Environmental controls on nitric oxide emission from northern Chihuahuan desert soils

A survey of nitric oxide (NO) emission from Chihuahuan desert soils found mean NO fluxes <0.1 ng NO-N cm^–2h^–1 during the dry season. These fluxes were at thelower end of the range reported for temperate grassland and woodlandecosystems. NO fluxes from wet or watered soils were higher(0.1–35 ng NO-N cm^–2 h^–1).Watering of black grama grassland soils produced an initial pulse of 12ng cm^–2 h^–1 (12-h after 1-cm watering)with high fluxes sustained over 4 days with repeated watering. Initialpulses from shrubland soils were lower (maximum 5 ngcm^–2 h^–1), and fluxes declined withrepeated watering. Repeated watering of creosotebush soils depleted thesoil NH ₄ ⁺ pool, and NO emissions weredirectly related to soil NH ₄ ⁺ concentrationsat the end of the experiment. In watered andNH ₄ ⁺ -fertilized creosotebush soils, NO fluxeswere positively related to potential net nitrification rates.NH ₄ ⁺ -fertilization boosted the initial NOpulse 15 times in the shrubland and 5 times in black grama grasslandrelative to watered controls. These experimental results point towardgreater substrate limitation in shrublands. In this desert basin, NOemission averaged 0.12 kg N ha^–1 y^–1in untreated soil and 0.76 kg N ha^–1y^–1 in watered soil. We multiplied these averages bythe distribution of grassland and shrubland vegetation within a58,600-ha area of the Jornada del Muerto basin to estimate regionallosses of 0.15–0.38 kg NO-N ha^–1y^–1 for this area of the Chihuahuan desert.     

Fate of 15N labelled urine on four soil types

A field lysimeter experiment was conducted over a 406 day period to determine the effect of different soil types on the fate of synthetic urinary nitrogen (N). Soil types included a sandy loam, silty loam, clay and peat. Synthetic urine was applied at 1000 kg N ha^-1, during a winter season, to intact soil cores in lysimeters. Leaching losses, nitrous oxide (N₂O) emissions, and plant uptake of N were monitored, with soil ¹⁵N content determined upon destructive sampling of the lysimeters. Plant uptake of urine-N ranged from 21.6 to 31.4%. Soil type influenced timing and form of inorganic-N leaching. Macropore flow occurred in the structured silt and clay soils resulting in the leaching of urea. Ammonium (NH ₄ ⁺ –N), nitrite (NO ₂ ^- –N) and nitrate (NO₃ ^-–N) all occurred in the leachates with maximum concentrations, varying with soil type and ranging from 2.3–31.4 g NH ₄ ⁺ –N mL^-1, 2.4–35.6 g NO ₂ ^- –N mL^-1, and 62–102 g NO ₃ ^- –N mL^-1, respectively. Leachates from the peat and clay soils contained high concentrations of NO ₂ ^- –N. Gaseous losses of N₂O were low (<2% of N applied) over a 112 day measurement period. An associated experiment showed the ratio of N₂–N:N₂O–N ranged from 6.2 to 33.2. Unrecovered ¹⁵N was presumed to have been lost predominantly as gaseous N₂. It is postulated that the high levels of NO ₂ ^- –N could have contributed to chemodenitrification mechanisms in the peat soil.     

Nitrification and denitrification in a system of reciprocating jet bioreactor

Concept of separation of stages coupled with novel design of reciprocating jet bioreactor have been incorporated in this research for the development of high efficiency treatment system for contaminated wastewaters.Evaluation of pilot plant data reveals that a three stage reciprocating jet bioreactor system could be effectively employed for nitrification and denitrification of highly polluted wastewater obtained from Berlin wastewater treatment plant. Such a system with COD destruction stage (residence time 1–3 hours) followed by nitrification stage (residence time 3–4.5 hours) and denitrification stage (residence time 0.3 hours) gives COD destruction rate upto 58 kg COD/(m³ day), nitrification rate upto 3.2 NH ₄ ⁺ -N/(m³ day) and denitrification rate upto 28 kg NO ₃ ^– -N/(m³ day) while providing COD, NH ₄ ⁺ -N and NO ₃ ^– -N conversion of more than 90%.Nitrification and denitrification of wastewater at such a short residence time is possible mainly due to the employment of reciprocating jet bioreactor system.Paper presented at the Third Joint Schlesinger Seminar on Transport phenomena and processes in biological systems, Technion — Israel Institute of Technology, Haifa, Israel, May 8–9, 1990     

Rapid immobilization of applied nitrogen in saline–alkaline soils

The dynamics of inorganic N are important in soil, and this applies particularly to the saline–alkaline soils of the former lake Texcoco in Mexico with high pH and salinity where a forestation program was started in the 1970s. In soils of lake Texcoco, in Mexico, more than 50% of applied N could not be accounted for one day after application of 200 mg kg^–1 soil along with glucose amendment. It was not clear whether this was due to abiotic or biotic processes, the form of inorganic N applied or the result of applying an easily decomposable substrate. We investigated this by adding glucose and 200 mg kg^–1 soil as (NH₄)₂SO₄-N or KNO₃-N to sterilized and unsterilized soil. The changes in inorganic and ninhydrin N, microbial biomass C and production of CO₂ were then monitored. Between the time of applying N and extraction with 0.5 M K₂SO₄, i.e., after ca 2 h, approximately 110 mg NH₄ ⁺-N kg^–1 dry soil could not be accounted for in the unsterilized and sterilized soil and that remained so for the entire incubation in the sterilized soil. After 1 day this increased to 140 mg NH₄ ⁺-N kg^–1 dry soil in the unsterilized control and 170 mg NH₄ ⁺-N kg^–1 dry soil in C amended soil. Volatilization of NH₃ accounted for 56 mg NH₄ ⁺-N kg^–1 so the rest appeared to be adsorbed on the soil matrix. The NH₃ volatilization and NH₄ ⁺ fixed in the soil matrix remained constant over time and no oxidation to NO₂ ^– or NO₃ ^– had occurred, so unaccounted N in unsterilized soil was probably incorporated into the microbial biomass in excess of what was required for metabolic activity. The unaccounted N was ca 70 mg NO₃ ^––N in nitrate amended soil after 3 days and 138 NO₃ ^––N when glucose was additionally added. Losses through abiotic processes were absent as inferred from changes in sterilized soil and the aerobic incubation inhibited possible losses through denitrification. It was inferred that NO₃ ^– that could not be accounted for was taken up by micro-organisms in excess of what was required for metabolic activity.     

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]

Summary Relationships between root zone temperature, concentrations and uptake rates of NH ₄ ⁺ and NO ₃ ^– were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH ₄ ⁺ and NO ₃ ^– uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (C_min) range of 100–150 M uptake rates of NH ₄ ⁺ were 3–4 times higher than those of NO ₃ ^– The preference for NH ₄ ⁺ uptake was also reflected in the minimum concentration (C_min) values. Supplying NH₄NO₃, the rate of NO ₃ ^– uptake was very low until the NH ₄ ⁺ concentrations had fallen below about 100 M. The shift from NH ₄ ⁺ to NO ₃ ^– uptake was correlated with a corresponding shift from net H⁺ production to net H⁺ consumption in the external solution. The uptake rates of NH ₄ ⁺ were correlated with equimolar net production of H⁺. With NO ₃ ^– nutrition net consumption of H⁺ was approximately twice as high as uptake rates of NO ₃ ^– In the forest stand the NO ₃ ^– concentration in the soil solution was more than 10 times higher than the NH ₄ ⁺ concentration (<100 M), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO ₃ ^– uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.     

Phosphorus and nitrogen retention in five Precambrian shield wetlands

Phosphorus and nitrogen mass balances of five wetlands (two beaver ponds, two conifer-Sphagnum swamps and one sedge fen) situated in three catchments in central Ontario, Canada, were measured. Monthly and annual input-output budgets of total phosphorus (TP), total nitrogen (TN), total organic nitrogen (TON), total inorganic nitrogen (TIN), ammonium ion (NH₄ ⁺ -N), nitrate (NO ₃ ^– -N) and dissolved organic carbon (DOC) were estimated for the five wetlands during the 1982–83 and 1983–84 water years. Except for the deepest beaver pond (3.2 m) which had annual TP retention of –44% (–0.030 ± 0.015 g m^–2 yr^–1), the wetlands retained < 0.001 to 0.015 g M^–2 yr^–1 ; however, this wasless than 20% of the inputs and the estimated budget uncertainties were equal to or greater than the retention rates. Annual TN retentions ranged from –0.44 to 0.56 g m^–2 yr^–1 (–12 to 4%) but were not significantly different from zero. The wetlands transformed nitrogen by retaining TIN (16 to 80% RT) and exporting an equivalent amount as TON (–7 to 102% RT). The beaver ponds, however, retained NO ₃ ^– while NH ₄ ⁺ was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH ₄ ⁺ and NO ₃ ^– . DOC fluxes into and out of the beaver ponds were equal (–18 and 4% RT) but output from the conifer swamps exceeded input by > 90%. Marked seasonal trends in nutrient retention were observed. Nutrient retention coincided with low stream flow, increased evapotranspiration and biotic uptake during the summer. Net nutrient export occurred during the winter and spring when stream flows were highest and biotic uptake was low.     

Activity of nitrifying populations in grassland soil polluted by polychlorinated biphenyls (PCBs)

The activity of nitrification was studied in the period of 1992 – 1994 in two grassland plots from the surroundings of a municipal waste incinerator. The soil parameters were fully comparable in both plots and the soils differed in the level of polychlorinated biphenyls (PCBs). The concentration of PCBs found in Klajdovka-control plot (KL): 4.4 ng g_{dry soil} ^–1 can be regarded as a background value, while the polluted plot, Bílá Hora (BH), contained increased amount of PCBs: 14.0 ng g_{dry soil} ^–1.The following parameters of nitrifying activity were determined: field concentrations of N_inorg species, mineralization potentials, nitrifying activity during long-term laboratory incubations, and the potential activity of both ammonium and nitrite oxidizers in short-term incubations in soil slurries. Simultaneous application of all these methods appeared to be very suitable for reliable assessment of nitrifying activity in the field.In the case of the polluted plot, the abnormal accumulation of nitrite was observed both in the field (e.g. in September 1992: BH-656.8 ng NO _inf2 ^– -N g_{dry soil} ^–1; KL-208.2 ng NO _inf2 ^– -N g_{dry soil} ^–1) and in the laboratory incubations. Furthermore, the capability of the polluted plot to nitrify higher amount of ammonium nitrogen appeared to be significantly reduced due to detrimental changes in the activity of nitrite-oxidizing community. In contrast to the nitrification, the mineralization potential did not differ between the plots throughout the sampling period.     

The chemistry of wet deposition for a tallgrass prairie ecosystem: inputs and interactions with plant canopies

The chemistry and nutrient inputs of wet deposition, and the N chemistry of throughfall, were characterized for a tallgrass prairie in north-central Kansas. Dominant ions in wetfall were NH ₄ ⁺ , Ca²⁺, H⁺, NO ₃ ^- , and SO ₄ ^2- ; weighted mean pH was 4.79. Principal sources of ions appeared to be natural emissions and wind-blown soils. Concentrations of NO ₃ ^- -N, NH ₄ ⁺ -N, and organic N in wet deposition were 0.31, 0.30, and 0.17 mg/L, respectively, resulting in N inputs of 2.5, 2.5, and 1.4 kg · ha^-1 · yr^-1. Comparisons with bulk precipitation suggested that at least 50% of atmospheric N inputs were from dry deposition. Concentrations of NO ₃ ^- -N, NH ₄ ⁺ -N, and organic N in unburned prairie throughfall were 0.27, 0.28, and 1.28 mg/L, and in burned prairie throughfall were 0.33, 0.37, and 0.91 mg/L, respectively. The prairie canopy intercepted up to 48% of incident precipitation. Lower inorganic N and higher organic N concentrations in throughfall relative to wet deposition probably resulted from leaf uptake of N and immobilization by microbes associated with the standing dead plant materials of the prairie canopy. The removal of these materials by fire is important in maintaining N availability for tallgrass prairie. Much of the N immobilization appeared to have been of N that was supplied to the prairie canopy by dry deposition.     

Loss of nitrogen in compacted grassland soil by simultaneous nitrification and denitrification

The soils of mid-Wales in grazed permanent pasture usually exhibit stagnogley features in the top 4–10 cm even though on sloping sites, they are freely drained. Nitrogen is often poorly recovered under these conditions. Our previous studies suggest that continuing loss of available N through concurrent nitrification and denitrification might provide an explanation for poor response to fertilizer N. The work described was designated to further test this proposition. When NH ₄ ⁺ –N was applied to the surface of intact cores, equilibrated at –5kPa matric potential, about 70% of NH ₄ ⁺ –N initially present was lost within 56 days of incubation. Study of different sections of the cores showed a rise in NO ₃ ^- level in the surface 0–2.5 cm soil layer but no significant changes below this depth. The imbalance between NO ₃ ^- accumulation and NH ₄ ⁺ disappearance during the study indicated a simultaneous nitrification and denitrification in the system. Furthermore, the denitrification potential of the soil was 3–4 times greater than nitrification potential so no major build-up of NO ₃ ^- would be expected when two processes occur simultaneously in micro-scale. When nitrification was inhibited by nitrapyrin, a substantial amount of NH ₄ ⁺ –N remained in the soil and persisted till the end of the incubation. The apparent recovery of applied N increased and of the total amount of N applied, 50% more was recovered relative to without nitrapyrin. It appears that addition of nitrapyrin inhibited nitrification, and consequently denitrification, by limiting the supply of NO ₃ ^- for denitrifying organisms. Emission of N₂O from the NH ₄ ⁺ amended soil cores further confirmed that loss of applied N was the result of both nitrification and denitrification, which occurred simultaneously in adjacent sites at shallow depths. This N loss could account for the poor response to fertilizer N often observed in pastoral agriculture in western areas of the UK.     

Net nitrogen mineralization and net nitrification along a tropical forest-to-pasture chronosequence

Soil inorganic nitrogen pools, net mineralization and net nitrification rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the western Brazilian Amazon Basin state of Rondônia to investigate the influence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrogen pools than pasture soils. In the forest, NO ₃ ^– pools equaled or exceeded NH ₄ ⁺ pools, while pasture inorganic N pools consisted almost exclusively of NH ₄ ⁺ . Rates of net N mineralization and net nitrification in seven -day laboratory incubations were higher in the seven - day forest than in the pastures. Net N mineralization rates did not differ significantly among different-aged pastures, but net nitrification rates were significantly lower in the 20-year-old pasture. Higher net N mineralization and net nitrification rates were measured in laboratory and in situ incubations of sieved soil, compared with in situ incubations of intact soil cores. Rates calculated in seven-day incubations were higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil was the most useful index for comparing N availability across the chronosequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO ₃ ^– losses either through leaching or gaseous emissions.     

Solution chemistry profiles of mixed-conifer forests before and after fire

Solution chemistry profiles of mixed-conifer forests in granitic catchments of the Sierra Nevada were measured for three years before (1987–1990) and three years after (1990–1993) prescribed fire. Wet deposition, throughfall and soil solution samplers were installed in both white-fir and giant-sequoia dominated forest stands underlain by poorly developed inceptisols. Stream water chemistry was monitored as part of an ongoing study of catchment outputs. Calcium, NO ₃ ^– and Cl^– were the major ions in precipitation. Canopy leaching increased mean concentrations of all major ions, especially K⁺ and Ca²⁺. Water flux through the soil occurred largely during spring snowmelt. Forest floor leachate represented the most concentrated solutions of major ions. Interaction with the mineral soil decreased mean concentrations of most species and the average composition of soil solutions closely resembled stream water at baseflow. Bicarbonate alkalinity, Ca²⁺, Mg²⁺, and Na⁺ were enriched in stream water relative to precipitation whereas inputs of H⁺, NH ₄ ⁺ , NO ₃ ^– and SO ₄ ^2– were retained within the catchments.Burning of the forest understory and litter layer increased solute concentrations in soil solution and stream water. Mean soil solution Ca²⁺, Mg²⁺ and K⁺ concentrations increased more than 10 fold, but the relative predominance of these cations was not affected by burning. Sulfate concentration, which was very low in soil solutions of undisturbed stands (<25 mmol_c m^–3), increased more than 100 times following fire. Ammonium concentration exhibited a rapid, short-term increase and then a decrease below pre-burn levels. Changes in soil solution chemistry were reflected in catchment outputs.Corresponding author.     

NO 2 − Efflux and its regulation in cyanobacterium Nostoc MAC

NO ₂ ^– efflux and its regulation have been studied in the cyanobacterium Nostoc MAC. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU), carbonyl cyanide-m-chlorophenyl hydrazone (CCCP), sodium azide, p-chloromercuribenzoate (PCMB), and dicyclohexylcarbodiimide (DCCD), a specific inhibitor of bacterial ATPase, inhibited the NO ₂ ^– efflux activity singificantly. No NO ₂ ^– efflux activity was observed under dark-aerobic as well as under dark-anaerobic conditions; however, the addition of ATP resulted in NO ₂ ^– efflux even under dark-aerobic condition. Maximum NO ₂ ^– efflux activity was observed when NO ₃ ^– served as the sole nitrogen source. However, NH ₄ ⁺ ions inhibited the NO ₂ ^– efflux activity when both NO ₃ ^– and NH ₄ ⁺ wer simulatneously available to the cells. The NO ₂ ^– efflux was freed from NH ₄ ⁺ repression by l-methionine-dl-sulfoximine (MSX), an irreversible inhibitor of glutamine synthetase (GS). Chloramphenicol, a protein synthesis inhibitor, inhibited the derepression of NO ₂ ^– efflux system when NH ₄ ⁺ -incubated cells were transferred to NO ₃ ^– medium. Tungstate-treated cells lacking functional NO ₃ ^– reductase but having NO ₃ ^– uptake activity also lacked NO ₂ ^– efflux activity. These results suggest that (i) NO ₂ ^– efflux in Nostoc MAC is NO ₃ ^– dependent and an energy-dependent process that can be regulated at the levels of NO ₃ ^– uptake and NO ₃ ^– reductase; (ii) NO ₂ ^– efflux system is NH ₄ ⁺ repressible; however, the product of NH ₄ ⁺ assimilation via GS is being required for repression to occur; (iii) de novo protein synthesis is required for derepression of the NO ₂ ^– efflux system; and (iv) the catalytic activity of NO ₂ ^– reductase also seems to play an important role in the regulation of NO ₂ ^– efflux system.     

Contemporary and pre-industrial global reactive nitrogen budgets

Increases and expansion of anthropogenic emissions of both oxidized nitrogen compounds, NO_x, and a reduced nitrogen compound, NH₃, have driven an increase in nitrogen deposition. We estimate global NO_x and NH₃ emissions and use a model of the global troposphere, MOGUNTIA, to examine the pre-industrial and contemporary quantities and spatial patterns of wet and dry NO_y and NH_x deposition. Pre-industrial wet plus dry NO_x and NH_x deposition was greatest for tropical ecosystems, related to soil emissions, biomass burning and lightning emissions. Contemporary NO_y+NH_x wet and dry deposition onto Northern Hemisphere (NH) temperate ecosystems averages more than four times that of preindustrial N deposition and far exceeds contemporary tropical N deposition. All temperate and tropical biomes receive more N via deposition today than pre-industrially. Comparison of contemporary wet deposition model estimates to measurements of wet deposition reveal that modeled and measured wet deposition for both NO ₃ ^– and NH ₄ ⁺ were quite similar over the U.S. Over Western Europe, the model tended to underestimate wet deposition of NO ₃ ^– and NH ₄ ⁺ but bulk deposition measurements were comparable to modeled total deposition. For the U.S. and Western Europe, we also estimated N emission and deposition budgets. In the U.S., estimated emissions exceed interpolated total deposition by 3-6 Tg N, suggesting that substantial N is transported offshore and/or the remote and rural location of the sites may fail to capture the deposition of urban emissions. In Europe, by contrast, interpolated total N deposition balances estimated emissions within the uncertainty of each.Abbreviations EMEP European Monitoring and Evaluation Program - GEIA Global Emissions Inventory Activity - NADP/NTN National Atmospheric Deposition Program/National Trends Network in the US - NH Northern Hemisphere - NH_x=NH₃+NH ₊ ⁴ NO_x=NO+NO₂ NO_y total odd nitrogen=NO_x+HNO₃+HONO+HO₂NO₂+NO₃+radical (NO₃ ^.)+Peroxyacetyl nitrates+N₂O₅+organic nitrates - SH Southern Hemisphere - Gg 10⁹ g - Tg 10¹² g     

Nitrate or ammonium nutrition in french bean

Summary Bean Plants were grown in a greenhouse in sand irrigated with nutrient solutions containing either 2 mM NO ₃ ^– or 2 mM NH ₄ ⁺ . After 45 days fresh weight of NH ₄ ⁺ plants was half that of NO ₃ ^– plants. Cation concentration in NH ₄ ⁺ plants was 30% less than in NO ₃ ^– plants. Amino acids (SER, ASN, GLN) accummulated 3 to 10 times more in NH ₄ ⁺ plants. The concentration of organic acids (malic, malonic, citric) was 10 to 30 times higher in NO ₃ ^– plants. The ATP-costings for the synthesis of amino acids and organic acids in NH ₄ ⁺ plants was half that of NO ₃ ^– ones: therefore it could not account for the reduction of growth in the ammonium-fed plants.     

Fluxes and transformations of nitrogen in a high-elevation catchment,Sierra Nevada

The fluxes and transformations of nitrogen (N) were investigated from 1985 through 1987 at the Emerald Lake watershed (ELW), a 120 ha high-elevation catchment located in the southern Sierra Nevada, California, USA. Up to 90% of annual wet deposition of N was stored in the seasonal snowpack; NO ₃ ^– and NH ₄ ⁺ were released from storage in the form of an ionic pulse, where the first fraction of meltwater draining from the snowpack had concentrations of NO ₃ ^– and NH ₄ ⁺ as high as 28 eq L^–1 compared to bulk concentrations of <5 eq L^–1 in the snowpack. The soil reservoir of organic N (81 keq ha^–1) was about ten times the N storage in litter and biomass (12 keq ha^–1). Assimilation of N by vegetation was balanced by the release of N from soil mineralization, nitrification, and litter decay. Mineralization and nitrification processes produced 1.1 keq ha^–1 yr^–1 of inorganic N, about 3 1/2 times the loading of N from wet and dry deposition. Less than 1% of the NH ₄ ⁺ in wet and dry deposition was exported from the basin as NH ₄ ⁺ . Biological assimilation was primarily responsible for retention of NH ₄ ⁺ in the basin, releasing one mode of H⁺ for every mole of NH ₄ ⁺ retained and neutralizing about 25% of the annual acid neutralizing capacity produced by mineral weathering in the basin. Nitrate concentrations in stream waters reached an annual peak during the first part of snowmelt runoff, with maximum concentrations in stream water of 20 eq L^–1, more than 4 times the volume-weighted mean annual concentrations of NO ₃ ^– in wet deposition. This annual peak in stream water NO ₃ ^– was consistent with the release of NO ₃ ^– from the snowpack in the form of an ionic pulse; however soil processes occurring underneath the winter snowpack were another potential source of this NO ₃ ^– . Concentrations of stream water NO ₃ ^– during the summer growing season were always near or below detection limits (0.5 eq L^–1).     

Throughfall chemistry in a loblolly pine plantationunder elevated atmospheric CO2 concentrations

Accelerated tree growth under elevatedatmospheric CO₂ concentrations may influencenutrient cycling in forests by (i) increasingthe total leaf area, (ii) increasing the supplyof soluble carbohydrate in leaf tissue, and (iii) increasing nutrient-use efficiency. Here wereport the results of intensive sampling andlaboratory analyses of NH ₄ ⁺ , NO ₃ ^– , PO ₄ ^3– , H⁺, K⁺, Na⁺,Ca²⁺, Mg²⁺, Cl^–, SO ₄ ^2– , and dissolved organic carbon (DOC) in throughfallprecipitation during the first 2.5+ years of the DukeUniversity Free-Air CO₂ Enrichment (FACE)experiment. After two growing seasons, a largeincrease (i.e., 48%) in throughfall deposition of DOCand significant trends in throughfall volume and inthe deposition of NH ₄ ⁺ , NO ₃ ^– , H⁺, and K⁺ can be attributed to the elevatedCO₂ treatment. The substantial increase indeposition of DOC is most likely associated withincreased availability of soluble C in plant foliage,whereas accelerated canopy growth may account forsignificant trends toward decreasing throughfallvolume, decreasing deposition of NH ₄ ⁺ ,NO ₃ ^– , and H⁺, and increasing deposition of K⁺ under elevated CO₂. Despiteconsiderable year-to-year variability, there wereseasonal trends in net deposition of NO ₃ ^– ,H⁺, cations, and DOC associated with plant growthand leaf senescence. The altered chemical fluxes inthroughfall suggest that soil solution chemistry mayalso be substantially altered with continued increasesin atmospheric CO₂ concentrations in the future.     

Effect of ammonium on the regulation of nitrate and nitrite transport systems in roots of intact barley (Hordeum vulgare L.) seedlings

The effect of NH ₄ ⁺ on the regulation of NO ₃ ^– and NO ₂ ^– transport systems in roots of intact barley (Hordeum vulgareL.) seedlings grown in NO ₃ ^– or NO ₂ ^– was studied. Ammonium partially inhibited induction of both transport systems. The inhibition was less severe in NO ₂ ^– -fed than in NO ₃ ^– -fed seedlings, presumably due to lower uptake of NH ₄ ⁺ in the presence of NO ₂ ^– . In seedlings pretreated with NH ₄ ⁺ subsequent induction was inhibited only when NH ₄ ⁺ was also present during induction, even though pretreated roots accumulated high levels of NH ₄ ⁺ . This indicates that inhibition may be regulated by NH ₄ ⁺ concentration in the cytoplasm rather than its total accumulation in roots. L-Methionine sulfoximine did not relieve the inhibition by NH ₄ ⁺ , suggesting that inhibition is caused by NH ₄ ⁺ itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO ₃ ^– transport activity similarly in roots grown in 0.1, 1.0, or 10 mM NO ₃ ^– for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. Induction of the NO ₃ ^– transport system was about twice as sensitive to NH ₄ ⁺ as compared to the NO ₂ ^– transport system. This may relate to higher turnover rates of membraneassociated NO ₃ ^– -transport proteins.Abbreviations Mes 2(N-morpholino)ethanesulfonic acid - MSO L-methionine sulfoximine     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

Foliar N application reduces soil NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack>-N leaching loss in apple orchards

A comparison of the effects of foliar and soil N application was made in field-grown mature fruiting Gala/M9 apple trees (Malus domestica Borkh) in 2001 and 2002 growing seasons under Pacific Northwest growing conditions in southern British Columbia, Canada. The trees, six years old at the start of the experiment, were treated: (1) with 5 g/l urea sprays supplied every two weeks (7 times) from mid May to mid August (total about 50 g N/tree/year), (2) with the same amount of N applied to the soil with the same timing and quantity as for the foliar treatment, and (3) with no N (control). Leaf color (as SPAD readings) and N concentrations (mg/g), and soil NH₄⁺-N and NO₃^–-N were measured periodically throughout the two seasons. Leached NO₃^–-N was monitored monthly via an anion exchange probe from June to October in 2001 and from May to November in 2002. Shoot length was measured in October and N concentration of one-year-old wood and roots was determined in December of each growing s eason. Soil N application significantly increased shoot length relative to control or foliar N application. Leaf color, leaf N, and N concentration of one-year-old wood and roots were similarly increased relative to control by both soil and foliar N application. These treatments also increased fruit yield relative to control. There was no significant difference in yield and fruit quality between soil and foliar N applications. Soil N application increased soil NH₄⁺-N and NO₃^–-N content in the root zone, and also increased the NO₃^– leaching loss below the root zone especially late in the growing season. Our results suggested that tree N status and yield and fruit quality could be maintained by multiple urea sprays during the growing season in apple orchards, and foliar N application will reduce the risk of soil NO₃^–-N leaching.     

The relative importance of autotrophic and heterotrophic nitrification in a conifer forest soil as measured by 15N tracer and pool dilution techniques

The importance of heterotrophic nitrification was studied in soil from a mixed-conifer forest. Three sites in the forest were sampled: a clear cut area, a young stand and a mature stand. In the mature stand, the mineral soil (0–10 cm) and the organic layer were sampled separately. Gross rates of N mineralization and nitrification were measured by¹⁵NH ₄ ⁺ and¹⁵NO ₃ ^– isotopic pool dilution, respectively. The rates of autotrophic and heterotrophic nitrification were distinguished by use of acetylene as a specific inhibitor of autotrophic nitrification. In samples supplemented with¹⁵NH ₄ ⁺ and treated with acetylene, no¹⁵NO ₃ ^– was detectable showing that the acetylene treatment effectively blocked the autotrophic nitrification, and that NH ₄ ⁺ was not a substrate for heterotrophic nitrification. In the clear cut area, autotrophic nitrification was the most important NO ₃ ^– generating process with total nitrification (45 ug N kg^–1h^–1) accounting for about one-third of gross N mineralization (140 ug N kg^–1 h^–1). In the young and mature forested sites, gross nitrification rates were largely unaffected by acetylene treatment indicating that heterotrophic nitrification dominated the NO ₃ ^– generating process in these areas. In the mature forest mineral and organic soil, nitrification (heterotrophic) was equal to only about 5% of gross mineralization (gross mineralization rates of 90 ug N kg^–1 h^–1 mineral; 550 ug N kg^–1 h^–1 organic). The gross nitrification rate decreased from the clear cut area to the young forest area to the mineral soil of the mature forest (45; 17; 4.5 ug kg^–1 h^–1 respectively). The¹⁵N isotope pool dilution method, combined with acetylene as an inhibitor of autotrophic nitrification provided an effective technique for assessing the importance of heterotrophic nitrification in the N-cycle of this mixed-conifer ecosystem.