DOC Dynamics Research Articles

Influence of the microtopography of patagonian peatbogs on the fluxes of greenhouse gasses and dissolved carbon in porewater

Peatlands play an important role in global carbon cycling, as they act as a natural sink of carbon dioxide (CO2) or as a source of methane (CH4). The influence of microtopography (lawns and hummock-hollow complexes) in the biogeochemical dynamics of dissolved organic and inorganic carbon (DOC and DIC) and dissolved CH4 and CO2 is generally miss-considered. Southernmost Patagonia has huge areas of pristine peatlands, which are still in a largely natural state with scarce anthropogenic influence. In this study we provide foundational insights into the dynamics of greenhouse gasses (GHGs) and DOC in Sphagnum dominated peatlands of Southern Patagonia, assessing the impact of microtopography on these dynamics. The stocks of dissolved GHGs and DOC in hummock-hollows complex and lawns were analysed at three depths (25, 50, and 75 cm) in four ombrotrophic peatbogs. CH4, N2O and CO2 fluxes, net ecosystem exchange (NEE) and gross primary productivity (GPP) were also studied. CO2 and CH4 fluxes were strongly affected by microtopography. Hummock-hollows showed higher CO2 fluxes, and temperature and radiation were the main drivers for respiration and GPP, respectively. In addition, in this microtopography, higher DOC concentrations were observed at the top depth. In contrast, lawns acted as a source of CH4, with higher emission rates and high dissolved GHGs concentration throughout the depth profile.

Optical Composition and Potential Driving Factors of Chromophoric Dissolved Organic Matter in Large Lakes and Reservoirs in the Eastern Region of China

Intensified urbanization has been occurring in the eastern region of China in recent decades, and excessive industrial and household sewage has been discharged into lakes and reservoirs, which has directly lowered water quality and destructed the functions of aquatic ecosystems. Lakes and reservoirs are typically drinking water sources supplying water for metropolitan areas as well as large- and medium-sized cities. Chromophoric dissolved organic matter (CDOM) is the colored fraction of DOM, and its source and optical composition strongly affect water supply safety and the health of surrounding citizens. In April 2021, we collected 68 samples from Reservoir Changtan (n=11), Lake Taihu (n=25), Lake Hongze (n=18), and Lake Gaoyou (n=14), and we further carried out 28 days of laboratory bio-incubation, together with optical measurements and parallel factor analysis (EEMs-PARAFAC) to analyze the bio-degradability ω(BDOC), sources, and optical composition of CDOM in these waters. The results showed that after 28 days of laboratory bio-incubation, the bioavailability of dissolved organic carbon (BDOC) of the four lakes and reservoirs were all higher than 50%. PARAFAC results showed that CDOM collected from the four lakes was composed of four fluorescent components, including a terrestrial humic-like C1, a tryptophan-like C2, and tyrosine-like C3 and C4. Protein-like components (C2, C3, and C4) contributed importantly to the CDOM pool in the four waterbodies, and in Lake Hongze the contribution of C2-C4 was as high as (90.0±2.2)%. In Lake Hongze, total phosphorus (TP) correlated closely with C1-C3, indicating that those components can be used to trace the variability of TP. Among the four waterbodies, a254 was positively correlated with DOC (R2=0.96, P<0.01), indicating that a254 can be used to estimate the dynamics of DOC in these waters. In Lake Taihu, we found a low level of humification index (HIX) and a high level of biological index (BIX), indicating that autochthonous substances contributed importantly to the CDOM pool in this lake. In comparison, allochthonous sources contributed importantly to the CDOM pool in the remaining three waters. The enhanced monitoring of the discharge of industrial and agricultural effluents in the upstream watersheds of the four waters can help to protect the water quality and maintain healthy aquatic ecosystems.

Spatial properties affecting the sensitivity of soil water dissolved organic carbon long-term median concentrations and trends.

It is increasingly clear that increases in dissolved organic carbon in upland waters in recent decades have often been dominated by acid deposition, but reasons for substantial variation in rates of change remain unclear. This paper focuses on the extent to which spatial properties, such as variation in soil properties, atmospheric deposition and climate, affect the sensitivity of DOC concentrations in soil water. The purpose is to i) examine evidence for differences in site average concentrations and trends in soil water DOC between sites with contrasting ecosystem properties, i.e. vegetation cover and soil type, and ii) identify the wider combination of site characteristics that best explain variation in these DOC metrics between sites. We collated soil water and deposition chemistry, soil chemistry and meteorological data from 15 long-term UK monitoring sites (1992-2010) covering a range of soils, vegetation, climate and acid deposition levels. Mineral soils under forests showed the greatest range of long-term mean DOC concentrations and trends. Regression analysis indicated that acid and sea-salt deposition, and soil sensitivity to acidification were the factors most strongly associated with spatial variation in mean DOC concentrations. Spatial variation in DOC trends were best explained by Al saturation and water flux. Overall, the sensitivity of DOC release from soil to changes in pollutant deposition could be related to the type of vegetation cover and soils chemistry properties, such as Al saturation, divalent base cation content and hydrological regime. The identification of the ecosystem properties that appear most influential in modifying DOC production and responses to long-term drivers, helps elucidate potential mechanistic explanations for differences in DOC dynamics across seemingly similar ecosystems, and points to the importance of DOC mobility in regulating its dynamics.

Predicting dissolved organic carbon concentration in a dynamic salt marsh creek via machine learning

AbstractDissolved organic carbon (DOC) is a master variable in aquatic systems. Resolving DOC dynamics requires high‐temporal resolution data. However, DOC concentration cannot be directly measured in situ, and discrete sample collection and analysis becomes expensive as temporal resolution increases. To surmount this problem, an option is to predict site‐specific DOC concentration with linear modeling and optical data predictors collected from high‐cost, high‐maintenance in situ spectrophotometers. This study sought to improve upon the accuracy and field costs of linear predictive DOC methods by using machine learning modeling coupled to low‐to‐zero cost predictors. To do this, we collected 16 months of in situ data (e.g., spectrophotometer attenuation, salinity, temperature), assembled freely available predictors (e.g., point in year, rainfall), and collected samples for DOC analysis, all in a salt marsh creek. At seasonal timescales, machine learning (coefficient of determination [R2] = 0.90) modestly improved upon the accuracy of linear methods (R2 = 0.80) but offered substantial instrumentation cost reductions (~ 90%) by requiring only cost‐free predictors (online data) or cost‐free predictors paired with low‐cost in situ predictors (temperature, salinity, depth). At intertidal timescales, linear methods proved ill‐equipped to predict DOC concentration compared to machine learning, and again, machine learning offered a substantial instrumentation cost reduction (~ 90%). Although our models were developed for and applicable to a single site, the use of machine learning with low‐to‐zero cost predictors provides a blueprint for others trying to model DOC dynamics and other analytes in any complex aquatic system.

Interactive effects of microplastics and glyphosate on the dynamics of soil dissolved organic matter in a Chinese loess soil

The increased use of plastic films and pesticides on agricultural soil leads to the accumulation of plastic debris and pesticide residues in soil. This accumulation has become a serious environmental issue, as it threatens life of earthworms, inhibits the enzyme activities and microbial diversity, and contributes to the loss of soil microbial carbon and nitrogen. However, little information is available regarding the effects of pesticides on soil dissolved organic matter (DOM). It is also unknown how plastic debris, especially small-sized particles called microplastics, influences the effects of pesticides on soil DOM. In this study, we performed a 30-day soil incubation experiment. Three levels of the common herbicide glyphosate were applied to soil: 0 (control, CK), 3.6 kg ha−1 (G1) and 7.2 kg ha−1 (G2). We also tested four levels of glyphosate and microplastics (homopolymer polypropylene powder) co-addition: 3.6 kg ha−1 + 7% (w/w) (M1G1), 3.6 kg ha−1 + 28% (w/w) (M2G1), 7.2 kg ha−1 + 7% (w/w) (M1G2), and 7.2 kg ha−1 + 28% (w/w) (M2G2). Glyphosate addition slightly increased soil fluorescein diacetate hydrolase (FDAse) and phenol oxidase (PO) activities. Although the glyphosate addition significantly promoted the accumulation of dissolved organic phosphorus (DOP) within the first 14 days, the M2 treatment decreased DOP at day 30. M2G1 and M2G2 increased soil FDAse activity and promoted the accumulation of DOC and DOP relative to G1 and G2 respectively while M1G1 and M1G2 benefited DON accumulation. Our results highlighted that the interaction between glyphosate and low microplastics content negatively affected DOC and DOP dynamics, leading to the loss of bioavailable C and P loss. The interaction between glyphosate and high content microplastics negatively affected DON compared with glyphosate addition, possibly decreasing DON.

Heavy metal leaching and plant uptake in mudflat soils amended with sewage sludge.

The leaching and uptake of Cd, Cu, Pb, and Zn by maize (Zea mays L.) in mudflat saline-alkali soils amended by sewage sludge was examined using a greenhouse leaching column experiment. Application of sewage sludge caused decreased pH, increased DOC, and increased Cd, Cu, Pb, and Zn concentrations in leachates. The similar temporal dynamics of DOC and the metal concentrations in leachates suggested complexation of the metals with DOC. There was downward movement of metals as evidenced by the metal enrichment in the bottom layer (20-40-cm depth) of leaching columns (p < 0.05). The enrichment of metals was contributed by the acid-soluble/exchangeable fraction (EX), reducible fraction (RG), oxidizable fraction (OXI), and residual fraction (RES), indicating redistribution of leached metals from the top 20-cm layer. The sewage sludge application also enhanced plant uptake of metals. However, even under the greatest sludge application rate (150gkg-1), very small proportions, averagely 0.65% and 0.35%, of the applied metals were leached and taken up by maize, respectively, over the experimental period. Long-term field-scale research is warranted for further investigation of the effects of sewage sludge amendment on heavy metal fractionation and distribution in mudflat soil-plant-water system.

The response of metal leaching from soils to climate change and land management in a temperate lowland catchment

Changes in soil hydrology as a result of climate change or changes in land management may affect metal release and leaching from soils. The aim of this study is to assess the cascading response of SOM and DOC levels and metal leaching to climate change in the medium-sized lowland Dommel catchment in the southern part of the Netherlands. We implemented the CENTURY model in a spatial setting to simulate SOM, DOC, and water dynamics in topsoils of the Dutch portion of the Dommel catchment under various climate and land management scenarios. These CENTURY model outputs were subsequently used to calculate changes in the topsoil concentrations, solubility, and leaching of cadmium (Cd) and zinc (Zn) for current (1991–2010) and future (2081–2100) conditions using empirical partition-relations. Since the metal leaching model could not be evaluated quantitatively against measured values, we focus mainly on the trends in the projected metal concentrations and leaching rates for the different scenarios. Our results show that under all climate and land management scenarios, the SOM contents in the topsoil of the Dommel catchment are projected to increase by about 10% and the DOC concentrations to decrease by about 20% in the period from present to 2100. These changes in SOM and DOC only have a minor influence on metal concentrations and leaching rates under the climate change scenarios. Our scenario calculations show a considerable decrease in topsoil Cd concentrations in the next century as a result of increased percolation rates. Zinc, however, shows an increase due to agricultural inputs to soil via manure application. These trends are primarily controlled by the balance between atmospheric and agricultural inputs and output via leaching. While SOM and DOC are important controls on the spatial variation in metal mobility and leaching rates, climate-induced changes in SOM and DOC only have a minor influence on metal concentrations and leaching rates. The climate-induced changes in metal concentrations in both the topsoil and the soil leachate are primarily driven by changes in precipitation and associated water percolation rates.

A high-resolution carbon balance in a small temperate catchment: Insights from the Schwabach River, Germany

The present study is one of only few to address the stable carbon isotope (δ13C) and concentration dynamics of DIC, DOC, and POC over an entire hydrologic year, using a dataset of high sampling resolution (4–11 samples retrieved per month). This research was performed in the catchment of the Schwabach River, a typical mid-latitude small headwater stream in Germany emplaced mainly in karstic bedrock. The DIC data indicated the dominance of mineral weathering as a DIC source, with a noticeable dilution effect during periods of high river flow. A weakly negative relationship between discharge and δ13CDIC hints at a greater importance of plant-derived organic matter during flooding events, likely transported to river waters via overland runoff and intermediate flow. DOC inputs appeared to be enhanced during periods of high discharge, indicating a greater importance of overland runoff as a DOC source. POC concentrations seem unaffected by changes in discharge, although a slight negative correlation between δ13CPOC and discharge may be derived from increased inputs of C4 plant material.Estimated CO2 concentrations were in excess of ambient atmospheric values throughout the year, confirming that the surface waters of the Schwabach River are a net CO2 source. The total riverine carbon flux was dominated by DIC (70%), followed by CO2 outgassing (21%), DOC (7%), and POC (2%). While the selection of a bi-monthly sampling scheme yielded a broadly similar carbon flux estimate to that utilizing the entire dataset, the use of a monthly sampling interval differed by as much as 19% from values using the high-resolution data set. This discrepancy is due to the inability of a monthly sampling scheme to capture sudden and large variations in river discharge and associated changes in dissolved/particulate carbon concentrations, such as those observed during flooding. We suggest that bi-monthly sampling is the minimum timeframe required to achieve an acceptable degree of accuracy in the calculation of carbon fluxes, at least during periods of high runoff. The application of high sampling frequencies and comprehensive DIC, DOC, and POC studies in future research would greatly reduce uncertainties in local riverine carbon budgets, and help clarify the role of smaller streams in the global carbon cycle.

Snowpack enhanced dissolved organic carbon export during a variety of hydrologic of events in an agricultural landscape, Midwestern USA

This study investigates the dynamics of riverine DOC concentrations during a series of stream discharge events (3–10days) following rainstorms of different intensity and duration. We examined six events, when high-frequency (hourly) water samples for DOC (n=321) were collected in spring (n=166) and autumn (n=155). Results identified three distinct water-mediated processes during stream discharges events that linked DOC source supply from agricultural land surfaces with sinks in a receiving river. These were as follows: 1) snowpack drives significant high DOC concentrations in base-flow during spring, 2) abundant organic matter in topsoil from crop residues determines a rapid DOC loading profile in the first flush, and 3) very large hydro-climatic events in snow-melting season over agricultural watersheds could increase the riverine DOC flux by 2.3 folds. These results revealed that ca. 76.5% of annual DOC was exported during a handful of storm-discharge events (78.9% for spring and 74.2% from autumn) over agricultural landscapes. Given the significant amount of riverine DOC exported from agricultural landscapes during severe weather events, our results suggest that changes in climate promote larger precipitation events that would likely enhance the export of terrestrial DOC to receiving water bodies. The study presents a semi-analytical model that is able to extrapolate the riverine DOC dynamics during storm discharge events of varying duration and intensity (R2 up to 0.9).

Scaling effects of riparian peatlands on stable isotopes in runoff and DOC mobilisation

We combined 13months of daily isotope measurements in stream water with daily DOC and 15 min FDOM (fluorescent component of dissolved organic matter) data at three nested scales to identify how riparian peatlands generate runoff and influence DOC dynamics in streams. We investigated how runoff generation processes in a small, riparian peatland-dominated headwater catchment (0.65km2) propagate to larger scales (3.2km2 and 31km2) with decreasing percentage of riparian peatland coverage. Isotope damping was most pronounced in the 0.65km2 headwater catchment due to high water storage in the organic soils encouraging tracer mixing. At the largest scale, stream flow and water isotope dynamics showed a more flashy response. The isotopic difference between the sites was most pronounced in the summer months when stream water signatures were enriched. During the winter months, the inter-site difference reduced. The isotopes also revealed evaporative fractionation in the peatland dominated catchment, in particular during summer low flows, which implied high hydrological connectivity in the form of constant seepage from the peatlands sustaining high baseflows at the headwater scale. This connectivity resulted in high DOC concentrations at the peatland site during baseflow (∼5mgl−1). In contrast, at the larger scales, DOC was minimal during low flows (∼2mgl−1) due to increased groundwater influence and the disconnection between DOC sources and the stream. High frequency data also revealed diel variability during low flows. Insights into event dynamics through the analysis of hysteresis loops showed slight dilution on the rising limb, the strong influence of dry antecedent conditions and a quick recovery between events at the riparian peatland site. Again, these dynamics are driven by the tight coupling and high connectivity of the landscape to the stream. At larger scales, the disconnection between the landscape units increases and the variable connectivity controls runoff generation and DOC dynamics. The results presented here suggest that the processes occurring in riparian peatlands in headwater catchments are less evident at larger scales which may have implications for the larger scale impact of peatland restoration projects.

Seasonal and storm flow dynamics of dissolved organic carbon in a Mediterranean mountain catchment (Vallcebre, eastern Pyrenees)

ABSTRACTTo improve understanding of DOC dynamics in seasonal Mediterranean environments, rainfall, soil water, groundwater and stream water samples were taken during a 27-month period in the Can Vila catchment (northeast Spain). Using these data, we characterized DOC dynamics in the different hydrological compartments and analysed the factors affecting them. We also analysed DOC dynamics during storm events and the factors that control DOC delivery to the stream. The results show some seasonality in rainwater and soil water DOC concentrations, while no clear seasonality was observed in stream water and groundwater, where DOC dynamics were strongly related to discharge and water table variations. For storm events with several discharge peaks, the slope of the discharge–DOC concentration relationship was higher for the first peak. The rather similar dynamics of stream water DOC concentration in all floods contrast with the observed diversity of hydrological processes. This raises the question of the origin of the observed rapid DOC increase.EDITOR M.C. AcremanASSOCIATE EDITOR K. Heal

Remineralization of terrestrial dissolved organic carbon in the Baltic Sea

To assess the remineralization dynamics of terrestrial dissolved organic carbon (tDOC) in Baltic Sea water a set of incubation experiments was performed. Baltic Sea water was spiked with low molecular weight (LMW) and high molecular weight (HMW) tDOC extracted from water of the rivers Vistula and Oder. In parallel, incubation experiments were performed with seawater in order to determine the remineralization dynamics of Baltic Sea DOC (mDOC) that is characteristic for the marine environment but not necessarily of marine origin. At 10 consecutive time steps in the course of an incubation period of 188days, the concentrations of O2 and total CO2 (CT) were measured to assess the O2 consumption and CO2 production. The latter was used as a measure for the DOC mineralization. Three DOC fractions having different reactivity were identified. The refractory fraction was predominant in all samples and amounted to 66% in mDOC and about 80% in the tDOC. Two fractions of bioavailable DOC, which differed significantly concerning their reactivity, were identified in both the mDOC and tDOC. The more reactive labile fraction contributed to about 3% to LMW and HMW tDOC whereas 10% of the mDOC consisted of labile DOC. The half-life time for labile DOC ranged between 3.2d and 6.4d. The less reactive bioavailable fraction, the semi-labile DOC, showed half-life times of weeks to months. The total bioavailable fraction of the mDOC (34%) was higher than that of tDOC by a factor of about 2. Furthermore, it was characterized by a bulk half-life time of 10.6d that was significantly lower than those for tDOC from the two rivers (32.5d and 21.7d).The measurements of the oxygen consumption and total CO2 production during the remineralization yielded a ΔO2:ΔCT molar ratio close to 1.0 for the terrestrial dissolved organic matter (DOM) whereas an increased ratio of 1.2–1.5 was observed during the remineralization of the Baltic Sea DOM. This indicates that tDOM consists of carbohydrate-like compounds. The higher O2 demand for the mineralization of mDOM can be explained by a higher proportion of lipid-like compounds and by nitrification.

Sensitivity of stream dissolved organic carbon to temperature and discharge: Implications of future climates

AbstractDissolved organic carbon (DOC) is a significant constituent in aquatic ecosystems with concentrations in streams influenced by both temperature and water flow pathway dynamics associated with changes in discharge (streamflow). We investigated the sensitivity of DOC concentrations in 12 high‐latitude headwater streams to changes in temperature and discharge using a mathematical model. The implications of differences in sensitivities were explored by using downscaled projections of air temperature and discharge to simulate possible trajectories of DOC concentrations in a changing climate. We found two distinct responses: (i) catchments where stream DOC sensitivity was high to temperature but low to discharge and (ii) catchments where stream DOC sensitivity was low to temperature but high to discharge. Streams with strong seasonal DOC dynamics were more sensitive to temperature changes than nonseasonal systems. In addition, stream DOC sensitivity to discharge was strongly correlated with vertical soil water DOC differences in the near‐stream zone. Simulations of possible future changes in DOC concentrations indicated median increases of about 4–24% compared to current levels when using projections of air temperature and discharge but even larger increases were observed for base flow concentrations (13–42%). Streams with high‐temperature sensitivity showed the largest increases in DOC concentrations. Our results suggest that future climatic changes could bring significant increases in surface water DOC concentrations in boreal and hemiboreal areas but that the response ultimately is dependent on vertical soil solution DOC differences and soil organic carbon distribution.

Application of D-SEM to a catchment in Central Amazonia: calibration and validation of the carbon and nitrogen cycles

We applied the D-SEM model for predicting carbon (C) and nitrogen (N) dynamics in a natural forest ecosystem in a Central Amazonian microcatchment. A continuous collection of hydrological and water quality data (2002–2010) was used for model calibration and validation. Comparisons between predicted and measured hydrologic variables showed that the model captured seasonal and interannual discharges and DOC concentrations in stream water. The model also reproduced the monthly dynamics of DOC in the soil pool and captured seasonal changes in nitrogen transformations following seasonal variations in rainfall, including increased mineralization, nitrification, lixiviation, and decreased denitrification. Model estimates of monthly DON and NH4 in stream water concentration were comparable to measured values (0.37±0.22; 0.065±0.06, respectively). Overall, this study demonstrated that the D-SEM model can predict water, C and N dynamics in drained forests under natural conditions in the Amazon. However, discrepancies between the magnitudes of the estimates suggest that the model requires refinement for further application in tropical forested catchments.

Experimental nitrogen deposition alters the quantity and quality of soil dissolved organic carbon in an alpine meadow on the Qinghai-Tibetan Plateau

Dissolved organic matter (DOM) plays a central role in driving biogeochemical processes in soils, but little information is available on the relation of soil DOM dynamics to microbial activity. The effects of NO3− and NH4+ deposition in grasslands on the amount and composition of soil DOM also remain largely unclear. In this study, a multi-form, low-dose N addition experiment was conducted in an alpine meadow on the Qinghai–Tibetan Plateau in 2007. Three N fertilizers, NH4Cl, (NH4)2SO4 and KNO3, were applied at four rates: 0, 10, 20 and 40kgNha−1yr−1. Soil samples from surface (0–10cm) and subsurface layers (10–20cm) were collected in 2011. Excitation/emission matrix fluorescence spectroscopy (EEM) was used to assess the composition and stability of soil DOM. Community-level physiological profile (CLPP, basing on the BIOLOG Ecoplate technique) was measured to evaluate the relationship between soil DOC dynamics and microbial utilization of C resources. Nitrogen (N) dose rather than N form significantly increased soil DOC contents in surface layer by 23.5%–35.1%, whereas it significantly decreased soil DOC contents in subsurface layer by 10.4%–23.8%. Continuous five-year N addition significantly increased the labile components and decreased recalcitrant components of soil DOM in surface layer, while an opposite pattern was observed in subsurface layer; however, the humification indices (HIX) of soil DOM was unaltered by various N treatments. Furthermore, N addition changed the amount and biodegradability of soil DOM through stimulating microbial metabolic activity and preferentially utilizing organic acids. These results suggest that microbial metabolic processes dominate the dynamics of soil DOC, and increasing atmospheric N deposition could be adverse to the accumulation of soil organic carbon pool in the alpine meadow on the Qinghai-Tibetan Plateau.

土壤溶解性有机物对CO2和N2O排放的影响

农田土壤是温室气体的重要排放源，溶解性有机物作为土壤微生物容易利用的基质，其含量变化与温室气体的产生和排放密切相关。基于室内培养试验，对溶解性有机物影响土壤CO₂、N₂O的排放过程进行了分析。设置空白（CK）、单施秸秆（S）、单施氮肥（N）、秸秆和氮肥（S+N）4个不同的处理，对添加不同物质条件下土壤溶解性有机碳（DOC）、溶解性有机氮（DON）和CO₂、N₂O的排放动态进行了研究，对DOC和DON影响CO₂、N₂O的排放过程进行了探讨。结果表明：不同处理的温室气体排放通量和土壤DOC、DON含量差异显著；各处理的CO₂排放通量和DOC动态随培养时间的延长呈现逐渐减小的趋势，S和S+N处理的N₂O排放和DON动态呈现先增大后减小的趋势； S+N处理的CO₂排放量最高，DON含量也显著高于其他处理，单施秸秆（S）处理的N₂O排放量和DOC含量显著高于其它处理，单施氮肥（N）对土壤CO₂的排放量和DOC含量的影响较小；土壤CO₂和N₂O的排放通量与土壤DOC和DON含量呈显著的相关性，相关系数（<em>R</em>²）达0.6以上，说明溶解性有机物的含量和动态对CO₂、N₂O的排放过程产生显著影响。;Agricultural soil is the main emission source of greenhouse gas. Dissolved organic matter (DOM) in agricultural soil is the labile component of organic matter and the most available substrates for microbial. Its concentration and dynamic change is closely related to the production and emission of greenhouse gas. The aim of this study was to estimate the effect of soil DOM concentration on the emission of CO₂ and N₂O after materials addition. And the relationship between the dynamics of DOM concentration and the emission of CO₂ and N₂O was evaluated after that. An incubation experiment was adopted in this study with the soil sampled from the tillage layer in the field at Yucheng comprehensive experimental station, Chinese Academy of Sciences. Four treatments, which were soil with or without material addition, i.e., straw only (S), nitrogen only (N), straw with nitrogen (S+N), and control (CK), respectively, were set. The amount of these added materials in this study was estimated based on the conventional application of N fertilizer and straw return in the North China Plain. The emissions of CO₂ and N₂O, and the concentrations of dissolved organic carbon and nitrogen (DOC/DON) were measured at different time interval during the 342 days incubation. The results showed that there was significant difference between these four treatments (<em>P</em> < 0.05) for the concentration of DOM and the emissions of CO₂ and N₂O. It presented a gradually decreasing trend in the emission of CO₂ and the concentration of DOC in soil for all these four treatments. However, the emission of N₂O and the concentration of DON increased at first and then decreased with time for these two treatments of S and S+N. In addition, the treatment of S+N showed the highest average concentration of DOC and DON. The lowest mean concentration of DOC and DON was found in the treatment of N and CK, respectively. In the treatment of S+N added with N fertilizer and corn straw, the cumulative CO₂ emission was 1668.11kg/hm² during the incubation period, which was much higher than the other three treatments. In the treatment of N added with N fertilizer only, the cumulative CO₂ emission was 196.01kg/hm², which was much lower than the other three treatments. The highest cumulative N₂O emission, 495.75g hm^-2, was found in the treatment of S with the addition of straw only, and the lowest cumulative N₂O emission, 136.93 g hm^-2, was found in the treatment of CK. A logarithmic equation was fitted between CO₂ emission and the time for each treatment (<em>R</em>² > 0.57, <em>P</em> < 0.05). There was a significant correlation between the emission of CO₂ and the concentration of DOC(<em>R</em>² > 0.65). It also happened between the emission of N₂O and the concentration of DON in soil (<em>R</em>² > 0.62), indicated that the concentration and dynamics of DOC and DON in soil caused by the addition of straw and N fertilizer had a significant influence on the emission of CO₂ and N₂O.

Microbial phototrophic fixation of atmospheric CO2 in China subtropical upland and paddy soils

Autotrophic microorganisms, which can fix atmospheric CO2 to synthesize organic carbon, are numerous and widespread in soils. However, the extent and the mechanism of CO2 fixation in soils remain poorly understood. We incubated five upland and five paddy soils from subtropical China in an enclosed, continuously 14CO2-labeled, atmosphere and measured 14CO2 incorporated into soil organic matter (SOC14) and microbial biomass (MBC14) after 110days. The five upland soils supported dominant crops soils (maize, wheat, sweet potato, and rapeseed) in the region, while all paddy soils were cultivated in a regime consisting of permanently-flooded double-cropping rice cultivation. The upland and paddy soils represented typical soil types (fluvisols and ultisols) and three landforms (upland, hill, and low mountain), ranging in total carbon from low (<10gkg−1 soil organic carbon) to medium (10–20gkg−1) to high (>20gkg−1). Substantial amounts of 14CO2 were fixed into SOC14 (mean 20.1±7.1mgCkg−1 in upland soil, 121.1±6.4mgCkg−1 in paddy soil) in illuminated soils (12h light/12h dark), whereas no 14C was fixed in soils incubated in continuous darkness. We concluded that the microbial CO2 fixation was almost entirely phototrophic rather than chemotrophic. The rate of SOC14 synthesis was significantly higher in paddy soils than in upland soils. The SOC14 comprised means of 0.15±0.01% (upland) and 0.65±0.03% (paddy) of SOC. The extent of 14C immobilized as MBC14 and that present as dissolved organic C (DOC14) differed between soil types, accounting for 15.69–38.76% and 5.54–18.37% in upland soils and 15.57–40.03% and 3.67–7.17% of SOC14 in paddy soils, respectively. The MBC14/MBC and DOC14/DOC were 1.76–5.70% and 1.69–5.17% in the upland soils and 4.23–28.73% and 5.65–14.30% in the paddy soils, respectively. Thus, the newly-incorporated C stimulated the dynamics of DOC and MBC more than the dynamics of SOC. The SOC14 and MBC14 concentrations were highly significantly correlated (r=0.946; P<0.0001). We conclude that CO2 uptake by phototrophic soil microorganisms can contribute significantly to carbon assimilation in soil, and so warrants further future study.

Adjacent catchments with similar patterns of land use and climate have markedly different dissolved organic carbon concentration and runoff dynamics

Temporal patterns in specific runoff, dissolved organic carbon concentrations [DOC] and fluxes were examined during two periods: 1994–1997 (period 1) and 2007–2009 (period 2) in five adjacent tributary catchments of Lake Simcoe, the largest lake in southern Ontario, Canada. The catchments displayed similar patterns of land use change with increases in urbanization (5–16%) and forest cover (0.2–4%) and declines in agriculture (4–8%) between 1994 and 2008. Climate in the catchments was similar; temperature increased slightly, but no significant change in precipitation was observed. Despite similar pattern of climate and land use, runoff responses and tributary [DOC] were different across the catchments. Following a very dry year (i.e. 1999), runoff increased steadily until the end of record. We observed increased variability in tributary [DOC] and higher DOC exports in period 2. This led to ~10% increase in [DOC] and a 13% increase in flux between the two study periods. Between the two periods, [DOC] increased by 15% in spring and 25% in summer, whereas flux increased by 17% in spring and 48% in summer. [DOC] was consistently higher in the growing (summer + autumn) than the dormant (winter + spring, minus spring melt months) seasons, but no unique pattern or simple linear flow/concentrations relationships existed. This suggests complex spatial and temporal pattern to runoff controls on DOC and flow dynamics in adjacent catchments. We therefore caution against extrapolating from monitored to unmonitored catchments. Copyright © 2012 John Wiley & Sons, Ltd.

Nitrate limitation and accumulation of dissolved organic carbon during a spring-summer cyanobacterial bloom in Lake Taihu (China)

Lake Taihu, high-molecular-weight dissolved organic matter (HMW-DOM), with sizes between 1 kDa and 0.2 μm, were collected using cross-flow ultrafiltration, from three different eutrophic regions. The DOC, and HMW-DOC concentrations, as well as environmental factors, including water temperature, nitrate, phosphate, and Chlorophyll-a (Chl-a), were analyzed. A significant negative relationship was observed between Chl-a and nitrate concentration, suggesting that cyanobacterial bloom was limited by nitrate. The high phosphate concentration can probably be attributed to phosphorus released from the sediment or can be a result of the accumulation of bloom. Furthermore, DOC concentration significantly increased with water temperature, and was negatively related to nitrate concentration, indicating that these two environmental factors are well correlated to DOC dynamics. DOC concentration did not significantly corresponded with the Chl-a concentration in Lake Taihu. Moreover, the relationship between HMW-DOC and Chl-a concentration was only observed in Gonghu Bay, suggesting that DOC and HMW-DOC are not solely derived from cyanobacterial bloom. Other organic carbon origins, such as terrestrial input, were also assumed to play an important role in Lake Taihu.

Modelling the long term impact of climate change on the carbon budget of Lake Simcoe, Ontario using INCA-C

This study presents a process-based model of dissolved organic carbon concentration ([DOC]) in catchments draining into Lake Simcoe, Ontario. INCA-C, the Integrated Catchment model for Carbon, incorporates carbon biogeochemical processes in a terrestrial system with hydrologic flow paths to simulate watershed wide [DOC]. The model successfully simulates present-day inter-annual and seasonal [DOC] dynamics in tributaries draining catchments with mixed or contrasting land cover in the Lake Simcoe watershed (LSW). The sensitivity of INCA-C to soil moisture, hydrologic controls and land uses within a watershed demonstrates its significance as a tool to explore pertinent environmental issues specific to the LSW. Projections of climate change under A1B and A2 SRES scenarios suggest a continuous monotonic increase in [DOC] in surface waters draining into Lake Simcoe. Large variations in seasonal DOC dynamics are predicted to occur during summer with a possibility of displacement of summer [DOC] maxima towards winter and a prolongation of summer [DOC] levels into the autumn. INCA-C also predicts possible increases in dissolved inorganic carbon in some tributaries with rising temperature suggesting increased CO2 emissions from rivers as climate changes.