The determination of NH3-concentration gradients in a spruce forest using a passive sampling technique

In order to estimate the influence of NH₃ on a forest in a clean atmosphere, the quality of the atmosphere in Sumava, South-Bohemia, was determined in September–October 1988. In this period the NO_X, SO₂, and aerosol concentrations were extremely low. The N-input into the forest margin amounted to 10–15 kg ha^?1 in these two months, based on a deposition velocity of 3 cm s^?1 and an average concentration of 9 μg m^?3 NH₃. The maximum hourly average ozone concentrations were found to be 50 to 100 μg m^?3. There exists a strong relation between the ambient NH₃ concentration profiles and the stable occupation. The concentration inside the stables exceeded the no-adverse-effect level for cows of 7.6 mg m^?3. This publication forms the basis for the estimation of the exposure of the forest to NH₃ and the interpretation of the observed effects as published elsewhere (Mejst?ík et al., unpublished).     

Symbiosis of Trifolium subterraneum with mycorrhizal fungi and Rhizobium trifolii as affected by ammonium sulphate and nitrification inhibitors

The symbioses between Trifolium subterraneum, mycorrhizal fungi and Rhizohium are affected by (NH₄)₂SO₄ and by the nitrification inhibitors 2-chloro-6 (trichloromethyl) pyridine (N-Serve) and 2-trichloromethyl pyridine (2TMP). At 50 μg · g^?1 soil N-Serve and 2TMP had toxic effects on plant growth, measured as leaf expansion, root length and dry weight. Lower concentrations of N-Serve also produced some toxic symptoms. The addition of (NH₄)₂SO₄ to the soil at 2 and 6 m-equiv NH⁺₄ per pot, resulted in reduced root length and nodulation. Shoot dry weight was reduced at 6 m-equiv NH⁺₄ per pot. In the presence of (NH₄)₂SO₄ the toxic effects of the nitrification inhibitors on plant growth were less.Both nitrification inhibitors reduced development of mycorrhizal entry-points and extent of root colonization (% infection). Percentage infection of the root system was also reduced by (NH₄)₂SO₄. Development of nodules on the lateral roots was increased in the presence of N-Serve at 5 and 15 μ · g^?1. This effect, however, was accompanied by a marked reduction in N₂ase activity. Smaller increases in nodulation were apparent with 2TMP and were associated with variable N₂ase activity.     

Growth,Nutrition, and Photosynthetic Response of Black Walnut to Varying Nitrogen Sources and Rates

ABSTRACT

Black walnut (Juglans nigra L.) half-sib 1+0 seedlings were exponentially fertilized with ammonium (NH₄ ⁺) as ammonium sulfate [(NH₄)₂SO₄], nitrate (NO₃ ^?) as sodium nitrate (NaNO₃), or a mixed nitrogen (N) source as ammonium nitrate (NH₄NO₃) at the rate of 0, 800, or 1600 mg N plant^?1 and grown for three months. One month following the final fertilization, N concentration, growth, and photosynthetic characteristics were assessed. Compared with unfertilized seedlings, N addition increased plant component N content, chlorophyll content, and photosynthetic gas exchange. Net photosynthesis ranged from 2.45 to 4.84 μmol m^?2 s^?1 for lower leaves but varied from 5.95 to 9.06 μmol m^?2 s^?1 for upper leaves. Plants responded more favorably to NH₄NO₃ than sole NH₄ ⁺ or NO₃ ^? fertilizers. These results suggest that N fertilization can be used to promote net photosynthesis as well as increase N storage in black walnut seedlings. The NH₄NO₃ appears to be the preferred N source to promote black walnut growth and physiology.     

Seasonal Evolution of Levels of Gaseous Pollutants in an Urban Area (Ciudad Real) in Central-Southern Spain: A Doas Study

Long term continuous monitoring measurements of urban atmospheric concentrations of O₃, NO₂, NO, and SO₂ were performed for the first time in Ciudad Real, a city in central-southern Spain. The measurements were carried out using the differential optical absorption spectroscopy (DOAS) technique, with a commercial system (OPSIS, Lund-Sweden), covering the summer and winter seasons (from 21st July 2000 to 23rd March 2001). Mean levels of O₃, NO₂ and SO₂ monitored during this period were: 27 μg m^?3, 50 μg m^?3 and 7 μg m^?3 respectively. The highest hourly averaged value of O₃ (160 μg m^?3) was measured during the summer period, while NO₂ was enhanced in wintertime (highest values 90 μg m^?3). In the coldest period, when central heating installations were operating, SO₂ showed maximum levels of 20 μg m^?3. The daily, weekly and seasonal analysis of the data shows that photochemical air pollution dominates in this urban atmosphere and is strongly influenced by levels of motor traffic and domestic heating system emissions. These measurements were compared with other studies in Spain and Europe. Also, the long path averaged DOAS measurements were compared with in situ observations made in Ciudad Real, from 23rd August 2000 to 25th September 2000, using a mobile air pollution control station. All gas concentrations reported in this paper are below the WHO guidelines and the different thresholds introduced by the European Environmental Legislation.     

Responses to sequential exposure to SO2 and salinity in soybean (Glycine max L.)

Soybean plants (Glycine max L. cv. Buchanan) were subjected to one of three levels of salinity preteatment (with electrical conductivities of 0.7, 4.4 and 6.5 dS m^?1) and then exposed to one of three concentrations of SO₂ (1, 145 and 300 bl l ^?1 for 5 h d^?1), or vice versa. Each stress episode lasted 3 weeks. Both salinity and SO₂ deecreased leaf area, root and shoot dry weight and the fresh weight of root nodules. SO₂ induced an increase in the shoot: root ratio and leaf chlorophyll concentrations. Low salinity pretreatment protected plant growth from SO₂ injury, probably by decreasing SO₂ uptake by increasing stomatal resistance. However, high salinity-treated plants, despite also showing stomatal closure, were severely injured by subsequent SO₂ exposure. Prior exposure to SO₂ caused plants to become more vulnerable to salt injury. Plants pretreated with high SO₂ were killed after 12 days of high salt stress. These data suggest that the compensatory mechanisms and predisposition characteristics of salinity and SO₂ largely depend upon the stress levels used.     

Specifics and Temporal Changes in Air Pollution in Areas Affected by Emissions from Oil Shale Industry, Estonia

Atmospheric air pollution levels and long-term effects on the environment caused by simultaneous presence of SO₂ and oil shale alkaline fly ash during the last five decades (since 1950) were investigated. The annual critical value of SO₂ for forest (20 µg m^?3) was surpassed in 1% (～35 km²) of the study area where the load was 30–40 µg m^?3. No effect of long-term SO₂ concentrations of up to 10–11 µg m^?3 (0.5-h max up to 270 µg m^?3) and simultaneous fly ash loads of up to 95 µg m^?3 (1000 µg m^?3) on the growth and needle longevity of Pinus sylvestris was established. The yearly deposition (average load up to 20–100 kg S ha^?1) was alkaline rather than acidic due to an elevated base cation deposition in 1960–1989. Since 1990, the proportion of SO₂ in the balance of components increased: about 70–85% of the total area was affected while the ratio of annual average concentrations of SO₂ to fly ash was over 1. The limit values of fly ash for Sphagnum mosses and conifers in the presence of SO₂ are recommended.     

Throughfall Chemistry in a Sitka Spruce Plantation in Response to Six Different Simulated Polluted Mist Treatments

A mixed provenance Sitka spruce plantation, planted in 1986 on a drained deep peat, has been exposed to 6 different simulated mist treatments in 4 replicated blocks since 1996. Treatments provided N and/or S at a concentration of 1.6 mol m^?3, supplying ca. 50 kg S and/or N ha^?1 yr^?1 as N (NH₄NO₃), S (Na₂SO₄), NS Acid (NH₄NO₃ + H₂SO₄ at pH 2.5), 2NS Acid (double dose by application at twice frequency), a control treatment supplied with additional rainwater only and a 'no treatment' set of plots. Throughfall, preserved with thymol in the field, was collected using gutters with a surface area of 1 m² in all the replicate plots, and was analysed for all major ions. Prior to treatment in 1999, S deposition in throughfall exceeded that in rain because of dry deposition of SO₂ and SO₄ ^2? to the canopy; NH₄ ⁺ and NO₃ ^? ions were both retained in the canopy. During treatment, only 20–40% of the applied N in the high-N treatments was retained in the canopy. Acidity in the applied mist was partly neutralised by the canopy, but not primarily through exchange of base cations, leading to the conclusion that weak organic acids, in solution or in situ in the canopy, contributed to the buffering of the H⁺ ion deposition in the acid treatments.     

Estimation of Dry Deposition by Using a Filter Pack Method at Chunchon, Korea

Atmospheric gases and particulates were collected using four-stage filter-pack in Chunchon from January through December in 1999. Particulate SO₄ ^2? and NO₃ ^?, and gaseous HNO₃, SO₂ and NH₃ were analyzed. Annual average concentration of SO₄ ^2?(S), NO₃ ^?(S), HNO₃ (g), SO₂(g) and NH₃(g) were 5.75µg/m³, 4.98µg/m³, 0.33ppb, 1.52ppb and 7.25ppb, respectively. Annual dry deposition fluxes were estimated using the measured concentration and dry deposition velocity published by other research group. Annual dry deposition of S was 287kg · (km)^?2·y^?1, which accounted for about 30% of total S deposition. For N deposition, dry deposition is predominant; about 70% of total N deposition was through dry process mostly as forms of NH₃ and HNO₃.     

Effects of NH4 + on the growth and K+ (86Rb) uptake of various ectomycorrhizal fungi in pure culture

As a result of considerable deposition of ammonium sulphate on vegetation and soil the NH₄/K ratios in the soil water extracts of pine forests in the Netherlands are increasing. Increasing the NH₄/K ratio from 1 to 40 in the solid nutrient medium of isolated ectomycorrhizal fungi will in general result in a 60 to 80% inhibition of the lateral growth. On increasing the NH4/K ratio the biomass of most of the fungi examined shows a maximum at ratios of approximately 10 to 20, although the colony diameter at those ratios remains rather small. NH₄ ⁺ also exerts a pronounced effect on the uptake rate of Rb⁺ inRhizopogon luteolus.At low Rb concentrations the inhibitory effect of NH₄ ⁺ is maximal, whereas on increasing the Rb+ concentration the inhibitory effect of NH₄ ⁺ is abolished. This means that NH₄ ⁺ competitively inhibits the Rb⁺ uptake inR. Iuteolus.     

Pollution sources and spreading of sulphur dioxide in the North-Eastern Estonia

The greatest sources of atmospheric emissions of SO₂ in Estonia are caused by power plants (TP) which use oil shale. Since 1990 the amount of SO ₂ discharges has continuously decreased due to fall in production of electric energy, and it was from TP as follows: in 1990–1991 about 180–200 thousand tons, in 1992 about 140 and in 1993–1994 about 100 thousand tons. In 1990 the annual mean emission intensity of SO ₂ from all North-East (NE) Estonian pollution sources was fixed to be about 6.kg/s, with a maximum of 9.5–11 kg/s in winter period. In 1992–1993 the corresponding values were 3.5–4.6 and 5.1–6.8 kg/s. The single maximum concentrations (MC, per 30 min.) of SO ₂ in the overground air layer would be in the ranges 25–450 μg/m ³ depending on emission intensity and wind parameters. The annual mean concentrations are below 25 μg/m ³ on the main territory, but may be up to 50–75 μg/m ³ near the power plants. In Kohtla-Järve town the annual mean values of 15.8–19.1 μg/m ³ and MC values of 271–442 μg/m ³ were fixed during 1991–1994 by automatic air monitoring system. Many arable lands, forest areas and wild-life preserves are subjected to relatively high sulphur precipitation loads, exceeding 0.5 g S/m² per year, of which the role of emissions from local sources is about 60–95%. On the basis of air pollution concentration maps, the landscape of NE Estonia is classified into zones of high, moderate and low pollution level.     

Response of Picea abies (L.) Karst. Provenances to sulphur dioxide and aluminum: A pilot study

Seedlings of six Picea abies provenances were exposed to Al, SO₂ and A1+SO₂ for 12 weeks. A molaric Ca/Al ratio of 0.05 was chosen by using 1.68 mM Al. On the average, concentration of SO₂ was 335 μg m^?3. Shoot and root growth were measured and needle discoloration was scored for different pollution regimes. Plant response to different environmental stresses differed significantly among the provenances used and showed marked provenance x treatment interactions. It is noteworthy that the high intraspecific variation of the traits investigated could cause misleading conclusions in nongenetic studies if inappropriate plant material is employed.     

Evidence of Long-Range Transport of Pollutants from the Size-Fractionated Ionic Composition of Aerosols in the Jeju Island of Korea

The ionic composition of total suspended particulate (TSP) and fine (PM_2.5) fractions was investigated from an 1,100 site in the middle of Mt. Halla in Jeju Island, Korea from March to November 2006. The sum concentrations of cation and anion species in TSP fraction were 205 ± 170 and 183 ± 164 neq m^?3, respectively, while those for PM_2.5 as 118 ± 129 and 88.5 ± 89.3 neq m^?3, respectively. In TSP, the concentration of the major ions changed in the order of SO₄ ^2? > NH₄ ⁺ > Ca²⁺ > Na⁺ > NO₃ ^? > Mg²⁺ > K⁺ > Cl^?, while its PM_2.5 counterpart as NH₄ ⁺ > SO₄ ^2? > Ca²⁺ > NO₃ ^? > Na⁺ > Mg²⁺ > K⁺ > Cl^?. Inspection of the temporal variabilities of ionic components indicated that most ions peaked in spring or fall months. The back trajectory analysis showed that the atmospheric composition of the major ionic species was affected fairly sensitively by long-range transport from China under the favorable meteorological conditions. In contrast, the lowest ionic concentration levels were seen most abundantly, when air masses passed from South Sea. Hence, the analysis of ionic concentration data suggests that their distributions are controlled by the combined effects of various source processes including the most prominent Chinese origin and the meteorological condition favorable for such transport.     

N,P, K and S uptake response to various levels of CO2 assimilation and growth rate in lettuce

Under conditions of limited nutrient supply, plant nutrient uptake is controlled by the external concentration of the ions. Limited information exists about the whole-plant regulation of nutrient uptake when the supply is adequate. To study the relationship between growth rate and carbon dioxide (CO₂) assimilation with nutrient uptake, growth chamber experiments were conducted with temperatures ranging from 10 to 35°C at medium (600 µmol m^?2 s^?1) and high (1200 µmol m^?2 s^?1) light intensities. Nutrient solution samples were collected every 24 hours and the concentration of ions was analyzed by Inductively coupled plasma -atomic emission spectroscopy (ICP-AES) and nitrate and ammonium (NO₃^?/NH₄⁺) conductivity. Leaf photo-synthesis was measured using a closed gas exchange system and the total amount of CO₂ assimilated was calculated from dry weight increases. The daily absorption of NO₃^?, Total nitrogen (N), dihydrogen phosphate (H₂PO₄^?) and potassium (K⁺) responded linearly to plant growth, while ammonium (NH₄⁺) and sulfate (SO₄^2?) uptake showed a curvilinear response. All the ions studied showed a curvilinear relation with CO₂ assimilation.     

Summertime study of acid deposition in the Detroit area

A comprehensive acid-deposition measuring station has been set up at the General Motors Technical Center site in Warren, Michigan. A second station is also being operated at a rural site near Lapeer, Michigan, which is approximately 54 km north of the Warren site. This report presents the results of this study for the June-September 1981 period. The rain composition was similar at both sites with the pH averaging 4.1 and the SO₄ ⁼/NO₃ ^? equivalence ratio averaging 2.1:1. This similarity suggests that local sources, i.e., relatively high emissions near the Warren site, and low emissions near the Lapeer site, have little effect on rain composition. The SO₄ ⁼/NO₃ ^? ratio for individual rain events in Warren reflected the SO₂/NO_x emission ratio in the area from which the event had come. Thus, the highest SO₄ ⁼/NO₃ ^? ratios were observed for rains that arrived from the southeast and the lowest ratios for rains from the southwest. No rain events arrived from a northwesterly direction during this sampling period. Measurements were carried out at both sites to estimate the relative contribution of dry deposition. The ambient particulate was acidic about half the time and basic half the time. The acidity in the particulate was due to acid SO₄ ⁼ and the basicity was due to soil-related materials. Gaseous HNO₃ averaged 2.0 μg m^?3while the basic gas, NH₃, averaged 0.83 μg m^?3. Based on these measurements, it was estimated that dry deposition of particles and HNO₃ contributed less than 10% of the total deposition of acidity during this study period.     

Interactive Effects of Nitrogen Source and Salinity on Growth Indices and Ion Content of Indian Mustard

ABSTRACT

The interactions between salinity and different nitrogen (N) sources nitrate (NO₃ ^?), ammonium (NH₄ ⁺), and NO₃ ^? + NH₄ ⁺ were investigated on Indian mustard (Brassica juncea cv. RH30). Treatments were added to observe the combined effect of two salinity levels (8 and 12 ds m^{? 1}) and three nitrogen sources (NO₃ ^?, NH₄ ⁺, and NO₃ ^? + NH₄ ⁺) on different growth parameters and mineral composition in different plant parts, i.e., leaves, stem, and root. Salinity has been known to affect the uptake and assimilation of various essential nutrients required for normal growth and development. Different growth parameters, i.e., leaf area, dry weight of different plant parts, absolute growth rate (AGR), relative growth rate (RGR), and net assimilation rate (NAR) declined markedly by salinity at pre-flowering and flowering stages. All growth indices were less sensitive to salinity (12 d s m^{? 1}) with the nitrate form of nitrogen. It is pertinent mention that a high dose (120 kg ha^{? 1}) of nitrogen in ammonium form NH₄ ⁺, acted synergistically with salinity in inhibiting growth. Plants fed with combined nitrogen (NO₃ ^? + NH₄ ⁺) had an edge over individual forms in ameliorating the adverse effects of salinity on growth and yield. Under salt stress, different nutrient elements such as N, phosphorus (P), potassium (K⁺), and magnesium (Mg^{2 +}) were decreased in different plant parts (leaves, stem, and root). The maximum and minimum reduction was observed with ammoniacal and combined form of nitrogen, respectively, while the reverse was true of calcium (Ca^{2 +}), sodium (Na⁺), chloride (Cl^?), and sulfate (SO₄ ^2?) at harvest. Nitrogen application (120 Kg ha^{? 1}) in combined form had been found to maintain highest concentrations of N, P, Mg^{2 +}, and Ca^{2 +} along with reduced concentrations of Na⁺, Cl^?, and SO₄ ^{2 ?}. However, reverse was true with ammoniacal form of nitrogen.     

Growth responses of maize,soybean, and tomato to sulphur dioxide and ozone exposure patterns utilizing an incomplete block factorial experimental design

Maize (Zea mays L.), soybean (Glycine max L.), and tomato (Lycopersicon esculentum Mill.) plants were grown in a controlled environment and exposed for 6 hr daily for 7 days to O₃ at 0.15 μL L^?1 and/or SO₂ at 0.30 μL L^?1 (daily exposures). Some plants exposed daily to O₃ were also exposed to SO₂ for 6 hr on the first, third, fifth, or seventh day of O₃ exposure (variable exposures) and some plants exposed daily to SO₂ were treated similarly with O₃ to determine the growth effects of O₃ or S0₂ pre- and/or post-treatments on S0₂ and O₃ mixture response. Growth sensitivity to 6 hr S0₂ or 6 hr O₃ treatments was generally affected by the previous history of O₃ or SO₂ exposure, respectively. Species differed in the number of days of O₃ or SO₂ treatments required to elicit maximum sensitivity to a single 6 hr O₃ and SO₂ treatment. Linear contrasts compared variable with daily exposures for the S0₂ and O₃ regimes. Plants exposed to the gas mixture for a single day (variable exposures) tended to be smaller than those exposed to the gas mixture daily, with the exception of soybean exposed to SO₂ during daily O₃. The six treatments were carried out in eight exposure chambers, as a partially balanced incomplete block design in blocks of four due to separate environmental control of the exposure facilities. The partially balanced incomplete block design proved to be about 2.6 times as efficient as a complete block design. The inclusion of covariates further increased precision.     

Casuarina and Eucalyptus response to single and multiple gaseous air pollutants

Casuarina cunninghamiana and Eucalyptus camadulensis (Egyptian var.) plants were exposed to 0.20 and 0.40 μL L^?1 O₃, SO₂ or NO₂ for 6 hr daily for 10 days. Eucalyptus plants were very sensitive to SO₂ and NO₂ and less sensitive to O₃. Casuarina plants were insensitive to the 3 gases. The rate of sorption of the 3 gases was estimated over a 10 day exposure to 0.20 μL L^?1 pollutant concentration singly and in a 3-gas mixture. Casuarina plants removed air pollutants more efficiently than Eucalyptus plants. Leaves of both species generally sorbed about the same volume of a given gas from the mixture and from the same single gas. The sorption rate over the 10 day exposure was almost constant after a higher sorption rate during the first day for both species.     

Evalution of Acid Deposition in Korea

This study was carried out to evaluate acid depositions and to understand their effect. Wet precipitation has been collected at twenty-four sites in Korea for one year of 1999. The ion concentrations such as H⁺, Na⁺, K⁺, Mg²⁺, NH₄ ⁺, Ca²⁺, Cl^?, NO₃ ^? and SO₄ ^2? were chemically analyzed and determined. Precipitation had wide range of pH(3.5～8.5), and volume-weighted average was 5.2. The contribution amounts of Cl^?, SO₄ ^2? and NO₃ ^? in anion were shown to be 54%, 32%, and 14%, respectively and those of Na⁺ and NH₄ ⁺ in cation were 32% and 25%. The ratios of Cl^? and Mg²⁺ to Na⁺ in precipitation were similar to those of seawater, which imply that great amount of Cl^? and Mg²⁺ in precipitation could be originated from seawater. The concentration of H⁺ is little related with SO₄ ^2?, NO₃ ^? and Cl^? ions, whereas nss^?SO₄ ^2? and NO₃ ^? are highly correlated with NH₄ ⁺, which could suggest that great amount of SO₄ ^2? and NO₃ ^? exist in the form of ammonium associated salt. The annual wet deposition amounts (g m^?2year^?1) of SO₄ ^2?, NO₃ ^?, Cl^?, H⁺, NH₄ ⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ were estimated as 0.88～4.89, 0.49～4.37, 0.30～9.80, 0.001～0.031, 0.06～2.15, 0.27～4.27, 0.10～3.81, 0.23～1.59 and 0.03～0.63.     

Heterotrophic N2-fixation in arid soil crusts

The cryptogamic soil crusts of the Great Basin Artemisia, Ceratoides, and Atriplex plant communities contain a significant heterotrophic N₂-fixing microbial population in addition to the predominating filamentous cyanobacteria. The bacterial association with the cyanobacteria exhibits a phycosphere-like effect. Heterotrophically fixed N gains reached 17.5 μg N· g^?1 of soil (23.1% increase above the initial soil N content) and 45.9 μg N·g^?1 of soil (57.4% increase) after 3 and 5 weeks, respectively. (NH₄)₂SO₄ and native plant material amendments to soil resulted in a 41–100% reduction in N₂-fixation. The potential input of N to soil crusts may be reduced in the presence of shrub-produced allelochemic agents and by concurrent denitrification.     

Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.