Interannual patterns of water table height and groundwater derived nitrate in nearshore sediments

Nitrate concentration and microbial nitrogen transformations in ground-water-affected sediments of Great South Bay, NY were examined over several annual cycles. Nitrate concentrations are typically higher at 40 cm depth than at the surface, while salinity generally decreases with depth. Denitrification occurs through the sediment core and is organic substrate limited at depth while being nitrate limited near the sediment-water interface. Denitrification accounts for about 50% of the biological NO₃ ^- decrease between 40 and 15 cm depth interval. Higher than average annual rainfall during 1983 and 1984 was reflected in an elevated water table as well as lower Bay salinities. Conversely, extremely low rainfall occurred in 1985 and 1986, and the water table reached an extreme low in Sep. 1986. Interestingly, the amounts of nitrate in the sediment column of our primary station varied directly with water table height and, presumably, the discharge rate of nitrate enriched groundwater. We suggest that this may be a result of the more efficient removal of advected nitrate by denitrification during low flow conditions.     

Sediment Nitrification, Denitrification, and Nitrous Oxide Production in a Deep Arctic Lake

We used a combination of ¹⁵N tracer methods and a C₂H₂ blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol · cm⁻³, increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times lower, and nitrate was only detectable within the top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zone underlain by more reduced conditions. The lake water never became anoxic. Sediment Eh values ranged from −7 to 484 mV, decreasing with depth, whereas pH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrification rate (49 ng of N · cm⁻³ · day⁻¹) was similar to the average denitrification rate (44 ng of N · cm⁻³ · day⁻¹). In situ N₂O production from nitrification and denitrification ranged from 0 to 25 ng of N · cm⁻³ · day⁻¹. Denitrification appears to depend on the supply of nitrate by nitrification, such that the two processes are coupled functionally in this sediment system. However, the low rates result in only a small nitrogen loss.     

Denitrification in Aquifer Soil and Nearshore Marine Sediments Influenced by Groundwater Nitrate

We estimated rates of denitrification at various depths in sediments known to be affected by submarine discharge of groundwater, and also in the parent aquifer. Surface denitrification was only measured in the autumn; at 40-cm depth, where groundwater-imported nitrate has been measured, denitrification occurred consistently throughout the year, at rates from 0.14 to 2.8 ng-atom of N g⁻¹ day⁻¹. Denitrification consistently occurred below the zone of sulfate reduction and was sometimes comparable to it in magnitude. Denitrification occurred deep (14 to 40 cm) in the sediments along 30 km of shoreline, with highest rates occurring where groundwater input was greatest. Denitrification rates decreased with distance offshore, as does groundwater influx. Added glucose greatly stimulated denitrification at depth, but added nitrate did not. High rates of denitrification were measured in the aquifer (17 ng-atom of N g⁻¹ day⁻¹), and added nitrate did stimulate denitrification there. The denitrification measured was enough to remove 46% of the nitrate decrease observed between 40- and 14-cm depth in the sediment.     

Nitrate Reduction Functional Genes and Nitrate Reduction Potentials Persist in Deeper Estuarine Sediments. Why?

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are processes occurring simultaneously under oxygen-limited or anaerobic conditions, where both compete for nitrate and organic carbon. Despite their ecological importance, there has been little investigation of how denitrification and DNRA potentials and related functional genes vary vertically with sediment depth. Nitrate reduction potentials measured in sediment depth profiles along the Colne estuary were in the upper range of nitrate reduction rates reported from other sediments and showed the existence of strong decreasing trends both with increasing depth and along the estuary. Denitrification potential decreased along the estuary, decreasing more rapidly with depth towards the estuary mouth. In contrast, DNRA potential increased along the estuary. Significant decreases in copy numbers of 16S rRNA and nitrate reducing genes were observed along the estuary and from surface to deeper sediments. Both metabolic potentials and functional genes persisted at sediment depths where porewater nitrate was absent. Transport of nitrate by bioturbation, based on macrofauna distributions, could only account for the upper 10 cm depth of sediment. A several fold higher combined freeze-lysable KCl-extractable nitrate pool compared to porewater nitrate was detected. We hypothesised that his could be attributed to intracellular nitrate pools from nitrate accumulating microorganisms like Thioploca or Beggiatoa. However, pyrosequencing analysis did not detect any such organisms, leaving other bacteria, microbenthic algae, or foraminiferans which have also been shown to accumulate nitrate, as possible candidates. The importance and bioavailability of a KCl-extractable nitrate sediment pool remains to be tested. The significant variation in the vertical pattern and abundance of the various nitrate reducing genes phylotypes reasonably suggests differences in their activity throughout the sediment column. This raises interesting questions as to what the alternative metabolic roles for the various nitrate reductases could be, analogous to the alternative metabolic roles found for nitrite reductases.     

The dark side of the hyporheic zone: depth profiles of nitrogen and its processing in stream sediments

1. Although it is well known that sediments can be hot spots for nitrogen transformation in streams, many previous studies have confined measurements of denitrification and nitrate retention to shallow sediments (<5 cm deep). We determined the extent of nitrate processing in deeper sediments of a sand plains stream (Emmons Creek) by measuring denitrification in core sections to a depth of 25 cm and by assessing vertical nitrate profiles, with peepers and piezometers, to a depth of 70 cm. 2. Denitrification rates of sediment slurries based on acetylene block were higher in shallower core sections. However, core sections deeper than 5 cm accounted for 68% of the mean depth‐integrated denitrification rate. 3. Vertical hydraulic gradient and vertical profiles of pore water chloride concentration suggested that deep ground water upwelled through shallow sediments before discharging to the stream channel. The results of a two‐source mixing model based on chloride concentrations suggested that the hyporheic zone was very shallow (<5 cm) in Emmons Creek. 4. Vertical profiles showed that nitrate concentration in shallow ground water was about 10–60% of the nitrate concentration of deep ground water. The mean nitrate concentrations of deep and shallow ground water were 2.17 and 0.73 mg NO₃‐N L^?1, respectively. 5. Deep ground water tended to be oxic (6.9 mg O₂ L^?1) but approached anoxia (0.8 mg O₂ L^?1) after passing through shallow, organic carbon‐rich sediments, which suggests that the decline in the nitrate concentrations of upwelling ground water was because of denitrification. 6. Collectively, our results suggest that there is substantial nitrate removal occurring in deep sediments, below the hyporheic zone, in Emmons Creek. Our findings suggest that not accounting for nitrate removal in deep sediments could lead to underestimates of nitrogen processing in streams and catchments.     

Denitrification, Nitrate Reduction, and Oxygen Consumption in Coastal and Estuarine Sediments

Denitrification and consumption of oxygen and nitrate in sediments from Tama Estuary, Odawa Bay, and Tokyo Bay were measured in an experimental sediment-water system. Filtered seawater containing [¹⁵N]nitrate flowed continuously over undisturbed sediments, and the concentrations of O₂, nitrate, and nitrite in the influent and effluent and of ¹⁵N₂ in the effluent were monitored. Under steady-state conditions, the rate of nitrate consumption was the same order of magnitude as the rate of oxygen consumption in Tama Estuary sediments, whereas the former rate was one order of magnitude lower than the latter rate in Odawa Bay and Tokyo Bay sediments. Denitrification accounted for 27 to 57% of the nitrate consumption.     

Presence of Nitrate-Accumulating Sulfur Bacteria and Their Influence on Nitrogen Cycling in a Shallow Coastal Marine Sediment

Nitrate flux between sediment and water, nitrate concentration profile at the sediment-water interface, and in situ sediment denitrification activity were measured seasonally at the innermost part of Tokyo Bay, Japan. For the determination of sediment nitrate concentration, undisturbed sediment cores were sectioned into 5-mm depth intervals and each segment was stored frozen at −30°C. The nitrate concentration was determined for the supernatants after centrifuging the frozen and thawed sediments. Nitrate in the uppermost sediment showed a remarkable seasonal change, and its seasonal maximum of up to 400 μM was found in October. The directions of the diffusive nitrate fluxes predicted from the interfacial concentration gradients were out of the sediment throughout the year. In contrast, the directions of the total nitrate fluxes measured by the whole-core incubation were into the sediment at all seasons. This contradiction between directions indicates that a large part of the nitrate pool extracted from the frozen surface sediments is not a pore water constituent, and preliminary examinations demonstrated that the nitrate was contained in the intracellular vacuoles of filamentous sulfur bacteria dwelling on or in the surface sediment. Based on the comparison between in situ sediment denitrification activity and total nitrate flux, it is suggested that intracellular nitrate cannot be directly utilized by sediment denitrification, and the probable fate of the intracellular nitrate is hypothesized to be dissimilatory reduction to ammonium. The presence of nitrate-accumulating sulfur bacteria therefore may lower nature's self-purification capacity (denitrification) and exacerbate eutrophication in shallow coastal marine environments.     

Hydrogeochemistry of Lake Gallocanta (Aragón,NE Spain)

Lake Gallocanta has undergone drastic changes during the last thirteen years. Water level changed from a high level (Z_max = 2 m) to total dryness in 1985. From 1986 to 1988 slow refilling occurred. The water volume fluctuations have been studied in relation to climatic variations recorded for that period. Variations in the major dissolved ions were related to water volume fluctuations from data at two different stages, one corresponding to the drying phase and another to the refilling phase. Mineralogical composition of the salts precipitated at different stages was examined by X-ray diffraction. Interstitial water and mineralogical composition of recent sediments were also studied along a transect through the lake.The water column decrease from 1977 to 1985 is related to decreasing annual rainfall (500-250 mm respectively). The refilling in 1986–1988 is due to high annual rainfall (537 mm). In addition to these fluctuations, seasonal changes of the water level between 20 and 50 cm occurred every year.Gallocanta is a Na-Mg-Cl-(SO₄) type lake. During the drying period a typical salt enrichment occurs with linear relationships between TDS, Cl, Na and K. Alkalinity is linearly correlated with Ca at relatively low salinities. As salinity increases a linear relationship between Ca and SO₄ is observed. Minerals formed from the brine are halite, bischofite, epsomite, hexahydrite, mirabilite, gypsum, aragonite, calcite and dolomite. The molar ratio Mg/Ca of the interstitial water changes from 1.5 along the shorelines, where calcite and aragonite precipitate, to 40 in the center of the lake. Sediment cores from the central part of the lake show aragonite in the top layers, magnesian calcite and low proportions of quartz and illite, while at 20 cm depth a high proportion of gypsum is present. In contrast, cores from the shore of the lake are mainly composed of low magnesium calcite in the top layers and low magnesium calcite together high magnesium calcite and dolomite between 30 and 70 cm depth. Gypsum deposits only occur in significant proportions at 80–100 cm depth.The refilling process showed relationships between volume and salt concentration following the Langbein model. The salt mass in solution decreased about 50% from the drying to the refilling phase. However, Mg content decreased about 70 % for the same period, suggesting a contribution of this element to the dolomite formation.     

Net sediment N2 fluxes in a southern New England estuary: variations in space and time

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N₂ across the sediment–water interface over an annual cycle using the N₂/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N₂–N m^?2 h^?1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (?5 to ?650 μmol N₂–N m^?2 h^?1) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.     

Denitrification in deep subsurface sediments

Dissimilatory nitrate reduction (denitrification) in subsurface sediments by indigenous microflora was investigated in samples obtained over a range of depths from 0 to 289 m. Denitrifying activity in sediment samples retrieved from similar stratigraphic horizons at four different sites was determined by measuring the accumulation of N₂O using the acetylene blockage technique. Denitrification was detected in unamended samples which received only prereduced deionized water at almost all depths in all sediments sampled. The surface sediments showed the highest denitrification activity. In the deeper sediments, denitrifying activity was much higher in saturated sandy samples and lower or absent in drier clay samples. Addition of nitrate enhanced denitrification activity in all samples from below the water table down to the maximum depth sampled (289 m), while addition of a carbon (succinate) source in general had no stimulatory effect. These results show that denitrifying microorganisms were present in all of the deep subsurface sediments tested in this study. Furthermore, these results suggest that adequate supplies of metabolizable organic carbon were available to support denitrifying activity. However, denitrification may be limited by inadequate supplies of nitrate in the sediments.     

Differences among maize cultivars in the utilization of soil nitrate and the related losses of nitrate through leaching

In a 2-year field experiment conducted on a Gleyic Luvisol in Stuttgart-Hohenheim one experimental and nine commercial maize cultivars were compared for their ability to utilize soil nitrate and to reduce related losses of nitrate through leaching. Soil nitrate was monitored periodically in CaCl₂ extracts and in suction cup water. Nitrate concentrations in suction water were generally higher than in CaCl₂ extracts. Both methods revealed that all cultivars examined were able to extract nitrate down to a soil depth of at least 120 cm (1988 season) or 150 cm (1987 season). Significant differences among the cultivars existed in nitrate depletion particularly in the subsoil. At harvest, residual nitrate in the upper 150 cm of the profile ranged from 73–110 kg N ha^–1 in 1987 and from 59–119 kg N ha^–1 in 1988. Residual nitrate was closely correlated with nitrate losses by leaching because water infiltration at 120 cm soil depth started 4 weeks after harvest (1987) or immediately after harvest (1988) and continued until early summer of the following year. The calculated amount of nitrate lost by leaching was strongly influenced by the method of calculation. During the winter of 1987/88 nitrate leaching ranged from 57–84 kg N ha^–1 (suction cups) and 40–55 kg N ha^–1 (CaCl₂ extracts), respectively. The corresponding values for the winter of 1988/89 were 47–79 and 20–39 kg N ha^–1, respectively. ei]Section editor: B E Clothier     

宁夏引黄灌区秸秆还田对麦田土壤硝态氮淋失的影响

以宁夏引黄灌区为例,探索秸秆还田条件下冬小麦土壤硝态氮淋失规律。试验设置常规施肥(CK)、常规施肥条件下施用4500kg/hm2(T1,半量还田)和9000 kg/hm2(T2,全量还田)秸秆3个处理。利用树脂芯法吸附10、20、30、60cm和90cm土层的硝态氮流失量。结果表明:硝态氮(纯N)淋失量6.26—12.85 kg/hm2,是冬小麦施用化肥氮量的2.78%—5.71%。与对照CK相比,T1和T2在10cm土层减少0.09%和3.97%;20cm土层减少8.51%和9.81%;30cm土层减少2.25%和10.34%;60cm土层减少23.85%和13.08%;90cm土层减少27.65%和20.73%。10cm和20cm土层,处理与对照以及处理之间均未到显著性差异(P0.05);30cm处理,T1与CK以及T1与T2未达到显著性差异,但T2与CK达到显著性差异表明全量还田效果最好;60cm土层,处理与对照、以及处理之间均达到显著性差异;90cm土层,处理与对照之间达到显著性差异,处理之间未达到显著性差异。硝态氮淋失主要发生在冬小麦返青至灌浆期间,占全生育期淋失量的52.95%—67.79%。T1、T2冬小麦产量增产率分别为10.11%与11.51%。可见,稻秆还田能够减少灌区土壤硝态氮淋失量。     

Denitrification in a Sand and Gravel Aquifer

Denitrification was assayed by the acetylene blockage technique in slurried core material obtained from a freshwater sand and gravel aquifer. The aquifer, which has been contaminated with treated sewage for more than 50 years, had a contaminant plume greater than 3.5-km long. Near the contaminant source, groundwater nitrate concentrations were greater than 1 mM, whereas 0.25 km downgradient the central portion of the contaminant plume was anoxic and contained no detectable nitrate. Samples were obtained along the longitudinal axis of the plume (0 to 0.25 km) at several depths from four sites. Denitrification was evident at in situ nitrate concentrations at all sites tested; rates ranged from 2.3 to 260 pmol of N₂O produced (g of wet sediment)⁻¹ h⁻¹. Rates were highest nearest the contaminant source and decreased with increasing distance downgradient. Denitrification was the predominant nitrate-reducing activity; no evidence was found for nitrate reduction to ammonium at any site. Denitrifying activity was carbon limited and not nitrate limited, except when the ambient nitrate level was less than the detection limit, in which case, even when amended with high concentrations of glucose and nitrate, the capacity to denitrify on a short-term basis was lacking. These results demonstrate that denitrification can occur in groundwater systems and, thereby, serve as a mechanism for nitrate removal from groundwater.     

Measurement of denitrification in estuarine sediment using membrane inlet mass spectrometry

Abstract Denitrification was measured in intact sediment cores and in homogenised slurries using membrane inlet mass spectrometry. Dissolved concentrations of O₂, N₂, N₂O and CO₂ were simultaneously monitored. Using a 0.8 mm diameter needle probe, a comparison was made of the gas profiles of intact cores obtained under different conditions, i.e. with air or argon as the headspace gas and after the addition of nitrate and/or a carbon source to the sediment surface. O₂ was detectable to a depth of 1 cm under a headspace of air and the depth at which the maxima of denitrification products occurred was 1.5–2 cm. Denitrification products (N₂O, N₂) occurred in the surface layers where O₂ was above the minimum level of detectability (> 0.25 μM): diffusion of N₂ and N₂O upwards from the anoxic zone, local anaerobic microenvironments or aerobic denitrification are alternative explanations for this observation. The addition of nitrate and/or acetate increased the concentrations of N₂, N₂O and CO₂ in the sediment core. In sediment slurries, the pH, nitrate concentration, carbon source and the depth from which the sample was taken affected the rate of denitrification. Nitrogen was the sole detectable end product. Maximum denitrification occurred at pH 7.5 and at 20 mM nitrate. Denitrification was at a maximum in those slurries prepared from sections of core at 1–2 cm depth.     

Arctic sediments (Svalbard): pore water and solid phase distributions of C,N, P and Si

Pore water and solid phase distributions of C, N, P and Si in sediments of the Arctic Ocean (Svalbard area) have been investigated. Concentrations of organic carbon (C_org) in the solid phase of the sediment varied from 1.3 to 2.8% (mean 1.9%), with highest concentrations found at shallow stations south/southwest of Svalbard. Relatively low concentrations were obtained at the deeper stations north/northeast of Svalbard. Atomic carbon to nitrogen ratios in the surface sediment ranged from below 8 to above 10. For some stations, high C/N ratios together with high concentrations of C_org suggest that sedimentary organic matter is mainly of terrigenous origin and not from overall biological activity in the water column. Organic matter reactivity (defined as the total sediment oxygen consumption rate normalized to the organic carbon content of the surface sediment) correlated with water depth at all investigated stations. However, the stations could be divided into two separate groups with different reactivity characteristics, representing the two most dominant hydrographic regimes: the region west of Svalbard mainly influenced by the West Spitsbergen Current, and the area east of Svalbard where Arctic polar water set the environmental conditions. Decreasing sediment reactivity with water depth was confirmed by the partitioning between organic and inorganic carbon of the surface sediment. The ratio between organic and inorganic carbon at the sediment-water interface decreased exponentially with water depth: from indefinite values at shallow stations in the central Barents Sea, to approximately 1 at deep stations north of Svalbard. At stations east of Svalbard there was an inverse linear correlation between the organic matter reactivity (as defined above) and concentration of dissolved organic carbon (DOC) in the pore water. The more reactive the sediment, the less DOC existed in the pore water and the more total carbonate (C_t or ΣCO₂) was present. This observation suggests that DOC produced in reactive sediments is easily metabolizable to CO₂. Sediment accumulation rates of opaline silica ranged from 0.35 to 5.7 μmol SiO₂ m⁻²d⁻¹ (mean 1.3 μmol SiO₂ m⁻²d⁻¹), i.e. almost 300 times lower than rates previously reported for the Ross Sea, Antarctica. Concentrations of ammonium and nitrate in the pore water at the sediment-water interface were related to organic matter input and water depth. In shallow regions with highly reactive organic matter, a pool of ammonium was present in the pore water, while nitrate conoentrations were low. In areas where less reactive organic matter was deposited at the sediment surface, the deeper zone of nitrification caused a build-up of nitrate in the pore water while ammonium was almost depleted. Nitrate penetrated from 1.8 to ≥ 5.8 cm into the investigated sediments. Significantly higher concentrations of “total” dissolved nitrogen (defined as the sum of NO₃, NO₂, NH₄ and urea) in sediment pore water were found west compared to east of Svalbard. The differences in organic matter reactivity, as well as in pore water distribution patterns of “total” dissolved nitrogen between the two areas, probably reflect hydrographic factors (such as ice coverage and production/import of particulate organic material) related to the dominant water mass (Atlantic or Arctic Polar) in each of the two areas. The data presented were collected during the European “Polarstern” Study (Arctic EPOS) sponsored by the European Science Foundation     

Factors Controlling Sediment Denitrification in Midwestern Streams of Varying Land Use

We investigated controls on stream sediment denitrification in nine headwater streams in the Kalamazoo River Watershed, Michigan, USA. Factors influencing denitrification were determined by using experimental assays based on the chloramphenicol-amended acetylene inhibition technique. Using a coring technique, we found that sediment denitrification was highest in the top 5 cm of the benthos and was positively related to sediment organic content. To determine the effect of overlying water quality on sediment denitrification, first-order stream sediments were assayed with water from second- and third-order downstream reaches, and often showed higher denitrification rates relative to assays using site-specific water from the first-order stream reach. Denitrification was positively related to nitrate (NO₃ ⁻) concentration, suggesting that sediments may have been nutrient-limited. Using stream-incubated inorganic substrata of varying size classes, we found that finer-grained sand showed higher rates of denitrification compared to large pebbles, likely due to increased surface area per volume of substratum. Denitrification was measurable on both inorganic substrata and fine particulate organic matter loosely associated with inorganic particles, and denitrification rates were related to organic content. Using nutrient-amended denitrification assays, we found that sediment denitrification was limited by NO₃ ⁻ or dissolved organic carbon (DOC, as dextrose) variably throughout the year. The frequency and type of limitation differed with land use in the watershed: forested streams were NO₃ ⁻-limited or co-limited by both NO₃ ⁻ and DOC 92% of the time, urban streams were more often NO₃ ⁻-limited than DOC-limited, whereas agricultural stream sediments were DOC-limited or co-limited but not frequently limited by NO₃ ⁻ alone.     

Denitrification in the top and sub soil of grassland on peat soils

Denitrification is an important process in the nitrogen (N) balance of intensively managed grassland, especially on poorly drained peat soils. Aim of this study was to quantify the N loss through denitrification in the top and sub soil of grassland on peat soils. Sampling took place at 2 sites with both control (0 N) and N fertilised (+ N) treatments. Main difference between the sites was the ground water level. Denitrification was measured on a weekly basis for 2 years with a soil core incubation technique using acetylene (C₂H₂) inhibition. Soil cores were taken from the top soil (0–20 cm depth) and the sub soil (20–40 cm depth) and incubated in containers for 24 hours. The denitrification rate was calculated from the nitrous oxide production between 4 and 24 hours of incubation. Denitrification capacities of the soils and the soil layers were also determined.The top soil was the major layer for denitrification with losses ranging from 9 to 26 kg N ha^–1 yr^–1 from the O N treatment. Losses from the top soil of the + N treatment ranged from 13 to 49 kg N ha^–1 yr^–1. The sub soil contributed, on average, 20% of the total denitrification losses from the 0–40 layer. Losses from the 0–40 cm layer were 2 times higher on the + N treatment than on the O N treatment and totalled up to 70 kg N ha^–1 yr^–1. Significant correlation coefficients were found between denitrification activity on the one hand, and ground water level, water filled pore space and nitrate content on the other, in the top soil but not in the sub soil. The denitrification capacity experiment showed that the availability of easily decomposable organic carbon was an important limiting factor for the denitrification activity in the sub soil of these peat soils.     

Hydrological variability, organic matter supply and denitrification in the Garonne River ecosystem

1. Groundwater nitrate contamination has become a worldwide problem as increasing amounts of nitrogen fertilisers are used in agriculture. Alluvial groundwater is uniquely juxtaposed between soils and streams. Hydrological connections among these subsystems regulate nutrient cycling. 2. We measured denitrification using an in situ acetylene‐block assay in a nitrate‐contaminated portion of the Garonne River catchment along a gradient of surface water–ground water mixing during high (snowmelt) and low flow. 3. During high flow (mid‐April to early June) the water table rose an average of 35 cm and river water penetrated the subsurface to a great extent in monitoring wells. Denitrification rates averaged 5.40 μgN₂O L^?1 min^?1 during the high flow period, nearly double the average rate (2.91 μgN₂O L^?1 min^?1) measured during base flow. This was driven by a strong increase in denitrification in groundwater under native riparian vegetation. Nitrate concentrations were significantly lower during high flow compared with base flow. Riparian patches had higher dissolved organic carbon concentrations that were more aromatic compared with the gravel bar patch closest to the river. 4. Multiple linear regression showed that the rate of denitrification was best predicted by the concentration of low molecular weight organic acids. These molecules are probably derived from decomposition of soil organic matter and are an important energy source for anaerobic respiratory processes like denitrification. The second best predictor was per cent surface water, reflecting higher denitrification rates during spring when hydrological connection between surface water and ground water was greatest. 5. Our results indicate that, while denitrification rates in Garonne River alluvium were spatially and temporally variable, denitrification was a significant NO₃ sink during transport from the NO₃‐contaminated floodplain to the river. DOC availability and river–floodplain connectivity were important factors influencing observed spatial and temporal patterns.     

Environmental effects on CO2 efflux from riparian tundra in the northern foothills of the Brooks Range,Alaska, USA

Summary Carbon dioxide efflux and soil microenvironmental factors were measured diurnally in Carex aquatilus-and Eriophorum angustifolium-dominated riparian tundra communities to determine the relative importance of soil environmental factors controlling ecosystem carbon dioxide exchange with the atmosphere. Measurements were made weekly between 18 June and 24 July 1990. Diurnal patterns in carbon dioxide efflux were best explained by changes in soil temperature, while seasonal changes in efflux were correlated with changes in depth to water table, depth to frozen soil and soil moisture. Carbon dioxide efflux rates were lowest early in the growing season when high water tables and low soil temperatures limited microbial and root activity. Individual rainfall events that raised the water table were found to strongly reduce carbon dioxide efflux. As the growing season progressed, rainfall was low and depth to water table and soil temperatures increased. In response, carbon dioxide efflux increased strongly, attaining rates late in the season of approximately 10 g CO₂ m^–2 day^–1. These rates are as high as maxima recorded for other arctic sites. A mathematical model is developed which demonstrates that soil temperature and depth to water table may be used as efficient predictors of ecosystem CO₂ efflux in this habitat. In parallel with the field measurements of CO₂ efflux, microbial respiration was studied in the laboratory as a function of temperature and water content. Estimates of microbial respiration per square meter under field conditions were made by adjusting for potential respiring soil volume as water table changed and using measured soil temperatures. The results indicate that the effect of these factors on microbial respiration may explain a large part of the diurnal and seasonal variation observed in CO₂ efflux. As in coastal tundra sites, environmental changes that alter water table depth in riparian tundra communities will have large effects on ecosystem CO₂ efflux and carbon balance.     

Sediment-water fluxes of nutrients in an Antarctic coastal environment: influence of bioturbation

Rates of exchanges of nitrate and ammonium across the sediment-water interface were measured in an inshore marine environment at Signy Island, South Orkney Islands, Antarctica, over 6 months from August 1991 to February 1992. The sediment was a source of ammonium to the water column but a sink of nitrate, although nitrate exchange rates were very variable. Concentration profiles of nitrate and ammonium in the sediment porewater corroborated the measured vertical exchanges. Bioturbation, by a largely amphipod benthic infauna which was confined to the top 2 cm of sediment, was investigated experimentally. Removal of bioturbation depressed sedimentary O₂ uptake by 33% and sedimentary release of NH₄ ⁺ by 50%. In contrast, in the absence of bioturbation, the removal of NO₃ ^– from the water column by the sediment increased in rate. The measured fluxes of ammonium and nitrate from the sediment did not match with the amount of nitrogen mineralised within the sediment, and urea may account for the difference. It is suggested that the export of nitrogen from the bottom sediment may be significant in sustaining primary production in the Antarctic inshore environment. Ammonium and urea are preferred to nitrate as a nitrogen source by phytoplankton. The nitrate concentrations in the sediment porewater were low, but a large pool of nitrate was identified in the top 0–2 cm layer, which was released by KCl extraction or by freezing of the sediment. This extractable pool of nitrate did not equilibrate with the soluble nitrate pool in the sediment, but seemed to be released from components of the benthic infauna, which were also largely confined to the top 0–2 cm. The physiological role of this nitrate is unknown.