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1.
In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH4+), causes significant nitrate (NO3?) leaching from well‐drained acid forest soils on hill slopes (HS), whereas significant retention of NO3? occurs in near‐stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ15N and δ18O) of nitrate (NO3?) in soil pore water along a hydrological continuum in the N‐saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH4+ and production of NO3? in soils on HS were associated with a significant decrease in δ15N‐NO3?, suggesting efficient nitrification despite low soil pH. The concentration of NO3? declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ15N and δ18O of residual NO3?, providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent 15N enrichment factor (ε) of NO3? of about ?5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ15N and δ18O‐NO3? in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ15N‐NO3? values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N‐saturated, subtropical forests with important implications for regional N budgets.  相似文献   

2.
This study examines the role of tree canopies in processing atmospheric nitrogen (Ndep) for four forests in the United Kingdom subjected to different Ndep: Scots pine and beech stands under high Ndep (HN, 13–19 kg N ha?1 yr?1), compared to Scots pine and beech stands under low Ndep (LN, 9 kg N ha?1 yr?1). Changes of NO3‐N and NH4‐N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ18O, Δ17O and δ15N in NO3? and δ15N in NH4+, were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4‐N and NO3‐N concentrations in RF compared to the LN sites. Similar values of δ15N‐NO3? and δ18O in RF suggested similar source of atmospheric NO3? (i.e. local traffic), while more positive values for δ15N‐NH4+ at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N‐forms changed after interacting with tree canopies. Indeed, 15N‐enriched NH4+ in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndep also at the low Ndep site. Using two independent methods, based on δ18O and Δ17O, we quantified for the first time the proportion of NO3? in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ17O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of Ndep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ17O.  相似文献   

3.
We present the first estimates of net anthropogenic nitrogen input (NANI) in European boreal catchments. In Swedish catchments, nitrogen (N) deposition is a major N input (31–94%). Hence, we used two different N deposition inputs to calculate NANI for 36 major Swedish catchments. The relationship between riverine N export and NANI was strongest when using only oxidized deposition (NOy) as atmospheric input (r2 = 0.70) rather than total deposition (i.e., both oxidized and reduced nitrogen, NOy + NHx deposition, r2 = 0.62). The y-intercept (NANI = 0) for the NANI calculated with NOy is significantly different from zero (p = 0.0042*) and indicates a background flux from the catchment of some 100 kg N km?2 year?1 in addition to anthropogenic inputs. This agrees with similar results from North American boreal catchments. The slope of the linear regressions was 0.25 for both N deposition inputs (NOy and NOy + NHx), suggesting that on average, 25% of the anthropogenic N inputs is exported by rivers to the Baltic Sea. Agricultural catchments in central and southern Sweden have increased their riverine N export up to tenfold compared to the inferred background flux. Although the relatively unperturbed northernmost catchments receive significant N loads from atmospheric deposition, these catchments do not show significantly elevated riverine N export. The fact that nitrogen export in Swedish catchments appears to be higher in proportion to NANI at higher loads suggests that N retention may be saturating as loading rates increase. In northern and western Sweden the export of nitrogen is largely controlled by the hydraulic load, i.e., the riverine discharge normalized by water surface area, which has units of distance time?1. Besides hydraulic load the percent total forest cover also affects the nitrogen export primarily in the northern and western catchments.  相似文献   

4.
Urban areas are expanding rapidly in tropical regions, with potential to alter ecosystem dynamics. In particular, exotic grasses and atmospheric nitrogen (N) deposition simultaneously affect tropical urbanized landscapes, with unknown effects on properties like soil carbon (C) storage. We hypothesized that (H1) soil nitrate (NO3?) is elevated nearer to the urban core, reflecting N deposition gradients. (H2) Exotic grasslands have elevated soil NO3? and decreased soil C relative to secondary forests, with higher N promoting decomposer activity. (H3) Exotic grasslands have greater seasonality in soil NO3? vs. secondary forests, due to higher sensitivity of grassland soil moisture to rainfall. We predicted that NO3? would be positively related to dissolved organic C (DOC) production via changes in decomposer activity. We measured six paired grassland/secondary forest sites along a tropical urban‐to‐rural gradient during the three dominant seasons (hurricane, dry, and early wet). We found that (1) soil NO3? was generally elevated nearer to the urban core, with particularly clear spatial trends for grasslands. (2) Exotic grasslands had lower soil C than secondary forests, which was related to elevated decomposer enzyme activities and soil respiration. Unexpectedly, soil NO3? was negatively related to enzyme activities, and was lower in grasslands than forests. (3) Grasslands had greater soil NO3? seasonality vs. forests, but this was not strongly linked to shifts in soil moisture or DOC. Our results suggest that exotic grasses in tropical regions are likely to drastically reduce soil C storage, but that N deposition may have an opposite effect via suppression of enzyme activities. However, soil NO3? accumulation here was higher in urban forests than grasslands, potentially related to of aboveground N interception. Net urban effects on C storage across tropical landscapes will likely vary depending on the mosaic of grass cover, rates of N deposition, and responses by local decomposer communities.  相似文献   

5.
The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5–8?kg?N?ha?1?y?1) or low (<2?kg?N?ha?1?y?1) inputs of atmospheric N deposition. The nitrate (NO3 ?) concentration was significantly greater in high-deposition lakes (11.3?μmol?l?1) compared to low-deposition lakes (3.3?μmol?l?1). Background denitrification was positively related to NO3 ? concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose–response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis–Menten kinetics, the maximum denitrification rate and half-saturation NO3 ? concentration did not differ between deposition regions and were 765?μmol?N?m?2?h?1 and 293?μmol?l?1?NO3 ?, respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification.  相似文献   

6.
Biogeochemical responses to changing climate and atmospheric deposition were investigated using nitrogen (N) and sulfur (S) mass balances, including dry deposition and organic solutes in the Arbutus Lake watershed in the Adirondack Mountains, New York State. Long‐term monitoring of wet‐only precipitation (NADP/NTN, 1983–2001) and dry deposition (AIRMoN, 1990–2001) at sites adjacent to the watershed showed that concentrations of SO42? in precipitation, SO42? in particles,and SO2 vapor all declined substantially (P<0.005) in contrast to no marked temporal changes observed for most N constituents (NH4+ in precipitation, HNO3 vapor, and particulate NO3?), except for NO3? in precipitation, which showed a small decrease in the late 1990s. From 1983 to 2001, concentrations of SO42? in the lake outlet significantly decreased (?2.1 μeq L?1 yr?1, P<0.0001), whereas NO3? and dissolved organic N (DON) concentrations showed no consistent temporal trends. With the inclusion of dry deposition and DON fluxes into the mass balance, the retained portion of atmospheric N inputs within the main subcatchment increased from 37% to 60%. Sulfur outputs greatly exceeded inputs even with the inclusion of dry S deposition, while organic S flux represented another source of S output, implying substantial internal S sources. A significant relationship between the annual mean concentrations of SO42? in lake discharge and wet deposition over the last two decades (r=0.64, P<0.01) suggested a considerable influence of declining S deposition on surface water SO42? concentrations, despite substantial internal S sources. By contrast, interannual variations in both NO3? concentrations and fluxes in lake discharge were significantly related to year‐to‐year changes in air temperature and runoff. Snowmelt responses to winter temperature fluctuations were crucial in explaining large portions of interannual variations in watershed NO3? export during the months preceding spring snowmelt (especially, January–March). Distinctive response patterns of monthly mean concentrations of NO3? and DON in the major lake inlet to seasonal changes in air temperature also suggested climatic regulation of seasonal patterns in watershed release of both N forms. The sensitive response of N drainage losses to climatic variability might explain the synchronous patterns of decadal variations in watershed NO3? export across the northeastern USA.  相似文献   

7.
Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO3 ?) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O2, NO3 ?, NH4 +, CO2, N2O, and CH4) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO3 ? concentrations (using N15 isotope tracing) interact to affect N cycling processes. At the lowest NO3 ? concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO3 ? concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O2, NO3 ?, NH4 + fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO3 ? concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.  相似文献   

8.
Knowledge of import, export, and transport of nitrogen (N) in headwater catchments is essential for understanding ecosystem function and water quality in mountain ecosystems, especially as these ecosystems experience increased anthropogenic N deposition. In this study, we link spatially explicit soil and stream data at the landscape scale to investigate import, export and transport of N in a 0.89?km2 site at the alpine-subalpine ecotone in the Front Range of the Rocky Mountains, Colorado, U.S.A. For two of the major N inputs to our site, N deposition in the snowpack and N fixation, a complementary relationship was found across the study site, with greater abundance of N-fixing plants in areas with less snow and substantial snow inputs in areas with low N fixer abundance. During the initial phases of snowmelt, mixing model end members for oxygen isotopes in nitrate (NO3 ?) indicated that a substantial quantity of NO3 ? is transported downhill into the forested subalpine without being assimilated by soil microbes. After this initial pulse, much less NO3 ? entered the stream and most but not all of it was microbial in origin. Rising δ15N in stream NO3 ? indicated greater influence of fractionating processes such as denitrification later in the season. NO3 ? from both atmospheric and microbial sources was not exported from our site because it was consumed within the first several hundred meters of the stream; ultimately, N exports were in the form of dissolved organic nitrogen (DON) and particulate N (PN). The results of this study suggest that the highest elevation dry alpine meadows rely more heavily on N fixation as an N source and experience less of the effects of anthropogenic N deposition than mid and lower elevation areas that have more snow. Our data also suggest that mid-elevation krummholz, moist meadows, and talus slopes are exporting N as NO3 ? shortly after the onset of snowmelt, but that this NO3 ? is rapidly consumed as the stream flows through the subalpine forest. This consumption by assimilation and/or denitrification currently provides a buffer against increased inorganic N availability downstream.  相似文献   

9.
The influx of atmospheric nitrogen to soils and surfaces in arid environments is of growing concern due to increased N emissions and N usage associated with urbanization. Atmospheric nitrogen inputs to the critical zone can occur as wet (rain or snow) or dry (dust or aerosols) deposition, and can lead to eutrophication, soil acidification, and groundwater contamination through leaching of excess nitrate. The objective of this research was to use the δ15N, δ18O, and Δ17O values of atmospheric nitrate (NO3 ?) (precipitation and aerosols) and NO3 ? in runoff to assess the importance of N deposition and turnover in semi-arid urban watersheds. Data show that the fractions of atmospheric NO3 ? exported from all the urban catchments, throughout the study period, were substantially higher than in nearly all other ecosystems studied with mean atmospheric contributions of 38% (min 0% and max 82%). These results suggest that catchment and stream channel imperviousness enhance atmospheric NO3 ? export due to inefficient N cycling and retention. In contrast, catchment and stream channel perviousness allow for enhanced N processing and therefore reduced atmospheric NO3 ? export. Overall high fractions of atmospheric NO3 ? were primarily attributed to slow N turn over in arid/semi-arid ecosystems. A relatively high fraction of nitrification NO3 ? (~30%) was found in runoff from a nearly completely impervious watershed (91%). This was attributed to nitrification of atmospheric NH4 + in dry-deposited dust, suggesting that N nitrifiers have adapted to urban micro niches. Gross nitrification rates based on NO3 ? Δ17O values ranged from a low 3.04 ± 2 kg NO3-N km?2 day?1 in highly impervious catchments to a high of 10.15 ± 1 kg NO3-N km?2 day?1 in the low density urban catchment. These low gross nitrification rates were attributed to low soil C:N ratios that control gross autotrophic nitrification by regulating gross NH4 + production rates.  相似文献   

10.
Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N2) and nitrous oxide (N2O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 μM of 15N enriched nitrate (NO3 ?, ~98 at. %). Effluxes of both N2 and N2O from sediment to the overlying water increased with increasing NO3 ? load. Although the ratio of N2O to N2 increased with increasing NO3 ? load, it remained below 0.04, N2 always being the main product. At the NO3 ? concentrations most frequently found in the studied river water (10–100 μM), up to 8% of the NO3 ? was removed in denitrification, whereas with the highest concentration (300 μM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO3 ? was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO3 ? load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO3 ? was assimilated by microbes during their growth. The 15N content in ammonium (NH4 +) in the efflux was low, suggesting that reduction of NO3 ? to NH4 + was not the reason for the NO3 ? removal. This study provides the first published information on denitrification and N2O fluxes and their regulation by NO3 ? load in eutrophic high latitude rivers.  相似文献   

11.
There is considerable uncertainty in the estimates of indirect N2O emissions as defined by the Intergovernmental Panel on Climate Change's (IPCC) methodology. Direct measurements of N2O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N2O budget. The objectives of this research were to measure the N2O fluxes from a spring‐fed river, relate these fluxes to the dissolved N2O concentrations and NO3‐N loading of the river, and to try to define the indirect emission factor (EF5‐r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL N2O L?1. River NO3‐N and dissolved N2O concentrations ranged from 2.5 to 5.3 mg L?1 and 0.4 to 1.9 μg N2O‐N L?1, respectively, with N2O saturation reaching 404%. Floating headspace chambers were used to sample N2O fluxes. N2O‐N fluxes were significantly related to dissolved N2O‐N concentrations (r2=0.31) but not to NO3‐N concentrations. The N2O‐N fluxes ranged from 38 to 501 μg m?2 h?1, averaging 171 μg m?2 h?1 (±SD 85) overall. The measured N2O‐N fluxes equated to an EF5‐r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N2O present in the groundwater that fed the river.  相似文献   

12.
There is considerable uncertainty in the estimates of indirect N2O emissions as defined by the intergovernmental panel on climate change's (IPCC) methodology. Direct measurements of N2O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N2O budget. The objectives of this research were to measure the N2O fluxes from a spring‐fed river, relate these fluxes to the dissolved N2O concentrations and NO3‐N loading of the river, and to try and define the indirect emission factor (EF5‐r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL N2O L?1. River NO3‐N and dissolved N2O concentrations ranged from 2.5 to 5.3 mg L?1 and 0.4 to 1.9 μg N2O‐N L?1, respectively, with N2O saturation reaching 404%. Floating headspace chambers were used to sample N2O fluxes. N2O‐N fluxes were significantly related to dissolved N2O‐N concentrations (r2=30.6) but not to NO3‐N concentrations. The N2O‐N fluxes ranged from 38–501 μg m?2 h?1, averaging 171 μg m?2 h?1 (±SD 85) overall. The measured N2O‐N fluxes equated to an EF5‐r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N2O present in the groundwater that fed the river.  相似文献   

13.
Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3 ??CN, NH4 +?CN and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?C0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 ? during the passage of rain water through the ecosystem and bulk ??15N values in soil to detect N transformations. Depletion of 15N in NO3 ? and increased NO3 ??CN fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 ? concentrations progressively decreased and were enriched in 15N but did not reach the ??15N values of solid phase organic matter (??15N = 5.6?C6.7??). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the ??15N value of NO3 ? in litter leachate was smaller (??15N = ?1.58??) than in the other quarters (??15N = ?9.38 ± SE 0.46??) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 ? between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3 ??CN from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 ? gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.  相似文献   

14.
Salinity intrusion caused by land subsidence resulting from increasing groundwater abstraction, decreasing river sediment loads and increasing sea level because of climate change has caused widespread soil salinization in coastal ecosystems. Soil salinization may greatly alter nitrogen (N) cycling in coastal ecosystems. However, a comprehensive understanding of the effects of soil salinization on ecosystem N pools, cycling processes and fluxes is not available for coastal ecosystems. Therefore, we compiled data from 551 observations from 21 peer‐reviewed papers and conducted a meta‐analysis of experimental soil salinization effects on 19 variables related to N pools, cycling processes and fluxes in coastal ecosystems. Our results showed that the effects of soil salinization varied across different ecosystem types and salinity levels. Soil salinization increased plant N content (18%), soil NH4+ (12%) and soil total N (210%), although it decreased soil NO3? (2%) and soil microbial biomass N (74%). Increasing soil salinity stimulated soil N2O fluxes as well as hydrological NH4+ and NO2? fluxes more than threefold, although it decreased the hydrological dissolved organic nitrogen (DON) flux (59%). Soil salinization also increased the net N mineralization by 70%, although salinization effects were not observed on the net nitrification, denitrification and dissimilatory nitrate reduction to ammonium in this meta‐analysis. Overall, this meta‐analysis improves our understanding of the responses of ecosystem N cycling to soil salinization, identifies knowledge gaps and highlights the urgent need for studies on the effects of soil salinization on coastal agro‐ecosystem and microbial N immobilization. Additional increases in knowledge are critical for designing sustainable adaptation measures to the predicted intrusion of salinity intrusion so that the productivity of coastal agro‐ecosystems can be maintained or improved and the N losses and pollution of the natural environment can be minimized.  相似文献   

15.
Vertical distribution of redox zones, concentrations of redox‐sensitive constituents, numbers of aerobic heterotrophic bacteria, and potential denitrification activity were studied in 1‐m cores taken at the transition between the oxidized and reduced layers in two Danish clayey subsoils. Based on the matrix soil colors, a redox sequence of oxidized, suboxic, and reduced zones was identified at both sites. The geochemical composition of the oxidized brown colored zone (to depths of 2.6 and 3.2 m) was characterized by high concentrations of NO3 ? and low amounts of total organic carbon, exchangeable forms of NH4 +, Fe2+, and Mn2+, and structural Fe(II) in the clay minerals. In the underlying 20‐ to 30‐cm‐deep suboxic zone, decreasing NO3 ? concentrations were observed together with increasing amount of exchangeable forms of Fe2+ and Mn2+, and structural Fe(II). Finally, in the reduced grey zone, NO3 ? was no longer present and maximum concentrations of other redox sensitive constituents occurred. Throughout the subsoils, the distribution of exchangeable Fe2+ corresponded most closely to changes in the colors of redox zones. The low‐organic Havrebjerg site displayed geochemical profiles indicating that NO3 ? was chemically reduced by structural Fe(II) in the clay minerals of the suboxic zone, and that the Fe(II) formed a geochemical barrier for the downward progression of NO3 ?. Aerobic heterotrophic bacteria occurred only in low numbers at this site and potential denitrification activity was very low. In contrast, the Sparresholm site had a significant population of bacteria in the suboxic zone, which also contained a heterogeneous distribution of potential denitrification activity. Specific microsites with facilitated transport of soluble organic substrates are proposed to support the denitrification activity in a heterogeneous distribution, constituting a microbial barrier for downward progression of NO3 ? in this subsoil.  相似文献   

16.
Atmospheric deposition of nitrogen (N) compounds is the major source of anthropogenic N to most upland ecosystems, where leaching of nitrate (NO 3 ? ) into surface waters contributes to eutrophication and acidification as well as indicating an excess of N in the terrestrial catchment ecosystems. Natural abundance stable isotopes ratios, 15N/14N and 18O/16O (the “dual isotope” technique) have previously been used in biogeochemical studies of alpine and forested ecosystems to demonstrate that most of the NO 3 ? in upland surface waters has been microbially produced. Here we present an application of the technique to four moorland catchments in the British uplands including a comparison of lakes and their stream inflows at two sites. The NO 3 ? concentrations of bulk deposition and surface waters at three sites are very similar. While noting the constraints imposed by uncertainty in the precise δ18O value for microbial NO 3 ? , however, we estimate that 79–98% of the annual mean NO 3 ? has been microbially produced. Direct leaching of atmospheric NO 3 ? is a minor component of catchment NO 3 ? export, although greater than in many similar studies in forested watersheds. A greater proportion of atmospheric NO 3 ? is seen in the two lake sites relative to their inflow streams, demonstrating the importance of direct NO 3 ? deposition to lake surfaces in catchments where terrestrial ecosystems intercept a large proportion of deposited N. The dominance of microbial sources of NO 3 ? in upland waters suggests that reduced and oxidised N deposition may have similar implications in terms of contributing to NO 3 ? leaching.  相似文献   

17.
We measured net nitrate retention by mass balance in a 700-m upwelling reach of a third-order sand plains stream, Emmons Creek, from January 2007 to November 2008. Surface water and groundwater fluxes of nitrate were determined from continuous records of discharge and from nitrate concentrations based on weekly and biweekly sampling at three surface water stations and in 23 in-stream piezometers, respectively. Surface water nitrate concentration in Emmons Creek was relatively high (mean of 2.25 mg NO3?CN l?1) and exhibited strong seasonal variation. Net nitrate retention averaged 429 mg NO3?CN m?2 d?1 and about 2% of nitrate inputs to the reach. Net nitrate retention was highest during the spring and autumn when groundwater discharge was elevated. Groundwater discharge explained 57?C65% of the variation in areal net nitrate retention. Specific discharge and groundwater nitrate concentration varied spatially. Weighting groundwater solute concentrations by specific discharge improved the water balance and resulted in higher estimates of nitrate retention. Our results suggest that groundwater inputs of nitrate can drive nitrate retention in streams with high groundwater discharge.  相似文献   

18.
The method of fluorescent probes has been an important technique for detection of nitrite (NO2?). As an important inorganic salt, excessive nitrite would threaten humans and the environment. In this paper, a colorimetric fluorescent probe P‐N (1,2‐diaminoanthraquinone) with rapid response and high selectivity, which could detect NO2? by visual colour changes and fluorescence spectroscopy is presented. The probe P‐N solution (pH 1) changed from pink to colourless with the addition of NO2? and fluorescence intensity at 639 nm clearly decreased. Good linear exists between fluorescence intensities and NO2? concentrations for the range 0–16 μM, and the detection limit was 54 nM (based on a 3σ/slope). Moreover, probe P‐N could also detect NO2? in real water samples, and results were all satisfactory. Probe P‐N shows great practical application value for detecting NO2? in the environment.  相似文献   

19.
There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ15N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha?1 yr?1, while the throughfall covered a larger range of 11.7–65.1 kg N ha?1 yr?1. Foliar N concentration, NO3? leaching to stream, and soil NO3? concentration were low and NO3? production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO3? leaching to stream, and negatively with pH values in soil and stream water. Foliar δ15N was between ?6.6‰ and 0.7‰, and was negatively correlated with soil NO3? concentration and NO3? leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ15N values. We proposed several potential mechanism that could contribute to the δ15N pattern, including (1) increased plant uptake of 15N‐depleted soil NO3?, (2) foliage uptake of 15N‐depleted NH4+, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.  相似文献   

20.
1. Riparian zones function as important ecotones that reduce nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. 2. In this study, riparian zone nitrogen retention was examined in a high permafrost catchment (approximately 53% of land area underlain by permafrost) and a low permafrost catchment (approximately 3%). To estimate the contribution of the riparian zone to catchment nitrogen retention, we analysed groundwater chemistry using an end‐member mixing model. 3. Stream nitrate concentration was over twofold greater in the low permafrost catchment than the high permafrost catchment. Riparian groundwater was not significantly different between catchments, averaging 13 μm overall. Nitrogen retention, measured using the end‐member mixing model, averaged 0.75 and 0.22 mmol N m?2 day?1 in low and high permafrost catchments, respectively, over the summer. The retention rate of nitrogen in the riparian zone was 10–15% of the export in stream flow. 4. Our results indicate that the riparian zone functions as an important sink for groundwater nitrate and dissolved organic carbon (DOC). However, differences in stream nitrate and DOC concentrations between catchments cannot be explained by solute inputs from riparian groundwater to the stream and differences between streams are probably attributable to deeper groundwater inputs or flows from springs that bypass the riparian zone.  相似文献   

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