The effect of total inorganic nitrogen and the balance between its ionic forms on adventitious bud formation and callus growth of ‘Węgierka Zwykła’ plum (<Emphasis Type="Italic">Prunus domestica</Emphasis> L.)

The effect of different ammonium NH ₄ ⁺ and nitrate NO ₃ ^? ratios (4:1, 2:1, 1:1, 1:2, 1:4, 1:6) on organogenesis of ‘W?gierka Zwyk?a’ leaf explants cultivated on media with nitrogen levels equalling full- or half-MS was investigated. On media with total nitrogen equal to ½ MS, explant regeneration increased significantly and was highest on media with 1:2 or 1:4 NH ₄ ⁺ :NO ₃ ^? ratio. An excess of ammonium versus nitrate ions had a negative effect on both regeneration and biomass. Addition of potassium to the medium increased the fresh weight of explants and the number of adventitious buds.     

Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^? ) as an inorganic nitrogen (N) source. ¹⁵N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH ₄ ⁺ and NO ₃ ^? by tea roots. The total ¹⁵N was detected, and kinetic parameters were calculated after feeding ¹⁵NH ₄ ⁺ or ¹⁵NO ₃ ^? to tea plants. The process of N assimilation was studied by monitoring the dynamic ¹⁵N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with ¹⁵NH ₄ ⁺ absorbed significantly more ¹⁵N than those supplied with ¹⁵NO ₃ ^? . The kinetics of ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V _max value for NH ₄ ⁺ uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO ₃ ^? (39.3 nmol/(g dry wt min)). K_M estimates were approximately 0.06 mM for NH ₄ ⁺ and 0.16 mM for NO ₃ ^? , indicating a higher rate of NH ₄ ⁺ absorption by tea plant roots. Tea plants fed with ¹⁵NH ₄ ⁺ accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with ¹⁵NO ₃ ^? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with ¹⁵N under both conditions. The rate of N assimilation into Thea in the roots of NO ₃ ^? -supplied tea plants was quicker than in NH ₄ ⁺ -supplied tea plants. NO ₃ ^? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO ₃ ^? as the sole N source. The NH ₄ ⁺ absorbed by tea plants directly, as well as that produced by NO ₃ ^? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The ¹⁵N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH ₄ ⁺ .     

Carbon and nitrogen in the ground waters of the Tellerman forest (European southern forest steppe)

Under the upland forests growing on the Dnieper loamy soils at 150–160 m above sea level, sandy aquifers are found at ~120 and ~140 m above sea level. The general mineralization of groundwaters (GWs) and the concentration of SO ₄ ^-2 increase upon approaching the foundation of fluvial glacial sediments. The concentration of C supplied to the local GWs is 10–12 mg L^–1 per year^–1. The concentration of C(CO ₃ ^-2 ; HCO ₃ ^-1 ; NO ₃ ^-1 ; NO ₂ ^-1 ) in the DSW is consistent with the C balance of upper biogeocenoses and the rate of groundwater migration. The concentrations of N(nh ₄ ⁺¹ ) are unrelated to the filtration of biogeocenosis moisture. They are identical both under sodic grassy glades and highly productive tree stands. The main part of N-containing ions in situ is synthesized and destroyed by anaerobic microorganisms living in the GW.     

Involvement of phosphoenolpyruvate carboxylase and antioxydants enzymes in nitrogen nutrition tolerance in <Emphasis Type="Italic">Sorghum bicolor</Emphasis> plants

Plants of Sorghum bicolor (C₄ species) were grown at different nitrate or ammonium concentrations (0.5, 5, 20 and 50 mM) in order to examine the effect of nitrogen nutrition on growth, phosphoenolpyruvate carboxylase (PEPC) and antioxidant enzymes activities in both roots and leaves of 30-day-old plants. At high NO₃^? levels (20 and 50 mM) the fresh weight was significantly higher. When the nitrogen source was in ammonium form, the leaf and root mass increased drastically at low concentration 5 mM and significantly at 20 mM, however similar fresh weight was found at high level of ammonium (50 mM). The leaves catalase (CAT), guaiacol peroxidase (POD), glutathione reductase (GR), and glutathione S-transferase (GST) activities and the roots glutathione reductase and glutathione S-transferase activities were significantly higher in the NH₄⁺-fed plants than those grown in the nitrate medium. Activity and proteins levels of phosphoenolpyruvate carboxylase in both leaves and roots of sorghum plants were increased progressively with increasing external nitrogen concentration. This increase was more pronounced at high level of ammonium (50 mM), being 2-fold at 50 mM of NO₃^? and 3-fold at 50 mM of NH₄⁺. Our results suggested that antioxidant enzymes activities and PEPC play a key role in ammonium detoxification and tolerance in sorghum plants.     

Nitrosomonas sp. Based biosensor for ammonium nitrogen measurement in wastewater

A bacterial culture of Nitrosomonas sp. was isolated from a nitrifying biofilm to construct a biosensor for ammonium nitrogen (NH ₄ ⁺ ?N) measurements in high ammonia wastewaters. The pure culture of microorganisms was immobilized into agarose gel matrix to attain a stable biosensor with a long service life. Biosensors were calibrated using (NH₄)₂SO₄ solution and a steady-state method. Subsequently, several experiments with synthetic and industrial wastewaters were conducted. A linear range up to 20 mg/L of NH ₄ ⁺ ?N, and sensitivities between 0.030 and 0.036 were gained with biosensors. During 14 days of stable service life of the Nitrosomonas sp. biosensors, variation of the signal was less than 7%. Response times of biosensors were 15 ～ 25 min, while recovery times were up to 25 min. Measurements with high ammonia content synthetic and industrial wastewaters were conducted, and 8.3 and 5.6% over estimation of NH ₄ ⁺ ?N was gained, respectively, compared with results of Nessler method. In spite of the small overestimation, the biosensor based on a pure culture of Nitrosomonas sp. and calibrated with (NH₄)₂SO₄ is suitable for the analysis of NH ₄ ⁺ ?N in high ammonia content wastewaters.     

Synthesis of high-molecular-mass polyhydroxybutyrate from methanol in <Emphasis Type="Italic">Methyloligella halotolerans</Emphasis> C2

The influence of the concentrations of carbon and energy sources (methanol) and nitrogen (NH ₄ ⁺ ) on the yield and molecular mass (Mm) of poly-β-hydroxybutyrate (PHB) synthesized by the novel methylotroph Methyloligella halotolerans C2 was investigated. It was shown that the maximum concentrations of NH ₄ ⁺ and methanol in the medium for PHB biosynthesis were 0.15 g/L and 1 ± 0.2 mL/L, respectively. A unique high-molecular (8000–10000 kDa) PHB was obtained with NH ₄ ⁺ and methanol concentrations of 0.12 g/L and 1.25 mL/L, respectively, in a fermenter. The main physicochemical and mechanical properties of the obtained bioplastic with a molecular mass of 10000 kDa were determined.     

Relative CO2/NH3 Permeabilities of Human RhAG, RhBG and RhCG

Mammalian glycosylated rhesus (Rh) proteins include the erythroid RhAG and the nonerythroid RhBG and RhCG. RhBG and RhCG are expressed in multiple tissues, including hepatocytes and the collecting duct (CD) of the kidney. Here, we expressed human RhAG, RhBG and RhCG in Xenopus oocytes (vs. H₂O-injected control oocytes) and used microelectrodes to monitor the maximum transient change in surface pH (ΔpH_S) caused by exposing the same oocyte to 5 % CO₂/33 mM HCO₃ ^? (an increase) or 0.5 mM NH₃/NH₄ ⁺ (a decrease). Subtracting the respective values for day-matched, H₂O-injected control oocytes yielded channel-specific values (*). $({\Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{CO}}_{ 2} }}$ and $({ - \Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{NH}}_{ 3} }}$ were each significantly >0 for all channels, indicating that RhBG and RhCG—like RhAG—can carry CO₂ and NH₃. We also investigated the role of a conserved aspartate residue, which was reported to inhibit NH₃ transport. However, surface biotinylation experiments indicate the mutants RhBG_D178N and RhCG_D177N have at most a very low abundance in the oocyte plasma membrane. We demonstrate for the first time that RhBG and RhCG—like RhAG—have significant CO₂ permeability, and we confirm that RhAG, RhBG and RhCG all have significant NH₃ permeability. However, as evidenced by $({\Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{CO}}_{ 2} }} /({ - \Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{NH}}_{ 3} }}$ values, we could not distinguish among the CO₂/NH₃ permeability ratios for RhAG, RhBG and RhCG. Finally, we propose a mechanism whereby RhBG and RhCG contribute to acid secretion in the CD by enhancing the transport of not only NH₃ but also CO₂ across the membranes of CD cells.     

Effect of ammonium and other ions on the glucose-dependent active transport of l-histidine in slices of rat-brain cortex

1. The influence of cations on the active transport into cells of rat-brain-cortex slices of l-histidine, an amino acid that is not metabolized by this tissue, has been studied. 2. Like other amino acids, l-histidine accumulated in the cells in the presence of glucose in concentrations up to over double that in the incubation medium. 3. The active transport of l-histidine was highest in a medium containing Ca²⁺ (3mm). The addition of K⁺ (27mm) led to a marked decrease in the intracellular concentration of l-histidine, though the oxygen uptake of the slices was higher. 4. The active l-histidine transport was inhibited by NH₄⁺. The inhibitory effect increased with the NH₄⁺ concentration, being about 25% at 8mm, 65% at 20mm, and 90% at 27 and 50mm. The oxygen uptake of the brain slices was depressed by only 25% by the highest NH₄⁺ concentration used, and less by lower concentrations.     

Ammonium nutrition increases water absorption in rice seedlings (Oryza sativa L.) under water stress

Water stress is a primary limitation on plant growth. In previous studies, it has been found that ammonium enhances the tolerance of rice plants to water stress, but how water is related to nitrogen form and water stress remains unknown. To study the effects of nitrogen form (NH ₄ ⁺ , NO ₃ ^? , and a mixture of NH ₄ ⁺ and NO ₃ ^? ) on the growth and water absorption of rice (Oryza sativa L.) seedlings, a hydroponic experiment with water stress, simulated by the addition of polyethylene glycol (PEG, 10% w/v, MW 6000), was conducted in a greenhouse. The results showed that, compared with non-water stress, under water stress, the fresh weight of rice seedlings increased by 14% with NH ₄ ⁺ nutrition, whereas it had decreased by about 20% with either NO ₃ ^? or mixed nitrogen nutrition. No significant difference was found in the transpiration rate of excised shoots or in xylem exudation of excised roots in NH ₄ ⁺ supply between the two water situations, whereas xylem flow decreased by 57% and 24% under water stress in NO ₃ ^? and mixed nutrition, and root hydraulic conductivity decreased by 29% and 54% in plants in NH ₄ ⁺ and NO ₃ ^? nutrition conditions, respectively. Although water absorption ability decreased in both NH ₄ ⁺ and NO ₃ ^? nutrition, aquaporin activity was higher in NH ₄ ⁺ than in NO ₃ ^? nutrition, regardless of water stress. We conclude that NH ₄ ⁺ nutrition can improve water handling in rice seedlings and subsequently enhance their resistance to drought.     

The Ammonia Transport, Retention and Futile Cycling Problem in Cyanobacteria

Ammonia is the preferred nitrogen source for many algae including the cyanobacterium Synechococcus elongatis (Synechococcus R-2; PCC 7942). Modelling ammonia uptake by cells is not straightforward because it exists in solution as NH₃ and NH ₄ ⁺ . NH₃ is readily diffusible not only via the lipid bilayer but also through aquaporins and other more specific porins. On the other hand, NH ₄ ⁺ requires cationic transporters to cross a membrane. Significant intracellular ammonia pools (≈1–10 mol?m^?3) are essential for the synthesis of amino acids from ammonia. The most common model envisaged for how cells take up ammonia and use it as a nitrogen source is the “pump–leak model” where uptake occurs through a simple diffusion of NH₃ or through an energy-driven NH ₄ ⁺ pump balancing a leak of NH₃ out of the cell. The flaw in such models is that cells maintain intracellular pools of ammonia much higher than predicted by such models. With caution, [¹⁴C]-methylamine can be used as an analogue tracer for ammonia and has been used to test various models of ammonia transport and metabolism. In this study, simple “proton trapping” accumulation by the diffusion of uncharged CH₃NH₂ has been compared to systems where CH₃NH ₃ ⁺ is taken up through channels, driven by the membrane potential (ΔU _i,o) or the electrochemical potential for Na⁺ (ΔμNa _i,o ⁺ ). No model can be reconciled with experimental data unless the permeability of CH₃NH₂ across the cell membrane is asymmetric: permeability into the cell is very high through gated porins, whereas permeability out of the cell is very low (≈40 nm?s^?1) and independent of the extracellular pH. The best model is a Na _in ⁺ /CH₃NH ₃ ⁺ _in co-porter driven by ΔμNa _i,o ⁺ balancing synthesis of methylglutamine and a slow leak governed by Ficks law, and so there is significant futile cycling of methylamine across the cell membrane to maintain intracellular methylamine pools high enough for fixation by glutamine synthetase. The modified pump–leak model with asymmetric permeability of the uncharged form is a viable model for understanding ammonia uptake and retention in plants, free-living microbes and organisms in symbiotic relationships.     

Effects of exogenous polyamines on nitrate tolerance in cucumber

Putrescine (Put), spermidine (Spd), and spermine (Spm) are the major polyamines (PAs) in plant, which are not only involved in the regulation of plant developmental and physiological processes, but also play key roles in modulating the defense response of plants to diverse environmental stresses. In this study, Cucumis sativus L. seedlings were cultivated in nutrient solution and sprayed with three kinds of PAs (Put, Spd, and Spm). The effects of PAs were investigated on excess nitrate stress tolerance of C. sativus by measuring growth and nitrogen (N) metabolism parameters. The contents of NO_3-^?N, NH_4-⁺N, proline and soluble protein in leaves were increased; while plant height, leaf area, shoot fresh and dry weight, root fresh weight were decreased under 140 mM NO₃^? treatment for 7 d. In addition, the activities of nitrate reductase (NR), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) were significantly inhibited under 140 mM NO₃^? treatment for 7 d. With foliar treatment by 1 mM Spd or Spm under stress treatment, the contents of Spm, Put, and Spd in leaves increased significantly, except that Spm content decreased under Spd treatment. The activities of NR, glutamine synthetase (GS), GOGAT and GDH and plant height, leaf area, shoot fresh and dry weights were significantly increased. The contents of proline and soluble protein in leaves were significantly enhanced. In contrast, the accumulation of NO_3-^?N and NH_4-⁺N were significantly decreased. However, there were minor differences in activities of N metabolism enzymes and the content of osmotic adjustment substances under 1 mM Put treatment. These findings suggest that 1 mM exogenous Spm or Spd could enhance the capacity of N metabolism, promote growth and increase resistance to high concentrations of NO₃^?. The ameliorating effect of Spd was the best, and that of Put the worst.     

Seasonal and within-event dynamics of rainfall and throughfall chemistry in an open tropical rainforest in Rondônia,Brazil

Prolonged dry periods, and increasingly the generation of smoke and dust in partially-deforested regions, can influence the chemistry of rainfall and throughfall in moist tropical forests. We investigated rainfall and throughfall chemistry in a palm-rich open tropical rainforest in the southwestern Brazilian Amazon state of Rondônia, where precipitation averages 2300 mm year^?1 with a marked seasonal pattern, and where the fragmentation of remaining forest is severe. Covering the transition from dry to wet season (TDWS) and the wet season (WS) of 2004–2005, we sampled 42 rainfall events on event basis as well as 35 events on a within-event basis, and measured concentrations of DOC, Na⁺, K⁺, Ca²⁺, Mg²⁺, NH ₄ ⁺ , Cl^?, SO ₄ ^2? , NO ₃ ^? and pH in rainfall and throughfall. We found strong evidence of both seasonal and within-event solute rainfall concentration dynamics. Seasonal volume-weighted mean (VWM_S) concentrations in rainfall of DOC, K⁺, Ca²⁺, Mg²⁺, NH ₄ ⁺ , SO ₄ ^2? and NO ₃ ^? were significantly higher in the TDWS than the WS, while VWM_S concentrations in throughfall were significantly higher for all solutes except DOC. Patterns were generally similar within rain events, with solute concentrations declining sharply during the first few millimeters of rainfall. Rainfall and throughfall chemistry dynamics appeared to be strongly influenced by forest and pasture burning and a regional atmosphere rich in aerosols at the end of the dry season. These seasonal and within-event patterns of rainfall and throughfall chemistry were stronger than those recorded in central Amazônia, where the dry season is less pronounced and where regional deforestation is less severe. Fragmentation and fire in Rondônia now appear to be altering the patterns in which solutes are delivered to remaining moist tropical forests.     

Nitrogen uptake and allocation in <Emphasis Type="Italic">Populus simonii</Emphasis> in different seasons supplied with isotopically labeled ammonium or nitrate

Key message

The total uptake of ¹⁵ NO ₃ -N was twofold higher than that of ¹⁵ NH ₄ -N when supplied with ammonium and/or nitrate in different seasons; the seedlings fertilized with NO ₃ -N had good growth with high photosynthetic rate and total biomass.

Abstract

Appropriate fertilization is crucial for maximum plant growth and improving nitrogen use efficiency. Poplar is an important fast-growing tree species for biomass production, however, little is known about fertilizer management of poplar plantations growing on barren soil in different seasons. To understand nitrogen uptake and allocation of Populus simonii supplied with different forms of nitrogen in different seasons, we determined nitrogen uptake and allocation of P. simonii potted seedlings after a 4-day supply of ¹⁵NH₄-N, ¹⁵NO₃-N, ¹⁵NH₄NO₃, and NH ₄ ¹⁵ NO₃ in May, July, and September. The total ¹⁵N uptake was twofold higher when supplied with sole ¹⁵NO₃-N compared to sole ¹⁵NH₄-N in all the investigated seasons. In the presence of ammonium nitrate (¹⁵NH₄NO₃ and NH ₄ ¹⁵ NO₃), the total ¹⁵N uptake was two times higher when supplied with NH ₄ ¹⁵ NO₃ compared to ¹⁵NH₄NO₃. Per unit biomass, the ¹⁵N-uptake ability of fine roots was higher in May and July compared to that in September. ¹⁵N was present mainly in leaves in May and July, and was mainly stored in roots and stems in autumn. The effect of nitrogen on the growth of P. simonii seedlings was studied by fertilizing with NH₄-N, NO₃-N, and NH₄NO₃ for 8 weeks. The seedlings fertilized with NO₃-N had good growth with high photosynthetic rate and total biomass indicating that NO₃-N is crucial for P. simonii growth. These data contribute to understand the nitrogen uptake in different seasons in trees supplied with different forms of nitrogen. This provides important theoretical bases for fertilizer management of poplar plantations.

     

Integrated ecological research of catchment-lake ecosystems in the Bohemian Forest (Central Europe): A preface

The Bohemian Forest (?umava, Böhmerwald) is situated in Central Europe and is among the most acidified lake districts in the world. Deposition of S and N compounds in the area rapidly increased between 1950 and 1980, and reached a maximum in the 1980s. During the 1990s, acid deposition decreased substantially, and current levels are comparable to the early 20^th century for SO ₄ ^2? and NH ₄ ⁺ , and to the mid 1960s for NO ₃ ^? . These changes in acid deposition have led to a partial recovery of the Bohemian Forest lakes. This paper provides an overview of previous research, and details on the organization and aims of current research on the Bohemian Forest lakes. Available historical data and regular monitoring (since 1984) provide a valuable background for long-term ecological research of the catchment-lake ecosystems that currently focuses on (i) chemical reversal and biological recovery of the lakes, (ii) acidification impacts on in-lake nutrient cycling, (iii) climatic effects on water chemistry, and (iv) catchment processes, including soil biogeochemistry and acidification impacts on vegetation.     

Positive feedback between acidification and organic phosphate mineralization in the rhizosphere of maize (Zea mays L.)

To test the hypothesis that rhizosphere acidification would enhance the hydrolyzation of organic phosphates by increasing phosphatase activity. A Petri dish experiment with sterile agar and a pot experiment with a low P soil were used. In the Petri dish experiment, roots of each plant were cultured in two compartments, each of which contained agar with one of three nitrogen combinations: NH ₄ ⁺ /N0 (N0 = nitrogen free), NH ₄ ⁺ /NO ₃ ^- , and NO ₃ ^- /N0. Phytin was supplied as the sole phosphorus (P) source to all compartments. In the pot experiment, the soil in each pot was treated with N0, KNO₃, or (NH₄)₂SO₄) together with 0 or 75 mg kg^?1 phytin-P. Dry weight, P concentration, and P content of roots were highest in the NH ₄ ⁺ compartments in the Petri dish experiment. In the pot experiment, dry weight, P concentration, and P content of both shoots and roots were higher with NH ₄ ⁺ than with NO ₃ ^- . NH ₄ ⁺ treatments reduced rhizosphere pH, promoted the hydrolization of phytin, enhanced acid phosphatase activity in the rhizosphere, and increased phytin-P utilization relative to N0 and NO ₃ ^- treatments. Phosphatase activity was negatively correlated with rhizosphere pH but was positively correlated with plant P content in both experiments. Rhizosphere acidification optimized the activity of acid phosphatase excreted by maize roots and promoted phytin mineralization. NH ₄ ⁺ -induced acidification in the maize rhizosphere improved the growth of maize roots by improving P uptake from phytin; the improved growth, in turn, increased NH ₄ ⁺ uptake and acidification.     

Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition

In plants of wheat (Triticum aestivum L.) grown in the media with nitrate (NO ₃ ^? plants), ammonium (NH ₄ ⁺ plants), and without nitrogen (N-deficient plants), the response to oxidative stress induced by the addition of 300 mM NaCl to the nutrient solution was investigated. Three-day-long salinization induced chlorophyll degradation and accumulation of malondialdehyde (MDA) in the leaves. These signs of oxidative stress were clearly expressed in NO ₃ ^? and N-deficient plants and weakly manifested in NH ₄ ⁺ plants. In none of the treatments, salinization induced the accumulation of MDA in the roots. Depending on the conditions of N nutrition, salt stress was accompanied by diverse changes in the activity of antioxidant enzymes in the leaves and roots. Resistance of leaves of NH ₄ ⁺ plants to oxidative stress correlated with a considerable increase in the activities of ascorbate peroxidase and glutathione reductase. Thus, wheat plants grown on the NH ₄ ⁺ -containing medium were more resistant to the development of oxidative stress in the leaves than those supplied with nitrate.     

First-principles study of ammonium ions and their hydration in montmorillonites

Density functional theory calculations were performed to investigate the adsorption and hydration of an ammonium ion (NH₄ ⁺) confined in the interlayer space of montmorillonites (MMT). NH₄ ⁺ is trapped in the six-oxygen-ring on the internal surface and forms a strong binding with the surface O atoms. The hydration of NH₄ ⁺ is affected significantly by the surface. Water molecules prefer the surface sites, and do not bind with the NH₄ ⁺ unless enough water molecules are supplied. Moreover, the water molecules involved in NH₄ ⁺ hydration tend to bind with the surface simultaneously. The hydration energy increases with the intercalated water molecules, in contrast to that in gas phase. In addition, the hydration leads to the extension of MMT basal spacing.

Element fluxes in watershed-lake ecosystems recovering from acidification: Plešné Lake,the Bohemian Forest, 2001–2005

Fluxes of major ions and nutrients were measured in the watershed-lake ecosystem of a strongly acidified lake, Ple?né jezero (Ple?né Lake), in the Czech Republic in hydrological years from 2001 through 2005. The lake is situated in a Norway spruce forest and has a steep watershed between elevations of 1090 and 1378 m. The average water input and output from the ecosystem was 1372 mm and 1157 mm (37 L km^?2 s^?1), respectively, and the water residence time averaged 306 days. Despite ecosystem recovery from acidification occurring since the late 1980s, the Ple?né watershed was an average net source of 25 mmol SO ₄ ^2? m^?2 yr^?1. Nitrogen saturation of the watershed caused low retention of the deposited inorganic N (< 44% on average) before 2004. Then, the watershed became a net source of 28–32 mmol m^?2 yr^?1 of inorganic N in the form of NO ₃ ^? due to climatic effects (a dry summer in 2003 and a cold winter in 2004) and forest dieback caused by a bark beetle attack in 2004. Nitrogen transformations and SO ₄ ^2? release were the dominant terrestrial sources of H⁺ (72 and 49 mmol m^?2 yr^?1, respectively) and the watershed was a net source of 24 mmol H⁺ m^?2 yr^?1. Ionic composition of surface inlets showed seasonal variations, with the most pronounced changes in NO ₃ ^? , ionic Al (Al_i), and DOC concentrations, while the composition of subsurface inlets was more stable. The in-lake biogeochemical processes reduced on average 59% of the incoming H⁺ (251 mmol H⁺ m^?2 yr^?1 on a lake-area basis). NO ₃ ^? assimilation and denitrification, photochemical and microbial decomposition of allochthonous organic acids, and SO ₄ ^2? reduction in the sediments were the most important aquatic H⁺ consuming processes (358, 121, and 59 mmol H⁺ m^?2 yr^?1, respectively), while hydrolysis of Al_i was the dominant in-lake H⁺ generating process (233 mmol H⁺ m^?2 yr^?1). Photochemical liberation from organic complexes was an additional in-lake source of Al_i. The net in-lake retention or removal of total phosphorus, total nitrogen, and silica were on average 50%, 27%, and 23%, respectively. The lake was a net source of NH ₄ ⁺ due to a cease in nitrification (pH < 5) and from NH ₄ ⁺ production by dissimilation exceeding its removal by assimilation.