Comparison of nitrification rates in three branches of the lower river Rhine

The nitrogen cycle in the lower river Rhine was analysed, using data on concentrations of ammonium, nitrite and nitrate, measured in the period from 1972 to 1986. The massive discharge of ammonium in densely populated areas in the Federal Republic of Germany led to microbial nitrification, detectable as decreases in ammonium and nitrite concentrations in the lower river Rhine over reaches 85–133 km long. The distribution of the nitrogen-rich Rhine waters over three different branches in the Netherlands permits some of the factors governing microbial nitrification in the river bed to be discriminated. In the fast-flowing main channel, intensively used by ships, nitrification is more important than in the smaller branches, despite the short residence time of the water in the main channel. Differences in the flow rate of water, in grain size distribution of sediments, and in intensity of shipping (aeration, turbulence) seemed to be responsible for the different rates of nitrification.     

Assessment of Nitrification Potential in Ground Water Using Short Term, Single-Well Injection Experiments

Nitrification was measured within a sand and gravel aquifer on Cape Cod, MA, using a series of single-well injection tests. The aquifer contained a wastewater-derived contaminant plume, the core of which was anoxic and contained ammonium. The study was conducted near the downgradient end of the ammonium zone, which was characterized by inversely trending vertical gradients of oxygen (270 to 0 μM) and ammonium (19 to 625 μM) and appeared to be a potentially active zone for nitrification. The tests were conducted by injecting a tracer solution (ambient ground water + added constituents) into selected locations within the gradients using multilevel samplers. After injection, the tracers moved by natural ground water flow and were sampled with time from the injection port. Rates of nitrification were determined from changes in nitrate and nitrite concentration relative to bromide. Initial tests were conducted with ¹⁵N-enriched ammonium; subsequent tests examined the effect of adding ammonium, nitrite, or oxygen above background concentrations and of adding difluoromethane, a nitrification inhibitor. In situ net nitrate production exceeded net nitrite production by 3- to 6- fold and production rates of both decreased in the presence of difluoromethane. Nitrification rates were 0.02–0.28 μmol (L aquifer)⁻¹ h⁻¹ with in situ oxygen concentrations and up to 0.81 μmol (L aquifer)⁻¹ h⁻¹ with non-limiting substrate concentrations. Geochemical considerations indicate that the rates derived from single-well injection tests yielded overestimates of in situ rates, possibly because the injections promoted small-scale mixing within a transport-limited reaction zone. Nonetheless, these tests were useful for characterizing ground water nitrification in situ and for comparing potential rates of activity when the tracer cloud included non-limiting ammonium and oxygen concentrations.     

Soil nitrogen turnover in proximal and distal stem areas of European beech trees

In European beech (Fagus sylvatica L.) forests, a large proportion of the water and ion input to the soil results from stemflow which creates a soil microsite of high element fluxes proximal to the tree trunk. The soil proximal to the stem is considered to have different rates of nitrogen turnover which might influence the estimation of N-turnover rates at the stand scale. In a previous study we reported high nitrate fluxes with seepage proximal to the stems in a forest dominated by European beech in Steigerwald, Germany. Here, we investigated the soil nitrogen turnover in the top 15 cm soil in proximal (defined as 1 m² around beech stems) and distal stem areas. Laboratory incubations and in situ sequential coring incubations were used to determine the net rates of ammonification, nitrification, and root uptake of mineral nitrogen. In the laboratory incubations higher rates of net nitrogen mineralization and nitrification were found in the forest floor proximal to the stem as compared to distal stem areas. No stem related differences were observed in case of mineral soil samples. In contrast, the in situ incubations revealed higher rates of nitrification in the mineral soil in proximal stem areas, while net nitrogen mineralization was equal in proximal and distal areas. In the in situ incubations the average ratio of nitrification/ammonification was 0.85 in proximal and 0.34 in distal stem areas. The net nitrogen mineralization was 4.4 g N m^-2 90 day^-1 in both areas. Mineralized nitrogen was almost completely taken up by tree roots with ammonium as the dominant nitrogen species. The average ratio of nitrate/ammonium uptake was 0.69 in proximal and 0.20 in distal areas. The higher water content of the soil in proximal stem areas is considered to be the major reason for the increased rates of nitrification. Different nitrogen turnover rates in proximal stem areas had no influence on the nitrogen turnover rates in soil at the stand scale. Consequently, the observed high nitrate fluxes with seepage proximal to stems are attributed to the high nitrogen input by stemflow rather than to soil nitrogen turnover. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quantitative traits associated with adaptation of three barley (Hordeum vulgare L) cultivars to suboptimal iron supply

A laboratory method was developed that allows determination of in situ net nitrification with high sensitivity and at high temporal resolution. Nitrate in soils is quantitatively converted into nitrous oxide under strictly anaerobic conditions in the presence of 10 kPa acetylene by the soil endogenous denitrifier population, with the N₂O detected by a gas chromatograph equipped with a ⁶³Ni electron capture detector. Thus, even low net nitrification rates, i.e. small net increases in soil nitrate concentrations can easily be detected. Comparison of results using this method with results obtained using the classical in situ incubation method (buried bag soil incubation) revealed excellent agreement. Application of the new method allowed both determination of the seasonal pattern of net nitrification as well as correlation analysis between in situ NO and N₂O flux rates and in situ net nitrification rates of the forest soils studied. Regardless of the forest site studied (spruce, spruce limed, beech), and during each year of a 3 years period (1995–1997), net nitrification varied strongly with season and was least during winter and greatest during summer. The long-term annual, mean rate of net nitrification for the untreated spruce site, the limed spruce site and the beech site were 1.54 ± 0.27 mg N kg^–1 sdw d^–1, 1.92 ± 0.23 mg N kg^–1 sdw d^–1 and 1.31 ± 0.23 mg N kg^–1 sdw d^–1, respectively. In situ rates of nitrification and NO and N₂O emission were strongly correlated for all sites suggesting that nitrification was the dominate source of NO as well as N₂O.     

Inorganic nitrogen dynamics in the River Seine downstream from Paris (France)

The River Seine, below Paris, receives the effluents from a large sewage treatment plant, increasing the ammonium concentration up to 6 mgN.1⁻ in late summer. Careful measurement of ammonium, nitrate and organic nitrogen during the downriver travel of the water masses over 100 km below the outfall, along with direct determination of nitrification and benthic fluxes, allowed to establish a budget of nitrogen transport and transformations in this reach of the river. Nitrification is shown to start after a distinct period of several days required for the growth of a significant nitrifying bacterial population. Denitrification is active in the upper layer of bottom sediments but absent from the water column. Comparison of our data with those published for the period 1973–1976 shows that the nitrate load carried by the river has increased not only because of higher runoff of agricultural nitrate in the upstream part of the watershed, but also as a result of the severe reduction in the rate of denitrification processes, owing to the restoration of better oxygen conditions.     

Remediation of Nitrogen-Contaminated Sediment Using Bioreactive,Thin-layer Capping with Biozeolite

In situ amendment of nitrogen-contaminated sediment using bioreactive, thin-layer capping (BTC) with biozeolite (i.e., zeolite with heterotrophic nitrifiers as well as aerobic denitrifiers attached) was studied herein. BTC with biozeolite for nitrogen-contaminated sediment management was evaluated through long-term (170 days) sediment incubation laboratory experiments. The results showed that BTC with relatively small dose rates (<10 kg m^?2) of biozeolite reduced the total nitrogen (TN) concentration in overlying water by over 90%, so it was effective in reducing the amount of N released from sediment. Higher-dose rates of biozeolite capping achieved an even higher removal efficiency. With the DO concentration of 1.5 ～ 6.5 mg L^?1 in overlying water, the reduction efficiency of TN in overlying water using BTC was higher than that less than 1 mg L^?1. In BTC systems, biological regeneration (i.e., heterotrophic nitrifiers attached to zeolite can regenerate the zeolite ion exchange capacity for ammonium) occurred in biozeolite which was saturated with ammonium during the nitrification period. In addition, TN contents in surface sediment in BTC systems were reduced at different levels after the experiment. These findings indicate that the BTC can be a feasible remedial approach to reduce N in overlying water and sediment in eutrophic water bodies. In the BTC, N load was reduced by the added biozeolite through adsorbing ammonium (NH₄⁺-N), converting NH₄⁺-N into nitrate nitrogen (NO₃^?-N) and nitrogen gas (N₂), and assimilating inorganic nitrogen.     

Significance of archaeal nitrification in hypoxic waters of the Baltic Sea

Ammonia-oxidizing archaea (AOA) of the phylum Thaumarchaeota are widespread, and their abundance in many terrestrial and aquatic ecosystems suggests a prominent role in nitrification. AOA also occur in high numbers in oxygen-deficient marine environments, such as the pelagic redox gradients of the central Baltic Sea; however, data on archaeal nitrification rates are scarce and little is known about the factors, for example sulfide, that regulate nitrification in this system. In the present work, we assessed the contribution of AOA to ammonia oxidation rates in Baltic deep basins and elucidated the impact of sulfide on this process. Rate measurements with ¹⁵N-labeled ammonium, CO₂ dark fixation measurements and quantification of AOA by catalyzed reporter deposition–fluorescence in situ hybridization revealed that among the three investigated sites the highest potential nitrification rates (122–884 nmol l⁻¹per day) were measured within gradients of decreasing oxygen, where thaumarchaeotal abundance was maximal (2.5–6.9 × 10⁵ cells per ml) and CO₂ fixation elevated. In the presence of the archaeal-specific inhibitor GC₇, nitrification was reduced by 86–100%, confirming the assumed dominance of AOA in this process. In samples spiked with sulfide at concentrations similar to those of in situ conditions, nitrification activity was inhibited but persisted at reduced rates. This result together with the substantial nitrification potential detected in sulfidic waters suggests the tolerance of AOA to periodic mixing of anoxic and sulfidic waters. It begs the question of whether the globally distributed Thaumarchaeota respond similarly in other stratified water columns or whether the observed robustness against sulfide is a specific feature of the thaumarchaeotal subcluster present in the Baltic Deeps.     

Nitrification and nitrate reductase activity along a secondary successional gradient

The control of nitrification was studied in a secondary successional gradient on Nantucket Island, MA. It was hypothesized that 1) variability in nitrification along the gradient is controlled by litter primary and secondary chemistry, and 2) differences in nitrate availability along the gradient are reflected in potential nitrate assimilation rates in plant tissue. Nitrification varied significantly (p<0.05) by successional stage in all study sites, generally increasing with successional age. The ratio of nitrification to total N mineralization did not vary significantly between successional stages, suggesting substrate limitation of nitrification. Litter terpenoid resin concentration was a significant predictor (p<0.05) of nitrification rate, but soil %C, %N, and water content also contributed significantly to a stepwise regression model predicting nitrification. Nitrate reductase activity (NRA), an index of potential nitrate assimilation, was measured in an assay species (Schizachyrium scoparium). Although there was no significant correlation with nitrification, NRA was significantly (p<0.05) negatively correlated with soil ammonium concentration along the successional gradient at one site, suggesting that plants preferentially utilized ammonium in this system.     

INHIBITION OF NITRIFICATION BY CLIMAX ECOSYSTEMS. II. ADDITIONAL EVIDENCE AND POSSIBLE ROLE OF TANNINS

The same nine plots were used in this study as in our previous study on inhibition of nitrification (Rice and Pancholy, 1972). These consisted of three stands representing two stages of old field succession and the climax in each of three vegetation types in Oklahoma: tall grass prairie, post oak-blackjack oak forest, and oak-pine forest. Soil samples were analyzed three times during the growing season of 1972 for exchangeable ammonium nitrogen, nitrate, and numbers of Nitrosomonas and Nitrobacter. Results were similar to those obtained during the entire year of 1971. The amount of ammonium nitrogen was lowest in the first successional stage, intermediate in the intermediate successional stage, and highest in the climax. The amount of nitrate was highest in the first successional stage, intermediate in the intermediate successional stage, and lowest in the climax. The numbers of nitrifiers were highest in the first successional stage usually and decreased to a very low number in the climax. These data furnish additional evidence that the nitrifiers are inhibited in the climax so that ammonium nitrogen is not oxidized to nitrate as readily in the climax as in the successional stages. This would aid in the conservation of nitrogen and energy in the climax ecosystem. Some inhibition of nitrification occurred in the intermediate stage of succession also. Previous studies of tannins indicated that these are inhibitory to nitrification, so all important plant species in the intermediate successional stage and the climax were analyzed for total tannin content. A method for extracting and quantifying condensed tannins from soils was developed and the amounts of tannins were determined in each 15-cm level down to 60 cm in the same two plots in each vegetation type. Gallic and ellagic acids, which result from the digestion of hydrolyzable tannins in oak species, were also extracted and quantified in the climax oak-pine forest. All the important herbaceous species, including the grasses, were found to have considerable amounts of condensed tannins. The highest amounts of tannins occurred in the oaks and pine, however. Condensed tannins, hydrolyzable tannins, ellagic acid, gallic acid, digallic acid, and commercial tannic acid (hydrolyzable tannin), in very small concentrations, were all found to completely inhibit nitrification by Nitrosomonas in soil suspensions for 3 weeks, the duration of the tests. Slightly larger concentrations were required to inhibit nitrification by Nitrobacter under similar conditions. The concentrations of tannins, gallic acid, and ellagic acid found in the soil of the research plots were several times higher than the minimum concentrations necessary to completely inhibit nitrification. The inhibition of nitrification was always greater in the climax stand than in the intermediate successional stage in each vegetation type, and the concentration of tannins in the top 15 cm of soil was always higher in the climax stand than in the intermediate successional stage. Moreover, the amounts of tannins calculated to be added to each plot each year are much less than the amounts found in the soil, indicating that the tannins accumulate over a period of time. Thus, it appears that the tannins and tannin derivatives may play a continuous and rather prominent role in the inhibition of nitrification by vegetation.     

Nitrification and mineralization potentials in a limed Ultisol in the humid tropics

Summary We studied the effect of liming on the rates of mineralization and nitrification in a coarse-textured kaolinitic Ultisol. Soil samples were taken from field plots which received lime rates from 0 to 4mt/ha three years prior to the study. The pH of the soil samples varied from 4.2 to 6.1. Ammonification of soil organic N and added urea source proceeded readily and was not affected by lime rate. Nitrification occurred in both limed and unlimed soils but the rate of nitrification depended upon the rate of lime application. Soil pH, exchangeable Ca and exchangeable A1 were significantly correlated with the amount of NO₃-N accumulated at the end of the 65 days incubation period. Nitrification of NH₄-N from ammonium sulfate was absent in soils receiving lower rates of lime which gave pH values ranging from 4.2 to 4.8. Added ammonium source was nitrified readily after a 3-week delay period in the soil (pH 6.1) which received a higher rate of lime (4 mt/ha).     

Effect of simazine,lindane and ceresan on soil respiration and nitrification rates

Summary The effect of three pesticidesviz, simazine, lindane and ceresan upto 100-fold of field application was studied on organic matter mineralization as well as on nitrification rates in Delhi alluvial soil. The normal rate of simazine (2 ppm) did not influence CO₂ evolution but higher concentrations suppressed it. Lindane and ceresan inhibited CO₂ production from soil by normal concentrations but the same CO₂ production was enhanced in berseem roots treated soil upto 10 ppm of ceresan.The nitrification of ammonium sulphate in soil due to these pesticides was impaired significantly for varying periods. The inhibitory effect of higher concentrations was marked only upto 3 weeks, subsequently the rate of nitrification was restored. Among the nitrifiers, Nitrobacter were more susceptible as compared to Nitrosomonas.     

Low Nitrification Rates in Acid Scots Pine Forest Soils Are Due to pH-Related Factors

In a previous study, ammonia-oxidizing bacteria (AOB)-like sequences were detected in the fragmentation layer of acid Scots pine (Pinus sylvestris L.) forest soils (pH 2.9–3.4) with high nitrification rates (>11.0 μg g⁻¹ dry soil week⁻¹), but were not detected in soils with low nitrification rates (<0.5 μg g⁻¹ dry soil week⁻¹). In the present study, we investigated whether this low nitrification rate has a biotic cause (complete absence of AOB) or an abiotic cause (unfavorable environmental conditions). Therefore, two soils strongly differing in net nitrification were compared: one soil with a low nitrification rate (location Schoorl) and another soil with a high nitrification rate (location Wekerom) were subjected to liming and/or ammonium amendment treatments. Nitrification was assessed by analysis of dynamics in NH₄ ⁺-N and NO₃ ⁻-N concentrations, whereas the presence and composition of AOB communities were assessed by polymerase chain reaction–denaturing gradient gel electrophoresis and sequencing of the ammonia monooxygenase (amoA) gene. Liming, rather than ammonium amendment, stimulated the growth of AOB and their nitrifying activity in Schoorl soil. The retrieved amoA sequences from limed (without and with N amendment) Schoorl and Wekerom soils exclusively belong to Nitrosospira cluster 2. Our study suggests that low nitrification rates in acidic Scots pine forest soils are due to pH-related factors. Nitrosospira cluster 2 detected in these soils is presumably a urease-positive cluster type of AOB.     

Effects of a clearcut on the net rates of nitrification and N mineralization in a northern hardwood forest,Catskill Mountains,New York,USA

The Catskill Mountains of southeastern New York receive among the highest rates of atmospheric nitrogen (N) deposition in eastern North America, and ecosystems in the region may be sensitive to human disturbances that affect the N cycle. We studied the effects of a clearcut in a northern hardwood forest within a 24-ha Catskill watershed on the net rates of N mineralization and nitrification in soil plots during 6 years (1994–1999) that encompassed 3-year pre- and post-harvesting periods. Despite stream NO₃^– concentrations that increased by more than 1400 mol l^–1 within 5 months after the clearcut, and three measures of NO₃^– availability in soil that increased 6- to 8-fold during the 1^st year after harvest, the net rates of N mineralization and nitrification as measured by in situ incubation in the soil remained unchanged. The net N-mineralization rate in O-horizon soil was 1– 2 mg N kg^–1 day^–1 and the net nitrification rate was about 1 mg N kg^–1 day^–1, and rates in B-horizon soil were only one-fifth to one-tenth those of the O-horizon. These rates were obtained in single 625 m² plots in the clearcut watershed and reference area, and were confirmed by rate measurements at 6 plots in 1999 that showed little difference in N-mineralization and nitrification rates between the treatment and reference areas. Soil temperature increased 1 ± 0.8 °C in a clearcut study plot relative to a reference plot during the post-harvest period, and soil moisture in the clearcut plot was indistinguishable from that in the reference plot. These results are contrary to the initial hypothesis that the clearcut would cause net rates of these N-cycling processes to increase sharply. The in situ incubation method used in this study isolated the samples from ambient roots and thereby prevented plant N uptake; therefore, the increases in stream NO₃^– concentrations and export following harvest largely reflect diminished uptake. Changes in temperature and moisture after the clearcut were insufficient to measurably affect the net rates of N mineralization and nitrification in the absence of plant uptake. Soil acidification resulting from the harvest may have acted in part to inhibit the rates of these processes. The US Governments right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Effect of continuous Cd feeding on the performance of a nitrification reactor

The inhibitory effect of Cd on nitrification was investigated in a continuous-flow system with enriched nitrifying bacteria. The maximum specific ammonium utilization rate and the half-saturation constant were found as 671 mg NH₄–N/g VSS day and 0.48 mg/l, respectively. In the case of continuous Cd input at 1 and 2.5 mg/l, nitrification was inhibited by 30% and 47%, respectively. Inhibition ranged from 20% to 40% and no further increase in inhibition was exhibited in new runs except at 10 mg/l influent Cd. At 10 mg/l influent Cd, specific ammonium utilization and nitrate production rates were inhibited by 90%. On the contrary, a serious nitrite accumulation was not observed during this period. When Cd feeding was stopped, recovery from inhibition was observed after 37 day which was seen by the improvement in ammonium utilization and nitrate production rates. A shift in microbial population from the initial Nitrosomonas sp. to the Cd-tolerant Nitrosospira sp. was observed in the recovery period from severe Cd inhibition. After the domination of Nitrosospira species, redosing at 10 mg/l and then at 15 mg/l did not affect the performance as before.     

Ammonium uptake in alpine streams in the High Tatra Mountains (Slovakia)

Uptake of NH _inf4 ^sup+ -N by streambed biota of mountain brooks was studied in the alpine zone of the High Tatra Mountains. Experiments were performed involving in situ dosing of ammonium directly to the acidified stream and incubations of ammonium and streambed bryophytes in enclosures within a range of pH from 4.45 to 8.10.NH _inf4 ^sup+ -N uptake length decreased with decreasing stream discharge, while comparable values of discharge-normalized uptake lengths were found during two in situ experiments.Maximum uptake rates of NH _inf4 ^sup+ -N obtained during the incubation of bryophytes (6 to 11 mg m^–2 h^–1) were comparable with results of two in situ experiments (8 and 12 mg m^–2 h^–1). The average NH _inf4 ^sup+ -N uptake rates observed during incubations lasting 3 to 5 hours (4.3 mg m^–2 h^–1) were not related to the pH of stream water. Nitrification of about 50% of the NH _inf4 ^sup+ -N added was observed in non-acidified streams, but was negligible in acidified streams. Significant photoinhibition of nitrification was observed in non-acidified streams during enclosure experiments.     

曝气充氧条件下污染河道氨挥发特性模拟

以污染河道为研究对象,模拟研究污染河道在曝气充氧(底泥曝气,ES组;水曝气,EW组)条件下氨挥发的特性,探讨主要影响因素及其作用过程.研究表明,污染河道水体具有一定氨挥发潜力,在实验室模拟条件下,氨挥发速率平均为2.51mg·m-2·h-1,相当于0.50 kgN· hm-2·d-1;曝气污染河道水体的氨挥发有一定的促进作用,与对照相比(EC组)氨挥发速率和累积氨挥发量存在显著差异(P＜ 0.05);不同曝气方式对氨挥发过程影响不同,氨挥发速率存在显著差异(P＜0.05);至实验结束,EW组的累积挥发量为2809.76 mg/m2,分别是ES组和EC组的1.17和2.25倍;各实验组的氨挥发累积量用一级动力学方程能很好地拟合,根据模型可以预测氨挥发量;同一温度条件下,pH值、铵氮浓度和通气频率是影响氨挥发的主要因素;曝气可以通过增加通气频率和提高水体pH值来促进氨挥发进行;在曝气作用下随着硝化过程的进行对氨挥发有一定的限制作用;曝气条件下,氨挥发作用在硝化过程启动阶段最为明显.     

The role of nitrogen in promoting the toxic cyanophyte Cylindrospermopsis raciborskii in a subtropical water reservoir

1. The relative contribution of dissolved and atmospheric nitrogen to promoting dominance of the toxic nitrogen‐fixing cyanobacterium, Cylindrospermopsis raciborskii was examined in a subtropical water reservoir, North Pine Reservoir. 2. A combination of process studies in situ and analysis of historical water quality data suggests that nitrogen fixation was not the principal mechanism for acquiring nitrogen and unlikely to be the mechanism whereby C. raciborskii gains a competitive advantage. Ammonium was the preferred nitrogen source, followed by nitrate then nitrogen fixation. 3. Ammonium uptake rates in the euphotic zone were higher in the summer and autumn months compared with winter and spring coinciding with lower ammonium concentrations. Nitrate uptake rates did not appear to vary seasonally and were lower than those for ammonium in the summer, but similar in winter. Nitrate concentrations were higher in winter than summer and generally higher than ammonium concentrations. 4. Ammonium and nitrate uptake rates were similar at light intensities between 10% and 100% of surface light, contrasting with primary productivity which peaked between about 10 to 20% of surface light. Thus the phytoplankton population was adapted to low light conditions but remained able to utilise dissolved inorganic nitrogen over a wide range of light conditions. 5. The ammonium pool in the surface waters was relatively small compared with the phytoplankton uptake rates, and ammonium must therefore be rapidly recycled through the food web over periods of less than 1 h. Short‐term depletion may result, during which time the higher concentrations of nitrate are likely to provide a supplementary supply of nitrogen. 6. The dominance of C. raciborskii in this reservoir is more likely to be due to a superior ability to scavenge and store the low concentrations of phosphate, and a superior adaptation to the low light conditions exacerbated by artificial mixing.     

Competition for ammonium between plant roots and nitrifying and heterotrophic bacteria and the effects of protozoan grazing

The competition for limiting amounts of ammonium between the chemolithotrophic ammonium-oxidizing species Nitrosomonas europaea, the heterotrophic species Arthrobacter globiformis and roots of Plantago lanceolata (Ribwort plantain) was studied in a series of model systems of increasing complexity, i.e. energy-limited continuous cultures, non-water-saturated continuously percolated soil columns and pots with γ-sterilized soil planted with axetic P. lanceolata seedlings. The effects of bacterial grazing by the flagellate species Adriamonas peritocrescens on the competition for ammonium were also investigated in the three model systems. It was found that N. europaea was a weaker competitor for ammonium than either A. globiformis or plant roots of P. lanceolata. It is assumed that the heterotrophic bacteria have a higher affinity for ammonium than the nitrifying bacteria, whereas growing plant roots have a greater capacity to exploit the soil for ammonium than the immobile nitrifying bacteria. It is not very likely that allelochemicals were involved in suppressing the nitrification process. Four reasons are given for this assumption. Presence of the flagellates strongly stimulated the potential nitrification rate in all model systems. It is assumed that there is a more even distribution over the soil of either nitrifying bacteria or their substrate ammonium in the presence of flagellates. In addition to the distribution effect, there is a stimulation of the potential ammonium oxidation rate. The results are discussed in the light of the function of nitrate as nitrogen sink in the biogeochemical nitrogen cycle.     

Measurement of in situ rates of nitrification in sediment

A method has been developed for the measurement of nitrification rates in intact sediment cores without disturbing the concentration gradients of oxygen and ammonium. N-serve (2-chloro-6-trichloromethyl-pyridine), a specific inhibitor of the autotrophic ammonium oxidation, was injected into a 0–2 cm surface layer of the sediment (20 ppm) and added to the water column of sediment cores (5 ppm). N-serve in these concentrations was sufficient to inhibit nitrification, but did not change the rate of ammonium production or incorporation in sediment suspensions, which were incubated aerobically and anaerobically. The ammonium accumulation in cores injected with N-serve was thus equal to the amount of ammonium which was oxidized to nitrate in the control cores. Nitrification rates were in the range of 0–3 mmol N m^{–2 –1}     

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.