stream DOC during storms and contributed less than 20% of the total export.     

Potential for long‐term transfer of dissolved organic carbon from riparian zones to streams in boreal catchments

Boreal regions store most of the global terrestrial carbon, which can be transferred as dissolved organic carbon (DOC) to inland waters with implications for both aquatic ecology and carbon budgets. Headwater riparian zones (RZ) are important sources of DOC, and often just a narrow ‘dominant source layer’ (DSL) within the riparian profile is responsible for most of the DOC export. Two important questions arise: how long boreal RZ could sustain lateral DOC fluxes as the sole source of exported carbon and how its hydromorphological variability influences this role. We estimate theoretical turnover times by comparing carbon pools and lateral exports in the DSL of 13 riparian profiles distributed over a 69 km² catchment in northern Sweden. The thickness of the DSL was 36 ± 18 (average ± SD) cm. Thus, only about one‐third of the 1‐m‐deep riparian profile contributed 90% of the lateral DOC flux. The 13 RZ exported 8.7 ± 6.5 g C m^?2 year^?1, covering the whole range of boreal stream DOC exports. The variation could be explained by local hydromorphological characteristics including RZ width (R² = 0.90). The estimated theoretical turnover times were hundreds to a few thousands of years, that is there is a potential long‐lasting supply of DOC. Estimates of net ecosystem production in the RZ suggest that lateral fluxes, including both organic and inorganic C, could be maintained without drawing down the riparian pools. This was supported by measurements of stream DO¹⁴C that indicated modern carbon as the predominant fraction exported, including streams disturbed by ditching. The transfer of DOC into boreal inland waters from new and old carbon sources has a major influence on surface water quality and global carbon balances. This study highlights the importance of local variations in RZ hydromorphology and DSL extent for future DOC fluxes under a changing climate.