Nitrogen retention in wetlands,lakes and rivers

As human activities continue to alter the global nitrogen cycle, the ability to predict the impact of increased nitrogen loading to freshwater systems is becoming more and more important. Nitrogen retention is of particular interest because it is through its combined processes (denitrification, nitrogen sedimentation and uptake by aquatic plants) that local and downstream nitrogen concentrations are reduced. Here, we compare the magnitude of nitrogen retention and its components in wetlands, lakes and rivers. We show that wetlands retain the highest proportion of total nitrogen loading, followed by lakes and then rivers. The differences in the proportion of N retained among systems is explained almost entirely by differences in water discharge. Denitrification is the primary mechanism of nitrogen retention, followed by nitrogen sedimentation and uptake by aquatic plants.     

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH₄NO₃) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m^-2 y^-1), high N (15 g N m^-2 y^-1), and low N + sulfur (5 g N m^-2 y^-1 plus 7.4 g S m^-2 y^-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of ¹⁵N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996. Received 2 April 1999; Accepted 29 October 1999.     

Plasticity in Resource Allocation and Nitrogen-use Efficiency in Riparian Vegetation: Implications for Nitrogen Retention

In this work, we summarize our current understanding of the function of riparian zones and describe an investigation of changes in the production per unit nitrogen (N) taken up, or nitrogen-use efficiency (NUE) and resource allocation of a riparian shrub in response to changes in N availability. Empirical work included measuring leaf %N and root-to-shoot ratios (R:S) of individual riparian shrubs (Baccharis salicifolia, or seepwillow) growing at a range of N availabilities in the field and growing in fertilized and unfertilized plots in a field fertilization experiment. In both observational and experimental work, N availability was related positively to %N of plant tissues and negatively to R:S. We used a simulation model to investigate feedbacks between seepwillow responses to and effects on N availability. In the model, plasticity in resource allocation and NUE in response to changes in N led to lower productivity at low N supply and higher productivity and lower retention at high N supply than was observed in plants constrained to a constant %N and R:S. Furthermore, uptake became relatively more important as a retention mechanism when plants responded to high N supply. These feedbacks could have significant effects on N retention by riparian zones in watersheds receiving large fertilizer inputs of N or on ecosystems exposed to high rates of atmospheric N deposition.     

Nitrogen Fluxes and Retention in Urban Watershed Ecosystems

Although the watershed approach has long been used to study whole-ecosystem function, it has seldom been applied to study human-dominated systems, especially those dominated by urban and suburban land uses. Here we present 3 years of data on nitrogen (N) losses from one completely forested, one agricultural, and six urban/suburban watersheds, and input–output N budgets for suburban, forested, and agricultural watersheds. The work is a product of the Baltimore Ecosystem Study, a long-term study of urban and suburban ecosystems, and a component of the US National Science Foundations long-term ecological research (LTER) network. As expected, urban and suburban watersheds had much higher N losses than did the completely forested watershed, with N yields ranging from 2.9 to 7.9 kg N ha^–1 y^–1 in the urban and suburban watersheds compared with less than 1 kg N ha^–1 y^–1 in the completely forested watershed. Yields from urban and suburban watersheds were lower than those from an agricultural watershed (13–19.8 kg N ha^–1 y^–1). Retention of N in the suburban watershed was surprisingly high, 75% of inputs, which were dominated by home lawn fertilizer (14.4 kg N ha^–1 y^–1) and atmospheric deposition (11.2 kg N ha^–1 y^–1). Detailed analysis of mechanisms of N retention, which must occur in the significant amounts of pervious surface present in urban and suburban watersheds, and which include storage in soils and vegetation and gaseous loss, is clearly warranted.     

Sources of Nitrogen to the Riparian Zone of a Desert Stream: Implications for Riparian Vegetation and Nitrogen Retention

Riparian zones effectively remove nitrogen (N) from water flowing through riparian soils, particularly in agricultural watersheds. The mechanism of N removal is still unclear, especially the role of vegetation. Uptake and denitrification are the two most commonly studied mechanisms. Retention of groundwater N by plant uptake is often inferred from measurements of N in net incremental biomass. However, this assumes other sources of N are not contributing to the N demand of plants. The purpose of this work was to investigate the relative importance of three sources of available N to riparian trees in a desert stream—input in stream water during floods, input during baseflow, and mineralization of N from soil organic matter. Two approaches were used; a mass balance approach in which the mass of available N from each source was estimated, and a correlational approach in which indexes of each source were compared to leaf N for individual willow trees. Total N from all sources was 396 kg ha⁻¹ y⁻¹, with 172 kg ha⁻¹ y⁻¹ from mineralization, 214 kg ha⁻¹ y⁻¹ from the stream during baseflow, and 9.6 kg ha⁻¹ y⁻¹ from floods. Leaf N was significantly related to N mineralization rates and flood inputs; it was not related to baseflow inputs. We conclude that mineralization is a major source of available N for willow trees, subsidized by input of N from floods. Baseflow inputs are most likely removed by rapid denitrification at the stream–riparian edge, while higher rates of flood supply exceed the capacity of this “filter.” Received 18 January 2001; accepted 15 June 2001.     

陆地生态系统氮饱和对植物影响的生理生态机制

由于化石燃料的燃烧、含氮化肥的使用以及畜牧业等人类活动的影响,向大气中排放的含氮化合物数量不断上升,从而引起大气氮沉降的增加,使得某些陆地生态系统出现氮饱和现象。丈章综述了全球氮沉降与陆地生态系统氮饱和现状,探讨了氮饱和对植物光合作用、养分平衡和抗逆性的影响机制。     

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO₃ ^–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO₃ ^– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO₃ ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO₃ ^-. At the regional scale, we observed high variability in the magnitude of NO₃ ^- losses: while six of the surveyed watersheds showed accelerated rates of NO₃ ^– loss, three did not. Moreover, in contrast to the strong linear relationship between NO₃ ^– loss and crown damage within HBEF watersheds [r ²: (W1 = 0.91, W6 = 0.85)], stream water NO₃ ^– concentrations were weakly related to crown damage (r ² = 0.17) across our regional sites. The efflux of NO₃ ^– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO₃ ^– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA.     

Factors Affecting Nitrous Oxide Production: A Comparison of Biological Nitrogen Removal Processes with Partial and Complete Nitrification

Nitrous oxide (N₂O) emission from biological nitrogen removal (BNR) processes has recently received more research attention. In this study, two lab-scale BNR systems were used to investigate the effects of various operating parameters including the carbon to nitrogen (C/N) ratio, ammonia loading, and the hydraulic retention time on N₂O production. The first system was operated in a conventional BNR mode known as the Ludzack–Ettinger (LE) process, consisting of complete denitrification and nitrification reactors, while the second one was operated in a shortcut BNR (SBNR) mode employing partial nitrification and shortcut denitrification, which requires less oxygen and carbon sources. As the C/N ratio was decreased, a significant increase in N₂O production was observed only in the anoxic reactor of the LE process, indicating that N₂O was released as an intermediate of the denitrification reaction under the carbon-limited condition. However, the SBNR process did not produce significant N₂O even at the lowest C/N ratio of 0.5. When the SBNR process was subjected to increasing concentrations of ammonia, N₂O production from the aerobic reactor was rapidly increased. Furthermore, the increasing production of N₂O was observed mostly in the aerobic reactor of the SBNR process with a decline in hydraulic retention time. These experimental findings indicated that the increase in N₂O production was closely related to the accumulation of free ammonia, which was caused by an abrupt increase of the ammonium loading. Consequently, the partial nitrification was more susceptible to shock loading conditions, resulting in a high production of N₂O, although the SBNR process was more efficient with respect to nitrogen removals as well as carbon and oxygen requirements.     

脱氮功能菌去除市政废水中氮素的研究

为了探讨实验室筛选获得的氨氧化细菌CM-NRO14和反硝化细菌CM-NRD3联合去除市政废水中氮素的应用价值,采用了两级A/O工艺进行菌株去除废水中氮素的小试实验,最后将菌株用于废水脱氮工程中。结果表明,脱氮功能菌实现了短程硝化-反硝化,氨氮去除率在98%以上,总氮去除率在75%以上, COD (化学需氧量)去除率大于90%,出水各项指标均低于城镇污水处理厂污染物排放一级(A)标准。脱氮功能菌在去除市政废水中氮素方面有很高的应用价值,可用于城镇污水处理厂提标改造等。     

Rapid Cycling of Organic Nitrogen in Taiga Forest Ecosystems

ABSTRACT We examined the dynamics of organic nitrogen (N) turnover in situ across a primary successional sequence in interior Alaska, USA, in an attempt to understand the magnitude of these fluxes in cold, seasonally frozen soils. Through a combination of soil extraction procedures and measurements of ¹³C-enriched CO₂ efflux from soils amended in the field with ¹³C-labeled amino acids, we were able to trace the fate of this N form. Amino acid turnover in situ at soil temperatures of 10°C or below show that amino acids represent a highly dynamic soil N pool with turnover times of approximately 3–6 h. The rapid turnover of free amino acids is associated with high soil proteolytic activity, which in turn is tightly correlated with soil protein concentration. Moreover, these estimates of soil amino acid turnover in the field correspond well with measurements of amino acid turnover under equivalent temperatures in the laboratory. The gross flux of amino acid-N over the growing season greatly exceeded the annual vegetation N requirement, suggesting that microbial biomass represent a significant sink for this organic N. Depending on the strength of this sink, N flow via free soil amino acids can potentially account for the entire N demand of vegetation in the absence of net N mineralization. These relationships underscore the important biogeochemical role of labile DON fractions in high-latitude forest ecosystems.     

Organic Nitrogen Retention in the Atchafalaya River Swamp

Freshwater diversions from the lower Mississippi River into the region’s wetlands have been considered an alternative means for reducing nitrogen loading. The Atchafalaya River Swamp, the largest freshwater swamp in North America, carries the entire discharge of the Red River and 30% of the discharge of the Mississippi River, but it is largely unknown how much nitrogen actually can be retained from the overflowing waters of the Mississippi–Atchafalaya River system. Nitrogen discharge from the upper Mississippi River Basin has been implicated as the major cause for the hypoxia in the Northern Gulf of Mexico, which threatens not only the aquatic ecosystem health, but also Louisiana’s fishery industry, among other problems. This study was conducted to determine the change in organic nitrogen mass as water flows through the Atchafalaya River Swamp and into the Gulf of Mexico. By utilizing the river’s long-term discharge and water quality data (1978–2002), monthly and annual organic nitrogen fluxes were quantified, and their relationships with the basin’s hydrologic conditions were investigated. A total Kjeldahl nitrogen (TKN) mass input–output balance between the upstream (Simmesport) and downstream (Morgan City and Wax Lake Outlet) locations was established to examine the organic nitrogen removal potential for this large swamp. The results showed that on average, TKN input into the Atchafalaya was 200 323 tons year⁻¹ and TKN output leaving the basin was 145 917 tons year⁻¹, resulting in a 27% removal rate of organic nitrogen. Monthly TKN input and output in the basin were highest from March to June (input vs. output: 25 000 vs. 18 000 tons month⁻¹) and lowest from August to November (8000 vs. 6000 tons month⁻¹). There was a large variation in both annual and inter-annual organic nitrogen removals. The variability was positively correlated with the amount of inflow water at Simmesport, suggesting that regulating the river’s inflow at the Old River flood control structures may help reduce nitrogen loading of the Mississippi River to the Gulf of Mexico. Furthermore, the in-stream loss of organic nitrogen indicates that previous studies may have overestimated nitrogen discharge from the Mississippi–Atchafalaya River system.     

Nitrogen Controls on Fine Root Substrate Quality in Temperate Forest Ecosystems

Nitrogen controls on fine root substrate quality (that is, nitrogen and carbon-fraction concentrations) were assessed using nitrogen availability gradients in the Harvard Forest chronic nitrogen addition plots, University of Wisconsin Arboretum, Blackhawk Island, Wisconsin, and New England spruce-fir transect. The 27 study sites encompassed within these four areas collectively represented a wide range of nitrogen availability (both quantity and form), soil types, species composition, aboveground net primary production, and climatic regimes. Changes in fine root substrate quality among sites were most frequently and strongly correlated with nitrate availability. For the combined data set, fine root nitrogen concentration increased (adjusted R ² = 0.46, P < 0.0001) with increasing site nitrate availability. Fine root “extractive” carbon-fraction concentrations decreased (adjusted R ² = 0.32, P < 0.0002), “acid-soluble” compounds increased (adjusted R ² = 0.35, P < 0.0001), and the “acid-insoluble” carbon fraction remained relatively high and stable (combined mean of 48.7 ± 3.1% for all sites) with increasing nitrate availability. Consequently, the ratio of acid-insoluble C–total N decreased (adjusted R ² = 0.40, P < 0.0001) along gradients of increasing nitrate availability. The coefficients of determination for significant linear regressions between site nitrate availability and fine root nitrogen and carbon-fraction concentrations were generally higher for sites within each of the four study areas. Within individual study sites, tissue substrate quality varied between roots in different soil horizons and between roots of different size classes. However, the temporal variation of fine root substrate quality indices within specific horizons was relatively low. The results of this study indicate that fine root substrate quality increases with increasing nitrogen availability and thus supports the substrate quality component of a hypothesized conceptual model of nitrogen controls on fine root dynamics that maintains that fine root production, mortality, substrate quality, and decomposition increase with nitrogen availability in forest ecosystems in a manner that is analogous to foliage.     

N Retention in Urbanizing Headwater Catchments

Urbanization can potentially alter watershed nitrogen (N) retention via combined changes in N loading, water runoff, and N processing potential. We examined N export and retention for two headwater catchments (∼4 km²) of contrasting land use (16% vs. 79% urban) in the Plum Island Ecosystem (PIE-LTER) watershed, MA. The study period included a dry year (2001–2002 water year) and a wet year (2002–2003 water year). We generalized results by comparing dissolved inorganic nitrogen (DIN) concentrations from 16 additional headwater catchments (0.6–4.2 km²) across a range of urbanization (6–90%). Water runoff was 25–40% higher in the urban compared to the forested catchment, corresponding with an increased proportion of impervious surfaces (25% vs. 8%). Estimated N loading was 45% higher and N flux 6.5 times higher in the urban than in the forested catchment. N retention (1 − measured stream export / estimated loading) was 65–85% in the urban site and 93–97% in the forested site, with lower retention rates during the wetter year. The mechanisms by which N retention stays relatively high in urban systems are poorly known. We show that N retention is related to the amount of impervious surface in a catchment because of associated changes in N loading (maximized at moderate levels of imperviousness), runoff (which continues to increase with imperviousness), and biological processes that retain N. Continued declines in N retention due to urbanization have important negative implications for downstream aquatic systems including the coastal zone.     

Nitrogen Loss and Denitrification as Studied in Relation to Reductions in Nitrogen Loading in a Shallow,Hypertrophic Lake (Lake Søbygård,Denmark)

A detailed mass balance on nitrogen was carried out in shallow and hypertrophic Lake Søbygård during 4.5 years before through 2.5 years after a 36 % reduction in nitrogen loading. Annual mean loss rate of nitrogen was 159–229 mg N m⁻² d⁻¹ before the loading reduction and 125 mg N m⁻² d⁻¹ after. In spite of a short hydraulic retention time (18–27 days) the proportion of nitrogen loading lost in the lake was high (38–53 %) and not affected by changes in loading. Calculated denitrification accounted for 86–93% of the loss rate, while 7–14% was permanently buried. Marked seasonal variations in the loss percentage were found during the season, ranging from 23 % in first quarter to 65 % in third quarter. The seasonal variation in the loss percentage of nitrogen showed a hysteresis like relationship to temperature, with a high percentage in fourth quarter. This suggests that the amount of available substrate, which mainly consists of sedimentated phytoplankton, accumulated during summer, is an important regulating factor. The ability of various published input-output models to predict the observed changes in in-lake nitrogen concentration in Lake Søbygård was tested. This study has further confirmed that small lakes with short retention and high nitrogen loading may significantly reduce the nitrogen loading of downstream aquatic environments.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

Lotic meiofaunal community dynamics: colonisation, resilience and persistence in a spatially and temporally heterogeneous environment

1.  The spatial and temporal dynamics of lotic meiofaunal communities were examined with a focus on colonisation, maintenance of populations in lotic environments and persistence of meiofaunal communities.
2.  Lotic meiofaunal colonisation of new habitats may take place via a number of mechanisms and is rapid at both the patch scale (within hours–days) and the drainage basin scale (within 20 years). Successional patterns in lotic meiofaunal communities are evident although data are extremely limited.
3.  Lotic meiofaunal communities appear to be resilient to high flow disturbances. Resilience is moderated by the availability of in-stream refugia and habitat hydrology. Lotic meiofauna may also adopt a 'refuge as habitat' approach to such disturbances.
4.  Lotic cyclopoid copepods possess a common suite of life history characteristics that confers resilience to disturbances. Compared to pelagic planktonic species they have short generation times, many descendants per reproductive cycle and long lifespans. Females in 'source' populations are likely to survive disturbances and so could continuously reproduce over a long period of time producing large numbers of offspring which develop rapidly and recolonise 'sink' areas of the stream.
5.  Persistence of lotic meiofaunal communities is highly variable. Meiofaunal persistence does not increase with increasing proportions of in-stream flow refugia.     

白洋淀水陆交错带土壤对磷氮截留容量的初步研究

１　白洋淀水陆交错带的景观特征和土壤白洋淀地处冀中凹陷,是处于发育后期的草型富营养化湖泊。在整个湖区３６６ｋｍ２的范围内,以芦苇植被为景观特征的水陆交错带约占３６％,分布在围堤内湖边的洼地上和大小淀泊之间。其精细景观类型可划分成苇园和苇地二种类型;其中苇园约占１／３［１］,主要分布在淀泊之间和近围,特征是沟壕纵横,管理精细,景观斑块类型较小,匀一化程度高;苇地约占２／３,分布在苇园外围地势较高处,连大片分布,斑块嵌接,微景观结构较复杂,多样性程度较高［１］。水陆交错带处于该湖泊和陆地系统间,是重…     

Nitrifier genomics and evolution of the nitrogen cycle.

Advances in technology have tremendously increased high throughput whole genome-sequencing efforts, many of which have included prokaryotes that facilitate processes in the extant nitrogen cycle. Molecular genetic and evolutionary analyses of these genomes paired with advances in postgenomics, biochemical and physiological experimentation have enabled scientists to reevaluate existing geochemical and oceanographic data for improved characterization of the extant nitrogen cycle as well as its evolution since the primordial era of planet Earth. Based on the literature and extensive new data relevant to aerobic and anaerobic ammonia oxidation (ANAMMOX), the natural history of the nitrogen-cycle has been redrawn with emphasis on the early roles of incomplete denitrification and ammonification as driving forces for emergence of ANAMMOX as the foundation for a complete nitrogen cycle, and concluding with emergence of nitrification in the oxic era.     

Nitrogen Cycles: Past, Present, and Future

This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N₂ to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to Highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially disconnected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N₂ by denitrification.