Mechanisms behind plant diversity effects on inorganic and organic N leaching from temperate grassland

Higher plant diversity reduces nitrate leaching by complementary resource use, while its relation to leaching of other N species is unclear. We determined the effects of plant species richness, functional group richness, and the presence of specific functional groups on ammonium, dissolved organic N (DON), and total dissolved N (TDN) leaching from grassland in the first 4 years after conversion from fertilized arable land to unfertilized grassland. On 62 experimental plots in Jena, Germany, with 1–60 plant species and 1–4 functional groups (legumes, grasses, tall herbs, small herbs), nitrate, ammonium, and TDN concentrations in soil solution (0–0.3 m soil layer) were measured fortnightly during 4 years. DON concentrations were calculated by subtracting inorganic N from TDN. Nitrogen concentrations were multiplied with modeled downward water fluxes to obtain N leaching. DON leaching contributed most to TDN leaching (64 ± SD 4% of TDN). Ammonium leaching was unaffected by plant diversity. Increasing species richness decreased DON leaching in the fourth year. We attribute this finding to enhanced use of DON as a C and N source and enhanced mineralization of DON by soil microorganisms. An increase of species richness decreased TDN leaching likely driven by the complementary use of nitrate by diverse mixtures. Legumes increased DON and TDN leaching likely because of their N\(_{2}\)-fixing ability and higher litter production. Grasses decreased TDN leaching because of more exhaustive use of nitrate and water. Our results demonstrate that increasing plant species richness decreases leaching of DON and TDN.     

The Long-term Effects of Disturbance on Organic and Inorganic Nitrogen Export in the White Mountains, New Hampshire

Traditional biogeochemical theories suggest that ecosystem nitrogen retention is controlled by biotic N limitation, that stream N losses should increase with successional age, and that increasing N deposition will accelerate this process. These theories ignore the role of dissolved organic nitrogen (DON) as a mechanism of N loss. We examined patterns of organic and inorganic N export from sets of old-growth and historically (80–110 years ago) logged and burned watersheds in the northeastern US, a region of moderate, elevated N deposition. Stream nitrate concentrations were strongly seasonal, and mean (± SD) nitrate export from old-growth watersheds (1.4 ± 0.6 kg N ha⁻¹ y⁻¹) was four times greater than from disturbed watersheds (0.3 ± 0.3 kg N ha⁻¹ y⁻¹), suggesting that biotic control over nitrate loss can persist for a century. DON loss averaged 0.7 (± 0.2) kg N ha⁻¹ y⁻¹ and accounted for 28–87% of total dissolved N (TDN) export. DON concentrations did not vary seasonally or with successional status, but correlated with dissolved organic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N demand but instead were consistent with expected differences in DOC production and sorption. Despite decades of moderate N deposition, TDN export was low, and even old-growth forests retained at least 65% of N inputs. The reasons for this high N retention are unclear: if due to a large capacity for N storage or biological removal, N saturation may require several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner. Received 27 April 1999; accepted 30 May 2000.     

Does Anthropogenic Nitrogen Enrichment Increase Organic Nitrogen Concentrations in Runoff from Forested and Human-dominated Watersheds?

Although the effects of anthropogenic nitrogen (N) inputs on the dynamics of inorganic N in watersheds have been studied extensively, “the influence of N enrichment on organic N loss” is not as well understood. We compiled and synthesized data on surface water N concentrations from 348 forested and human-dominated watersheds with a range of N loads (from less than 100 to 7,100 kg N km⁻² y⁻¹) to evaluate the effects of N loading via atmospheric deposition, fertilization, and wastewater on dissolved organic N (DON) concentrations. Our results indicate that, on average, DON accounts for half of the total dissolved N (TDN) concentrations from forested watersheds, but it accounts for a smaller fraction of TDN in runoff from urban and agricultural watersheds with higher N loading. A significant but weak correlation (r ² = 0.06) suggests that N loading has little influence on DON concentrations in forested watersheds. This result contrasts with observations from some plot-scale N fertilization studies and suggests that variability in watershed characteristics and climate among forested watersheds may be a more important control on DON losses than N loading from atmospheric sources. Mean DON concentrations were positively correlated, however, with N load across the entire land-use gradient (r ² = 0.37, P < 0.01), with the highest concentrations found in agricultural and urban watersheds. We hypothesize that both direct contributions of DON from wastewater and agricultural amendments and indirect transformations of inorganic N to organic N represent important sources of DON to surface waters in human-dominated watersheds. We conclude that DON is an important component of N loss in surface waters draining forested and human-dominated watersheds and suggest several research priorities that may be useful in elucidating the role of N enrichment in watershed DON dynamics.     

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH₄-N + NO₃-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN– DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kgha^{minus 1} yr^{minus 1}) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO₃-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO₃-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

Reproductive patterns and secondary production of <Emphasis Type="Italic">Gammaropsis japonicus</Emphasis> (Crustacea,Amphipoda) on the seagrass <Emphasis Type="Italic">Zostera marina</Emphasis> of Korea

It is essential to know the nutrient limitation status of biofilms to understand how they may buffer uptake and export of nutrients from polluted watersheds. We tested the effects of nutrient additions on biofilm biomass (chlorophyll a, ash free dry mass (AFDM), and autotrophic index (AI, AFDM/chl a)) and metabolism via nutrient-diffusing substrate bioassays (control, nitrogen (N), phosphorus (P), and N + P treatments) at 11 sites in the Upper Snake River basin (southeast Idaho, USA) that differed in the magnitude and extent of human-caused impacts. Water temperature, turbidity, and dissolved inorganic N concentrations all changed seasonally at the study sites, while turbidity and dissolved inorganic N and P also varied with impact level. Chl a and AI on control treatments suggested that the most heavily impacted sites supported more autotrophic biofilms than less-impacted sites, and that across all sites biofilms were more heterotrophic in autumn than in summer. Nutrient stimulation or suppression of biofilm biomass was observed for chl a in 59% of the experiments and for AFDM in 33%, and the most frequent response noted across all study sites was N limitation. P suppression of chl a was observed only at the most-impacted sites, while AFDM was never suppressed by nutrients. When nutrient additions did have significant effects on metabolism, they were driven by differences in biomass rather than by changes in metabolic rates. Our study demonstrated that biofilms in southeast Idaho rivers were primarily limited by N, but nutrient limitation was more frequent at sites with good water quality than at those with poor water quality. Additionally, heterotrophic and autotrophic biofilm components may respond differently to nutrient enrichment, and nutrient limitation of biofilm biomass should not be considered a surrogate for metabolism in these rivers. Handling editor: D. Ryder     

Inverse relationship of epilithic algae and pelagic phosphorus in unproductive lakes: Roles of N2 fixers and light

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Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Do cyanobacteria dominate in eutrophic lakes because they fix atmospheric nitrogen?

1. The sources of nitrogen for phytoplankton were determined for a bloom‐prone lake as a means of assessing the hypothesis that cyanobacteria dominate in eutrophic lakes because of their ability to fix nitrogen when the nitrogen : phosphorous (N : P) supply ratio is low and nitrogen a limiting resource. 2. Nitrogen fixation rates, estimated through acetylene reduction with ¹⁵N calibration, were compared with ¹⁵N‐tracer estimates of ammonium and nitrate uptake monthly during the ice‐free season of 1999. In addition, the natural N stable isotope composition of phytoplankton, nitrate and ammonium were measured biweekly and the contribution of N₂ to the phytoplankton signature estimated with a mixing model. 3. Although cyanobacteria made up 81–98% of phytoplankton biomass during summer and autumn, both assays suggested minimal N acquisition through fixation (<9% for the in‐situ incubations; <2% for stable isotope analysis). Phytoplankton acquired N primarily as ammonium (82–98%), and secondarily as nitrate (15–18% in spring and autumn, but <5% in summer). Heterocyst densities of <3 per 100 fixer cells confirmed low reliance on fixation. 4. The lake showed symptoms of both light and nitrogen limitation. Cyanobacteria may have dominated by monopolizing benthic sources of ammonium, or by forming surface scums that shaded other algae.     

EFFECT OF IRON NUTRITION ON THE MARINE CYANOBACTERIUM SYNECHOCOCCUS GROWN ON DIFFERENT N SOURCES AND IRRADIANCES1

The effects of Fe deficiency on the marine cyanobacterium Synechococcus sp. were examined in batch cultures grown on nitrate or ammonium as a sole nitrogen source under two different irradiances. Fe-stressed cells showed lower chlorophyll a content and cellular C and N quotas. Light limitation increased the critical iron concentration below which both suppression of growth rate and changes in cellular composition were observed. At a limiting irradiance (26 μmol.m⁻².s⁻¹), this critical value was ∼10 nM, a 10 times increase compared to high-light cultures. Moreover, at low light the cellular chlorophyll a concentration was higher than at saturating light (110 μmol.m⁻².s⁻¹), this difference being most pronounced under Fe-stressed conditions. Cells grown on ammonium showed a lower half-saturation constant for Fe (K_s) compared to cells grown on nitrate, indicating Synechococcus sp. has the ability to grow faster on ammonium than on nitrate in a low Fe environment at high light. Consequently, in high-nutrient and low-chlorophyll regions where Fe limits new production, cyanobacteria most likely grow on regenerated ammonium, which requires less energy for assimilation. The K_s for growth on Fe at low light was significantly higher than at high light compared with the cells grown on the same N source, suggesting the cells require more Fe at low light. Therefore, if cells that are already Fe-limited also become light-limited, their iron stress level will increase even more. For cyanobacteria this is the first report of a study combining the interactions of Fe limitation, light limitation, and nitrogen source (NO₃⁻ vs. NH₄⁺).     

Nitrogen biogeochemistry of a mature Scots pine forest subjected to high nitrogen loads

Nitrogen (N) biogeochemistry of a mature Scots pine (Pinus sylvestris L.) stand subjected to an average total atmospheric N deposition of 48 kg ha^?1 year^?1 was studied during the period 1992–2007. The annual amount of dissolved inorganic nitrogen (DIN) in throughfall (TF) averaged 34 kg ha^?1 year^?1 over the 16-year monitoring period. The throughfall fluxes contained also considerable amounts of dissolved organic nitrogen (DON) (5–8.5 kg N ha^?1 year^?1), which should be incorporated in the estimate of N flux using throughfall collectors. Throughfall DIN fluxes declined at a rate of ?0.9 kg N ha^?1 year^?1, mainly due to the decreasing TF fluxes of ammonium (NH₄), which accounted for 70% to TF DIN. The decrease in TF DIN was accompanied by a decrease in DIN leaching in the seepage water (?1.6 kg N ha^?1 year^?1), which occurred exclusively as nitrate (NO₃ ^?). Nitrate losses in the leachate of the forest floor (LFH) equalled the TF NO₃ ^? delivered to the LFH-layer. On the contrary, about half of the TF NH₄ ⁺ was retained within the LFH-layer. Approximately 60% of the TF DIN fluxes were leached indicating that N inputs were far in excess of the N requirements of the forest. For DON, losses were only substantial from the LFH-layer, but no DON was leached in the seepage water. Despite the high N losses through nitrate leaching and NO _x emission, the forest was still accumulating N, especially in the aggrading LFH-layer. The forest stand, on the contrary, was found to be a poor N sink.     

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone,Green Lakes Valley,Colorado Front Range

Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH₄-N), nitrate-N (NO₃-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO₃-N decreased downstream from 4.3 kg ha⁻¹ in the alpine to 0.75 kg ha⁻¹ at treeline, while yields of DON were much less variable (0.40–0.34 kg ha⁻¹). Yields of NH₄-N and PON were low and showed little variation with basin area. NO₃-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R ² = 0.81, P < 0.001) Concentrations of NO₃-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry. Received 26 July 2001; accepted 6 May 2002.     

Nitrogen yields from undisturbed watersheds in the Americas

Yields of total fixed nitrogen and nitrogen fractions are summarized for thirty-one watersheds in which anthropogenic disturbance of the nitrogen cycle, either through land use or atmospheric deposition, is negligible or slight. These yields are taken as representative of background conditions over a broad range of watershed areas, elevations, and vegetation types. The data set focuses on watersheds of the American tropics, but also includes information on the Gambia River (Africa) and some small watersheds in the Sierra Nevada of California. For the tropical watersheds, total nitrogen yield averages 5.1 kg ha^–1 y^–1. On average, 30% of the total is particulate and 70% is dissolved. Of the dissolved fraction, an average of 50% is organic and 50% is inorganic, of which 20% is ammonium and 80% is nitrate. Yields are substantially lower than previously estimated for background conditions. Yields of all nitrogen fractions are strongly related to runoff, which also explains a large percentage of variance in yield of total nitrogen (r²=0.85). For total nitrogen and nitrogen fractions, yield increases at about two-thirds the rate of runoff; concentration decreases as runoff increases. There is a secondary but significant positive relationship between elevation and yield of DIN. Ratios DON/TDN and PN/TN both are related to watershed area rather than runoff; DON/TDN decreases and PN/TN increases toward higher stream orders. The analysis suggests for tropical watersheds the existence of mechanisms promoting strong homeostasis in the yield of N and its fractions for a given moisture regime, as well as predictable downstream change in proportionate representation N fractions. Yields and concentrations for small tropical watersheds are much larger than for the few temperate ones with which comparisons are possible.     

Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon

This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon, a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO₃−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH₄−N) concentrations were very low (usually < 0.005 mg L⁻¹) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged (defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds that were all older than 10 years still lost significantly more N, primarily as NO₃−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal scales in a region of relatively low anthropogenic N input.     

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N<Subscript>2</Subscript>-fixing Red Alder

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N₂-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r ² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha^–1 y^–1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.     

Nitrogen uptake and transformation in a midwestern U.S. stream: A stable isotope enrichment study

This study presents a comprehensive analysis ofnitrogen (N) cycling in a second-order forestedstream in southern Michigan that has moderatelyhigh concentrations of ammonium (mean,16 g N/L) and nitrate (17 g N/L). Awhole-stream ¹⁵NH₄ ⁺ addition wasperformed for 6 weeks in June and July, and thetracer ¹⁵N was measured downstream inammonium, nitrate, and detrital and livingbiomass. Ancillary measurements includedbiomass of organic matter, algae, bacteria andfungi, nutrient concentrations, hydrauliccharacteristics, whole-stream metabolism, andnutrient limitation assays. The resultsprovide insights into the heterotrophic natureof woodland streams and reveal the rates atwhich biological processes alter nitrogentransport through stream systems.Ammonium uptake lengths were 766–1349 m anduptake rates were 41–60 g N m^–2min^–1. Nitrate uptake could not bedetected. Nitrification rates were estimatedfrom the downstream increase in¹⁵N-enriched nitrate using a simulationmodel. The ammonium was removed bynitrification (57% of total uptake),heterotrophic bacteria and fungi associatedwith detritus (29%), and epilithic algae(14%). Growth of algae was likely limited bylight rather than nutrients, and dissolvedO₂ revealed that the stream metabolism washeterotrophic overall (P:R = 0.2). Incubationsof detritus in darkened chambers showed thatuptake of ¹⁵N was mostly heterotrophic.Microbial N in detritus and algal N inepilithon appeared to reach isotopic steadystate with the dissolved ammonium, but theisotopic enrichment of the bulk detritus andepilithon did not approach that of ammonium,probably due to a large fraction of organic Nin the bulk samples that was not turning over. The actively cycling fraction of total N inorganic compartments was estimated from theisotopic enrichment, assuming uptake ofammonium but not nitrate, to be 23% forepilithon, 1% for fine benthic organic matter,5% for small woody debris, and 7% for leaves. These percentages agree with independentestimates of epilithic algal biomass, whichwere based on carbon:chlorophyll ratios in bulksamples and in algal fractions separated bydensity-gradient centrifugation in colloidalsilica, and of microbial N in the detritus,which were based on N released by chloroformfumigations.     

High Organic Carbon Export Precludes Eutrophication Responses in Experimental Rocky Shore Communities

We studied the effect of nutrient inputs on the carbon (C) budget of rocky shore communities using a set of eight large experimental mesocosms. The mesocosms received a range of inorganic nitrogen (N) and phosphorus (P) additions, at an N:P ratio of 16. These additions were designed to elevate the background concentration, relative to that in eutrophic Oslofjord (Norway) waters, by 1, 2, 4, 8, 16, 32 μmol dissolved inorganic nitrogen (DIN)l⁻¹ (and the corresponding P increase). Two unamended mesocosms were used as controls. The nutrients were added continuously for 27 months before gross primary production (GPP), respiration (R), net community production (NCP), and dissolved organic carbon (DOC) production were assessed for the dominant algal species (Fucus serratus) and for the whole experimental ecosystem. Inputs and outputs of DOC and particulate organic carbon (POC) from the mesocosms were also quantified. The F. serratus communities were generally autotrophic (average P/R ratio = 1.33 ± 0.12), with the GPP independent of the nutrient inputs to the mesocosms, and maintained a high net DOC production during both day (0.026 ± 0.008 g C m⁻² h⁻¹) and night (0.015 ± 0.004 g C m⁻² h⁻¹). All the experimental rocky shore ecosystems were autotrophic (P/R ratio = 2.04 ± 0.28), and neither macroalgal biomass nor production varied significantly with increasing nutrient inputs. Most of the excess production from these autotrophic ecosystems was exported from the systems as DOC, which accounted for 69% and 58% of the NCP of the dominant community and the experimental ecosystem, respectively, the rest being lost as POC. High DOC release and subsequent export from the highly energetic environments occupied by rocky shore communities may prevent the development of eutrophication symptoms and render these communities resistant to eutrophication. Received 10 October 2001; accepted 18 July 2002.     

Does N2 fixation meet the nitrogen requirements of heterocystous blue-green algae in shallow eutrophic lakes?

Summary Phytoplankton net carbon uptake and nitrogen fixation were studied in two shallow, eutrophic lakes in South Sweden. Ranges of diurnal net carbon uptake were estimated by subtracting 24-h respiration rates corresponding to 5–20% of P _max, respectively, from daytime carbon uptake values. total nitrogen requirement of the phytoplankton assemblage was determined from the diurnal net carbon uptake, assuming a phytoplankton C:N ratio of 9.5:1. Nitrogen supplied by nitrogen fixation only occasionally corresponded to the demands of the total phytoplankton assemblage. When heterocystous algae made up a substantial proportion (10%) of the total phytoplankton biomass, nitrogen fixation could meet the requirements of heterocystous blue-green algae on c. 50% of the sampling occasions. Nitrogen deficiencies in heterocystous algae were most probably balanced by the simultaneous or sequential assimilation of dissolved inorganic nitrogen. It was concluded that uptake of ammonium or nitrate, regenerated from lake seston and sediment, is the main process by which growth of phytoplankton is maintained during summer in the lake ecosystems studied.     

Growth response of four freshwater algal species to dissolved organic nitrogen of different concentration and complexity

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Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE)

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free‐air CO₂ enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion‐exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the ¹³C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO₂ enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope ¹⁵N increased strongly. This indicates that the CO₂ enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO₂‐enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine‐root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO₂ concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.