CO2 Supersaturation and Net Heterotrophy in a Tropical Estuary (Cochin, India): Influence of Anthropogenic Effect

Carbon biogeochemistry of a tropical ecosystem (The Cochin Estuary, India) undergoing increased human intervention was studied during February (premonsoon), April (early monsoon) and September (monsoon) 2005. The Cochin estuary sustains high levels of pCO₂ (up to 6000 μatm) and CO₂ effluxes (up to 274 mmolC m^?2 d^?1) especially during monsoon. A first-order estimate of the carbon mass balance shows that net production of dissolved inorganic carbon is an order of magnitude higher than the net loss of dissolved and particulate organic carbon from the estuary. This imbalance is attributed to the organic inputs to the estuary through anthropogenic supplies. The bacteria-mediated mineralization of organic matter is mainly responsible for the build-up of pCO₂ and increased CO₂ emission to the atmosphere indicating heterotrophy. The linear correlation between excess CO₂ and apparent oxygen utilization indicates respiration as the chief mechanism for CO₂ supersaturation. An increase in the net negative ecosystem production (–ve NEP) between premonsoon (?136 mmolC m^?2 d^?1 or ?376 MgC d^?1) and monsoon (?541 mmolC m^?2 d^?1 or ?1500 MgC d^?1) is supported by a corresponding increase in O₂ influxes from 17 mmol O₂ m^?2 d^?1 (126 MgC d^?1) to ?128 mmol O₂ m^?2 d^?1 (?946 MgC d^?1) and CO₂ emissions from 65 mmolC m^?2 d^?1 (180 MgC d^?1) to 267 mmolC m^?2 d^?1 (740 MgC d^?1). There is a significant north-south gradient in metabolic rates and CO₂ fluxes attributable to the varying flow patterns and anthropogenic inputs into the estuary. The study reveals that the Cochin estuary, a previously autotrophic (CO₂ sink) system, has been transformed to a heterotrophic (CO₂ source) system following rapid urbanization and industrialization. Moreover, the export fluxes from the Cochin estuary appear to be quite important in sustaining net heterotrophy in the southeastern Arabian Sea.     

Benthic decomposition of Zostera marina roots: a controlled laboratory experiment

The initial benthic decomposition of Zostera marina roots was studied in a controlled flow-through chamber experiment for 23 days. Sediment chambers without added roots served as controls. The inflowing and outflowing artificial seawater (ASW) was analyzed for O₂, ΣCO₂, urea-N, NH₄⁺ and NO₂⁻+NO₃⁻. Sediment profiles of Eh, particulate organic carbon (POC) and nitrogen, dissolved organic nitrogen (DON), dissolved free amino acids (DFAA), urea-N, NH₄⁺, DFAA and urea turnover rates, sulfate reduction and counts of total anaerobic heterotrophic bacteria and different functional groups were determined. Fluxes of O₂, ΣCO₂, urea-N and NH₄⁺ were stimulated during root decomposition compared to the unamended control. There were indications of stimulated bacterial growth based on counts of total anaerobic heterotrophic bacteria, anaerobic phosphatase utilizers, ammonifyers and sulfate reducers. Independent estimates of nitrogen and carbon incorporation into bacterial biomass during root decomposition indicate that a major fraction of the nitrogen for microbial growth was mobilized from the indigenous particulate organic nitrogen (PON) pool, whereas the energy source for bacterial growth was mainly obtained from the added eelgrass roots. Most of the nitrogen mineralized during root decomposition was incorporated into the bacterial biomass resulting in a low efflux of urea-N and inorganic nitrogen from the sediment to the water column.     

Carbon mass balance methodology to characterize the growth of pigmented marine bacteria under conditions of light cycling

A carbon mass balance methodology employing minimal measurements was applied to heterotrophic and photoheterotrophic marine bacteria grown under constant dilution and exposed to 12-h intervals of light or darkness. Carbon mass balance calculations using measurements taken every 3 h closed to within 93–103% using dissolved organic carbon, biomass carbon and CO₂ production data only, indicating that background interference from dissolved inorganic carbon variations in the amended seawater medium was not significant. Neither strain was observed to sustain a net CO₂ fixation using paramagnetic measurement of oxygen uptake rates (OUR), indicating a need for more sensitive on-line measurement techniques for OUR. Photoheterotrophic growth demonstrated lower carbon-mole biomass yields (0.41±0.026 vs. 0.64±0.013 mol mol^–1) despite higher specific glucose uptake rates (0.025 vs. 0.02 mol mol^–1 h^–1), suggesting that bioreactor-based study of marine bacteria can present growth modes that are different from those encountered in the marine environment.     

Influence of carbon availability on the production of NO, N2O, N2 and CO2 by soil cores during anaerobic incubation

Net productions of permanent soil atmosphere gases (N₂, CO₂, O₂) and temporary gases (N₂O, NO) were monitored in soil cores using a non-interfering, fully automated measuring technique allowing highly time resolved measurements over prolonged periods. The influence of changes in available organic carbon on CO₂, N₂O, NO and N₂ production was studied by changing the soil carbon content through aerobic preincubations of different length, up to 21 days.The aerobic preincubation caused an increase in NO₃ ^- concentration and a decrease in available carbon content. Available carbon content dominated both CO₂ and total N gas (N₂+N₂O+NO) production during anaerobiosis. Both CO₂ and total N gas production rates decreased with increasing length of the previous aerobic preincubation, this in spite of the higher initial NO₃ ^- concentration.Total denitrification rates were closely related to the anaerobic CO₂ production rates. No relation was found between water soluble carbon content and total denitrification. The N₂O/N₂ ratio could be explained by an interaction of carbon availability, NO₃ ^- concentration and enzyme status. Net N₂O consumption was monitored. The balance between cumulative total N gas production and NO₃ ^- consumption varied according to the different treatments. Cumulative N₂O production exceeded cumulative N₂ production for 0 up to 5 days.     

Potential rates of nitrification and denitrification in an oligotrophic freshwater sediment system

Potential rates of nitrification and denitrification were measured in an oligotrophic sediment system. Nitrification potential was estimated using the CO oxidation technique, and potential denitrification was measured by the acetylene blockage technique. The sediments demonstrated both nitrifying and denitrifying activity. E_h, O₂, and organic C profiles showed two distinct types of sediment. One type was low in organic C, had high O₂ and E_h, and had rates of denitrification 1,000 times lower than the other which had high organic C, low O₂, and low E_h. Potential nitrification and denitrification rates were negatively correlated with E_h. This suggests that environmental heterogeneity in denitrifier and nitrifier populations in oligotrophic sediment systems may be assessed using E_h before sampling protocols for nitrification or denitrification rates are established. There was no correlation between denitrification and nitrification rates or between either of these processes and NH₄ ⁺ or NO₃ ^– concentrations. The maximum rate of denitrification was 0.969 nmole N cm^–3 hour^–1, and the maximum rate of nitrification was 23.6 nmole cm^–3 hour^–1, suggesting nitrification does not limit denitrification in these oligotrophic sediments. Some sediment cores had mean concentrations of 6.0 mg O₂/liter and still showed both nitrification and denitrification activity.     

Carbon turnover in peatland mesocosms exposed to different water table levels

Changes of water table position influence carbon cycling in peatlands, but effects on the sources and sinks of carbon are difficult to isolate and quantify in field investigations due to seasonal dynamics and covariance of variables. We thus investigated carbon fluxes and dissolved carbon production in peatland mesocosms from two acidic and oligotrophic peatlands under steady state conditions at two different water table positions. Exchange rates and CO₂, CH₄ and DOC production rates were simultaneously determined in the peat from diffusive-advective mass-balances of dissolved CO₂, CH₄ and DOC in the pore water. Incubation experiments were used to quantify potential CO₂, CH₄, and DOC production rates. The carbon turnover in the saturated peat was dominated by the production of DOC (10–15 mmol m^–2 d^–1) with lower rates of DIC (6.1–8.5 mmol m^–2 d^–1) and CH₄ (2.2–4.2 mmol m^–2 d^–1) production. All production rates strongly decreased with depth indicating the importance of fresh plant tissue for dissolved C release. A lower water table decreased area based rates of photosynthesis (24–42%), CH₄ production (factor 2.5–3.5) and emission, increased rates of soil respiration and microbial biomass C, and did not change DOC release. Due to the changes in process rates the C net balance of the mesocosms shifted by 36 mmol m^–2 d^–1. According to our estimates the change in C mineralization contributed most to this change. Anaerobic rates of CO₂ production rates deeper in the peat increased significantly by a factor of 2–3.5 (DOC), 2.9–3.9 (CO₂), and 3–14 (CH₄) when the water table was lowered by 30 cm. This phenomenon might have been caused by easing an inhibiting effect by the accumulation of CO₂ and CH₄ when the water table was at the moss surface.     

Soil moisture surpasses elevated CO2 and temperature as a control on soil carbon dynamics in a multi-factor climate change experiment

Some single-factor experiments suggest that elevated CO₂ concentrations can increase soil carbon, but few experiments have examined the effects of interacting environmental factors on soil carbon dynamics. We undertook studies of soil carbon and nitrogen in a multi-factor (CO₂ × temperature × soil moisture) climate change experiment on a constructed old-field ecosystem. After four growing seasons, elevated CO₂ had no measurable effect on carbon and nitrogen concentrations in whole soil, particulate organic matter (POM), and mineral-associated organic matter (MOM). Analysis of stable carbon isotopes, under elevated CO₂, indicated between 14 and 19% new soil carbon under two different watering treatments with as much as 48% new carbon in POM. Despite significant belowground inputs of new organic matter, soil carbon concentrations and stocks in POM declined over four years under soil moisture conditions that corresponded to prevailing precipitation inputs (1,300 mm yr^?1). Changes over time in soil carbon and nitrogen under a drought treatment (approximately 20% lower soil water content) were not statistically significant. Reduced soil moisture lowered soil CO₂ efflux and slowed soil carbon cycling in the POM pool. In this experiment, soil moisture (produced by different watering treatments) was more important than elevated CO₂ and temperature as a control on soil carbon dynamics.     

The flux of carbon from rivers: the case for flux from England and Wales

This study uses the extensive monitoring datasets of the Environment Agency of England and Wales to calculate the flux of dissolved organic carbon (DOC); particulate organic carbon (POC); and excess dissolved CO₂ through English and Welsh rivers. The innovation of this study’s approach is to account for the losses of carbon within the fluvial system as well as fluxes at the catchment outlet. In order to make this assessment this study considers: the biochemical oxygen demand (BOD) as a measure of the degradation of DOC; and the dissolved CO₂ concentration of groundwater as calculated and apportioned into surface waters on the basis of Ca concentrations. The study shows that the best estimate of carbon export, via rivers, from England and Wales is 10.34 Mg C/km²/year, with 4.19 Mg C/km²/year of this going to the atmosphere. The mapping of the carbon export shows that there are regional hotspots of carbon export and in a small number of cases rivers could be net sinks of carbon due to their low dissolved CO₂ content relative to the atmosphere. The flux calculated by this approach is probably still an underestimate of the carbon flux through fluvial systems but the scale of the export is greater than that previously reported and there is evidence that the fluvial flux of carbon is increasing on a decadal scale.     

Abiotic synthesis of organic molecules from minerals containing traces of dissolved H2O,CO2 and N2 part I: Basic principles

Oxides and silicates which crystallize in the presence of H₂O, CO₂ and N₂, especially at high pressures, are expected to dissolve traces of these fluid components. In doing so atomic and electronic rearrangements occur between the host lattice and the dissolved molecules which lead to different solute species. These may be simply hydroxyl OH^–, carbonate CO₃ ^2– of similar ions, but other species can also form which are chemically reduced such as molecular H₂, CO₂ ^2– and CO^– or oxidized such as peroxy ions, O₂ ^2–. When the minerals containing such solute species are decompressed and cooled, they eventually become supersaturated with respect to their dissolved impurities. If the relative mobilities of the different solute species are different, diffusion and subsurface segregation lead to spatial separation: the surface and subsurface zone become enriched in the reduced species while the oxidized species remain in the bulk. Upon heating or during chemical dissolution (weathering) the reduced species contained in the surface react leading to the formation of a wide variety of H–C–O–N molecules: not only H₂O, CO₂ and possibly N₂ but also organic molecules. No external reduced atmosphere is needed, according to this hypothesis.     

Microbial carbon oxidation rates and pathways in sediments of two Tanzanian mangrove forests

Temporal and spatial variations in benthic metabolism and anaerobic carbon oxidation pathways were assessed in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. The objectives were (1) to evaluate how benthic metabolism is affected by organic carbon availability; (2) to determine the validity of diffusive release of CO₂ as a measure benthic carbon oxidation; and (3) to assess the partitioning of anaerobic carbon pathways and factors controlling the availability of electron acceptors (e.g. oxidized iron). Microbial carbon oxidation measured as diffusive exchange of O₂ and CO₂ (32?C67 and 28?C115 mmol m^?2 day^?1, respectively) showed no specific temporal patterns. Low intertidal sediments at Mtoni fed by labile algal carbon of anthropogenic origin had higher diffusive CO₂ release than high intertidal sediments that primarily received less reactive mangrove detritus. Diffusive release of CO₂ apparently underestimated total sediment carbon oxidation due to CO₂ loss from deep sediments via emission through biogenic structures (i.e. crab burrows and pneumatophores) and porewater seepage into creeks. We propose that diffusive fluxes in the present mangrove sediments are roughly equivalent to depth-integrated reactions occurring in the upper 12 cm. Anaerobic carbon oxidation was dominated by FeR irrespective of anthropogenic influence in sediments where the oxidizing effects of biogenic structures increased the Fe(III) level. More than 80% of the anaerobic carbon oxidation in Mtoni and Ras Dege sediments was due to FeR when reactive Fe(III) exceeded 30 ??mol cm^?3. The anthropogenic influence at Mtoni was primarily noted as up to one order of magnitude higher denitrification than at Ras Dege, but this process always accounted for less than 1% of total carbon oxidation. It is noteworthy that organic and nutrient enrichment of anthropogenic origin in Mtoni has no measurable effect on microbial processes, other than carbon oxidation in the low intertidal area and denitrification throughout the forest, and indicates a strong resilience of mangrove environments towards disturbances.     

Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

The Catskill Mountains of southeastern New York State have among thehighest rates of atmospheric nitrogen deposition in the United States. Somestreams draining Catskill catchments have shown dramatic increases in nitrateconcentrations while others have maintained low nitrate concentrations. Streamsin which exchange occurs between surface and subsurface (i.e. hyporheic) watersare thought to be conducive to nitrate removal via microbial assimilationand/ordenitrification. Hyporheic exchange was documented in the Neversink River inthesouthern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate(NO₃ ^–) losses along hyporheic flowpaths werenegligible. In this study, Neversink River water was amended with natural,bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series ofexperimental mesocosms that simulated hyporheic flowpaths. DOC and N dynamicswere examined before and throughout a three week BDOC amendment. In addition,bacterial production, dissolved oxygen demand, denitrification, and sixextracellular enzyme activities were measured to arrive at a mechanisticunderstanding of potential DOC and NO₃ ^– removalalong hyporheic flowpaths. There were marked declines in DOC and completeremoval of nitrate in the BDOC amended mesocosms. Independent approaches wereused to partition NO₃ ^– loss into two fractions:denitrification and assimilation. Microbial assimilation appears to be thepredominant process explaining N loss. These results suggest that variabilityinBDOC may contribute to temporal differences in NO₃ ^–export from streams in the Catskill Mountains.     

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon,Chilika, India)

An effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO₂ (pCO₂) and CO₂ air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L^?1 d^?1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L^?1 d^?1 and from 14.1 to 389.4 µg C L^?1 d^?1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO₂ (10,134 µatm) and CO₂ air–water flux (714 mm m^?2 d^?1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO₂ supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.     

Denitrification by Alcaligenes denitrificans in a closed rotating biological contactor

Biological denitrification using a pure culture of Alcaligenes denitrificans was investigated in a closed rotating biological contactor, which operated with a hydraulic retention time of 2 h, a carbon/nitrogen ratio of 2:1, with a dissolved O₂ concentration below 6 mg l^–1 and under three different phosphate concentrations. Alcaligenes denitrificans was not repressed by O₂ limitation and the removal of nitrate was about 30% more efficient at the intermediate phosphate concentration (20 mg P l^–1).     

Influence of elevated CO2 and nitrogen nutrition on rice plant growth,soil microbial biomass,dissolved organic carbon and dissolved CH4

Rice (Oryza sativa) was grown in six sunlit, semi-closed growth chambers for two seasons at 350 L L^–1 (ambient) and 650 L L^–1 (elevated) CO₂ and different levels of nitrogen (N) supplement. The objective of this research was to study the influence of CO₂ enrichment and N nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon (DOC) and dissolved CH₄. Elevated CO₂ concentration ([CO₂]) demonstrated a wide range of enhancement to both above- and below-ground plant biomass, in particular to stems and roots (for roots when N was not limiting) in the mid-season (80 days after transplanting) and stems/ears at the final harvest, depending on season and the level of N supplement. Elevated [CO₂] significantly increased microbial biomass carbon in the surface 5 cm soil when N (90 kg ha^–1) was in sufficient supply. Low N supplement (30 kg ha^–1) limited the enhancement of root growth by elevated [CO₂], leading consequently to diminished response of soil microbial biomass carbon to CO₂ enrichment. The concentration of dissolved CH₄ (as well as soil DOC, but to a lesser degree) was observed to be positively related to elevated [CO₂], especially at high rate of N application (120 kg ha^–1) or at 10 cm depth (versus 5 cm depth) in the later half of the growing season (at 80 kg N ha^–1). Root senescence in the late season complicated the assessment of the effect of elevated [CO₂] on root growth and soil organic carbon turnover and thus caution should be taken when interpreting respective high CO₂ results.     

Effects of Methane Metabolism on Nitrification and Nitrous Oxide Production in Polluted Freshwater Sediment

We report the effect of CH₄ and of CH₄ oxidation on nitrification in freshwater sediment from Hamilton Harbour, Ontario, Canada, a highly polluted ecosystem. Aerobic slurry experiments showed a high potential for aerobic N₂O production in some sites. It was suppressed by C₂H₂, correlated to NO₃^- production, and stimulated by NH₄⁺ concentration, supporting the hypothesis of a nitrification-dependent source for this N₂O production. Diluted sediment slurries supplemented with CH₄ (1 to 24 μM) showed earlier and enhanced nitrification and N₂O production compared with unsupplemented slurries (≤1 μM CH₄). This suggests that nitrification by methanotrophs may be significant in freshwater sediment under certain conditions. Suppression of nitrification was observed at CH₄ concentrations of 84 μM and greater, possibly through competition for O₂ between methanotrophs and NH₄⁺ -oxidizing bacteria and/or competition for mineral N between these two groups of organisms. In Hamilton Harbour sediment, the very high CH₄ concentrations (1.02 to 6.83 mM) which exist would probably suppress nitrification and favor NH₄⁺ accumulation in the pore water. Indeed, NH₄⁺ concentrations in Hamilton Harbour sediment are higher than those found in other lakes. We conclude that the impact of CH₄ metabolism on N cycling processes in freshwater ecosystems should be given more attention.     

Modeling nitrogen removal in water hyacinth ponds receiving effluent from waste stabilization ponds

We developed a dynamic model to predict nitrogen removal in water hyacinth ponds (WHPs) receiving effluent from waste stabilization ponds (WSPs). The model is based on the biofilm reaction on the root surface of plant and pond walls. The model consists of mass balances of six main substrates including: particulate organic nitrogen (PON), dissolved organic nitrogen (DON), ammonium (NH₄⁺), nitrite and nitrate (NOx), soluble chemical oxygen demand (SCOD), and particulate chemical oxygen demand (PCOD). The model, incorporating major nitrogen transformation mechanisms such as hydrolysis, mineralization, and nitrification–denitrification, accounts also for carbon consumption and plant uptake. The model's application to a pilot plant showed good agreement between measured and predicted values. According to the modeling results, in the WHPs, nitrification and denitrification were the predominant nitrogen removal processes occurring simultaneously. Temperature and hydraulic retention time (HRT) had a profound effect on the performance of nitrogen removal while an algae biomass (PCOD) accumulated in the WHPs, was a useful carbon source for denitrification.     

Metabolic Quotient Measured by Free-Water Method in Six Enclosures with Different Silver Carp Densities

Quasi-continuous DO and pH measurements (total 47 days) were conducted during enclosure experiments (6 enclosures; 5 × 5 × 2.5 m), in which a biomass gradient of silver carp was created. After subtracting the air–water exchanges of O₂ and CO₂, the chemical and biochemical changes in DO (dissolved oxygen) and DIC (dissolved inorganic carbon) were estimated in order to evaluate MQ (metabolic quotient: DO change divided by DIC change) at intervals of 1 hour. By removing small absolute changes below the threshold value (0.01 mM h^?1), the averaged values of the 24 MQ means for the respective 1-hour periods ranged from 0.96 to 1.20 in the six enclosures. Because the MQs in the daytime inversely correlated well with the ratio of NH⁺ ₄–N to (NH⁺ ₄–N + NO^? ₃–N), not the ecosystems, i.e., density of fish, community structure of zooplankton and phytoplankton, but the form of nitrogen uptaken for primary production principally determined the MQs. The higher MQs observed in the daytime compared with the nighttime (from 14% to 21% except 3% for one enclosure) could not be explained by the denitrification and/or dissolution of CaCO₃ in the sediments, therefore suggesting the selectively faster decomposition of part of the organic matter provided through primary production, in other words, an accumulation of another part of the organic matter in the diurnal and/or daily time scale.     

N<Subscript>2</Subscript>O production by denitrification in an urban river: evidence from isotopes,functional genes,and dissolved organic matter

Rivers are important sources of N₂O emissions into the atmosphere. Nevertheless, N₂O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N₂O, NH₄ ⁺, NO₂ ^?, and NO₃ ^? in surface water to identify the microbial processes of N₂O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N₂O production processes. The observed nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) of N₂O were within the expected isotopic range of N₂O produced by nitrate reduction, indicating that N₂O was dominantly produced by denitrification. The positive significant correlation between N₂O_Net concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N₂O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N₂O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N₂O. Measurement of isotope ratios of N₂O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N₂O production along the Tama River.     

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO₂ to the atmosphere. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the rates of CO₂ production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO₂ in the soil air, but the sources of this CO₂ and the spatiotemporal dynamics of CO₂ production throughout the soil profile are poorly understood. We studied CO₂ dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO₂ concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO₂ production with soil profile depth. The CO₂ diffusion constants were checked for accuracy using measured soil air ²²²Rn activities. The modeled net CO₂ production rates were compared with CO₂ fluxes measured at the soil surface. In general, the modeled and measured net CO₂ fluxes were very similar although the model consistently underestimated CO₂ production rates in the surficial soil horizons when the soils were moist. Profile CO₂ production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO₂ production in both the surface and subsurface soil horizons. Whole profile CO₂ production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO₂ flux from the soil profiles are relatively low (4–11 mg C– CO₂ m⁻² h⁻¹), 20%–50% of the CO₂ diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance ¹⁴C signatures of the CO₂ and soil organic C suggest that the subsurface CO₂ is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.     

Fluxes of dissolved carbon dioxide and inorganic carbon from an upland peat catchment: implications for soil respiration

This study uses long-term water chemistry records for a circum-neutral peat stream to reconstruct a 7-year record of dissolved CO₂ and DIC flux from the catchment. Combining catchment flux with a knowledge of in-stream metabolism and gas evasion from the stream surface enables an estimate of the dissolved CO₂ content of water emerging from the peat profile to be made; furthermore, these can be used to estimate soil CO₂ respiration. In this way multi-annual records of CO₂ production can be reconstructed, and therefore inter-annual controls on production examined. The results suggest that:(i) Stream evasion of CO₂ within the catchment varied between 80 and 220 g C/m of stream/yr, while in-stream metabolism produces between 1.0 and 2.9 g C/m of stream/yr;Export of dissolved CO₂ emerging from the soil profile, above that expected at equilibrium with the atmosphere, varies between 9.6 and 25.6 tonnes,C/km²/yr; andThe export of dissolved CO₂ implies a soil respiration rate of between 64.2 and 94.9 tonnes C/km²/yr.The inter-annual variation in both dissolved CO₂ flux and soil CO₂ respiration suggests that severe drought has no long-term effect on CO₂ production and that temperature-based models of soil CO₂ respiration will be adequate in all but the severest of summer droughts. The inter-annual variation in CO₂ flux shows that CO₂ production is decoupled from dissolved organic carbon (DOC) production. The decoupling of DOC and dissolved CO₂ production shows that enzymatic-latch production of DOC is an anaerobic process and will not increase soil CO₂ respiration.