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Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: II. Temporal variations and mixing processes  [PDF]
L. Ruiz,S. Abiven,C. Martin,P. Durand
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2002,
Abstract: In catchments with impervious bedrock, the nitrate concentrations in streamwater often show marked seasonal and small inter-annual variations. The inter-annual trends are usually attributed to changes in nitrogen inputs, due to changes in land use or in nitrogen deposition whereas seasonal patterns are explained in terms of availability of soil nitrate for leaching and of seasonality of nitrogen biotransformations. The companion paper showed that inter-annual variations of nitrogen in streamwater are not directly related to the variations of land use. The aim of this study is to describe nitrate concentration variations in a set of very small adjacent catchments, and to discuss the origin of the inter-annual and seasonal trends. Data from four catchments at the Kerbernez site (South Western Brittany, France) were used in this study. Nitrate concentrations in streamwater were monitored for eight years (1992 to 1999) at the outlet of the catchments. They exhibit contrasting inter-annual and seasonal patterns. An extensive survey of agricultural practices during this period allowed assessment of the amount of nitrogen available for leaching. The discharges measured since 1997 show similar specific fluxes but very different seasonal dynamics between the catchments. A simple, lumped linear store model is proposed as an initial explanation of the differences in discharge and nitrate concentration patterns between the catchments. The base flow at the outlet of each catchment is considered as a mixture of water from two linear reservoirs with different time constants. Each reservoir comprises two water stores, one mobile contributing to discharge, the other, immobile, where nitrate moves only by diffusion. The storm flow, which accounts for less than 10% of the annual flux, is not considered here. Six parameters were adjusted for each catchment to fit the observed data: the proportion of deep losses of water, the proportion of the two reservoirs and the size and initial concentration of the two immobile stores. The model simulates the discharge and nitrate concentration dynamics well. It suggests that the groundwater store plays a very important role in the control of nitrate concentration in streamwater, and that the pattern of the seasonal variation of nitrate concentration may result from the long term evolution of nitrogen losses by leaching. Keywords: nitrate, diffuse pollution, groundwater, seasonal variations, agricultural catchment, simulation model
Estimation of nitrogen budgets for contrasting catchments at the landscape scale
E. Vogt, C. F. Braban, U. Dragosits, M. R. Theobald, M. F. Billett, A. J. Dore, Y. S. Tang, N. van Dijk, R. M. Rees, C. McDonald, S. Murray, U. M. Skiba,M. A. Sutton
Biogeosciences (BG) & Discussions (BGD) , 2013,
Abstract: A comprehensive assessment of nitrogen (N) flows at the landscape scale is fundamental to understand spatial interactions in the N cascade and to inform the development of locally optimised N management strategies. To explore these interactions, complete N budgets were estimated for two contrasting hydrological catchments (dominated by agricultural grassland vs. semi-natural peat-dominated moorland), forming part of an intensively studied landscape in southern Scotland. Local scale atmospheric dispersion modelling and detailed farm and field inventories provided high resolution estimations of input fluxes. Direct agricultural inputs (i.e. grazing excreta, N2 fixation, organic and synthetic fertiliser) accounted for most of the catchment N inputs, representing 82% in the grassland and 62% in the moorland catchment, while atmospheric deposition made a significant contribution, particularly in the moorland catchment, contributing 38% of the N inputs. The estimated catchment N budgets highlighted areas of key uncertainty, particularly N2 exchange and stream N export. The resulting N balances suggest that the study catchments have a limited capacity to store N within soils, vegetation and groundwater. The "catchment N retention", i.e. the amount of N which is either stored within the catchment or lost through atmospheric emissions, was estimated to be 13% of the net anthropogenic input in the moorland and 61% in the grassland catchment. These values contrast with regional scale estimates: Catchment retentions of net anthropogenic input estimated within Europe at the regional scale range from 50% to 90%, with an average of 82% (Billen et al., 2011). This study emphasises the need for detailed budget analyses to identify the N status of European landscapes.
Estimation of nitrogen budgets for contrasting catchments at the landscape scale  [PDF]
E. Vogt,C. F. Braban,U. Dragosits,M. R. Theobald
Biogeosciences Discussions , 2012, DOI: 10.5194/bgd-9-8989-2012
Abstract: A comprehensive assessment of nitrogen (N) flows at the landscape scale is fundamental to understand spatial interactions in the N cascade and to inform the development of locally optimised N management strategies. To explore this interactions, complete N budgets were estimated for two contrasting hydrological catchments (dominated by agricultural grassland vs. semi-natural peat-dominated moorland), forming part of an intensively studied landscape in southern Scotland. Local scale atmospheric dispersion modelling and detailed farm and field inventories provided high resolution estimations of input fluxes. Agricultural inputs (i.e. grazing excreta, organic and synthetic fertiliser) accounted for most of the catchment N inputs with 80% in the grassland and 57% in the moorland catchment, while atmospheric deposition made a significant contribution, particularly in the moorland catchment with 38% of the N inputs. The estimated catchment N budgets highlighted areas of key uncertainty, particularly N2 emissions from denitrification and stream N export. The resulting N balances suggest that the study catchments have a limited capacity to store N within soils, vegetation and groundwater. The "catchment N retention", i.e. the amount of N which is either stored within the catchment or lost through atmospheric emissions, was estimated to be 3% of the net anthropogenic input in the moorland and 55% in the grassland catchment. These values contrast with regional scale estimates: catchment retentions of net anthropogenic input estimated within Europe at the regional scale range from 50% to 90% with an average of 82% (Billen et al., 2011). This study emphasises the need for detailed budget analyses to identify the N status of European landscapes.
Solute transport dynamics in small, shallow groundwater-dominated agricultural catchments: insights from a high-frequency, multisolute 10 yr-long monitoring study
A. H. Aubert, C. Gascuel-Odoux, G. Gruau, N. Akkal, M. Faucheux, Y. Fauvel, C. Grimaldi, Y. Hamon, A. Jaffrézic, M. Lecoz-Boutnik, J. Molénat, P. Petitjean, L. Ruiz,P. Merot
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2013,
Abstract: High-frequency, long-term and multisolute measurements are required to assess the impact of human pressures on water quality due to (i) the high temporal and spatial variability of climate and human activity and (ii) the fact that chemical solutes combine short- and long-term dynamics. Such data series are scarce. This study, based on an original and unpublished time series from the Kervidy-Naizin headwater catchment (Brittany, France), aims to determine solute transfer processes and dynamics that characterise this strongly human-impacted catchment. The Kervidy-Naizin catchment is a temperate, intensive agricultural catchment, hydrologically controlled by shallow groundwater. Over 10 yr, five solutes (nitrate, sulphate, chloride, and dissolved organic and inorganic carbon) were monitored daily at the catchment outlet and roughly every four months in the shallow groundwater. The concentrations of all five solutes showed seasonal variations but the patterns of the variations differed from one solute to another. Nitrate and chloride exhibit rather smooth variations. In contrast, sulphate as well as organic and inorganic carbon is dominated by flood flushes. The observed nitrate and chloride patterns are typical of an intensive agricultural catchment hydrologically controlled by shallow groundwater. Nitrate and chloride originating mainly from organic fertilisers accumulated over several years in the shallow groundwater. They are seasonally exported when upland groundwater connects with the stream during the wet season. Conversely, sulphate as well as organic and inorganic carbon patterns are not specific to agricultural catchments. These solutes do not come from fertilisers and do not accumulate in soil or shallow groundwater; instead, they are biogeochemically produced in the catchment. The results allowed development of a generic classification system based on the specific temporal patterns and source locations of each solute. It also considers the stocking period and the dominant process that limits transport to the stream, i.e. the connectivity of the stocking compartment. This mechanistic classification can be applied to any chemical solute to help assess its origin, storage or production location and transfer mechanism in similar catchments.
Source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments
M. Voss, B. Deutsch, R. Elmgren, C. Humborg, P. Kuuppo, M. Pastuszak, C. Rolff,U. Schulte
Biogeosciences (BG) & Discussions (BGD) , 2006,
Abstract: Nitrate input to a river is largely controlled by land use in its catchment. We compared the information carried by the isotopic signatures of nitrate in 12 Baltic rivers, in relation to the vegetation cover, land use, and fertilization of agricultural land of their catchments. We found isotope values in nitrate ranging from 2 to 14‰ for δ15N and 8 to 25‰ for δ18O. The annual variability of riverine nitrate isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, the Vistula and Oder. Nordic rivers with relatively pristine vegetation in their catchments show not only low δ15N values and high δ18O-NO3 but also lower annual variability than rivers draining densely populated land. Seasonal signals were found in all the rivers. We used load weighted nitrate isotope data and data from the three major N sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results of an isotope mixing model (IMM-1) agree reasonably well with the same estimates for agricultural land derived from a Global Land Cover (GLC) data base, with a deviation varying from 16% to +26%. The comparison with an emission model (EM) reveals relatively good agreements for intensively used catchments ( 18 to +18% deviation). Rather unsatisfactory agreement was found between the IMM-1 and GLC calculations for pristine catchments ( 36 to +50% deviation). Advantages and limitations of the tested model are discussed.
Determination of bromide, chloride, fluoride, nitrate and sulphate by ion chromatography: comparisons of methodologies for rainfall, cloud water and river waters at the Plynlimon catchments of mid-Wales
M. Neal, C. Neal, H. Wickham,S. Harman
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2007,
Abstract: The results of determination of bromide, chloride, fluoride, nitrate and sulphate using ion chromatography (IC) are compared with those obtained by colorimetric and inductively coupled plasma optical emission spectroscopy (ICPOES) for rainfall, cloud water and stream waters in the Plynlimon experimental catchments of mid-Wales. For bromide, the concentrations determined by IC are lower than those for the colorimetric method used; the colorimetric method probably determined bromide plus organo-bromine compounds. It is suggested that the values determined by the colorimetric method be termed dissolved labile bromine (DLBr). The study shows that sulphate is the overriding form of sulphur in the waters. For chloride and nitrate, measurements by both methods approach a 1:1 relationship that is barely statistically significantly different from unity. For fluoride, the IC method gives lower values than the colorimetric, especially for the stream waters. However, the colorimetric method determines total fluorine so that a difference is to be expected (for example, fluoride strongly complexes with aluminium that is present, especially in the streamwater).
River biogeochemistry and source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments  [PDF]
M. Voss,B. Deutsch,R. Elmgren,C. Humborg
Biogeosciences Discussions , 2006,
Abstract: Nitrate input to a river is largely controlled by land use in its catchment. We compared the information carried by the isotopic signatures of nitrate in 12 Baltic rivers, in relation to the vegetation cover of their catchments. We found isotope values in nitrate ranging from 2 to 14 for δ15N and 8 to 25 per mil for δ18O. Seasonal signals were evident in all rivers. The annual variability of riverine isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, Vistula and Oder. Nordic rivers with relatively pristine vegetation in its catchments show not only low δ15N values and high δ18O-NO3 but also lower annual variability than rivers draining densely populated land. Seasonal signals could be found in all of the rivers. We used load weighted nitrate isotope data and data from the three major sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results agree well with same estimates derived from a Global Land Cover data base. The comparison with an emission model (EM) reveals good agreements for intensively used catchments and rather bad ones for pristine catchments. Advantages and limitations of the tested model types are discussed.
Upward nitrate transport by phytoplankton in oceanic waters: balancing nutrient budgets in oligotrophic seas  [PDF]
Tracy A Villareal,Cynthia H. Pilskaln,Joseph P. Montoya,Mark Dennett
PeerJ , 2015, DOI: 10.7287/peerj.preprints.189v1
Abstract: In oceanic gyres, primary producers are numerically dominated by small (1-5 μm diameter) pro- and eukaryotic cells that primarily utilize recycled nutrients produced by rapid grazing turnover in a highly efficient microbial loop. Continuous losses of nitrogen to depth by sinking, either as single cells, aggregates or fecal pellets, are balanced by both nitrate inputs at the base of the euphotic zone and nitrogen-fixation. This input of N (new nitrogen) to balance export losses (the biological pump) is a fundamental aspect of nitrogen cycling and central to understanding carbon fluxes in the ocean. In the Pacific Ocean, detailed nitrogen budgets at the time-series station HOT require upward transport of nitrate from the nutricline (80-100 m) into the surface layer (~0-40 m) to balance productivity and export needs. However, concentration gradients are negligible and cannot support the fluxes. Physical processes can inject nitrate into the base of the euphotic zone, but the mechanisms for transporting this nitrate into the surface layer across many 10s of m in highly stratified systems are unknown. In these seas, vertical migration by the very largest 102-103 μm diameter) phytoplankton is common as a survival strategy to obtain nitrogen from sub-euphotic zone depths. This vertical migration is driven by buoyancy changes rather than by flagellated movement and can provide upward nitrogen transport as nitrate (mM concentrations) in the cells. However, the contribution of vertical migration to nitrate transport has been difficult to quantify over the required basin scales. In this study, we use towed optical systems and isotopic tracers to show that migrating diatom (Rhizosolenia) mats are widespread in the N. Pacific Ocean from 140°W to 175°E and together with other migrating phytoplankton (Ethmodiscus, Halosphaera, Pyrocystis, and solitary Rhizosolenia) can mediate time-averaged transport of N (235 μmol N m-2 d-1) equivalent to eddy nitrate injections (242 μmol NO3- m-2 d-1). This upward biotic transport can close nitrate budgets in the upper 250 m of the central Pacific Ocean and together with diazotrophy creates a surface zone where biological nutrient inputs rather than physical processes dominate the new N flux. In addition to these numerically rare large migrators, there is extensive evidence in the literature of ascending behavior in small phytoplankton that contributes to upward flux as well. Although passive downward movement has dominated models of phytoplankton flux, there is now sufficient evidence to require a rethinking of this paradigm.
Upward nitrate transport by phytoplankton in oceanic waters: balancing nutrient budgets in oligotrophic seas  [PDF]
Tracy A. Villareal,Cynthia H. Pilskaln,Joseph P. Montoya,Mark Dennett
PeerJ , 2015, DOI: 10.7717/peerj.302
Abstract: In oceanic subtropical gyres, primary producers are numerically dominated by small (1–5 μm diameter) pro- and eukaryotic cells that primarily utilize recycled nutrients produced by rapid grazing turnover in a highly efficient microbial loop. Continuous losses of nitrogen (N) to depth by sinking, either as single cells, aggregates or fecal pellets, are balanced by both nitrate inputs at the base of the euphotic zone and N2-fixation. This input of new N to balance export losses (the biological pump) is a fundamental aspect of N cycling and central to understanding carbon fluxes in the ocean. In the Pacific Ocean, detailed N budgets at the time-series station HOT require upward transport of nitrate from the nutricline (80–100 m) into the surface layer (~0–40 m) to balance productivity and export needs. However, concentration gradients are negligible and cannot support the fluxes. Physical processes can inject nitrate into the base of the euphotic zone, but the mechanisms for transporting this nitrate into the surface layer across many 10s of m in highly stratified systems are unknown. In these seas, vertical migration by the very largest (102–103 μm diameter) phytoplankton is common as a survival strategy to obtain N from sub-euphotic zone depths. This vertical migration is driven by buoyancy changes rather than by flagellated movement and can provide upward N transport as nitrate (mM concentrations) in the cells. However, the contribution of vertical migration to nitrate transport has been difficult to quantify over the required basin scales. In this study, we use towed optical systems and isotopic tracers to show that migrating diatom (Rhizosolenia) mats are widespread in the N. Pacific Ocean from 140°W to 175°E and together with other migrating phytoplankton (Ethmodiscus, Halosphaera, Pyrocystis, and solitary Rhizosolenia) can mediate time-averaged transport of N (235 μmol N m-2 d-1) equivalent to eddy nitrate injections (242 μmol NO3 m-2 d-1). This upward biotic transport can close N budgets in the upper 250 m of the central Pacific Ocean and together with diazotrophy creates a surface zone where biological nutrient inputs rather than physical processes dominate the new N flux. In addition to these numerically rare large migrators, there is evidence in the literature of ascending behavior in small phytoplankton that could contribute to upward flux as well. Although passive downward movement has dominated models of phytoplankton flux, there is now sufficient evidence to require a rethinking of this paradigm. Quantifying these fluxes is a challenge for
European nitrogen policies, nitrate in rivers and the use of the INCA model  [PDF]
R. Skeffington
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2002,
Abstract: This paper is concerned with nitrogen inputs to European catchments, how they are likely to change in future, and the implications for the INCA model. National N budgets show that the fifteen countries currently in the European Union (the EU-15 countries) probably have positive N balances – that is, N inputs exceed outputs. The major sources are atmospheric deposition, fertilisers and animal feed, the relative importance of which varies between countries. The magnitude of the fluxes which determine the transport and retention of N in catchments is also very variable in both space and time. The most important of these fluxes are parameterised directly or indirectly in the INCA Model, though it is doubtful whether the present version of the model is flexible enough to encompass short-term (daily) variations in inputs or longer-term (decadal) changes in soil parameters. As an aid to predicting future changes in deposition, international legislation relating to atmospheric N inputs and nitrate in rivers is reviewed briefly. Atmospheric N deposition and fertiliser use are likely to decrease over the next 10 years, but probably not sufficiently to balance national N budgets. Keywords: nitrogen deposition, nitrogen fertilisers, nitrogen budgets, nitrogen balance, nitrate leaching, INCA Model, environmental legislation, EU directives, air pollution, water pollution
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