An alternative explanation for the post-disturbance NO3– flush in some forest ecosystems

The appearance of soil NO₃^– after forest disturbance is commonly ascribed to a higher availability of NH₄⁺ to autotrophic nitrifiers, or to a reduction in available-C resulting in lower microbial assimilation of NO₃^–. Alternatively, it has been proposed that increasing NH₄⁺ pools following disturbance could increase net nitrification by reducing microbial assimilation of NO₃^–. Forest floor material was collected from shelterwood harvest plots which displayed both low available-C and low NH₄⁺ pools, and where previous experiments had suggested the prevalence of heterotrophic nitrification. Subsamples were amended with incremental rates of glucose-C or NH₄⁺, and gross NO₃^– transformation rates were measured by isotope dilution. Glucose-C additions had little effect on the net difference between gross NO₃^– production and consumption rates. On the other hand, NH₄⁺ additions caused gross NO₃^– consumption processes to decrease sharply, while gross NO₃^– production processes remained constant. The results suggest that NH₄⁺ can have an immediate positive effect on net nitrification rates by suppressing NO₃^– assimilation and uptake systems.     

Variation in nitrate uptake and assimilation between two ecotypes of Lotus japonicus and their recombinant inbred lines

A large genetic variability was observed in the shoot NO₃^– content of recombinant inbred lines (RILs) of Lotus japonicus . To determine the cause of this variability, we have studied some aspects of nitrate uptake and assimilation in the two parental ecotypes (Gifu and Funakura) and four representatives of the RILs population differing both in their shoot biomass and shoot NO₃^– content. Higher shoot NO₃^–content was mainly due to an increase in the uptake of the ion regardless of the plant biomass production. The positive correlation observed between the shoot NO₃^– content and the steady state level of mRNA encoding high affinity NO₃^– transporters suggests that the higher NO₃^– influx is due to enhanced expression of the transporters. In contrast, neither the level of nitrate reductase mRNA, nor the potential enzyme activity in vivo in the different lines was correlated with the shoot NO₃^– content. This indicates that NO₃^– transport in Lotus is one of the main checkpoints controlling shoot NO₃^– accumulation. In addition, this study shows that at least in Lotus , it is possible, through breeding strategies, to lower the NO₃^– content without affecting biomass production.     

A comparison of NH4+ and NO3– net fluxes along roots of rice and maize

Net fluxes of NH₄⁺ and NO₃^– along adventitious roots of rice ( Oryza sativa L.) and the primary seminal root of maize ( Zea mays L.) were investigated under nonperturbing conditions using ion-selective microelectrodes. The roots of rice contained a layer of sclerenchymatous fibres on the external side of the cortex, whereas this structure was absent in maize. Net uptake of NH₄⁺ was faster than that of NO₃^– at 1 mm behind the apex of both rice and maize roots when these ions were supplied together, each at 0·1 mol m^–3. In rice, NH₄⁺ net uptake declined in the more basal regions, whereas NO₃^– net uptake increased to a maximum at 21 mm behind the apex and then it also declined. Similar patterns of net uptake were observed when NH₄⁺ or NO₃^– was the sole nitrogen source, although the rates of NO₃^– net uptake were faster in the absence of NH₄⁺. In contrast to rice, rates of NH₄⁺ and NO₃^– net uptake in the more basal regions of maize roots were similar to those near the root apex. Hence, the layer of sclerenchymatous fibres may have limited ion absorption in the older regions of rice roots.     

Role of phosphoenolpyruvate carboxylase in anaplerosis in the green microalga Dunaliella salina cultured under different nitrogen regimes

Anaplerosis plays a very important role in providing C for N assimilation. In green algae and higher plants, phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) is the main anaplerotic carboxylase. On this basis we hypothesize that N availability affects PEPC expression. In order to test this hypothesis, the model organism Dunaliella salina was cultured under a variety of N growth regimes. Our results show that the level of PEC activity was unaffected by the N form in which N was supplied to the cells, when N concentration was low (0.5–0.01 m M ). When cells were adapted to growth at 5 m M N, however, PEPC activity on a per cell basis was substantially higher in NH₄⁺-adapted cells as compared to their NO₃^–-adapted counterparts; however, the same difference was not observed on a protein basis. This notwithstanding, even at low N, PEPC of cells cultured in the presence of either NH₄⁺ or NO₃^– appeared to differ in their molecular masses. These results suggest that cells adapted to different N-form express distinct PEPC isoforms. In addition to this, we observed that, in algae adapted to high (5 m M ) NH₄⁺ concentration, a PEPC isoform was induced that differed from the isoforms observed in algae adapted to lower concentrations of the same N-source. These findings lead us to conclude that the expression of PEPC isoforms in D. salina responds to the variation in the C-skeleton demand deriving from changes in the chemical form and availability of N.     

Microbial community structure in autotrophic nitrifying granules characterized by experimental and simulation analyses

This study evaluates the community structure in nitrifying granules (average diameter of 1600 μm) produced in an aerobic reactor fed with ammonia as the sole energy source by a multivalent approach combining molecular techniques, microelectrode measurements and mathematical modelling. Fluorescence in situ hybridization revealed that ammonia-oxidizing bacteria dominated within the first 200 μm below the granule surface, nitrite-oxidizing bacteria a deeper layer between 200 and 300 μm, while heterotrophic bacteria were present in the core of the nitrifying granule. Presence of these groups also became evident from a 16S rRNA clone library. Microprofiles of NH₄⁺, NO₂^–, NO₃^– and O₂ concentrations measured with microelectrodes showed good agreement with the spatial organization of nitrifying bacteria. One- and two-dimensional numerical biofilm models were constructed to explain the observed granule development as a result of the multiple bacteria–substrate interactions. The interaction between nitrifying and heterotrophic bacteria was evaluated by assuming three types of heterotrophic bacterial growth on soluble microbial products from nitrifying bacteria. The models described well the bacterial distribution obtained by fluorescence in situ hybridization analysis, as well as the measured oxygen, nitrite, nitrate and ammonium concentration profiles. Results of this study are important because they show that a combination of simulation and experimental techniques can better explain the interaction between nitrifying bacteria and heterotrophic bacteria in the granules than individual approaches alone.     

Root ammonium transport efficiency as a determinant in forest colonization patterns: an hypothesis

Ratios of ammonium (NH₄⁺) to nitrate (NO₃^–) in soils are known to increase during forest succession. Using evidence from several previous studies, we hypothesize that a malfunction in NH₄⁺ transport at the membrane level might limit the persistence of early successional tree species in later seral stages. In those studies, ¹³N radiotracing was used to determine unidirectional fluxes and pool sizes of NH₄⁺ and NO₃^– in seedlings of the late-successional species white spruce ( Picea glauca ) and in the early successional species Douglas-fir ( Pseudotsuga menziesii var. glauca ) and trembling aspen ( Populus tremuloides ). At high external NH₄⁺, the two early successional species accumulated excessive NH₄⁺ in the root cytosol, and exhibited high-velocity, low-efficiency (15% to 22%), membrane fluxes of NH₄⁺. In sharp contrast, white spruce had low cytosolic NH₄⁺ accumulation, and lower-velocity but much higher-efficiency (65%), NH₄⁺ fluxes. Because these divergent responses parallel known differences in tolerance and toxicity to NH₄⁺ amongst these species, we propose that they constitute a significant driving force in forest succession, complementing the discrimination against NO₃^– documented in white spruce (Kronzucker et al. 1997).     

Time-course of inorganic nitrogen uptake and incorporation by natural populations of freshwater phytoplankton

SUMMARY. 1. Time-course measurements of NH₄⁺ and NO₃⁻uptake were made on the natural phytoplankton populations in a eutrophic lake at a time when these nutrients were at their lowest annual concentration.
2. Both NH₄⁺ and NO₃⁻ uptake was increased at least five-fold during the first 5 min of incubation following near saturating pulses of these nutrients.
3. Elevated uptake was also observed following low level (∼2μg N 1⁻¹) pulses of NH₄⁺ and NO₃⁻, but substrate depletion during the first hour of incubation may have been partially responsible for this apparent enhancement.
4. Incorporation of ^I5N into TCA-insoluble material (protein) following the saturating NH₄⁺ pulse was increased less than total cellular ¹⁵N uptake, whereas no elevation of ¹⁵N incorporation into protein was observed following a saturating NO₃⁻pulse.
5. The percentage of ^I5N incorporated into protein, with respect to total cellular uptake, was ∼32% and ∼12% for NH₄⁺ and NO₃⁻, respectively, following 5 h of incubation.     

Terrestrial export of highly bioavailable carbon from small boreal catchments in spring floods

1. We assessed the terrestrial export of organic carbon, which effectively supported aquatic bacterial production (BP), from small boreal catchments during spring flood. We analysed stream runoff from nine small catchments with different proportions of peat mires and coniferous forests by monitoring the dissolved organic carbon (DOC) flux in combination with conducting bacterial bioassays.
2. Multiple linear regression analysis showed that BP during 7-day-dark bioassays (BP₇; μg C L⁻¹day⁻¹) was explained by both the quantity and quality (low-molecular weight fractions) of the DOC. BP₇ can be used as a measure of export of terrestrial organic carbon that is highly bioavailable.
3. Total export of DOC during spring flood from the different catchments ranged from 20 to 27 kg ha⁻¹ and was negatively correlated to forest cover (%). However, the export of BP₇ carbon was positively correlated to forest cover and varied from about 0.1 kg ha⁻¹ in mire-dominated streams to about 0.2 kg ha⁻¹ in forest-dominated streams.
4. The high bioavailability of forest carbon suggests that forests are the main contributors of BP-supporting carbon in boreal streams although mires have higher area-specific export of DOC.     

Microbial activity associated with seston in headwater streams: effects of nitrogen, phosphorus and temperature

SUMMARY 1. The influences of temperature and dissolved nitrates and phosphates on microbial activity associated with suspended fine particulate organic matter (seston) were evaluated in four headwater streams in the southern Appalachian Mountains.
2. Temperature manipulations of ± 5°C always induced significant changes in [¹⁴C] glucose mineralization (ANOVA; P <0.05) and [³H]thymidine incorporation (ANOVA; P <0.05).
3. Nutrient amendments of 1.0 mg NO₃ I⁻¹ and 0.05 mg PO₄I⁻¹ induced no significant alterations in bacterial mineralization of [¹⁴C]glucose (ANOVA; P >0.05) or incorporation of [³H]thymidine (ANOVA; P >0.05) in short-term (i.e. 3 h) experiments.
4. Microorganisms attached to refractory particulate organic matter do not appear to be limited by nitrogen or phosphorus even in streams with ambient nutrient concentrations as low as 0.06 mg NO₃ I⁻¹ and <0.03 mg PO₄ I⁻¹.
5. Our results indicate that variations in water temperature resulting from diurnal and seasonal temperature fluctuations, forest clear-cutting, and catchment elevation and aspect can have marked effects upon microbial activity and production, while short-term alterations in nutrient regime appear to have no significant effect on microbial activity associated with seston.     

Impact of gaseous nitrogen deposition on plant functioning

Dry deposition of NH₃ and NO_x (NO and NO₂) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH₃ dissolves to form NH₄⁺, whereas NO₂ dissolves to form NO₃⁻ and NO₂⁻. The latter compound can also be formed after exposure to NO. There is evidence that NH₃-N and NO_x-N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO₃⁻ and NO₂⁻ can be reduced, and NH₄⁺ can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with low concentrations of atmospheric NH₃ increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH₃ and in vitro GDH activity. The contribution of atmospheric NH₃ and NO₂ deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates ( RGRs ). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small.     

Simultaneous Measurement of Nitrite and Nitrate Levels as Indices of Nitric Oxide Release in the Cerebellum of Conscious Rats

Abstract: We examined the modulation of nitric oxide production in vivo by measuring levels of nitrite (NO₂⁻) and nitrate (NO₃⁻) in the dialysate of the cerebellum in conscious rats, by using an in vivo brain microdialysis technique. The levels of both NO₂⁻ and NO₃⁻ were decreased by the intraperitoneal injection of N ^G-nitro- l -arginine methyl ester, an inhibitor of nitric oxide synthase, whereas N ^G-nitro- d -arginine methyl ester had no effect. l -Arginine by itself increased NO₂⁻ and NO₃⁻ levels and diminished the reduction of their levels caused by N ^G-nitro- l -arginine methyl ester. Direct infusion of l -glutamate, N -methyl- d -aspartate, or KCl into the cerebellum through a dialysis probe resulted in an increase in NO₂⁻ and/or NO₃⁻ levels. The effects of N -methyl- d -aspartate and KCl were dependent on extracellular calcium. Furthermore, the stimulatory effects of l -glutamate and N -methyl- d -aspartate were inhibited by N ^G-nitro- l -arginine methyl ester and (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), an N -methyl- d -aspartate receptor antagonist. These results suggest that NO₂⁻ and NO₃⁻ levels may be related to nitric oxide production in vivo.     

Spatio-temporal variation in macroinvertebrate assemblages of glacial streams in the Swiss Alps

1. Changes in water chemistry, benthic organic matter (BOM), and macroinvertebrates were examined in four different glacial streams over an annual cycle. The streams experienced strong seasonal changes in water chemistry that reflected temporal changes in the influence from the source glacier, especially in water turbidity, particulate phosphorus and conductivity.
2. Nitrogen concentrations were high (nitrate-N values were 130–274 μg L^–1), especially during spring snowmelt runoff. Benthic organic matter attained >600 g m^–2 dry mass at certain times, peaks being associated with seasonal blooms of the alga Hydrurus foetidus .
3. Macroinvertebrate taxon richness was two to three times higher (also numbers and biomass) in winter than summer suggesting winter may be a more favourable period for these animals. Benthic densities averaged 1140–3820 ind. m^–2, although peaking as high as 9000 ind. m^–2. Average annual biomass ranged from 102 to 721 mg m^–2, and reached >2000 mg m^–2 at one site in autumn.
4. Taxa common to all sites included the dipterans Diamesa spp. and Rhypholophus sp., the plecopterans Leuctra spp. and Rhabdiopteryx alpina , and the ephemeropterans Baetis alpinus and Rhithrogena spp. Principal components analysis clearly separated winter assemblages from those found in summer.     

Contribution of inorganic components to osmotic adjustment and leaf folding for drought tolerance in pearl millet

Pearl millet, Pennisetum glaucum , is capable of adapting to severely dry environmental conditions. In order to elucidate the mechanism of adaptation to highly dehydrated conditions, we selected both tolerant (IP8210) and susceptible (IP8949) accessions from a total of 15 pearl millet accessions and characterized their morphological and physiological responses to severe drought stress. When these selected accessions were stressed with a severe drought treatment, the leaves of IP8210 exhibited upright folding, a response that effectively reduces the evaporative surface area of the canopy. On the contrary, the leaves of IP8949 exhibited wilting and did not appear to adapt to the drought stress. In comparison with IP8949, the capacity of osmotic adjustment (OA) was greater in both younger leaves and stems of IP8210, while their decrease in relative water content was different. IP8210 accumulated higher concentrations of NO₃^– than IP8949 in response to drought stress. In addition to inorganic solutes, several organic components such as sucrose, glucose, quaternary ammonium compounds, and amino acids including proline were also accumulated. IP8210 tended to accumulate more amino acids, typically due to the accumulation of asparagine and proline, while IP8949 accumulated more soluble sugars. While it is possible that K⁺ and NO₃^– were the major components contributing to osmotic regulations, sugars and amino acids might also function as a cytoprotectant, in addition to their role as osmoprotectants. Collectively, these results demonstrate that the morphological adaptation of leaf folding, OA in both the younger leaves and the stem, and the accumulation of NO₃^– and amino acids during earlier stress period contribute to superior drought tolerance that was exhibited in IP8210 of pearl millet.     

Simulated chronic nitrogen deposition increases carbon storage in Northern Temperate forests

High levels of atmospheric nitrogen (N) deposition in Europe and North America were maintained throughout the 1990s, and global N deposition is expected to increase by a factor of 2.5 over the next century. Available soil N limits primary production in many terrestrial ecosystems, and some computer simulation models have predicted that increasing atmospheric N deposition may result in greater terrestrial carbon (C) storage in woody biomass. However, empirical evidence demonstrating widespread increases in woody biomass C storage due to atmospheric N deposition is uncommon. Increased C storage in soil organic matter due to chronic N inputs has rarely been reported and is often not considered in computer simulation models of N deposition effects. Since 1994, we have experimentally simulated chronic N deposition by adding 3 g N m⁻² yr⁻¹ to four different northern hardwood forests, which span a 500 km geographic gradient in Michigan. Each year we measured tree growth. In 2004, we also examined soil C content to a depth of 70 cm. When we compared the control treatment with the NO₃⁻ deposition treatment after a decade of experimentation, ecosystem C storage had significantly increased in both woody biomass (500 g C m⁻²) and surface soil (0–10 cm) organic matter (690 g C m⁻²). The increase in surface soil C storage was apparently driven by altered rates of organic matter decomposition, rather than an increase in detrital inputs to soil. Our results, for study locations stretching across hundreds of kilometers, support the hypothesis that chronic N deposition may increase C storage in northern forests, potentially contributing to a sink for anthropogenic CO₂ in the northern Hemisphere.     

Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR_L), concentration of B, nitrate (NO₃⁻), ammonium (NH₄⁺), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR_L, organic N, soluble proteins, and NR and NiR activities. The lowest NO₃⁻ and NH₄⁺ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO₃⁻ reduction and increases NH₄⁺ assimilation in tomato plants.     

Effects of spring acid episodes on macroinvertebrates revealed by population data and in situ toxicity tests

1. While streams in Europe and North America are now recovering chemically from chronic acidification, severe episodic acidification continues to threaten sensitive biota. To appraise further the biological importance of episodic acidification, we surveyed the distribution of the mayfly Baetis alpinus in streams in the Southern Alps (Canton Ticino, Switzerland) in relation to runoff acidity during spring floods. Moreover, to improve mechanistic understanding, in situ toxicity assays were carried out on nymphal B. alpinus during low flows and spring floods, both in streams prone to acid episodes and in well‐buffered controls. 2. Streams surveyed for invertebrates represented three groups which contrasted in susceptibility to episodic acidity. Group one included streams that were acid (alkalinity <0) in spring; group two streams were susceptible to acid episodes because of low base‐flow alkalinity (<200 μeq L^?1); and group three streams were well‐buffered and unlikely ever to be acid. The abundance of B. alpinus was similar among groups during stable flows in winter and summer, but was significantly lower in groups one and two following spring snowmelt. 3. During the bioassays, control streams remained circumneutral to alkaline (pH >6.4). By contrast, episodic streams were circumneutral at low flows, but became acid (pH 4.5–5.6 and total dissolved aluminium to 276 μg L^?1) during intense spring snowmelt. After 15‐day exposures, nymphal B. alpinus survival in the circumneutral control streams exceeded 92% irrespective of flow. In the episodic streams, survival matched the controls during low flows, but declined significantly to 10–20% during acid episodes in spring. Shorter exposure (2–4 days) to pH 5.5–5.7 did not cause significant mortality either during the exposure or over the following 7 days, indicating that B. alpinus might recover from short acid episodes. 4. Our data suggest that the spring distribution of B. alpinus in acid sensitive parts of the Alps directly reflects the toxicity of acid runoff during snowmelt. Our study illustrates that even mild episodic acidification can have significant consequences in Alpine streams for one of the most important invertebrate indicators of acidity.     

Can filter‐feeding fishes improve water quality in lakes?

1.  In this paper we examine the potential of a cichlid fish species ( Sarotherodon galilaeus ) to both maintain positive growth rates through filter-feeding on phytoplankton and improve water quality in Lake Kinneret through suppression of dinoflagellate ( Peridinium gatunense ) blooms.
2.  Seasonal plankton consumption by S. galilaeus from Lake Kinneret was examined experimentally by monitoring changes in plankton assemblages during 24 h in 5-m³ mesocosms containing varying densities of fish. Taxon-specific grazing rates ranged from 0 to 17 mg g_fish day^–1, with mean total consumption of 1.6% fish body weight per day. During the spring bloom of P. gatunense , S. galilaeus consumed mostly (94%) netphytoplankton (≥20 μm). The remaining 6% consisted mostly of nanophytoplankton (<20 μm). During the summer and fall, net- and nanophytoplankton accounted for 54 and 42%, respectively, of the diet of S. galilaeus . Zooplankton and flagellated and ciliated protozoans made up the remaining 4%.
3.  Simulations using a fish bioenergetics model indicated that consumption rates ( C ) were near maximum in spring (90% C _max), while consumption was reduced in summer-fall (59% C _max). Sarotherodon galilaeus obtains sufficient energy through filter-feeding year-round, although most growth (≥60%) occurs during the spring P. gatunense bloom.
4.  Despite efficient feeding on P. gatunense and nanophytoplankton by S. galilaeus , estimates of instantaneous plankton mortality caused by ingestion were two orders of magnitude lower than maximum potential plankton growth rates. Thus the potential for the S. galilaeus population in Lake Kinneret to positively affect water quality through algal suppression is low.     

Effects of soil frost on nitrogen net mineralization, soil solution chemistry and seepage losses in a temperate forest soil

Freezing and thawing may alter element turnover and solute fluxes in soils by changing physical and biological soil properties. We simulated soil frost in replicated snow removal plots in a mountainous Norway spruce stand in the Fichtelgebirge area, Germany, and investigated N net mineralization, solute concentrations and fluxes of dissolved organic carbon (DOC) and of mineral ions (NH₄⁺, NO₃⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺). At the snow removal plots the minimum soil temperature was −5 °C at 5 cm depth, while the control plots were covered by snow and experienced no soil frost. The soil frost lasted for about 3 months and penetrated the soil to about 15 cm depth. In the 3 months after thawing, the in situ N net mineralization in the forest floor and upper mineral soil was not affected by soil frost. In late summer, NO₃⁻ concentrations increased in forest floor percolates and soil solutions at 20 cm soil depth in the snow removal plots relative to the control. The increase lasted for about 2–4 months at a time of low seepage water fluxes. Soil frost did not affect DOC concentrations and radiocarbon signatures of DOC. No specific frost effect was observed for K⁺, Ca²⁺ and Mg²⁺ in soil solutions, however, the Na⁺ concentrations in the upper mineral soil increased. In the 12 months following snowmelt, the solute fluxes of N, DOC, and mineral ions were not influenced by the previous soil frost at any depth. Our experiment did not support the hypothesis that moderate soil frost triggers solute losses of N, DOC, and mineral ions from temperate forest soils.     

Acidic episodes retard the biological recovery of upland British streams from chronic acidification

We tested two predictions required to support the hypothesis that anthropogenic acidic episodes might explain the poor biological response of upland British streams otherwise recovering from acidification: (i) that invertebrate assemblages should differ between episodic and well-buffered streams and (ii) these effects should differentiate between sites with episodes caused by anthropogenic acidification as opposed to base-cation dilution or sea-salt deposition. Chronic and episodically acidic streams were widespread, and episodes reflected acid titration more than dilution. Nonmarine sulphate (16–18% vs. 5–9%), and nitrate (4–6% vs. 1–2%) contributed more to anion loading during episodes in Wales than Scotland, and Welsh streams also had a larger proportion of total stream sulphate from nonmarine sources (64–66% vs. 35–46%). Sea-salts were rarely a major cause of episodic ANC or pH reduction during the events sampled. By contrast, streams with episodes driven by strong anthropogenic acids had lower pH (5.0±0.6) and more dissolved aluminium (288±271 μg L⁻¹) during events than where episodes were caused by dilution (pH 5.4±0.6; 116±110 μg Al L⁻¹) or where streams remained circumneutral (pH 6.7±1.0; 50±45 μg Al L⁻¹). Both biological predictions were supported: invertebrate assemblages differed among sites with different episode chemistry while several acid-sensitive species were absent only where episodes reflected anthropogenic acidification. We conclude that strong acid anions – dominantly nonmarine sulphate – still cause significant episodic acidification in acid-sensitive areas of Britain and may be a sufficient explanation for slow biological recovery in many locations.     

Relationships between shoot to root ratio, growth and leaf soluble protein concentration of Pisum sativum, Phaseolus vulgaris and Triticum aestivum under different nutrient deficiencies

Relations between shoot to root dry weight ratio (S : R), total plant dry weight (DW), shoot and plant N concentration and leaf soluble protein concentration were examined for pea ( Pisum sativum L.), common bean ( Phaseolus vulgaris L.) and wheat ( Triticum aestivum L.) under different nutrient deficiencies. A regression model incorporating leaf soluble protein concentration and plant DW could explain greater than 80% of the variation in S : R within and between treatments for pea supplied different concentrations of NO₃^– or NH₄⁺ in solid substrate; pea and bean supplied different concentrations of N, P, K and Mg in liquid culture; and wheat supplied different concentrations of N, P, K, Mg, Ca and S in liquid culture. Addition of shoot or plant N concentration to the model explained little more of the variation in S : R. It is concluded that results are consistent with the proposal that macronutrient effects on S : R are primarily mediated through their effects on protein synthesis and growth.