Influences of different nitrogen sources on nitrogen- and water-use efficiency, and carbon isotope discrimination, in C3 Triticum aestivum L. and C4 Zea mays L. plants

The impacts of various nitrogen sources, i.e. NO⁻ ₃, NH₄ ⁺ or NH₄NO₃ in combination with gaseous NH₃, on nitrogen-, carbon- and water-use efficiency and ¹³C discrimination (δ¹³C) by plants of the C₃ species Triticum aestivum L. (wheat) and the C₄ species Zea mays L. (maize) were studied. Triticum aestivum and Z. mays were hydroponically grown with 2 mol · m⁻³ of N supplied as NO⁻ ₃, NH₄ ⁺ or NH₄NO₃ for 21 and 18 d, respectively, and thereafter exposed to gaseous NH₃ at 320 μg · m⁻³ or to ambient air for 7 d. In T. aestivum and Z. mays over a 7-d growth period, nitrogen-use efficiency (NUE) values were influenced by N-sources in the decreasing order NH₄NO₃-N > NO⁻ ₃-N > NH₄ ⁺-N and NO⁻ ₃-N > NH₄NO₃-N > NH₄ ⁺-N, respectively. Fumigation with NH₃ decreased the NUE values of plants grown with any of the N-forms. During 28- and 7-d growth periods, N-sources affected water-use efficiency (WUE) values in the decreasing order of NH₄ ⁺-N > NO⁻ ₃-N≈NH₄NO₃-N in non-fumigated T. aestivum, while fumigation with NH₃ increased the WUE of NO⁻ ₃-grown plants. There were insignificant effects of N-sources on WUE values of Z. mays over 25- and 7-d growth periods. Furthermore, δ¹³C values in plant tissues (leaves, stubble and roots) were higher (less negative) in NH₄ ⁺-grown plants of T. aestivum and Z. mays than in those supplied with NH₄NO₃ or NO⁻ ₃. Regardless of the N-form supplied to the roots of the plant species, exposure to NH₃ caused more-positive δ¹³C values in the plant tissues. These results indicate that the variations in N-source were associated with small but significant variations in δ¹³C values in plants of T. aestivum and Z. mays. These differences in δ¹³C values are in the direction expected from differences in WUE values over long or short growth periods and with differences in the extent of non-Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase, EC 4.1.1.39) carboxylate contribution to net C acquisition, as a function of N-source. Received: 12 September 1997 / Accepted: 13 January 1998     

Initiation of [36Cl]hexachlorobenzene dechlorination in three different soils under artificially induced anaerobic conditions

The potential for reductive dechlorination of hexachlorobenzene was investigated in samples of three different, naturally oxic soils held under conditions of high oxygen deficiency. The soils were water-saturated and the influence on dechlorination of adding different electron donors, a surfactant and an anaerobic microbial consortium was tested. The influence of supplied electron donors seems to depend on the organic matter content of the soils. Dechlorination in the organic-matter-rich soil from Maulach was not affected by amendment with organic electron donors. A release of about 40% chloride within 140 days was observed for this soil in all biotic-treated assays. By contrast, the organic-matter-poor soil of Eppingen showed no dechlorination in unamended assays. However, when it was supplemented with organic electron donors dechlorination of 2%–37% occurred within 140 days, depending on the type of electron donor. Complex substrate (wheat strawdust), from which carbon is slowly liberated, gave the best results. These two soils had an indigenous dechlorinating anaerobic microflora, whereas the third soil (Rastatt) required inoculation with an anaerobic consortium for dechlorination. The addition of electron donors alone did not cause dechlorination in this sandy soil. The addition of a surfactant (Tween 80) to increase the bioavailability of hexachlorobenzene did not enhance dechlorination. This process was not inhibited by inherent alternative electron acceptors in soil (NO³⁻, SO₄ ²⁻, Fe³⁺). The dechlorination did not require methanogenic conditions. Received: 12 December 1996 / Received revision: 14 March 1997 / Accepted: 15 March 1997     

Reduction of the nitrogen and carbon content in swine waste with algae and bacteria

Animal waste causes environmental problems like eutrophication of ground and surface water or the pollution of the atmosphere because of its high NH₄ ⁺ content. The aim of our study was to fix the nitrogen of swine waste as biomass. Therefore, an isolated alga, Chlorella sp., and bacteria naturally living in liquid manure were grown in batch cultures (containing diluted swine waste supplied with a nutrient solution) and continuous cultures (undiluted liquid manure) to achieve reduction of NH₄ ⁺ and total organic carbon (TOC) contents. For continuous cultivation, a photobioreactor of our own design was used. The batch cultivation of Chlorella sp. and bacteria in swine waste resulted in good growth of both groups of organisms and in a reduction of 25% NH₄ ⁺ and 80% TOC. In the continuous cultivation a steady state was not achieved owing to a change in the composition of the bacterial population. NH₄ ⁺ was totally removed, but NO₂ ⁻ (up to 100 mM) was transiently released. NO₃ ⁻ was not detected. These effects might be explained by the presence of heterotrophic nitrifiers, which are able to oxidize NH₄ ⁺ to NO₂ ⁻ and to reduce NO₂ ⁻ to gaseous compounds. Received: 21 January 1999 / Received revision: 9 March 1999 / Accepted: 14 March 1999     

Biomass Production with Willow and Poplar Short Rotation Coppices on Sensitive Areas—the Impact on Nitrate Leaching and Groundwater Recharge in a Drinking Water Catchment near Hanover,Germany

In a lowland drinking water catchment area, nitrate leaching as well as groundwater recharge (GWR) was investigated in willow and poplar short rotation coppice (SRC) plantations of different ages, soil preparation measures prior to planting and harvesting intervals. Significantly increased nitrate concentrations of 16.6 ± 1.6 mg NO₃-N L⁻¹ were measured in winter/spring 2010 on a poplar site, established in 2009 after deep plowing (90 cm) but then, subsequently decreased strongly to below 2 mg NO₃-N L⁻¹ in spring 2011. The fallow ground reference plot showed nitrate concentrations consistently below 1 mg L⁻¹ and estimated annual seepage output loss was only 1.36 ± 1.1 kg ha⁻¹ a⁻¹. Leaching loss from a neighboring willow plot from 2005 was 14.3 ± 6.6 kg NO₃-N ha⁻¹ during spring 2010 but decreased to 2.0 ± 1.5 kg NO₃-N ha⁻¹ during the subsequent drainage period. A second willow plot, not harvested since its establishment in 1994, showed continuously higher nitrate concentrations (10.2 ± 1.7 NO₃-N L⁻¹), while a neighboring poplar plot, twice harvested since 1994 showed significantly reduced nitrate concentrations. Water balance simulations, referenced by soil water tension and throughfall measurements, showed that at 655 mm annual rainfall, GWR from the reference plot (300 mm a⁻¹) was reduced by 40 % (to 180 mm a⁻¹) on the 2005 willow stand, mainly due to doubled rainfall interception losses. However, transpiration was limited by low soil water storage capacities, which in turn led to a moderate impact on GWR. We conclude that well-managed SRC on sensitive areas can prevent nitrate leaching, while impacts on GWR may be mitigated by management options.     

Effects of NO2? and NO3? on the Fe(III)EDTA reduction in a chemical absorption–biological reduction integrated NOx removal system

The biological reduction of Fe(III) ethylenediaminetetraacetic acid (EDTA) is a key step for NO removal in a chemical absorption–biological reduction integrated process. Since typical flue gas contain oxygen, NO₂ ⁻ and NO₃ ⁻ would be present in the absorption solution after NO absorption. In this paper, the interaction of NO₂ ⁻, NO₃ ⁻, and Fe(III)EDTA reduction was investigated. The experimental results indicate that the Fe(III)EDTA reduction rate decrease with the increase of NO₂ ⁻ or NO₃ ⁻ addition. In the presence of 10 mM NO₂ ⁻ or NO₃ ⁻, the average reduction rate of Fe(III)EDTA during the first 6-h reaction was 0.076 and 0.17 mM h⁻¹, respectively, compared with 1.07 mM h⁻¹ in the absence of NO₂ ⁻ and NO₃ ⁻. Fe(III)EDTA and either NO₂ ⁻ or NO₃ ⁻ reduction occurred simultaneously. Interestingly, the reduction rate of NO₂ ⁻ or NO₃ ⁻ was enhanced in presence of Fe(III)EDTA. The inhibition patterns observed during the effect of NO₂ ⁻ and NO₃ ⁻ on the Fe(III)EDTA reduction experiments suggest that Escherichia coli can utilize NO₂ ⁻, NO₃ ⁻, and Fe(III)EDTA as terminal electron acceptors.     

The role of ion regulation in the control of the distribution of Gammarus tigrinus (Sexton) in salt-polluted rivers

Fluctuating salinities at different sites on the German salt-polluted rivers Werra and Weser were compared with extracellular ion levels of specimens of Gammarus tigrinus (Sexton; Amphipoda, Crustacea), collected at the same sites. G. tigrinus regulated haemolymph concentrations of inorganic anions (Cl⁻, SO²⁻ ₄, PO³⁻ ₄) and cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) during fluctuations of salt pollution in the upper Weser. This capacity to regulate varying levels of salt pollution in the upper Weser, correlated well with the distribution of the brackish amphipods in this river ecosystem. G. tigrinus tolerated periods of Na⁺ and Cl⁻ stress (>380 mmol l⁻¹) without compensating these maxima by regulating extracellular Na⁺ and Cl⁻. However, during such bursts of Na⁺ and Cl⁻ stress in Werra and Weser, the ability to regulate extracellular [K⁺] at river water K⁺ stress of ≥6.0 mmol l⁻¹ may explain why this brackish species has been more successful in these rivers than its competitors like Gammarus pulex. The present investigation demonstrates that the water salinity affects the [NO⁻ ₃] in the haemolymph of G. tigrinus. With increasing hypo-osmotic stress the animals accumulate increasing amounts of NO⁻ ₃. A simultaneous increase in stream water [NO⁻ ₃] causes an additional accumulation of NO⁻ ₃ in the haemolymph. The high extent of accumulation indicates that active ion transport systems may be involved. The accumulation of NO⁻ ₃ in the haemolymph has low physiological consequences to G. tigrinus, but when hypo-osmotically stressed under anoxic conditions, nitrite formed by the reduction of nitrate may have an adverse affect on the metabolism of G. tigrinus. Accepted: 4 October 1999     

Acidianus manzaensis sp. nov., a Novel Thermoacidophilic Archaeon Growing Autotrophically by the Oxidation of H2 with the Reduction of Fe3+

A novel thermoacidophilic iron-reducing Archaeon, strain NA−1, was isolated from a hot fumarole in Manza, Japan. Strain NA-1 could grow autotrophically using H₂ or S⁰ as an electron donor and Fe³⁺ as an electron acceptor, and also could grow heterotrophically using some organic compounds. Fe³⁺ and O₂ served as electron acceptors for growth. However, S⁰, NO₃ ⁻, NO₂ ⁻, SO₄ ²⁻, Mn⁴⁺, fumarate, and Fe₂O₃ did not serve as electron acceptors. The ranges of growth temperature and pH were 60–90°C (optimum: 80°C) and pH 1.0–5.0 (optimum: pH 1.2–1.5), respectively. Cells were nearly regular cocci with an envelope comprised of the cytoplasmic membrane and a single outer S-layer. The crenarchaeal-specific quinone (cardariellaquinone) was detected, and the genomic DNA G + C content was 29.9 mol%. From 16S rDNA analysis, it was determined that strain NA-1 is closely related to Acidianus ambivalens (93.1%) and Acidianus infernus (93.0%). However, differences revealed by phylogenetic and phenotypic analyses clearly show that strain NA-1 represents a new species, Acidianus manzaensis, sp. nov., making it the first identified thermoacidophilic iron-reducing microorganism (strain NA-1^T = NBRC 100595 = ATCC BAA 1057). Strain NA-1 has been deposited in the culture collections of the National Institute of Technology and Evolution (NBRC 100595) and American Type Culture Collection (ATCC BAA 1057). The 16S rDNA sequence has been deposited at GenBank under accession number AB182498.     

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K⁺, Na⁺, Ca²⁺, Mg²⁺, NO₃ ⁻-N, NH₄ ⁺-N, PO₄ ³⁻-P, and SO₄ ²⁻ were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO₃ ⁻-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO₃ ⁻-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO₃ ⁻-N increased four fold, and streamwater NO₃ ⁻-N doubled. Significant changes also occurred in soilwater pH, DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO₃ ⁻-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams. Received 30 January 2001; accepted 25 July 2002.     

Nitrogen release from surface sand of a high energy beach along the southeastern coast of North Carolina,USA

This study examined changes in dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) in coastal seawater after exposure to sand along a high energy beach face over an annual cycle between April 2004 and July 2005. Dissolved organic nitrogen, NO₃ ⁻, and NH₄ ⁺ were released from sand to seawater in laboratory incubation experiments clearly demonstrating that they are a potential source of N to underlying groundwater or coastal seawater. DON increases in seawater, after exposure to surface sands in laboratory experiments, were positively correlated with in situ water column DON concentrations measured at the same time as sand collection. Increase in NO₃ ⁻ and NH₄ ⁺ were not correlated with their in situ concentrations. This suggests that DON released from beach sands is relatively more recalcitrant while NO₃ ⁻ and NH₄ ⁺ are utilized rapidly in the coastal ocean. The release of N was seasonal with carbon to nitrogen ratios indicating that recent primary productivity was responsible for the largest fluxes in summer while more degraded humic material contributed to lower fluxes in winter. Fluxes of total dissolved nitrogen (DON and DIN) from surface sand (2.1 × 10⁻⁴ mol m⁻² h⁻¹) were similar to that of groundwater and more than an order of magnitude larger than rain deposition indicating the potential importance of surface sand derived nitrogen to the coastal zone with a corresponding impact on primary productivity.     

Nitrate and phosphate ion removal from water by Phormidium laminosum immobilized on hollow fibres in a photobioreactor

Removal of nitrate and phosphate ions from water, by using the thermophilic cyanobacterium Phormidium laminosum, immobilized on cellulose hollow fibres in the tubular photobioreactor at 43 °C, was studied by continuously supplying dilute growth medium for 7 days and then secondarily treated sewage (STS) for 12 days. The concentrations of NO⁻ ₃ and PO³⁻ ₄ in the effluent from the dilute growth medium decreased from 5.0 mg N/l to 3.1 mg N/l, and from 0.75 mg P/l to 0.05 mg P/l respectively, after a residence time of 12 h. The concentrations of NO⁻ ₃ and PO³⁻ ₄ in the effluent from STS decreased from 11.7 mg N/l to 2.0 mg N/l, and from 6.62 mg P/l to 0.02 mg P/l respectively, after a residence time of 48 h. The removal rates of nitrogenous␣and phosphate ions from STS were 0.24 and 0.11 mmol day⁻¹ l reactor⁻¹ respectively, under the same conditions. Although, among nitrogenous ions, nitrate and ammonium ions were efficiently removed by P.␣laminosum, the nitrite ion was released into the effluent when STS was used as influent. Treatment of water with thermophilic P. laminosum immobilized on hollow fibres thus appears to be an appropriate means for the removal of inorganic nitrogen and phosphorus from treated wastewater. Received: 15 August 1997 / Received last revision: 18 November 1997 / Accepted: 29 November 1997     

Differential growth response of <Emphasis Type="Italic">Ulva lactuca</Emphasis> to ammonium and nitrate assimilation

Controlled cultivation of marine macroalgal biomass such as Ulva species, notably Ulva lactuca, is currently studied for production of biofuels or functional food ingredients. In a eutrophic environment, this macrophyte is exposed to varying types of nutrient supply, including different and fluctuating levels of nitrogen sources. Our understanding of the influences of this varying condition on the uptake and growth responses of U. lactuca is limited. In this present work, we examined the growth response of U. lactuca exposed to different sources of nitrogen (NH₄⁺; NO₃⁻; and the combination NH₄NO₃) by using photo-scanning technology for monitoring the growth kinetics of U. lactuca. The images revealed differential increases of the surface area of U. lactuca disks with time in response to different N-nutrient enrichments. The results showed a favorable growth response to ammonium as the nitrogen source. The NH₄Cl and NaNO₃ rich media (50 μM of N) accelerated U. lactuca growth to a maximum specific growth rate of 16.4 ± 0.18% day⁻¹ and 9.4 ± 0.72% day⁻¹, respectively. The highest biomass production rate obtained was 22.5 ± 0.24 mg DW m⁻²·day⁻¹. The presence of ammonium apparently discriminated the nitrate uptake by U. lactuca when exposed to NH₄NO₃. Apart from showing the significant differential growth response of U. lactuca to different nitrogen sources, the work exhibits the applicability of a photo-scanning approach for acquiring precise quantitative growth data for U. lactuca as exemplified by assessment of the growth response to two different N-sources.     

Sampling spatial and temporal variation in soil nitrogen availability

There are few studies in natural ecosystems on how spatial maps of soil attributes change within a growing season. In part, this is due to methodological difficulties associated with sampling the same spatial locations repeatedly over time. We describe the use of ion exchange membrane spikes, a relatively nondestructive way to measure how soil resources at a given point in space fluctuate over time. We used this method to examine spatial patterns of soil ammonium (NH⁺ ₄) and nitrate (NO⁻ ₃) availability in a mid-successional coastal dune for four periods of time during the growing season. For a single point in time, we also measured soil NH⁺ ₄ and NO⁻ ₃ concentrations from soil cores collected from the mid-successional dune and from an early and a late successional dune. Soil nitrogen concentrations were low and highly variable in dunes of all ages. Mean NH⁺ ₄ and NO⁻ ₃ concentrations increased with the age of the dune, whereas coefficients of variation for NH⁺ ₄ and NO⁻ ₃ concentrations decreased with the age of the dune. Soil NO⁻ ₃ concentration showed strong spatial structure, but soil NH⁺ ₄ concentration was not spatially structured. Plant-available NH⁺ ₄ and NO⁻ ₃ showed relatively little spatial structure: only NO⁻ ₃ availability in the second sampling period had significant patch structure. Spatial maps of NH⁺ ₄ and NO⁻ ₃ availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO⁻ ₃ availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO⁻ ₃ concentrations, providing some evidence that patches of plant-available NO⁻ ₃ may reappear at the same spatial locations at irregular points in time. Received: 20 February 1998 / Accepted: 23 November 1998     

Hydroxylamine oxidation and subsequent nitrous oxide production by the heterotrophic ammonia oxidizer Alcaligenes faecalis

Nitrous oxide (N₂O), a greenhouse gas, is emitted during autotrophic and heterotrophic ammonia oxidation. This emission may result from either coupling to aerobic denitrification, or it may be formed in the oxidation of hydroxylamine (NH₂OH) to nitrite (NO₂ ⁻). Therefore, the N₂O production during NH₂OH oxidation was studied with Alcaligenes faecalis strain TUD. Continuous cultures of A. faecalis showed increased N₂O production when supplemented with increasing NH₂OH concentrations. ¹⁵N-labeling experiments showed that this N₂O production was not due to aerobic denitrification of NO₂ ⁻. Addition of ¹⁵N-labeled NH₂OH indicated that N₂O was a direct by-product of NH₂OH oxidation, which was subsequently reduced to N₂. These observations are sustained by the fact that NO₂ ⁻ production was low (0.23 mM maximum) and did not increase significantly with increasing NH₂OH concentration in the feed. The NH₂OH-oxidizing capacity increased with increasing NH₂OH concentrations. The apparent V _max and K _m were 31 nmol min⁻¹ mg dry weight⁻¹ and 1.5 mM respectively. The culture did not increase its growth yield and was not able to use NH₂OH as the sole N source. A non-haem hydroxylamine oxidoreductase was partially purified from A. faecalis strain TUD. The enzyme could only use K₃Fe(CN)₆ as an electron acceptor and reacted with antibodies raised against the hydroxylamine oxidoreductase of Thiosphaera pantotropha. Received: 1 September 1998 / Received revision: 5 November 1998 / Accepted: 7 November 1998     

Natural 15N abundance in two nitrogen saturated forest ecosystems

Natural ¹⁵N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha⁻¹ year⁻¹). In the Douglas fir stand δ¹⁵N values of the vegetation ranged between −5.7 and −4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in ¹⁵N and varied from −3.3 to −1.2‰δ¹⁵N. At both sites δ¹⁵N values increased with soil depth, from −5.7‰ and −1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ¹⁵N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH₄ ⁺-N of −0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH₄ ⁺-N was enriched and NO₃ ⁻-N depleted in ¹⁵N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ¹⁵N values of NH₄ ⁺ and NO₃ ⁻ were very similar in the drainage water at 90 cm soil depth at both sites (−7.1 to −3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the ¹⁵N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ¹⁵N values for the N saturated ecosystems appeared particularly sensitive to the ¹⁵N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural ¹⁵N abundance values. The results indicated that the natural ¹⁵N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ¹⁵N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural ¹⁵N abundance values seems a promising technique for interpreting natural ¹⁵N abundance values found in these forest ecosystems. Received: 5 May 1996 / Accepted: 10 April 1997     

Structural and functional properties of sympagic communities in the annual sea ice at Terra Nova Bay (Ross Sea, Antarctica)

Studies on the chemical and biological properties of annual pack ice at a coastal station in Terra Nova Bay (74°41.72′S, 164°11.63′E) were carried out during austral spring at 3-day intervals from 5 November to 1 December 1997. Temporal changes of nutrient concentrations, algal biomasses, taxonomic composition, photosynthetic pigment spectra and P–E relationships were studied. Quantity, composition and degradation rates of organic matter in the intact sea ice were also investigated. In addition, microcosm experiments were carried out to evaluate photosynthetic and photo-acclimation processes of the sympagic flora in relation to different light regimes. High concentrations of ammonia were measured in four ice-cores (weighted mean values of the cores ranged from 4.3 ± 1.9 μM to 7.2 ± 3.4 μM), whereas nitrate and phosphate displayed high concentrations (up to 35.9 μM and 7.6 μM, respectively) only in the bottom layer (135–145 cm depth). Particulate carbohydrate and protein concentrations in the intact sea ice ranged from 0.5 to 2.3 mg l⁻¹ and 0.2 to 2.0 mg l⁻¹, respectively, displaying a notable accumulation of organic matter in the bottom colored layer, where bacterial enzymatic activities also reached the highest values. Aminopeptidase activity was extremely high (up to 19.7 μM l⁻¹ h⁻¹ ± 0.05 in the bottom layer), suggesting a rapid turnover rate of nitrogen–enriched organic compounds (e.g. proteins). By contrast, bacterial secondary production was low, suggesting that only a very small fraction of mobilized organic matter was converted into bacterial biomass (<0.01‰). The sympagic autotrophic biomass (in terms of chlorophaeopigments) of the bottom layer was high, increasing during the sampling period from 680 to 2480 μg l⁻¹. Analyses of pigments performed by HPLC, as well as microscope observations, indicated that diatoms dominated bottom communities. The most important species were Amphiprora sp. and Nitschia cfr. stellata. Bottom sympagic communities showed an average P ^B _max of 0.12 mgC mg Chl⁻¹ and low photoadaptation index (E _k=18 μE m⁻² s⁻¹, E _m=65 μE m⁻² s⁻¹). Results of the microcosm experiment also indicated that communities were photo-oxidized when irradiance exceeded 100 μE m⁻² s⁻¹. This result suggests that micro- autotrophs inhabiting sea ice might have a minor role in the pelagic algal blooms. Accepted: 4 August 1999     

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO₃⁻), aluminum (Al), and calcium (Ca²⁺) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO₃⁻ concentration was about 20 μmol l⁻¹ for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l⁻¹ about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Al_im) export increased by 4−fold during the first year after the harvest. Al_im mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca²⁺ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Al_im and NO₃⁻ concentrations were strongly correlated in B-horizon soil water after the clearcut (r ² = 0.96), especially at NO₃⁻ concentrations greater than 100 μmol l⁻¹. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO₃⁻ which mixed with acidic, high Al_im soil water and decreased the concentration of Al_im in streamwater after the harvest. Five years after the harvest soil water NO₃⁻ concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO₃⁻ concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO₃⁻ concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO₃⁻ and Al_im concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.     

Isolation and characterization of nitrite-reductase-deficient mutants of Chlorella sorokiniana (strain 211-8k)

 A method is presented to isolate mutants of Chlorella sorokiniana with defects in NO₃ ⁻ metabolism. Three nitrite-reductase (NIR; E.C.1.7.7.1)-deficient mutants were obtained from 500 pinpoint-colony-forming clones. The final screening was performed using NO₃ ⁻, NO₂ ⁻ or NH⁺ ₄ as N-source. The mutants isolated absorb NO₃ ⁻ with rates close to those measured for the wild type and they excrete NO₂ ⁻ into the medium. The ratio between NO₃ ⁻ uptake and NO₂ ⁻ excretion was 1:1. The sensitivity of NO₃ ⁻ uptake to NH⁺ ₄ was reduced in the mutant strains as it was in the N-starved wild type of Chlorella. Nitrate reductase (NR; EC 1.6.6.1) expression and NR activity were slightly reduced compared to the wild type due to feedback regulation in the mutant strains. No NIR protein was found in the three mutants. However, NIR activity was obtained (50% of the wild-type) for one mutant strain. The NIR-deficient mutants and the already available NR-deficient mutants will be promising tools for investigations of the nitrate assimilation pathway on the molecular level and for studies searching for signaling of C and N metabolism by inorganic N-compounds. Received: 8 October 1999 / Accepted: 25 January 2000     

Denitrification in the vadose zone and in surficial groundwater of a sandy entisol with citrus production

A portion of nitrate (NO ₃ ⁻ ), a final breakdown product of nitrogen (N) fertilizers, applied to soils and/or that produced upon decomposition of organic residues in soils may leach into groundwater. Nitrate levels in water excess of 10 mg L⁻¹ (NO₃–N) are undesirable as per drinking water quality standards. Nitrate concentrations in surficial groundwater can vary substantially within an area of citrus grove which receives uniform N rate and irrigation management practice. Therefore, differences in localized conditions which can contribute to variations in gaseous loss of NO ₃ ⁻ in the vadose zone and in the surficial aquifer can affect differential concentrations of NO₃–N in the groundwater at different points of sampling. The denitrification capacity and potential in a shallow vadose zone soil and in surficial groundwater were studied in two large blocks of a citrus grove of ‘Valencia’ orange trees (Citrus sinensis (L.) Obs.) on Rough lemon rootstock ( Citrus jambhiri (L.)) under a uniform N rate and irrigation program. The NO₃–N concentration in the surficial groundwater sampled from four monitoring wells (MW) within each block varied from 5.5- to 6.6-fold. Soil samples were collected from 0 to 30, 30 to 90, or 90 to 150 cm depths, and from the soil/groundwater interface (SGWI). Groundwater samples from the monitoring wells (MW) were collected prior to purging (stagnant water) and after purging five well volumes. Without the addition of either C or N, the denitrification capacity ranged from 0.5 to 1.53, and from 0.0 to 2.25 mg N₂O–N kg⁻¹ soil at the surface soil and at the soil/groundwater interface, respectively. The denitrification potential increased by 100-fold with the addition of 200 mg kg⁻¹ each of N and C. The denitrification potential in the groundwater also followed a pattern similar to that for the soil samples. Denitrification potential in the soil or in the groundwater was greatest near the monitor well with shallow depth of vadose zone (MW3). Cumulative N₂O–N emission (denitrification capacity) from the SGWI soil samples and from stagnant water samples strongly correlated to microbial most probable number (MPN) counts (r² = 0.84 – 0.89), and dissolved organic C (DOC) (r² = 0.96 – 0.97). Denitrification capacity of the SGWI samples moderately correlated to water-filled pore space (WFPS) (r² = 0.52). However, extractable NO₃-N content of the SGWI soil samples poorly (negative) correlated to denitrification capacity (r² = 0.35). However, addition C, N or both to the soil or water samples resulted in significant increase in cumulative N₂O emission. This study demonstrated that variation in denitrification capacity, as a result of differences in denitrifier population, and the amount of readily available carbon source significantly (at 95% probability level) influenced the variation in NO₃–N concentrations in the surficial groundwater samples collected from different monitoring wells within an area with uniform N management. This revised version was published online in June 2006 with corrections to the Cover Date.     

Accumulation of inorganic and organic osmolytes and their role in osmotic adjustment in NaCl-stressed vetiver grass seedlings

The accumulation of inorganic and organic osmolytes and their role in osmotic adjustment were investigated in roots and leaves of vetiver grass (Vetiveria zizanioides) seedlings stressed with 100, 200, and 300 mM NaCl for 9 days. The results showed that, although the contents of inorganic (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃⁻, SO₄²⁻ and H₂PO₃⁻)) and organic (soluble sugar, organic acids, and free amino acids) osmolytes all increased with NaCl concentration, the contribution of inorganic ions (mainly Na⁺, K⁺, and Cl⁻) to osmotic adjustment was higher (71.50–80.56% of total) than that of organic solutes (19.43–28.50%). The contribution of inorganic ions increased and that of organic solutes decreased in roots with the enhanced NaCl concentration, whereas the case in leaves was opposite. On the other hand, the osmotic adjustment was only effective for vetiver grass seedlings under moderate saline stress (less than 200 mM NaCl).     

Tree species and wood ash affect soil in Michigan’s Upper Peninsula

Tree species and wood ash application in plantations of short-rotation woody crops (SRWC) may have important effects on the soil productive capacity through their influence on soil organic matter (SOM) and exchangeable cations. An experiment was conducted to assess changes in soil C and N contents and pH within the 0–50 cm depth, and exchangeable cation (Ca²⁺, Mg²⁺, K⁺, and Na⁺) and extractable acidity concentrations within the 0–10 cm depth. The effects of different species (European larch [Larix decidua P. Mill.], aspen [Populus tremula L. × Populus tremuloides Michx.], and four poplar [Populus spp.] clones) and wood ash applications (0, 9, and 18 Mg ha⁻¹) on soil properties were evaluated, using a common garden experiment (N = 70 stands) over 7 years of management in Michigan’s Upper Peninsula. Soils were of the Onaway series (fine-loamy, mixed, active, frigid Inceptic Hapludalfs). The NM-6 poplar clone had the greatest soil C and N contents in almost all ash treatment levels. Soil C contents were 7.5, 19.4, and 10.7 Mg C ha⁻¹ greater under the NM-6 poplar than under larch in the ash-free, medium-, and high-level plots, respectively. Within the surface layer, ash application increased soil C and N contents (P < 0.05) through the addition of about 0.7 Mg C ha⁻¹ and 3 kg N ha⁻¹ with the 9 Mg ha⁻¹ ash application (twofold greater C and N amounts were added with the 18 Mg ha⁻¹ application). During a decadal time scale, tree species had no effects—except for K⁺—on the concentrations of the exchangeable cations, pH, and extractable acidity. In contrast, ash application increased soil pH and the concentration of Ca²⁺ (P < 0.05), from 5.2 ± 0.4 cmol_c kg⁻¹ (ash-free plots) to 8.6 ± 0.4 cmol_c kg⁻¹ (high-level ash plots), and tended to increase the concentration of Mg²⁺ (P < 0.1), while extractable acidity was reduced (P < 0.05) from 5.6 ± 0.2 cmol_c kg⁻¹ (ash-free plots) to 3.7 ± 0.2 cmol_c kg⁻¹ (high-level plots). Wood ash application, within certain limits, not only had a beneficial effect on soil properties important to the long-term productivity of fast-growing plantations but also enhanced long-term soil C sequestration.