Impacts of changing climate and atmospheric deposition on N and S drainage losses from a forested watershed of the Adirondack Mountains, New York State

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 SO₄^2? in precipitation, SO₄^2? 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 SO₄^2? 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 SO₄^2? 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 SO₄^2? 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.     

An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils

Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH₄ ⁺–N?+?NO₃ ^?–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25?cm depth. Fluxes of DIN were generally dominated by NO₃ ^?–N; but NH₄ ⁺–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH₄ ⁺–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH₄ ⁺–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p?<?0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p?<?0.01) and surface litter (p?<?0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively.     

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.     

Genetic engineering of sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis> L.) for resistance to necrosis disease through deployment of the TSV coat protein gene

Nitrogen is a major driver of plant growth and the nitrogen source can be critical to good growth in vitro. A response surface methodology mixture-component design and a data mining algorithm were applied to nitrogen (N) nutrition for improving the micropropagation of Prunus armeniaca Lam. Data taken on shoot cultures included a subjective quality rating, shoot number, shoot length, leaf characteristics and physiological disorders. Data were analyzed using the Classification and Regression Tree data mining algorithm. The best overall shoot quality as well as leaf color were on medium with NO₃^??>?25 mM and NH₄⁺/Ca⁺ >?0.8. Improving shoot length to15 mm required 25?<?NO₃^? ≤?35 mM with NH₄⁺/Ca²⁺ ≤?2.33. The most shoots (11.6) were produced with NO₃^? >?25 mM and NH₄⁺/Ca²⁺ ≤ 0.8, but there were 5–10 shoots at other NO₃^? concentrations regardless of NH₄⁺/Ca²⁺ proportion. Leaves increased in size with higher NO₃^? concentrations (>?55 mM). Physiological disorders were also influenced by the nitrogen components. Shoot tip necrosis was rarely present with NO₃^? > 45 mM. Callus production decreased somewhat with NH₄⁺/Ca²⁺ >?2.33. Suggested concentrations for an improved medium considering all of these growth characteristics would be 25?<?NO₃^? ≤?35 mM and NH₄⁺/Ca⁺ ≤ 0.8. Validation experiments comparing WPM and three trial media showed improvements in several shoot growth parameters on medium with optimized mesos and optimized nitrogen components.     

Contrasting stream water NO3− and Ca2+ in two nearly adjacent catchments: the role of soil Ca and forest vegetation

Two nearly adjacent subcatchments, located in the Adirondack Mountains of New York State, US, with similar atmospheric inputs of N (0.6 kmol ha^?1 yr^?1), but markedly different stream water solute concentrations, provided a unique opportunity to evaluate the mechanisms causing this variation. Subcatchment 14 (S14) had much greater stream water Ca²⁺ and NO₃^? concentrations (851 and 73 μmol_c L^?1, respectively) than Subcatchment 15 (S15) (427 and 26 μmol_c L^?1, respectively). To elucidate factors affecting the variability in stream water concentrations, soil and forest floor samples from each subcatchment were analyzed for total elemental cations and extractable N species. Mineral soil samples were also analyzed for exchangeable cations. Tree species composition was characterized in each subcatchment and potential differences in land use history and hydrology were also assessed. Compared with S15, soils in S14 had significantly higher total elemental Ca²⁺ in the forest floor (380 vs. 84 μmol g^?1), Bs horizon (e.g. 1361 vs. 576 μmol g^?1) and C horizon (1340 vs. 717 μmol g^?1). Exchangeable Ca²⁺ was also significantly higher in the mineral soil (64 μmol g^?1 in S14 vs. 8 μmol g^?1 in S15). Extractable NO₃^? was higher in S14 compared with S15 in both the forest floor (0.1 vs. 0.01 μmol g^?1) and Bs horizon (0.2 vs. 0.07 μmol g^?1) while extractable NH₄⁺ was higher in S14 vs. S15 in the forest floor (7 vs. 5 μmol g^?1). The total basal area of ‘base‐rich indicator’ tree species (e.g. sugar maple, American basswood, eastern hophornbeam) was significantly greater in S14 compared with S15, which had species characteristic of sites with lower base concentrations (e.g. American beech and eastern white pine). The disparity in stream water Ca²⁺ and NO₃^?, concentrations and fluxes between S14 and S15 were explained by differences in tree species composition and soil properties rather than differences in land use or hydrology. The marked difference in soil Ca²⁺ concentrations in S14 vs. S15 corresponded to the higher stream water Ca²⁺ and the larger contribution of base‐rich tree species to the overstory biomass in S14. Soil under such species is associated with higher net mineralization and nitrification and likely contributed to the higher NO₃^? concentrations in the drainage waters of S14 vs. S15. Studies investigating differences in spatial and temporal patterns of the effects of chronic N deposition on surface water chemistry need to account for changes in tree species composition and how vegetation composition is influenced by soil properties, as well as climatic and biotic changes.     

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

NH₄⁺ and NO₃^? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO₃^?up take followed saturable kinetics (K₂=4.9 μg-at N t^?1, V_max= 2.85 μg- at N, g(wet)^?1. h^?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO₃^? uptake was reduced one-half in the presence of NH₄⁺, but presence of NO₃^? had no effect on NH₄⁺ uptake. Darkness reduced both NO₃^? and NH₄⁺ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO₃^? uptake followed saturable kinetices: K₂=13.1 μg-at N. l^?1. V_max=3.05 μg-at N. g(wet)^?1. h^?1.NH₄⁺ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K₂=5.3 μg-at N.1–1, V_max= 2.38 μg-at N G (wet)^?1.h^?1: at higher concentration uptake increased lincarly with concentrations. NO₃^?and NH₄⁺ were taken up simulataneously: presence of one form did not affect uptake of the other.     

Alkalinity and acidity cycling and fluxes in an intermediate fen peatland in northern Ontario

Peatlands are important to global carbon (C) sequestration and surface water acid–base status, both of which are affected by peatland alkalinity and acidity cycling. Relationships among sulfate (SO₄ ^2?), nitrate (NO₃ ^?), organic acids (OA^?), base cations (i.e., Ca²⁺, Mg²⁺, K⁺, and Na⁺), proton (H⁺) acidity, and bicarbonate (HCO₃ ^?) alkalinity were investigated in an intermediate fen peatland in northern Ontario during 2004 (an average precipitation year) and 2005 (a dry summer). Potential evapotranspiration was higher and the water table, groundwater input from the uplands, and runoff were lower during 2005. Net inputs of base cations, HCO₃ ^?, SO₄ ^2?, and OA^?, and to a lesser degree NO₃ ^?, were lower during the drier year, mainly due to lower groundwater transfer to the fen. Fen porewater HCO₃ ^? concentration and net output were also lower in the drier year, whereas Ca²⁺, Mg²⁺, and SO₄ ^2? concentrations and net output were higher. During the climatically average year, N immobilization, carbonic acid (H₂CO₃) dissociation, and OA dissociation were equally important H⁺-producing reactions. Peat cation exchange accounted for 50% of the H⁺ sink, while SO₄ ^2? reduction and denitrification accounted for an additional 20 and 25% of the H⁺ sink, respectively. During the dry year, S oxidation accounted for 55% of the H⁺ net production, while that for H₂CO₃ dissociation was 70% lower than that during the climatically average year. Peat cation exchange consumed three times the acidity, and accounted for 92% of the H⁺ consumption during the dry year compared to the climatically average year. This was consistent with a three-fold higher net base cation export from the fen during the dry year. Based on the study results, a conceptual model was developed that describes the role of acidity formation and its implications to intermediate fen acidification.     

AMMONIUM,NITRATE, PHOSPHATE,AND INORGANIC CARBON UPTAKE IN AN OLIGOTROPHIC LAKE: SEASONAL VARIATIONS AMONG LIGHT RESPONSE VARIABLES1

The dependence of substrate saturated uptake of ¹⁵NH₄⁺, ¹⁵NO₃^?, ³²PO₄^3?, and ¹⁴CO₂ on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO₄^3? uptake was not dependent upon PPFD at any time of the year, whereas NH₄⁺, NO₃^?, and CO₂ uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH₄⁺, NO₃^? and CO₂ uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH₄⁺, NO₃^? and PO₄^3? were incorporated in the dark at measurable rates most of the year, whereas dark CO₂ uptake was always near 0 relative to light uptake. CO₂ and NO₃^? uptake were more strongly influenced by PPFD than was NH₄^3? uptake. The PPFD dependence of PO₄^3?, NH₄⁺, NO₃^? and CO₂ uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.     

Fifteen-year Change in Forest Floor Organic and Element Content and Cycling at the Turkey Lakes Watershed

To assess the long-term effects of atmospheric deposition on forest floor chemical composition, we took quantitative samplings of L-(Oi), F-(Oe), and H-(Oa) layers at an old-growth sugar maple–yellow birch stand on a till soil at the Turkey Lakes Watershed near Lake Superior, Ontario, Canada, in 1981 and 1996. We then assessed these samples for contents of organic matter (OM), total N, K, Ca, Mg, S, and Na, and exchangeable NH₄ ⁺, NO₃ ⁻, K⁺, Ca²⁺, Mg²⁺, SO₄ ²⁻, and Na⁺. Over the 15-year period, total OM and element contents remained unchanged, with the exception of N, which increased significantly from 61.3 kmol/ha in 1981 to 78.4 kmol/ha in 1996. On an area basis, there were significant increases in exchangeable Ca²⁺ (from 3.8 to 4.6 kmol/ha) and Na⁺ (from 0.05 to 0.08 kmol/ha) and decreases in exchangeable NH₄ ⁺-N (from 1.41 to 0.95 kmol/ha) and SO₄ ²⁻-S (from 1.29 to 0.96 kmol/ha). There were no significant differences in average annual litterfall OM, N, Ca, Mg, S or Na inputs between 1980 and 1985 and between 1992 and 1997. Average annual wet-only SO₄ ²⁻-S deposition during 1981–86 was 0.30; during 1992–97, it was 0.21 kmol/ha. Annual wet-only NO₃ ⁻-N averaged 0.33 kmol/ha during 1981–86 and was similar during 1992–97. Throughfall was less rich in SO₄ ²⁻ and Ca²⁺, Mg²⁺, and Na⁺ during 1992–97 than earlier. Throughfall NH₄ ⁺ and NO₃ ⁻ fluxes were unchanged. Efflux of cations from the forest floor reflected reduced throughput of SO₄ ²⁻. Overall, the results suggest that in spite of atmospheric inputs, active biological processes—including litter input, fine-root turnover, and tree uptake—serve to impart stability to the mineral composition of mature sugar maple forest floor. Received 5 October 1999; accepted 25 October 2000.     

Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O₂ m^?2 h^?1), ammonium (NH₄ ⁺), nitrate (NO₃ ^?), dissolved organic nitrogen, dinitrogen gas (N₂), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH₄ ⁺), NO₃ ^? and N₂ fluxes (annual means +14.6, +15.9 and 44.6 μmol N m^?2 h^?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O₂ m^?2 h^?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH₄ ⁺ flux which may have masked reductions in NH₄ ⁺ flux associated with BMA assimilation during the light. Peaks in net N₂ fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO₃ ^? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O₂ m^?2 h^?1.     

Contrasting responses of two Sitka spruce forest plots in Ireland to reductions in sulphur emissions: results of 20 years of monitoring

Long-term trends in ion concentrations of bulk precipitation, throughfall, forest floor leachate (humus water) and shallow and deep soil water were assessed at two Sitka spruce (Picea sitchensis) stands—one on an Atlantic peat bog in the west of Ireland (Cloosh), the other on the east coast on a peaty podzol (Roundwood). Deposition at Cloosh was dominated by marine ions (sodium, [Na⁺], chloride [Cl^?], and magnesium [Mg²⁺]), whereas bulk precipitation and throughfall at Roundwood was characterized by inputs of non-marine sulphate (nmSO₄ ^2?), acidity and inorganic nitrogen (NH₄ ⁺, NO₃ ^?). Significant declines in concentrations of nmSO₄ ^2? and acidity in bulk precipitation and throughfall were observed at both sites. The decline in throughfall nmSO₄ ^2? was significantly related to reductions in European sulphur dioxide (SO₂) emissions. At Roundwood, SO₄ ^2? declined significantly in humus, shallow and deep soil water. In deep soil water this was accompanied by a long-term increase in pH and a reduction in total aluminum (Al_tot). The recovery from acidification was delayed by high concentrations of NO₃ ^?, which strongly influenced acidity and Al_tot concentrations. At Cloosh, there was a significant decline in SO₄ ^2? in humus water but long-term trends were not evident in shallow or deep soil water; SO₄ ^2? concentrations at these depths fluctuated in response to drought-events. Marine ions strongly influenced soil water chemistry at both sites; at Cloosh soil water acidity was strongly related to Na⁺ and Cl^?, while at Roundwood, Na⁺, Cl^? and Mg²⁺ influenced Al_tot concentrations. Dissolved organic carbon increased significantly in humus and soil water at Roundwood, where it was associated with declining acidity. Soil water at both sites was influenced by a combination of anthropogenic sulphur (S) and nitrogen (N) deposition, drought and sea-salt events. The study highlights the value of long-term monitoring in assessing the response of forest soils to S and N deposition against a background of climate influences on soil water through drought and sea-salt events.     

Uptake and Transport of Calcium and Phosphorus in Lolium perenne in Response to N Supplied to Halves of a Divided Root System

The uptake and transport of Ca²⁺ and HPO₄^2? from roots of Lolium perenne L. was studied using variable N nutrition supplied to halves of a divided root system. Plants were grown for 4 weeks in solution containing 0.11 mM NO₃^?–N; then one-half of the root system was supplied with either 4.0 mM NO₃^?–N or 0.28 mM NH₄⁺–N while the other half of the root system remained in low-N solution. Uptake and transport of Ca²⁺ increased and uptake of HPO₄^2? declined in root halves supplied with high NO₃^?–N for 16 h. After supply of high NO₃^?–N or NH₄⁺–N to half the root system for 6 days, the roots supplied with high-N exhibited significantly higher rates of uptake and percentage transport to shoots of both Ca²⁺ and HPO₄^2?–. However, in neither the 16-h nor 6-day treatment did Ca²⁺ or HPO₄^2? uptake of the root half supplied with low N differ significantly from the control (low N supplied to both halves of the root). Significantly higher N concentrations were found in low-N supplied roots (compared to the control) as a result of internal translocation of N from high-N supplied roots to low-N supplied roots. Although N concentration in the low-N supplied roots increased, uptake rates of Ca²⁺ or HPO₄^2? did not change implying that external N concentration may be the important factor which influences or governs N mediated uptake responses. This would further suggest that the site of uptake regulation for Ca²⁺ and HPO₄^2? exists on the outer plasma membrane which is in direct contact with the external solution. Transport of Ca²⁺ and HPO₄^2? to the shoot was generally increased in low-N root halves after 6 days of high-N supply to the other half of the root. This implies that plant growth demand may be a major factor in regulating rates of Ca²⁺ and HPO₄^2? transport from roots to the shoot. It also reinforces the hypothesis that uptake and transport of ions out of the root are separately controlled or regulated in the plant.     

The effect of nutrient concentrations,nutrient ratios and temperature on photosynthesis and nutrient uptake by Ulva prolifera: implications for the explosion in green tides

The green-tide macroalga, Ulva prolifera, was tested in the laboratory to determine its nutrient uptake and photosynthesis under different conditions. In the nutrient concentration experiments U. prolifera showed a saturated uptake for nitrate but an escalating uptake in the tested range for phosphorus. Both N/P and NO₃ ^?/NH₄ ⁺ ratios influenced nutrient uptake significantly (p?<?0.05) while the PSII quantum yield [Y(II)] (p?>?0.05) remained unaffected. The maximum N uptake rate (33.9?±?0.8 μmol g^?1 DW h^?1) and P uptake rate (11.1?±?4.7) was detected at N/P ratios of 7.5 and 2.2, respectively. U. prolifera preferred NH₄ ⁺-N to NO₃ ^?-N when the NO₃ ^?-N/NH₄ ⁺-N ratio was less than 2.2 (p?<?0.05). But between ratios of 2.2 and 12.9, the uptake of NO₃ ^?-N surpassed that of NH₄ ⁺-N. In the temperature experiments, the highest N uptake rate and [Y(II)] were observed at 20 °C, while the lowest rates were detected at 5 °C. P uptake rates were correlated with increasing temperature.     

Kinetics of nitrate and ammonium absorption and accompanying H+ fluxes in roots of Lolium perenne L. and N2-fixing Trifolium repens L.

Kinetic parameters for NH₄⁺ and NO₃^? uptake were measured in intact roots of Lolium perenne and actively N₂-fixing Trifolium repens. Simultaneously, net H⁺ fluxes between the roots and the root medium were recorded, as were the net photosynthetic rate and transpiration of the leaves. A Michaelis–Menten-type high-affinity system operated in the concentration range up to about 500 mmol m^?3 NO₃^? or NH₄⁺. In L. perenne, the V_max of this system was 9–11 and 13–14 μmol g^?1 root FW h^?1 for NO₃^? and NH₄⁺, respectively. The corresponding values in T. repens were 5–7 and 2 μmol g^?1 root FW h^?1. The K_m for NH₄⁺ uptake was much lower in L. perenne than in T. repens (c. 40 compared with 170 mmol m^?3), while K_m values for NO₃^? absorption were roughly similar (around 130 mmol m^?3) in the two species. There were no indications of a significant efflux component in the net uptake of the two ions. The translocation rate to the shoots of nitrogen derived from absorbed NO₃^?-N was higher in T. repens than in L. perenne, while the opposite was the case for nitrogen absorbed as NH₄⁺. Trifolium repens had higher rates of transpiration and net photosynthesis than L. perenne. Measurements of net H⁺ fluxes between roots and nutrient solution showed that L. perenne absorbing NO₃^? had a net uptake of H⁺, while L. perenne with access to NH₄⁺ and T. repens, with access to NO₃^? or NH₄⁺, in all cases acidified the nutrient solution. Within the individual combinations of plant species and inorganic N form, the net H⁺ fluxes varied only a little with external N concentration and, hence, with the absorption rate of inorganic N. Based on assessment of the net H⁺ fluxes in T. repens, nitrogen absorption rate via N₂ fixation was similar to that of inorganic N and was not down-regulated by exposure to inorganic N for 2 h. It is concluded that L. perenne will have a competitive advantage over T. repens with respect to inorganic N acquisition.     

Nitrogen dynamics at the sediment–water interface in shallow, sub-tropical Florida Bay: why denitrification efficiency may decrease with increased eutrophication

Nitrogen (N) dynamics at the sediment–water interface were examined in four regions of Florida Bay to provide mechanistic information on the fate and effects of increased N inputs to shallow, subtropical, coastal environments. Dissimilatory nitrate (NO₃ ^?) reduction to ammonium (DNRA) was hypothesized to be a significant mechanism retaining bioreactive N in this warm, saline coastal ecosystem. Nitrogen dynamics, phosphorus (P) fluxes, and sediment oxygen demand (SOD) were measured in north-central (Rankin Key; eutrophic), north-eastern (Duck Key; high N to P seston ratios), north-western (Murray Key; low N to P ratios), and central (Rabbit Key; typical central site) Florida Bay in August 2004, January 2005, and November 2006. Site water was passed over intact sediment cores, and changes in oxygen (O₂), phosphate (o-PO₄ ^3?), ammonium (NH₄ ⁺), NO₃ ^?, nitrite (NO₂ ^?), and N₂ concentrations were measured, without and with addition of excess ¹⁵NO₃ ^? or ¹⁵NH₄ ⁺ to inflow water. These incubations provided estimates of SOD, nutrient fluxes, N₂ production, and potential DNRA rates. Denitrification rates were lowest in summer, when SOD was highest. DNRA rates and NH₄ ⁺ fluxes were high in summer at the eutrophic Rankin site, when denitrification rates were low and almost no N₂ came from added ¹⁵NO₃ ^?. Highest ¹⁵NH₄ ⁺ accumulation, resulting from DNRA, occurred at Rabbit Key during a picocyanobacteria bloom in November. ¹⁵NH₄ ⁺ accumulation rates among the stations correlated with SOD in August and January, but not in November during the algal bloom. These mechanistic results help explain why bioreactive N supply rates are sometimes high in Florida Bay and why denitrification efficiency may decrease with increased NO₃ ^? inputs in sub-tropical coastal environments.     

Nutrient hot spots in some sierra Nevada forest soils

Re-examination of data distributions from several forested sites in the eastern Sierra Nevada Mountains shows consistent, positive outliers and skew for NH₄ ⁺, NO₃ ^?, and mineral N in resin lysimeters and resin capsules, indicating that most values were low but hotspots of high N flux were present in most cases. Exact causes of these N flux hotspots is not known, but could include water flux hotspots (e.g., preferential flowpaths), microbial hotspots, and possibly the entry of N-enriched O horizon interflow. Soil and resin stake (PRS probe) data from one site (North Lake Tahoe) also showed consistent, positive outliers and skew for NH₄ ⁺, NO₃ ^? , and mineral N, suggesting the presence of microbially produced hotspots. Bicarbonate-P data from soils and ortho-P data from PRS probes also showed highly positive skew and extreme outliers, but Bray (HCl/NH₄F-extractable) P in soils did not. Other measured nutrients (extractable Mg²⁺, K⁺, SO₄ ^2?, and Ca²⁺) also showed positive skew and outliers, but less so than NH₄ ⁺, NO₃ ^?, and mineral N. Calcium stood out among measured nutrients as the most abundant nutrient with the least outliers and the lowest (sometimes negative) skew. The differences in distributions of NH₄ ⁺, NO₃ ^?, and mineral N and those of Ca²⁺ may reflect relative abundance: the most abundant ion, Ca²⁺, shows little evidence of hotspots whereas the much less abundant ions, NH₄ ⁺, NO₃ ^? consistently show evidence of hotspots. We hypothesize that the differing distributions of N and Ca reflect the relative biological competition for these nutrients and that positively-skewed distributions and hotspots will be characteristic of any other nutrient when it is in limited supply relative to biological demand.     

High NH4+ efflux from roots of the common alpine grass,Festuca nigrescens,at field-relevant concentrations restricts net uptake

Alpine meadows of high ecological value could be severely endangered by anthropogenic N enrichment, modifying the relationships between species and the environment. While a constraint exerted by N availability on alpine plant development has been demonstrated by some fertilization experiments, in others no effect was observed. Basically, the problem is that mineral N absorption has not been characterized in alpine plants. In growth chamber experiments, we investigated the component fluxes of ¹⁵NO₃^? and ¹⁵NH₄⁺ uptake in a tussock grass (Festuca nigrescens) very common and representative of the dominant plant growth form in European alpine meadows. Rates of influx supported data already published for low elevation herbaceous species. These rates were up to ten times higher for NH₄⁺ than for NO₃^? but rates of net uptake were similar for both ions demonstrating the occurrence of elevated NH₄⁺ efflux (80% of primary influx). An increase in external N in the range of field-relevant concentrations did not substantially enhance net uptake. Thus, the alpine plant which is assumed to be adapted to relatively high soil NH₄⁺ responded like an NH₄⁺-sensitive species: as if it was unable to use the incoming nitrogen. It is suggested that the ability of this typical alpine grass to respond to increasing N availability due to global changes is limited.     

The uptake and translocation of macro- and microelements in rape and wheat seedlings as affected by selenium supply level

Plant growth in saline soils may be increased by fertilisation, but little is known about the effect of different forms of N on wheat growth in soils with different salinity levels. The aim of this study was to investigate the response of wheat (Triticum aestivum L., cv Krichauff) to (NH₄)₂SO₄ or KNO₃ or NH₄NO₃ at 0 (N0), 50 (N50), 100 (N100) and 200 (N200) mg N?kg^?1 soil in a saline sandy loam. Salinity was induced using Na⁺ and Ca²⁺ salts to achieve three EC_e levels, 2.8, 6.6 and 11.8 dS m^?1 denoted S1, S2 and S3, respectively, while maintaining a low SAR (>1). Dry weights of shoot and root were reduced by salinity in all N treatments. Addition of N significantly increased shoot and root dry weights with significant differences between N forms. Under non-saline conditions (S1), addition of NO₃???N at rates higher than N50 had a negative effect, while N100 as NH₄???N or NH₄NO₃???N increased shoot and root dry weights. At N100, shoot concentrations of N and K were higher and P, Ca, Fe, Mn, Cu and Zn were lower with NO₃???N than with NH₄???N nutrition. The concentration of all nutrients however fell in ranges did not appear to be directly associated with poor plant growth with NO₃???N. At all N additions, calculations indicated that soil salinity was highest with N addition as NO₃???N and decreased in the following order: NO₃?N > NH₄?N > NH₄NO₃?N. Addition of greater than N50 as NO₃???N, compared to NH₄???N or NH₄???NO_3, increased soil salinity and reduced micronutrient uptake both of which likely limited plant growth. It can be concluded that in saline soils addition of 100 mg N?kg^?1 as NH₄???N or NH₄NO₃???N is beneficial for wheat growth, whereas NO₃???N can cause growth depression.     

Organic Acids and Ionic Balance in Xylem Exudate of Wheat during Nitrate or Sulfate Absorption

Experiments were designed to study the importance of organic acids as counterions for K⁺ translocation in the xylem during excess cation uptake. A comparison was made of xylem exudate from wheat seedlings treated 72 hours with either 1.0 millimolar KNO₃ or 0.5 millimolar K₂SO₄, both in the presence of 0.2 millimolar CaSO₄. Exudation from KNO₃ plants had twice the volume and twice the K⁺ and Ca²⁺ fluxes or rate of delivery to shoots, as K₂SO₄ plants. Malate flux was 25% higher in K₂SO₄ than in KNO₃ exudate. Malate was the principal anion accompanying K⁺ or Ca²⁺ in K₂SO₄ treatment, while in the KNO₃ treatment, NO₃⁻ was the principal anion. The contribution of SO₄²⁻ was negligible in both treatments. In a second experiment, exudate was collected every 4 hours during the daytime throughout a 72-hour treatment with KNO₃. Malate was the only anion present in exudate at first, just after the CaSO₄ pretreatment had ended. Malate concentration decreased and NO₃⁻ concentration increased with time and these concentrations were negatively correlated. By 62 hours, NO₃⁻ represented 80% of exudate anions. K⁺ and NO₃⁻ concentrations in exudate were strongly correlated with K⁺ and NO₃⁻ uptake, respectively. The first 36 hours of absorption from KNO₃ solution resembled the continuous absorption of K₂SO₄, in that malate was the principal counterion for translocation of K⁺.     

Large Canopy Exchange Fluxes of Inorganic and Organic Nitrogen and Preferential Retention of Nitrogen by Epiphytes in a Tropical Lowland Rainforest

Little is known about how tropical forest canopies interact with atmospheric nitrogen deposition and how this affects the internal nutrient dynamics and the processing of external nutrient inputs. The objectives of this study therefore were (1) to investigate gross and net canopy nitrogen (N) fluxes (retention and leaching) and (2) the effect of canopy components on net canopy N retention. Tracers were applied on detached branches in a tropical wet lowland rainforest, Costa Rica. A novel ¹⁵N pool dilution method showed that gross canopy fluxes (retention and leaching) of NO₃ ^?, NH₄ ⁺, and dissolved organic nitrogen (DON) were remarkably higher than net throughfall fluxes. Gross fluxes of NH₄ ⁺ and NO₃ ^? resulted in a negligible net flux whereas DON showed net uptake by the canopy. The highest quantity of ¹⁵N was recovered in epiphytic bryophytes (16.4%) although the largest biomass fraction was made up of leaves. The study demonstrates that tracer applications allow investigation of the dynamic and complex canopy exchange processes and that epiphytic communities play a major role in solute fluxes in tree canopies and therefore in the nutrient dynamics of tropical rain forests.