Nitrogen-fixing activity in peat soils from a raised bog

The nitrogenase (acetylene reductase) activity in monolithic and minced peat samples was found to be low, no more than 0.014-0.022 mg N/(kg h). Incorporation of the 15N2 isotope into organic compounds of peat soil was from 2.71-8.13 mg N/kg over 15 days. The nitrogen-fixing activity was the highest in a 10-20 cm layer of soil and much lower in the upper (under green moss) and deeper (20-30 cm) layers. The addition of glucose to soil samples stimulated nitrogen fixation considerably after 18-26 h. The maximum nitrogenase activity (3.5-3.8 mg N/(kg h)) observed after 60-70 h coincided with the peak of respiratory activity. A repeated addition of glucose after its exhaustion increased nitrogenase activity without a lag period to 8.5 mg N/(kg h). Investigation of the effect of environmental factors (temperature, pH, aeration, and light intensity) on potential nitrogen-fixing activity in peat samples revealed that nitrogen fixation could proceed in a wide range of pH values (from 3.0 to 7.5) and temperatures (from 5 to 35 degrees C). The nitrogen-fixing bacteria belonging to different trophic groups were enumerated by using nitrogen-free media with pH values and mineralization levels close to those in situ. In samples of peat soil, diazotrophic methanol-utilizing bacteria prevailed (2.0-2.5 x 10(6) cells/g); the second largest group was facultatively anaerobic bacteria of the family Enterobacteriaceae.     

Minimizing the effect of mineral nitrogen on biological nitrogen fixation in common bean by increasing nutrient levels

Although common bean (Phaseolus vulgaris L.) has good potential for N₂ fixation, some additional N provided through fertilizer usually is required for a maximum yield. In this study the suppressive effect of N on nodulation and N₂ fixation was evaluated in an unfertile soil under greenhouse conditions with different levels of soil fertility (low=no P, K and S additions; medium = 50, 63 and 10 mg kg^–1 soil and high = 200, 256 and 40 mg kg^–1 soil, respectively) and combined with 5, 15, 60 and 120 mg N kg^–1 soil of ¹⁵N-labelled urea. The overall average nodule number and weight increased under high fertility levels. At low N applications, nitrogen had a synergistic effect on N₂ fixation, by stimulating nodule formation, nitrogenase activity and plant growth. At high fertility and at the highest N rate (120 mg kg^–1 soil), the stimulatory effect of N fertilizer on N₂ fixation was still observed, increasing the amounts of N₂ fixed from 88 up to 375 mg N plant^–1. These results indicate that a suitable balance of soil nutrients is essential to obtain high N₂ fixation rates and yield in common beans.     

Changes in Asymbiotic, Heterotrophic Nitrogen Fixation on Leaf Litter of Metrosideros polymorpha with Long-Term Ecosystem Development in Hawaii

We measured nitrogenase activity (acetylene reduction) of asymbiotic, heterotrophic, nitrogen-fixing bacteria on leaf litter from the tree Metrosideros polymorpha collected from six sites on the Hawaiian archipelago. At all sites M. polymorpha was the dominant tree, and its litter was the most abundant on the forest floor. The sites spanned a soil chronosequence of 300 to 4.1 million y. We estimated potential nitrogen fixation associated with this leaf litter to be highest at the youngest site (1.25 kg ha^-1 y^-1), declining to between 0.05 and 0.22 kg ha^-1 y^-1 at the oldest four sites on the chronosequence. To investigate how the availability of weathered elements influences N fixation rates at different stages of soil development, we sampled M. polymorpha leaf litter from complete, factorial fertilization experiments located at the 300-y, 20,000-y and 4.1 million–y sites. At the youngest and oldest sites, nitrogenase activity on leaf litter increased significantly in the plots fertilized with phosphorus and “total” (all nutrients except N and P); no significant increases in nitrogenase activity were measured in leaf litter from treatments at the middle-aged site. The results suggest that the highest rates of N fixation are sustained during the “building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic) nutrients from weathering are substantial. Received 4 February 1999; accepted 29 March 2000.     

Nitrogen fixation,transfer and turnover in upland and lowland grass-clover swards,using15N isotope dilution

The stable isotope¹⁵N is particularly valuable in the field for measuring N fixation by isotope dilution. At the same time other soil-plant processes can be studied, including¹⁵N recovery, and nitrogen transfer between clover and grass. Three contrasting sites and soils were used in the present work: a lowland soil, an upland soil, and an upland peat. Nitrogen fixation varied from 12 gm^–2 on lowland soil to 2.7 gm^–2 on upland peat. Most N transfer occurred on upland soil (4.2 gm^–2) which, added to nitrogen fixed, made a total of 8.7 gm² input during summer 1985.¹⁵N recovery for the whole experiment was small, around 25%.Measurement of dead and dying leaves, stubble and roots, suggests that plant organ death is the first stage in N transfer from white clover to ryegrass, through the decomposer cycle. Decomposition was fastest on lowland soils, slowest on peat. On lowland soil this decomposer nitrogen is apparently subverted before transfer, probably by soil microbes.Variations in natural abundance of¹⁵N in plants were found in the two species on the different soils. These might be used to measure nitrogen fixation without adding isotope, but the need for many replicates and repeat samples would limit throughput.     

Field measurement of symbiotic nitrogen fixation in an established lucerne ley using15N and an acetylene reduction method

Summary Lucerne is an important forage legume in the south and south-east of Sweden on well-drained soils. However, data is lacking on the apparent amount of nitrogen derived through N₂ fixation by field-grown lucerne. This report provides basic information on the subject. The experiment was performed in a lucerne ley grown 40 km north of Uppsala. The input of nitrogen through fixation to the above-ground plant material of an established lucerne (Medicago sativa L.) ley was estimate by¹⁵N methodology during two successive years. The amount of fixed N was 242 kg N ha^–1 in 1982 and 319 kg N ha^–1 in 1983. The proportion of N derived from the atmosphere (%Ndfa) was 70% and 80% for the two years respectively. The first harvest in both years contained a lower proportion fixed N. Both N₂ fixation and dry matter production were enhanced during the second year, particularly in the first harvest. The Ndfa was 61% in the first harvest in 1982, compared to 72% Ndfa during the same period in 1983. This demonstrates the strong influence of environment on both dry matter production and N₂ fixation capacity of the lucerne.In addition anin situ acetylene reduction assay was used in 1982 to measure the seasonal distribution of the N₂ fixation and in 1983 to study the effect of soil moisture on the N₂ fixation process. The seasonal pattern showed great dependence on physiological development and harvest pattern of the lucerne ley. The maximum rate of N₂ fixation occurred at the bud or early flower stage of growth and was followed by a rapid decline as flowering proceeded. After harvest the nitrogenase activity markedly decreased and remained low during at least two weeks until regrowth of new shoots began. Irrigation doubled the nitrogenase activity of the lucerne in late summer 1983, when soil moisture content in the top soil was near wilting point. No changes in nitrogenase activity did occur in response to watering earlier during the summer, when the soil matric potential was around –0.30 MPa.     

N2-fixation in Erwinia herbicola

Four from 18 strains of Erwinia herbicola tested had nitrogenase activity and grew with N₂ as sole source of nitrogen under strict anaerobic conditions with a doubling time of 20–24 h. Nitrogenase activity started only 96–120 h after transfer to a special medium maintained under anaerobic conditions. A ten fold increase in protein per culture found after the maximum nitrogenase activity of 80–130 nmol C₂H₄. mg protein^-1·min^-1 was accompanied by a fall in pH of the medium (20 mM phosphate buffer and in 125 mM Tris-buffer) from pH 7.2 to 5.4 or less, but only to 6.8 in 100 mM phosphate buffer. In all cases we found a sharp curtailing of nitrogenase activity 48 h after the maximum. The bacteria utilized only 35–50% of the nitrogen fixed for growth. Erwinia herbicola strains differed from two strains of Enterobacter agglomerans in being unable to fix nitrogen on agar surfaces exposed to air. Specific nitrogenase activity in Erwinia herbicola is compared with data reported for other Enterobacteriaceae and is found to be higher than that reported for Klebsiella pneumoniae, Enterobacter cloacae or Citrobacter freundii.     

Denitrification in the top and sub soil of grassland on peat soils

Denitrification is an important process in the nitrogen (N) balance of intensively managed grassland, especially on poorly drained peat soils. Aim of this study was to quantify the N loss through denitrification in the top and sub soil of grassland on peat soils. Sampling took place at 2 sites with both control (0 N) and N fertilised (+ N) treatments. Main difference between the sites was the ground water level. Denitrification was measured on a weekly basis for 2 years with a soil core incubation technique using acetylene (C₂H₂) inhibition. Soil cores were taken from the top soil (0–20 cm depth) and the sub soil (20–40 cm depth) and incubated in containers for 24 hours. The denitrification rate was calculated from the nitrous oxide production between 4 and 24 hours of incubation. Denitrification capacities of the soils and the soil layers were also determined.The top soil was the major layer for denitrification with losses ranging from 9 to 26 kg N ha^–1 yr^–1 from the O N treatment. Losses from the top soil of the + N treatment ranged from 13 to 49 kg N ha^–1 yr^–1. The sub soil contributed, on average, 20% of the total denitrification losses from the 0–40 layer. Losses from the 0–40 cm layer were 2 times higher on the + N treatment than on the O N treatment and totalled up to 70 kg N ha^–1 yr^–1. Significant correlation coefficients were found between denitrification activity on the one hand, and ground water level, water filled pore space and nitrate content on the other, in the top soil but not in the sub soil. The denitrification capacity experiment showed that the availability of easily decomposable organic carbon was an important limiting factor for the denitrification activity in the sub soil of these peat soils.     

Methanotrophic Bacteria of Acidic Sphagnum Peat Bogs

Acidic Sphagnum peat bogs cover a considerable part of the territory of Russia and are an important natural source of biogenic methane, which is formed in their anaerobic layers. A considerable portion of this methane is consumed in the aerobic part of the bog profile by acidophilic methanotrophic bacteria, which comprise the methane filter of Sphagnum peat bogs and decrease CH₄ emission to the atmosphere. For a long time, these bacteria escaped isolation, which became possible only after the elucidation of the optimal conditions of their functioning in situ: pH 4.5–5.5; temperature, from 15 to 20°C; and low salt concentration in the solution. Imitation of these conditions and rejection of earlier used media with a high content of biogenic elements allowed methanotrophic bacteria of two new genera and species—Methylocella palustris and Methylocapsa acidiphila—to be isolated from the peat of Sphagnum peat bogs of European northern Russia and western Siberia. These bacteria are well adapted to the conditions in cold, acidic, oligotrophic Sphagnum peat bogs. They grow in a pH range of 4.2–7.5 with an optimum at 5.0–5.5, prefer moderate temperatures (15–25°C) and media with a low content of mineral salts (200–500 mg/l), and are capable of active dinitrogen fixation. Design of fluorescently labeled 16S rRNA–targeted oligonucleotide probes for the detection of Methylocella palustris and Methylocapsa acidiphila and their application to the analysis of sphagnum peat samples showed that these bacteria represent dominant populations of methanotrophs with a density of 10⁵–10⁶ cells/g peat. In addition to Methylocella and Methylocapsa populations, one more abundant population of methanotrophs was revealed (10⁶ cells/g peat), which were phylogenetically close to the genus Methylocystis.     

Nitrogenase activity of two genotypes of Acacia mangium as affected by phosphorus nutrition

The specific nodulation, nitrogenase activity (acetylene reduction) and budgets of carbon allocation to respiration by nodulated roots were examined in two provenances of Acacia mangium Willd. grown in a glasshouse for 17 weeks to investigate the effects of soil phosphorus and genotypes of the host plant on symbiotic nitrogen fixation. Application of phosphorus (0–80 mg P kg^-1 soil) increased specific nodulation (g nodule dry weight g^-1 plant dry weight) of provenance Ma11 by two-fold and the percentage of nodulated root respiration allocated to nitrogenase by 50%, but had no effect on specific activity of nitrogenase or specific respiration coupled with nitrogenase activity. Improved phosphorus nutrition increased the specific nitrogenase activity of provenance Ma9 by 2-fold, the percentage of nodulated root respiration allocated to nitrogenase, and specific nitrogenase-linked respiration by 50%, respectively, but had no effect on the specific nodulation. The percentage of respiration coupled with nitrogenase activity in nodulated root respiration by provenance Ma9 was 60–70% higher than that in provenance Ma11, regardless of phosphorus levels applied. At the optimal level of phosphorus addition (10 mg P kg^-1 soil), provenance Ma9 had a lower dry mass than provenance Ma11. This was accompanied by a lower nodulated root respiration and a higher percentage of nodulated root respiration allocated to nitrogenase activity in provenance Ma9.     

Effects of fertilization rate and crop rotation on diazotrophic cyanobacteria in paddy field

Application of chemical fertilizers at the recommended level (medium fertility) or lower stimulated growth of the diazotrophic cyanobacterial population and nitrogenase activity in a paddy field. High fertilizer levels proved to be inhibitory to nitrogen-fixing cyanobacteria indicating that indiscriminate use of chemical fertilizers for a longer period drastically disturbed the natural ecological balance. The rice–mustard–moong (RMM) crop rotation was observed to be more suitable for cyanobacterial nitrogen fixation than rice–wheat–maize rotation. The cropped plots had higher nitrogenase activity than fallow plots. The low fertility coupled with RMM rotation were found to be best suited for promoting nitrogen fixation by cyanobacteria to supply the rice plants. A top dressing of chemical nitrogenous fertilizer drastically suppressed the cyanobacterial nitrogenase activity (ARA) within 12 h; the magnitude of inhibition varied with respect to the cropping system. The inhibition was overcome by the 10th day and the ARA value reached the preapplication value or even higher in the case of low fertility and medium fertility level plots. A regression equation was established to predict nitrogen fixation in a given soil ecosystem.     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.     

Urea hydrolysis in some tea soils

The influence of application of hexachlorocyclohexane (HCH=gamma BHC), to a submerged tropical field soil at rates equivalent to recommended field rates (1–2.5kg a.i./ha) and twice this level, upon the rhizosphere soil nitrogenase, nitrogen fixers, and soil redox potential (Eh) was investigated. The rhizosphere soil from HCH-treated field exhibited significantly higher nitrogenase activity than that from untreated fields. HCH retarded the drop in redox potential of the field soil upto 80 days after transplantation under submerged conditions. Populations of nitrogen-fixingAzospirillum sp. and Azotobacter, to a greater extent, and anaerobic organisms, to a lesser extent, were stimulated in HCH-treated soils. Results indicate the stimulation of heterotrophic nitrogen-fixing bacteria by HCH in submerged paddy soils.     

Crop residue application increases nitrogen fixation and dry matter production in groundnut (Arachis hypogaea L.) grown on an acid sandy soil in Niger,West Africa

Field experiments were conducted during the rainy reasons of 1989, 1990 and 1991 on an acid sandy soil in Niger, West Africa, to assess the effect of millet straw application (+CR) on growth and N₂ fixation of groundnut (Arachis hypogaea L.).Three years of +CR (4 t ha^–1 yr^–1) increased symbiotic N₂ fixation, total dry matter production (haulm plus pods) by 83% and total nitrogen (N) accumulation by 100%. Concentration of N in the shoot dry matter and total N in the soil were only slightly affected by the +CR treatment.Crop residue application increased the concentration of potassium (K) and molybdenum (Mo) and decreased the concentrations of aluminium (Al) and manganese (Mn) distinctly, both in the plant (shoot and nodule dry matter) and in the soil.The increase in dry matter production and N uptake was mainly due to improved N₂ fixation reflected by enhanced formation and growth of nodules as well as nitrogenase activity. This was attributed to improved chemical soil conditions, particularly to the higher availability of Mo and the lowered content of available Al and Mn.Although with the application of 4 t CR ha^–1, 60 kg K were supplied, increased growth could not be attributed to the additional supply of K.ICRISAT Journal Article No. 1229.ICRISAT Journal Article No. 1229.     

In situ acetylene-ethylene assay of biological nitrogen fixation in lowland rice soils

Summary An in situ device for assaying biological nitrogen fixation in flooded rice soils, using the acetylene reduction method, was developed. Diurnal variations in acetylene reduction by an inoculated field plot and by laboratory-grown cultures of nitrogen-fixing algae showed a prominent single-peak pattern of nitrogenase activity. The peak occurred at mid-day for laboratory-grown algae and at late afternoon for the algae grown in the field plot. Some nitrogenase activity was noted during the night. Acetylene reduction studies in rice fields of Albay province, Philippines, showed an estimated fixation of 18.5 to 33.3 kg N/ha each cropping season for the fields of Puro soil and 2.3 to 5.7 kg N/ha each cropping season for the fields of Santo Domingo soil. re]19751202     

Epiphytic Cyanobacteria on Chara vulgaris Are the Main Contributors to N2 Fixation in Rice Fields

The distribution of nitrogenase activity in the rice-soil system and the possible contribution of epiphytic cyanobacteria on rice plants and other macrophytes to this activity were studied in two locations in the rice fields of Valencia, Spain, in two consecutive crop seasons. The largest proportion of photodependent N₂ fixation was associated with the macrophyte Chara vulgaris in both years and at both locations. The nitrogen fixation rate associated with Chara always represented more than 45% of the global nitrogenase activity measured in the rice field. The estimated average N₂ fixation rate associated with Chara was 27.53 kg of N ha⁻¹ crop⁻¹. The mean estimated N₂ fixation rates for the other parts of the system for all sampling periods were as follows: soil, 4.07 kg of N ha⁻¹ crop⁻¹; submerged parts of rice plants, 3.93 kg of N ha⁻¹ crop⁻¹; and roots, 0.28 kg of N ha⁻¹ crop⁻¹. Micrographic studies revealed the presence of epiphytic cyanobacteria on the surface of Chara. Three-dimensional reconstructions by confocal scanning laser microscopy revealed no cyanobacterial cells inside the Chara structures. Quantification of epiphytic cyanobacteria by image analysis revealed that cyanobacteria were more abundant in nodes than in internodes (on average, cyanobacteria covered 8.4% ± 4.4% and 6.2% ± 5.0% of the surface area in the nodes and internodes, respectively). Epiphytic cyanobacteria were also quantified by using a fluorometer. This made it possible to discriminate which algal groups were the source of chlorophyll a. Chlorophyll a measurements confirmed that cyanobacteria were more abundant in nodes than in internodes (on average, the chlorophyll a concentrations were 17.2 ± 28.0 and 4.0 ± 3.8 μg mg [dry weight] of Chara⁻¹ in the nodes and internodes, respectively). These results indicate that this macrophyte, which is usually considered a weed in the context of rice cultivation, may help maintain soil N fertility in the rice field ecosystem.     

NONSYMBIOTIC AND SYMBIOTIC NITROGEN FIXATION IN A WEAKLY MINEROTROPHIC PEATLAND

The acetylene reduction assay was used to measure nonsymbiotic and symbiotic nitrogen fixation in a weakly minerotrophic peatland throughout the ice-free season. Nonsymbiotic nitrogen fixation was found in surface materials and subsurface peat. In surface materials, nitrogenase activity measured in the field contributed about 0.6 kg N ha^-1 yr^-1, was closely associated with Sphagnum, but was not correlated with temperature between 12 and 27 C. No cyanobacteria were found in association with Sphagnum. In subsurface peat, nitrogenase activity measured in situ contributed no more than 0.4 kg N ha^-1 yr^-1 and was closely correlated with temperature between 7 and 21 C. There were uncertainites in these measurements due to presence of ethylene oxidizing activity and a long time lag. Symbiotic nitrogen fixation was found only in actinomycete-induced root nodules of Myrica gale L. Legumes were absent and the few lichens present lacked nitrogenase activity. Based on acetylene reduction assays, Myrica gale fixed about 35 kg N ha^-1 yr^-1. Nitrogenase activity in Myrica gale showed a strong seasonal pattern which varied little during three consecutive years even though water levels varied substantially. Nitrogen input to the peatland from nonsymbiotic nitrogen fixation was only 15% the amount contributed by bulk precipitation. Symbiotic fixation, in contrast, contributed approximately six times the amount in bulk precipitation.     

Quantitative effects of soil nitrate,growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.)

The influence of soil nitrate availability, crop growth rate and phenology on the activity of symbiotic nitrogen fixation (SNF) during the growth cycle of pea (Pisum sativum cv. Baccara) was investigated in the field under adequate water availability, applying various levels of fertiliser N at the time of sowing. Nitrate availability in the ploughed layer of the soil was shown to inhibit both SNF initiation and activity. Contribution of SNF to total nitrogen uptake (%Ndfa) over the growth cycle could be predicted as a linear function of mineral N content of the ploughed layer at sowing. Nitrate inhibition of SNF was absolute when mineral N at sowing was over 380 kg N ha^–1. Symbiotic nitrogen fixation was not initiated unless nitrate availability in the soil dropped below 56 kg N ha^–1. However, SNF could no longer be initiated after the beginning of seed filling (BSF). Other linear relationships were established between instantaneous %Ndfa and instantaneous nitrate availability in the ploughed layer of the soil until BSF. Instantaneous %Ndfa decreased linearly with soil nitrate availability and was nil above 48 and 34 kg N ha^–1 for the vegetative and reproductive stages, respectively, levels after which no SNF occurred. Moreover, SNF rate was shown to be closely related to the crop growth rate until BSF. The ratio of SNF rate over crop growth rate decreased linearly with thermal time. Maximum SNF rate was about 40 mg N m^–2 degree-day^–1, equivalent to 7 kg N ha^–1, regardless of the N treatment. From BSF to the end of the growth cycle, the high N requirements of the crop were supported by both SNF and nitrate root absorption but, of the two sources, nitrate root absorption seemed to be less affected by the presence of reproductive organs. However, since soil nitrate availability was low at the end of the growth cycle, SNF was the main source of nitrogen acquisition. The onset of SNF decrease at the end of the growth cycle seemed to be first due to nodule age and then associated to the slowing of the crop growth rate.     

A comparison of dinitrogen fixation rates in wood litter decayed by white-rot and brown-rot fungi

Nitrogen fixation rates, as estimated by the acetylene reduction technique, were determined in conifer wood litter being decayed by brown- and white-rot fungi. Average ethylene production rates were significantly higher in white-rotted wood (15.1 nmol g^–1 day^–1) than in brown-rotted wood (2.3 nmol g^–1 day^–1). This difference may be related to a higher soluble sugar content in white-versus brown-rotted wood. The nitrogen-fixing bacteriumAzospirillum was not detected in any of the decaying wood samples examined. Greater nitrogen additions from nitrogen-fixing bacteria may be a factor in the more rapid white-rot decay of hardwood litter, as compared to the slower brown-rot decay of conifer wood.     

Response of field-grown chickpea (Cicer arietinum L.) to phosphorus fertilization for yield and nitrogen fixation

Application of phosphorus at 40, 60, 80 and 100 kg P₂O₅ ha^–1 in the presence of a uniform dressing of nitrogen (N) and potash (K₂O) each applied at 20 and 24 kg ha^–1 to chickpea (CM-88) grown in sandy loam soil in a replicated field experiment improved the nodulation response of the crop, increased its grain yield (ka ha^–1) by 18, 59, 40 and 14 percent, biomass yield (ka ha^–1) by 32, 32, 54 and 14 percent, biomass N (kg ha^–1) by 31, 48, 49, 19 percent, and biomass P (kg ha^–1) by 26, 40, 41 and 11 percent, respectively. The effect of phosphorus on the nitrogenase activity of the excised roots of chickpea was, however, inconsistent.     

13C isotope discrimination correlates with biological nitrogen fixation in soybean (Glycine max (L.) Merrill)

Ammonium sulphate was applied at the rate of 300 kg N ha^–1 with or without the nitrification inhibitor 1-carbamoyl-3(5)-methylpyrazol (4 kg ha^–1) to plots measuring 1.5 × 1.5 m. The fertilizer and the inhibitor were washed into the top 15-cm layer of the soil, which was highly calcareous (55% CaCO₃), and the plots were kept bare. The process of nitrification was monitored by regular soil sampling. In the absence of the inhibitor, nitrification was completed in three weeks. In the presence of the inhibitor only 10% of applied N was nitrified by the end of the third week and 42% by the end of the eighth week. Average soil temperature at 5–, 10– and 20-cm depth over the first six weeks was 26.0, 24.8 and 24.2°C, respectively.