Nitrogenase (C2H2) Activity in Roots of Non-Cultivated and Cereal Plants: Influence of Nitrogen Fertilizer on Populations and Activity of Nitrogen-Fixing Bacteria

Root samples of 11 non-cultivated monocotyledonous and 7 dicotyledonous species taken during a wet summer had low mean nitrogenase activities of 10.2 and 7.1 nmol C₂H₄·g^?1 DW·h^?1 after preincubation at pO₂ 0.02, respectively. Maxima of 139–169 nmol·g^?1·h^?1 were observed with Agrostis vulgaris and Agropyron repens on a sandy soil poor in C_org. Three of 6 early, but none of 4 late fodder maize cultivars had a very low activity up to 0.5 nmol·g^?1h^?1. Oat, rye and wheat roots from plots with organic or mineral N fertilizers had activities between 1.3 and 7.3 nmol·g^?1h^?1 at flowering, which were not correlated with their Azospirillum populations (10²-10⁷·g^?1 after preincubation). Winter wheat and barley roots given 0, 40, 80 and 120 kg. ha^?1 NH₄NO₃-N in 0–3 applications had mean activities of 0.08, 4.06, 0.09 and 0.08 nmol or 1.77, 2.67, 0.36 and 0.23 nmol C₂H₄g^?1·h^?1 after flowering, respectively. An appreciable part of this activity could be removed by root washing. In preincubated rhizosphere soil of wheat and barley populations of N₂-fixing, facultative anaerobic Klebsiella and Enterobacter spp. were 10–100 times higher than those of Azospirillum sp., both being higher in O N than in 80 kg N·ha^?1 trials.     

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.     

Influence of various soil factors on nitrogen fixation by Azospirillum spp

The influence of soil pH, redox potential and added organic matter on N₂-fixation by Azospirillum was studied. Application of rice straw to alluvial, laterite and acid-sulphate Pokkali soils under submerged conditions enhanced the population of N₂-fixing Azospirillum spp. An acid-sulphate saline soil of extremely low pH (3.2) harboured Azospirillum spp with appreciable N₂-fixing activity. Enrichment cultures originating from soils with low pH (<4.0) possessed lower N₂-fixing activity compared to cultures from soils with higher pH values (upto 6.6). Azospirillum cultures from soils that had undergone prolonged waterlogging showed lower N₂-fixing activity than cultures isolated from soils submerged for a few days. A relationship was shown between the in vitro N₂-fixing activity of Azospirillum cultures and the redox status of the soil samples; activity was high when the soil redox potential was between ?50 to ?150mV. The results show that the N₂-fixing activity of Azospirillum cultures is governed by fluctuations in soil redox potential, pH and organic matter.     

Free-living nitrogen-fixing bacteria in temperate cropping systems: Influence of nitrogen source

Sustainable cropping systems rely on a minimum of external inputs. In these systems N is largely acquired in animal manures and leguminous green manures. Little is known of how these organic forms of N fertilizer influence the presence and activity of free-living N₂-fixing bacteria. High concentrations of inorganic N in soil inhibit N₂-fixation in cyanobacteria and Azotobacter spp. It is likely that manure and fertilizer applications would result in concentrations of inorganic N capable of inhibiting N₂ fixation and, ultimately, the presence of these organisms. We investigated the effect of synthetic and organic N fertilizer sources on the populations and N₂-fixation potential of free-living N₂-fixing bacteria in the Farming Systems Trial at the Rodale Research Institute. Field plots received the following N treatments prior to corn (Zea mays L.) production: (1) Legume rotations and green manures supplying about 165 kg N ha^-1; (2) beef cattle manure applied at a rate of 220 kg N ha^-1 (plus 60 kg N ha^-1 from 1994 hay plow-down); or (3) fertilizer N (urea and NH₄NO₃) applied at a rate of 145 kg N ha^-1. Soil samples were collected at two depths from corn plots four times during the growing season, and analyzed for soil moisture, soil pH, numbers of N₂-fixing cyanobacteria and Azotobacter spp., extractable NH _inf4 ^sup+ and NO _inf3 ^sup- , and potentially mineralizable N. Soil samples collected in mid-July were analyzed for nitrogenase activity (by C₂H₂ reduction) and total C and N. Populations of Azotobacter spp. and cyanobacteria were influenced only slightly by treatment; however, cyanobacteria species composition was notably influenced by treatment. Nitrogenase activity in surface soils was greatest in legume-N plots and in subsurface plots levels were greatest in fertilizer-N plots. Populations and activity of free-living N-fixing bacteria appeared to be somewhat reduced in all plots as a result of low soil pH levels and high concentrations of inorganic N across all treatments. Annual applications of N to all plots resulted in high levels of potentially mineralizable N that in turn may have reduced non-symbiotic N₂-fixation in all plots.     

Variation in nitrogen fixation among populations ofFrankia sp. andCeanothus sp. in actinorhizal association

Summary Wildland shrub improvement is needed for sound range and disturbed land revegetation practice. The possibility of selecting superior N₂-fixingFrankia-Ceanothus spp. actinorhizal associations was examined. Greenhouse tests were used to expose various soil-borne microsymbiont andCeanothus sp. population accessions in reciprocal combination. The acetylene reduction rate was used as a measure of N₂-fixation capacity. There was no significant interaction between host and microsymbiont regardless of source for all variables measured. The acetylene reduction rate, nodule number and mass, plant biomass, and root: shoot ratio were significantly different among soil sources. The acetylene reduction rate was not significantly different amongCeanothus sp. accessions. Neither was it strongly correlated with other variables. It was concluded that the N₂-fixation rate is more a function ofFrankia sp. than the hostCeanothus sp. in actinorhizal associations. It appears possible to select soil sources with superior N₂-fixing microsymbiont populations.The use of trade or firm names in this paper is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.This article was produced by government employees and is in the public domain and not subject to copyright. It may be freely reprinted with customary crediting of the source.     

Critical evaluation of the acetylene reduction test for estimating the activity of nitrogen-fixing bacteria associated with the roots of wheat and barley

The reliability of the C₂H₂ reduction test for estimating the activity of N₂-fixing bacteria associated with the roots of cereals has been evaluated in Scottish soils. Six wheat cultivars, including two chromosome substitution lines, and five barley cultivars were grown in a glasshouse in nine soils from the North East of Scotland. All the soils exhibited C₂H₄ oxidase activity which was completely inhibited by 0.0001–0.1 atm C₂H₂. Over-estimation of C₂H₂ reduction, resulting from the accumulation of endogenous C₂H₄, could, therefore, occur in assays of undisturbed plants, with the real possibility of deducing the existence of N₂-fixation where none existed. However, radiolabelled C₂H₂ reduction tests on undisturbed plants producing 2.4–18.0 μmol C₂H₄ day^?1, showed that all the C₂H₄ had been derived from the C₂H₂. With less active plants, the source of the C₂H₄ could not be accurately determined by this tracer method. These low rates of C₂H₄ production (< 2.4 μmol C₂H₄ day^?1), referred to as apparent C₂H₂ reduction, should, therefore, not be considered proof of N₂-fixation.The highest C₂H₂ reduction activities were observed in soils at maximum water holding capacity (MWHC). Roots removed from these soils reduced C₂H₂ immediately, if the initial partial pressure of O₂ (pO₂) was < 0.1 atm. Roots washed free of soil did not oxidize C₂H₄ during the 8 h assay. The C₂H₂ reduction activities of these excised roots could not be related to the activity of plants in soil for three reasons. (1) Development of C₂H₂ reduction was dependent on protein synthesis (inhibited by chloramphenicol), indicating re-establishment of activity destroyed by exposure to atmospheric pO₂, rather than continuation of the activity of undisturbed roots. (2) A lag period, dependent on the volume of the incubation vessel, was observed, indicating the involvement of root respiration in the assay. (3) Growth of N₂-fixing bacteria on compounds released from the roots (reducing sugars, amino acids and α-keto acids) occurred during the assay.Even with the possibility of over-estimation of N₂-fixation, the C₂H₂ reduction activities measured were considered to be too low to contribute significantly to the nitrogen requirement of the cereals grown under field conditions in Scotland.Some guidelines for screening programmes of N₂-fixation associated with the roots of grasses are suggested.     

Estimation of the total gaseous nitrogen losses from clay soils under laboratory and field conditions

Acetylene blockage was evaluated as a method for measuring losses of N₂O + N₂ from two Denchworth series clay soils. The denitrification potential in anaerobic, dark incubations at 20°C with nitrate (equivalent to 100 kg N ha^?1 0–20 cm depth), maximum water holding capacity, and acetylene (1%), was equivalent to 32 ± 11 and 39 ± 6 kg N ha^?1 per day for the two 0–20 cm soils and was positively correlated with carbon content (r= 0.98). After 4 days N₂O was reduced to N₂ in the presence of C₂H₂. In April 1980 following irrigation (24 mm) and applications of ammonium nitrate (70 kg N ha^?1) and acetylene, the mean nitrous oxide flux from soil under permanent grass was 0.05 ± 0.01 kg N₂O-N ha^?1 per day for 8 days. In June 1980, the losses of nitrogen from cultivated soils under winter wheat after irrigation (36 mm) and acetylene treatment were 0.006 ± 0.002 and 0.04–0.07 ± 0.01 kg N ha^?1 per day respectively before and after fertilizer application (70 kg N ha^?1). The nitrous oxide flux in the presence of acetylene decreased briefly, indicating that nitrification was rate determining in drying soil.     

Seasonal fluctuations in nitrogen fixation (acetylene reduction) by free-living bacteria in soils contaminated with cadmium, lead and zinc

Acetylene reduction by non-symbiotic, heterotrophic micro-organisms in a range of soils containing different concentrations of heavy metals was determined using intact soil cores. The suitability of this method for the soils used in this investigation was established. Samples were collected seasonally, and were incubated under standard conditions (darkness: 15°). Mean values of metal concentrations in the soil (μg g^?1) were: Cd: 1–200; Pb: 60–8000; Zn: 70–26000, Cu: 20–40. Rates of acetylene reduction were generally low, from 2800 to 50000 nmol C₂H₄, m^?2 day^?1. Assuming a 3:1 ratio of C₂H₂ reduction to N₂ fixation, this represents a rate of 0.3 to 5.0 g N fixed ha^?1 day^?1 in the surface 150 mm of soil. No consistent effect of heavy metal concentration was found. The most important factors determining activity were soil moisture content and possibly inorganic nitrogen concentration. It thus appears that the bacteria in polluted soils are capable of adapting to potentially toxic concentrations of heavy metals, or that these metals are present in the soils tested in unavailable or non-toxic forms.     

Acetylene reduction in soil and litter from pine and eucalypt forests in south-eastern Australia

Rates of C₂H₂-reduction in surface soil and litter from pine and eucalypt forests were measured for 1 yr. Rates of reduction increased significantly with moisture content, and mean rates (nmol kg^?1 h^?1) decreased in the order pine litter (339), eucalypt litter (220), eucalypt soil (54), pine soil (7). Asymbiotic N₂-fixation in litter and surface soil was estimated to be 108 mg m^?2 yr^?1 in eucalypt forest and 64 mg m^?2 yr^?1 in pine forest. About 80% of total fixation in eucalypt was in the soil, while 80% of the total in pine was in the litter. N₂ase was active in rotting wood but not in fresh foliage.     

Plant,Microbial and Soil Factors,Determining Nitrogen Fixation in the Rhizosphere

A genotype effect on associative (rhizosphere) N₂-fixation was observed with two cultivars of Sorghum bicolor (nutans) with a maximum rate of 8 μmol C₂H₄ · h^?1 · plant^?1 in one genotype compared to 0.9 μmol in the other. Characteristics of the high fixing genotype were a reduced transpiration rate, a lower number of stomata and increased root exudate production per gram root dry weight with higher concentration of dicarboxylic acids. The bacterial rhizosphere composition revealed a three times higher number of N₂-fixing bacteria, a tenfold reduction of actinomycetes and a threefold reduction of Arthrobacter associated with the high fixing cultivar compared to the low fixing genotype. From these and other plant rhizospheres two new nitrogen fixing bacteria, Pseudomonas stutzeri and Erwinia herbicola, were characterized. With the N₂-fixing bacteria Azospirillum brasilense and Klebsiella pneumoniae an enhancement of specific nitrogenase activity by aromatic compounds, for example phenolics, the herbicide alachlor and the insecticide carbofuran was demonstrated. An oscillating nitrogenase activity in Azospirillum brasilense under microaerobic conditions was found, resulting from an encystation and deencystation under those conditions. Experiments with wheat roots demonstrated that reduced oxygen tensions, essential for a maximum rhizosphere N₂-fixation, reduced root growth significantly and altered the N-metabolism of the roots.     

Nitrous oxide and methane fluxes during the growing season from cultivated peat soils,peaty marl and gyttja clay under different cropping systems

Drainage of peatlands affects the fluxes of greenhouse gases (GHGs). Organic soils used for agriculture contribute a large proportion of anthropogenic GHG emissions, and on-farm mitigation options are important. This field study investigated whether choice of a cropping system can be used to mitigate emissions of N₂O and influence CH₄ fluxes from cultivated organic and carbon-rich soils during the growing season. Ten different sites in southern Sweden representing peat soils, peaty marl and gyttja clay, with a range of different soil properties, were used for on-site measurements of N₂O and CH₄ fluxes. The fluxes during the growing season from soils under two different crops grown in the same field and same environmental conditions were monitored. Crop intensities varied from grasslands to intensive potato cultivation. The results showed no difference in median seasonal N₂O emissions between the two crops compared. Median seasonal emissions ranged from 0 to 919?µg?N₂O?m^?2?h^?1, with peaks on individual sampling occasions of up to 3317?µg?N₂O?m^?2?h^?1. Nitrous oxide emissions differed widely between sites, indicating that soil properties are a regulating factor. However, pH was the only soil factor that correlated with N₂O emissions (negative exponential correlation). The type of crop grown on the soil did not influence CH₄ fluxes. Median seasonal CH₄ flux from the different sites ranged from uptake of 36?µg CH₄?m^?2?h^?1 to release of 4.5?µg?CH₄?m^?2?h^?1. From our results, it was concluded that farmers cannot mitigate N₂O emissions during the growing season or influence CH₄ fluxes by changing the cropping system in the field.     

Effects of cyanobacteria strains selected for their bioconditioning and biofertilization potential on maize dry matter and soil nitrogen status in a South African soil

Abstract

Some cyanobacteria strains have biofertilization and/or bioconditioning effects in soils as a result of their ability to fix dinitrogen or produce exocellular polysaccharides. The objective of the present study was to screen indigenous cyanobacteria strains with the potential to improve the N fertility and structural stability of degraded soils, and evaluate their ameliorative effectiveness in semiarid soils of the Eastern Cape, South Africa. Soils from Guquka, Hertzog and Qunu villages, and Fort Cox College were used in the screening study. The results showed that only three cyanobacteria strains (3g, 3v and 7e) out of 97 isolated strains were heterocystous, with appreciable nitrogenase activity and the ability to produce exocellular polysaccharides. Nostoc strains 3g and 3v had a greater ability to produce exocellular polysaccharides, but low potential to fix dinitrogen (4.7 and 1.3?nmol C₂H₄?μg^?1?chl?h^?1, respectively). Strain 7e had the greatest ability to fix dinitrogen (16.1?nmol C₂H₄?μg^?1?chl?h^?1), but produced fewer exocellular polysaccharides. The ability of strains 3g and 7e to influence maize dry matter (DM) and soil C and N contents was tested in a nitrogen-poor soil with Nostoc strain 9v as a reference strain. Potted soils with and without growing maize plants were inoculated with the different cyanobacteria strains in a glasshouse at a rate of 6?g?m^?2 soon after maize emergence. Harvesting and soil sampling were done 6?weeks after inoculation. Inoculation with strains 3g and 7e increased maize DM and N uptake significantly, on par with the reference strain. These increases were consistent with increases in nitrate-N observed at harvest time in inoculated cropped and non-cropped soils. Strain 7e resulted in greater increases in soil nitrate-N, tissue N and uptake than strain 3g, perhaps because of its greater ability to fix dinitrogen. Cropping with maize reduced soil total C and N, possibly owing to its negative effects on cyanobacteria establishment. These results suggest that indigenous cyanobacteria strains screened for greater N₂-fixing ability have the potential to improve the productivity of N-poor soils in semiarid regions in South Africa.     

Enhancement of aerobic nitrogenase activity (acetylene reduction assay) by phenol in soils and the rhizosphere of cereals

By addition of phenol at concentrations between 0.1 and 10 mmol·l^?1, nitrogenase activity (acetylene reduction assay) is enhanced by a factor of 5 in the rhizosphere of Pennisetum glaucum (pearl millet) incubated under 20% O₂. No increase is found under microaerobic conditions. This enhancement effect is also noticed in a soil amended with a sucrose concentration of 20 mmol·l^?1. Under those conditions, however, an enhancement is found under aerobic as well as under microaerobic conditions and a further increase of the phenol added reduces the activity to almost zero. A 4-fold increase of N₂-fixation by phenol addition under aerobic conditions was determined with homogenous sediments from a fresh water lake while anaerobic N₂-fixation was already slightly reduced by the same concentration added. Excised roots of Sorghum nutans CSH 5 failed to show any phenol enhancement of nitrogenase activity. After a preincubation of 6h, inhibition of nitrogenase activity under air by addition of 1 mmol·l^?1 was much more pronounced than under microaerobic conditions.     

Effects of pentachlorophenol on asymbiotic nitrogen fixation in soil

The effect of 50, 100, 150, and 400 μg sodium pentachlorophenate (Na-PCP) per gram soil was studied in nonsterile soil incubated under aerobic and anaerobic conditions, and in sterilized soil inoculated withAzotobacter sp. isolated from the soil. N₂ fixation was determined by acetylene reduction. Pentachlorophenate at a concentration of 50 μg g^?1 had an inhibitory effect in nonsterile soil incubated aerobically while strong inhibition of dinitrogen fixation in nonsterile soil occurred in the presence of 100 μg g^?1 and above. The EC₅₀ values for the inhibition of nitrogenase activity in nonsterile soil incubated aerobically and anaerobically and in sterilized soil inoculated withAzotobacter sp. suspensions were 49.8±1.4 μg Na-PCP g^?1, 186.8±2.8 μg Na-PCP g^?1, and 660.8±29.3 μg Na-PCP g^?1, respectively.     

Ethylene production and decomposition in soils

Six soils differing in texture and use were investigated for their ability to produce and decompose ethylene. In addition, changes in methane and CO₂ concentrations were monitored. The effects of organic amendments and different water tensions were studied, and a method using low concentrations of acetylene as an inhibitor of ethylene degradation was tested. Possible reduction of acetylene to ethylene was identified by the use of CO or NH₄ ⁺-N, of which the latter turned out to be the more reliable method. This reduction only occurred in a grassland soil. Under aerobic soil conditions, gross ethylene production rates of up to 4.7pmol g^–1 h^–1 could be measured. Highest ethylene production and lowest ethylene decomposition was detected in a spruce forest soil. Fine textured soils produced more ethylene than coarse textured soils. Amended soils produced more ethylene at –100kPa and –5kPa than at 0kPa water tension. Ethylene decomposition was most effective in soils from deciduous woodlands and reached rates of up to 137pmol g^–1 h^–1. Parallels between ethylene and methane decomposition were observed. The addition of 5mgg^–1 glucose and 1mgg^–1 methionine not only promoted ethylene production but also inhibited ethylene decomposition. Received: 4 April 1997     

Nitrogen fixation by Azospirillium brasilense in soil and the rhizosphere under controlled environmental conditions

Summary Acetylene reduction activity by Azospirillum brasilense, either free-living in soils or associated with wheat roots, was determined in a sterilised root environment at controlled levels of O₂ tension and with different concentrations of mineral N. In an unplanted, inoculated soil nitrogenase activity remained low, at approximately 40 nmol C₂H₄ h^-1 per 2kg fresh soil, increasing to 300 nmol C₂H₄ h^-1 when malic acid was added as a C source via a dialyse tubing system. The N₂ fixation by A. brasilense in the rhizosphere of an actively growing plant was much less sensitive to the repressing influence of free O₂ than the free-living bacteria were. An optimum nitrogenase activity was observed at 10 kPa O₂, with a relatively high level of activity remaining even at an O₂ concentration of 20 kPa. Both NO _inf3 ^sup- and NH _inf4 ^sup+ repressed nitrogenase activity, which was less pronounced in the presence than in the absence of plants. The highest survival rates of inoculated A. brasilense and the highest rates of acetylene reduction were found in plants treated with azospirilli immediately after seedling emergence. Plants inoculated at a later stage of growth showed a lower bacterial density in the rhizosphere and, as a consequence, a lower N₂-fixing potential. Subsequent inoculations with A. brasilense during plant development did not increase root colonisation and did not stimulate the associated acetylene reduction. By using the ¹⁵N dilution method, the affect of inoculation with A. brasilense in terms of plant N was calculated as 0.067 mg N₂ fixed per plant, i.e., 3.3% of the N in the root and 1.6% in the plant shoot were of atmospheric origin. This ¹⁵N dilution was comparable to that seen in plants inoculated with non-N₂-fixing Psudomonas fluorescens.     

太湖地区主要土壤中的固定态铵及其有效性

测定了太湖地区主要土壤中固定态铵的储量和表土的固铵能力,并通过盆栽试验研究了它们的有效性。土壤的固定态铵含量和表土的固铵能力因母质而异,长江冲积物发育的土壤最高,次为黄土状物质的,第四纪红色粘土的最低。各土壤0—20厘米土层中的固定态铵总量平均占全氮总量的18.5%,0—100厘米土体中占34%。各土壤“固有的”固定态铵的有效性差异较大,视土层等的不同,在0—13%间。“新固定”的来自肥料或土壤有机质矿化释出的铵则很高,一般均能被当季作物完全吸收利用。渍水条件并不能提高固定态铵的有效性。讨论了铵的固定作用在土壤氮素肥力中的意义;指出,由于铵的固定作用和不同土壤的固铵能力各异,常用的淹水培育法所得到的土壤氮素矿化量值不但一般偏低,而且难于进行相互比较。     

Effects of nitrogen dioxide on selected soil processes

Nitrogen dioxide gas was rapidly absorbed by soil. After a 15 min incubation at 25°C, soil at a moisture content of 16% absorbed 99% of the NO₂ introduced into the gas-phase volume of a closed system. The presence of microorganisms hatl no influence on the rate of absorption of the gas by soil. The absorption of NO₂ by sandy clay loam soil was not an oxygen- or temperature-dependent process nor did it depend upon the moisture content of the soil. These physical factors acquired significance only in determining the initial rate of absorption of the gas and the rate at which NO₂ diffused through the soil. Exposure of soil to NO₂ resulted in substantial increases in the levels of NO _inf2 ^sup? N in the soil. Chemical oxidation of the NO _inf2 ^sup? N resulted in an increase in NO _inf3 ^sup? N levels. During a 14-day incubation, NO _inf2 ^sup? N concentrations in sterile soil exposed to an atmosphere containing 100 μg ml^?1 of NO₂ decreased from 190 μg g^?1 of soil to 105 μg g^?1 with an accompanying increase in NO _inf3 ^sup? N from 2 μg g^? ₁ to 63 μg g^?1 of soil. Nitrogen dioxide severely inhibited the growth of both aerobic and anaerobic asymbiotic N₂-fixing bacteria in soil. After a 48 h incubation at 25°C, soil aggregates exposed to an atmosphere containing 100 μg ml^?1 of NO₂ contained 88% and 98% fewer aerobic and anaerobic N₂-fixing bacteria, respectively. C₂H₂-reduction measurements showed that nitrogenase synthesis and activity in artificial soil aggregates amended with 2% glucose were inhibited by 20% and 48%, respectively, when exposed to atmospheric concentrations of 35 and 3.5 μg ml^?1 of NO₂, respectively.     

Contributions from different microbial processes to N2O emission from soil under different moisture regimes

Nitrous oxide emissions from a sandy-loam textured soil wetted to matric potentials of either-1.0 or-0.1 kPa were determined in laboratory experiments in which the soil was incubated in air (control), air plus 10 Pa C₂H₂ (to inhibit nitrification), 100 kPa O₂ (to suppress denitrification), 10 kPa C₂H₂ (to inhibit N₂O reduction to N₂ in denitrification) or following autoclaving. The total N₂O production, consumption and net N₂O emission from the soils together with the contributions to N₂O emission from different processes of N₂O production were estimated. The rate of N₂O production was significantly greater in the wetter soil (282 pmol N₂O g^-1 soil h^-1) than in the drier soil (192 pmol N₂O g^-1 soil h^-1), but because N₂O consumption by denitrifiers was also greater in the wetter soil, the net N₂O emissions from the wetter and the drier soils did not differ significantly. Non-biological sources made no significant contribution to N₂O emission under either moisture regime and biological processes other than denitrification and nitrification made only a small contribution (1% of the total N₂O production) in the wetter soil. Denitrifying nitrifiers were the predominant source of N₂O emitted from the drier soil and other (non-nitrifying) denitrifiers were the predominant source of N₂O emitted from the wetter soil.     

Non-symbiotic nitrogen fixation in the rhizospheres of rice,maize and different tropical grasses

Nitrogenase activity estimated in the rhizospheres of rice, maize and different tropical grasses grown under controlled laboratory conditions was shown to depend upon plant species. High nitrogenase activity (2000–6000 nmoles C₂H₄ h^?1 g^?1 dry root) occurred in rice rhizosphere, this activity being only 10 times lower than that of symbiotic systems; in the rhizosphere of many other grasses grown in a similar way nitrogenase activity was as low as 10 nmoles C₂H₄ h^?1 g^?1 dry root. The influence of soil type on nitrogenase activity was impressive; but the exact nature of the factors implicated could not be established. A rather weak flush of nitrogenase activity in the rhizosphere occurred in the early stage of the plant growth; it was probably due to the exudation of compounds from the seed and lasted 2 or 4 days according to the size of the seed. When the plant entered into its intense photosynthetic phase, the nitrogenase activity gradually increased. When the shoots were severed, nitrogenase activity in the rhizosphere ceased. Nitrogenase activity in the rhizosphere responded greatly to light intensity. Extrapolation of these laboratory findings to the field is discussed.