Estimation of symbiotically fixed nitrogen in field grown soybeans: An application of natural15N/14N abundance and a low level15N-tracer technique

Summary Plants from agricultural and natural upland ecosystem were investigated for¹⁵N content to evaluate the role of symbiotic N₂-fixation in the nitrogen nutrition of soybean. Increased yields and lower δ¹⁵N values of nodulating soybeansvs, non-nodulating isolines gave semi-quantitative estimates of N₂ fixation. A fairly large discrepancy was found between estimations by δ¹⁵N and by N yield at 0 kg N/ha of fertilizer. More precise estimates were made by following changes in plant δ¹⁵N when fertilizer δ¹⁵N was varied near¹⁵N natural abundance level. Clearcut linear relationships between δ¹⁵N values of whole plants and of fertilizer were obtained at 30 kg N/ha of fertilizer for three kinds of soils. In experimental field plots, nodulating soybeans obtained 13±1% of their nitrogen from fertilizer, 66±8% from N₂ fixation and 21±10% from soil nitrogen in Andosol brown soil; 30%, 16% and 54% in Andosol black soil; 7%, 77% and 16% in Alluvial soil, respectively. These values for N₂ fixation coincided with each corresponding estimation by N yield method. Other results include: 1)¹⁵N content in upland soils and plants was variable, and may reflect differences in the mode of mineralization of soil organics, and 2) nitrogen isotopic discrimination during fertilizer uptake (δ¹⁵N of plant minus fertilizer) ranged from −2.2 to +4.9‰ at 0–30 kg N/ha of fertilizer, depending on soil type and plant species. The proposed method can accurately and relatively simply establish the importance of symbiotic nitrogen fixation for soybeans growing in agricultural settings.     

15N-Determined effect of inoculation with N2 fixing bacteria on nitrogen assimilation in Western Canadian wheats

Summary A two-year field study was undertaken using¹⁵N isotope techniques to differentiate between stimulation of N uptake and N₂ fixation in Western Canadian cultivars of spring wheat (Triticum aestivum L. emend Thell) and durum (T. turgidum L. emend Bowden) in response to inoculation with N₂-fixing bacteria. Bacterial inoculation either had no effect or lowered the % N derived from the fertilizer and the fertilizer use efficiency. Despite the depression of fertilizer uptake, inoculants did not alter the relative uptake from soil and fertilizer-N pools indicating that bacterial inoculation did not alter rooting patterns. Nitrogen-15 isotope dilution indicated that N₂ fixation did occur. In 1984, % plant N derived from the atmosphere (% Ndfa) due to inoculation with Bacillus C-11-25 averaged 23.9% while that withAzospirillum brasilense ATCC 29729 (Cd) averaged 15.5%. In 1985, higher soil N levels reduced these values by approximately one-half. Cultivar x inoculant interactions, while significant, were not consistent across years. However, these interactions did not affect cultivars ‘Cadet’ and ‘Rescue’. In agreement with previous results, ‘Cadet’ performed well with all inoculants in both years while ‘Rescue’ performed poorly. Among 1984 treatments, the N increament in inoculated plants was positively correlated with % Ndfa but no such correlation existed in 1985. N₂ fixation averaged over all cultivars and strains was 17.9 and 6.7 kg N fixed ha⁻¹ in 1984 and 1985, respectively. Highest rates of N₂ fixation were estimated at 52.4 kg N ha⁻¹ for ‘Cadet’ in 1984 and 31.3 kg N ha⁻¹ for ‘Owens’ in 1985, both inoculated with Bacillus C-11-25, an isolate from southern Alberta soils. Inoculation with either ofAzospirillum brasilense strain Cd (ATCC29729) or 245 did not result in as consistent or as high N₂ fixation, suggesting that these wheats had not evolved genetic compatability with this exogenous microorganism. These agronomically significant amounts of N₂ fixation occurred under optimally controlled experimental conditions in the field. It is yet to be determined if N₂ fixation would occur in response to bacterial inoculation under dryland conditions commonly occurring in Western Canada. Contribution from Agriculture Canada Research Station, Lethbridge, Alberta, Canada.     

The course of 15N-ammonium nitrate in a spring barley cropping system

¹⁵N-labelled ammonium nitrate was applied to spring barley growing on a Cambisol soil in western Switzerland. Immobilization, plant uptake and disappearance of inorganic nitrogen were followed at frequent intervals. Fertilizer nitrogen disappeared shortly after its application, mainly through immobilization by soil microorganisms and absorption by the crop. Some of the added nitrogen was probably denitrified as a result of humid conditions during the first days after fertilizer application. At the end of the growing season, 31% of the added nitrogen was recovered from the aerial barley plants, and 56% was immobilized by microorganisms. Most of the fertilizer nitrogen not used by the crop was immobilized in the upper 0–30 cm soil layer. This prevented downward movement of nitrate and limited nitrogen losses. Fertilizer efficiency was mainly determined by the competition between crop uptake and microbial immobilization. Careful consideration of the time of fertilization, taking into account plant growth and weather conditions, can result in an increase in fertilizer efficiency and minimal pollution.     

Fertilizer vs. organic matter contributions to nitrogen leaching in cropping systems of the Pampas: 15N application in field lysimeters

Nitrogen (N) export from soils to streams and groundwater under the intensifying cropping schemes of the Pampas is modest compared to intensively cultivated basins of Europe and North America; however, a slow N enrichment of water resources has been suggested. We (1) analyzed the fate of fertilizer N and (2) evaluated the contribution of fertilizer and soil organic matter (SOM) to N leaching under the typical cropping conditions of the Pampas. Fertilizer N was applied as ¹⁵N-labeled ammonium sulfate to corn (in a corn/soybean rotation) sown under zero tillage in filled-in lysimeters containing two soils of different texture representative of the Pampean region (52 and 78 kg N ha^-1, added to the silt loam and sandy loam soil, respectively). Total fertilizer recovery at corn harvest averaged 84 and 64% for the silt loam and sandy loam lysimeters, respectively. Most fertilizer N was removed with plant biomass (39%) or remained immobilized in the soil (29 and 15%, for the silt loam and sandy loam soil, respectively) whereas its loss through drainage was negligible (<0.01%). We presume that the unaccounted fertilizer N losses were related to volatilization and denitrification. Throughout the corn growing season, subsequent fallow and soybean crop, which took place during an exceptionally dry period, the fertilizer N immobilized in the organic pool remained stable, and N leaching was scarce (7.5 kg N ha^-1), similar at both soils, and had a low contribution of fertilizer N (0–3.5%), implying that >96% of the leached N was derived from SOM mineralization. The inherent high SOM of Pampean soils and the favorable climatic conditions are likely to propitiate year-round production of nitrate, favoring its participation in crop nutrition and leaching. The presence of ¹⁵N in drainage water, however, suggests that fertilizer N leaching could become significant in situations with higher fertilization rates or more rainy seasons.     

Crop uptake and leaching losses of15N labelled fertilizer nitrogen in relation to waterlogging of clay and sandy loam soils

Summary Ammonium nitrate fertilizer, labelled with¹⁵N, was applied in spring to winter wheat growing in undisturbed monoliths of clay and sandy loam soil in lysimeters; the rates of application were respectively 95 and 102 kg N ha⁻¹ in the spring of 1976 and 1975. Crops of winter wheat, oilseed rape, peas and barley grown in the following 5 or 6 years were treated with unlabelled nitrogen fertilizer at rates recommended for maximum yields. During each year of the experiments the lysimeters were divided into treatments which were either freelydrained or subjected to periods of waterlogging. Another labelled nitrogen application was made in 1980 to a separate group of lysimeters with a clay soil and a winter wheat crop to study further the uptake of nitrogen fertilizer in relation to waterlogging. In the first growing season, shoots of the winter wheat at harvest contained 46 and 58% of the fertilizer nitrogen applied to the clay and sandy loam soils respectively. In the following year the crops contained a further 1–2% of the labelled fertilizer, and after 5 and 6 years the total recoveries of labelled fertilizer in the crops were 49 and 62% on the clay and sandy loam soils respectively. In the first winter after the labelled fertilizer was applied, less than 1% of the fertilizer was lost in the drainage water, and only about 2% of the total nitrogen (mainly nitrate) in the drainage water from both soils was derived from the fertilizer. Maximum annual loss occurred the following year but the proportion of tracer nitrogen in drainage was nevertheless smaller. Leaching losses over the 5 and 6 years from the clay and sandy loam soil were respectively 1.3 and 3.9% of the original application. On both soils the percentage of labelled nitrogen to the total crop nitrogen content was greater after a period of winter waterlogging than for freely-drained treatments. This was most marked on the clay soil; evidence points to winter waterlogging promoting denitrification and the consequent loss of soil nitrogen making the crop more dependent on spring fertilizer applications.     

Symbiotic N2 fixation in pea and field bean estimated by15N fertilizer dilution in field experiments with barley as a reference crop

Summary The total amount of nitrogen derived from symbiotic nitrogen fixation in two pea and one field bean cultivar, supplied with 50 kg N ha⁻¹ at sowing (‘starter’-N), was estimated to 165, 136, and 186 kg N ha⁻¹, respectively (three-year means). However, estimates varied considerably between the three years. At the full bloom/flat pod growth stage from 30 to 59 per cent of total N₂ fixation had taken place. The proportion of total N derived from N₂ fixation at maturity was higher in seeds than in vegetative plant parts and amounted to 59.5, 51.3 and 66.3 per cent of total above-ground plant N in the two pea cultivars and field bean, respectively (three-year means). The recovery of fertilizer N was 62.2, 70.2, 52.1, and 69.5 per cent in the two pea cultivars, field bean and barley, respectively. Growth analysis indicated that barley did not meet the claims for an ideal reference crop in the¹⁵N fertilizer dilution technique for estimating N₂ fixation in pea and field bean. ‘Starter’-N neither increased the seed yield nor the N content of the grain legumes.     

Evaluation of N2-fixation and nitrogen economy of a maize/cowpea intercrop system using15N dilution methods

Yields of above ground biomass and total N were determined in summer-grown maize and cowpea as sole crops or intercrops, with or without supplementary N fertilizer (25 kg N ha⁻¹, urea) at an irrigated site in Waroona, Western Australia over the period 1982–1985. Good agreement was obtained between estimates of N₂ fixation of sole or intercrop cowpea (1984/85 season) based on the¹⁵N natural abundance and¹⁵N fertilizer dilution techniques, both in the field and in a glasshouse pot study. Field-grown cowpea was estimated to have received 53–69% of its N supply from N₂-fixation, with N₂-fixation onlyslightly affected by intercropping or N fertilizer application. Proportional reliance on N₂-fixation of cowpea in glasshouse culture was lower (36–66%) than in the field study and more affected by applied N. Budgets for N were drawn up for the field intercrops, based on above-ground seed yields, return of crop residues, inputs of fixed N and fertilizer N. No account was taken of possible losses of N through volatilization, denitrification and leaching or gains of N in the soil from root biomass. N₂-fixation was estimated tobe 59 kg N ha⁻¹ in the plots receiving no fertilizer N, and 73 kg N ha⁻¹ in plots receiving 25 kg N ha⁻¹ as urea. Comparable fixation by sole cowpea was higher (87 and 82 kg N ha⁻¹ respectively) but this advantage was outweighed by greater land use efficiency by the intercrop than sole crops.     

The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. ¹⁵N-labelled urea, ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (80 kg N ha^-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0–26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop ¹⁵N uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha^-1 day^-1 at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. ¹⁵N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH₄ ⁺ and NO₃ ^- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.     

Fertilizer and soil nitrogen accumulation by irrigated barley grown on a red-brown earth in south-eastern Australia

¹⁵N labelled (NH₄)₂SO₄ was applied to barley at 5 g N m⁻² (50 kg N ha⁻¹) in microplots at sowing to study the timing of the N losses and the contribution of soil and fertilizer N to the plant. Water treatments included rainfed and irrigation at 45–50 mm deficit beginning in the spring. Recovery of¹⁵N in the plant increased to a maximum of about 20% within 91 days after sowing (DAS 91) and then remained constant. Approximately 16% (0.8 g N m⁻²) of the fertilizer was in the stem and leaves at DAS 91 and this N was subsequently redistributed to the head. At maturity, approximately 75% of the¹⁵N assimilated by the tops was recovered in the grain. Soil N contributed 3.6 g N m⁻² to the head; 2.2 g N m⁻² was remobilized from the stem and leaves, and the balance, approximately 1.4 g N m⁻², was taken up from the soil between DAS 69 to 91. Effects of irrigation treatments on N accumulation were not significant. Residual¹⁵N fertilizer in the soil decreased with time from sowing, and at maturity 40% of the applied N was recovered in the surface 0.15 m.¹⁵N movement to depth was limited and less than 5% of the fertilizer was recovered below 0.15 m. Irrigation had no effect on the¹⁵N recovery at depth. Total recovery of the¹⁵N varied between 60 and 67% and implies that 33–40% was lost from the soil-plant system. The total recovery in the soil and plant was not affected by time or irrigation in the interval DAS 39 to 134. Losses occurred before DAS 39 when crop uptake of N was small and soil mineral N content was high. There was an apparent loss of 1.9 g fertilizer N m⁻² (i.e. 38% of that applied) between DAS 1 and 15. This loss occurred before crop emergence when rainfall provided conditions suitable for denitrification.     

Uptake of carbon and nitrogen derived from carbon-13 and nitrogen-15 dual-labeled maize residue compost applied to radish, komatsuna, and chingensai for three consecutive croppings

A pot experiment was conducted to determine the effects of the application of ¹³C (1.256 atom%) and ¹⁵N (1.098 atom%) dual-labeled maize residue compost (MRC) on the nitrogen and carbon uptake by radish, komatsuna, and chingensai as compared with the effect of inorganic fertilizer (IF). The vegetables were grown over three consecutive growing seasons over 4 months; compost was applied at the rate of 24 g kg^–1 soil. Nonlabeled nitrogen fertilizer was applied to the compost treatments in the second and third crops to compare the effects of blends of compost with N fertilizer to fertilizer alone. The N uptake and yield of vegetables were significantly higher with the recommended inorganic N treatment. The vegetables took up significantly (P < 0.05) lower amounts of N from MRC than from IFs during the three cultivations. The values of the N uptake derived by fertilizer application to the plant exhibited significant differences among different vegetables. Nitrogen recovered by komatsuna and chingensai from MRC was 7.3 (6.6%), 2.7 (1.8%), and 2.3, (1.7%) in the first, second, and third crops, respectively. Radish, komatsuna, and chingensai recovered significant amounts of C from MRC in the first and second crops, with negligible C recovery in the third crop. The initial loss of fertilizer C in soil at the first crop indicates that the microbial decomposition decoupled substantial amounts of ¹³C/¹⁵N-labeled compounds early in plant development, thus giving the microorganisms a preemptive competitive advantage in the acquisition of easily available ¹³C/¹⁵N-labeled substrates. It is concluded that a combination of compost and inorganic N did not supply sufficient plant-available N to increase vegetables yields or N uptake over those of fertilizer alone. The data suggested that higher productivity of vegetables might be achieved after the accumulation of a certain amount of residual compost N.     

Short-range spatial variability of soil δ15N natural abundance – effects on symbiotic N2-fixation estimates in pea

The δ¹⁵N natural abundance (‰) of the total soil N pool varies at the landscape level, but knowledge on short-range variability and consequences for the reliability of isotopic methods are poorly understood. The short-range spatial variability of soil δ¹⁵N natural abundance as revealed by the ¹⁵N abundance in spring barley and N₂-fixing pea was measured within the 0.15–4 m scale at flowering and at maturity. The short-range spatial variability of soil δ¹⁵N natural abundance and symbiotic nitrogen fixation were high at both growth stages. Along a 4-m row, the δ¹⁵N natural abundance in barley reference plants varied up to 3.9‰, and sometimes this variability was observed even between plants grown only 30 cm apart. The δ¹⁵N natural abundance in pea varied up to 1.4‰ within the 4-m row. The estimated percentage of nitrogen derived from the atmosphere (%Ndfa) varied from 73–89% at flowering and from 57–95% at maturity. When increasing the sampling area from 0.01 m² (single plants) and up to 0.6 m² (14 plants) the %Ndfa coefficient of variation (CV) declined from 5 to 2% at flowering and from 12 to 2% at maturity. The implications of the short-range variability in δ¹⁵N natural-abundance are that estimates of symbiotic N₂-fixation can be obtained from the natural abundance method if at least half a square meter of crop and reference plants is sampled for the isotopic analysis. In fields with small amounts of representative reference crops (weeds) it might be necessary to sow in reference crop species to secure satisfying N₂-fixation estimates.     

Evaluating the effects of gross nitrogen mineralization,immobilization, and nitrification on nitrogen fertilizer availability in soil experimentally contaminated with diesel

Sandy clay loam soil was contaminated with 5000 mg kg⁻¹ diesel, and amended with nitrogen (15.98 atom% ¹⁵N) at 0, 250, 500, and 1000 mg kg⁻¹ to determine gross rates of nitrogen transformations during diesel biodegradation at varying soil water potentials. The observed water potential values were −0.20, −0.47, −0.85, and −1.50 MPa in the 0, 250, 500, and 1000 mg kg⁻¹ nitrogen treatments respectively. Highest microbial respiration occurred in the lowest nitrogen treatment suggesting an inhibitory osmotic effect from higher rates of nitrogen application. Microbial respiration rates of 185, 169, 131, and 116 mg O₂ kg⁻¹ soil day⁻¹ were observed in the 250, 500, control and 1000 mg kg⁻¹ nitrogen treatments, respectively. Gross nitrification was inversely related to water potential with rates of 0.2, 0.04, and 0.004 mg N kg⁻¹ soil day⁻¹ in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. Reduction in water potential did not inhibit gross nitrogen immobilization or mineralization, with respective immobilization rates of 2.2, 1.8, and 1.8 mg N kg⁻¹ soil day⁻¹, and mineralization rates of 0.5, 0.3, and 0.3 mg N kg⁻¹ soil day⁻¹ in the 1000, 500, and 250 mg kg⁻¹ nitrogen treatments, respectively. Based on nitrogen transformation rates, the duration of fertilizer contribution to the inorganic nitrogen pool was estimated at 0.9, 1.9, and 3.2 years in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. The estimation was conservative as ammonium fixation, gross nitrogen immobilization, and nitrification were considered losses of fertilizer with only gross mineralization of organic nitrogen contributing to the most active portion of the nitrogen pool.     

Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l^-1 CO₂) and temperature increase (+3 °C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser supplies (160 and 530 kg N ha^-1 y^-1) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water. During the growing season, swards from the control climate (350 μl l^-1 [CO₂] at outdoor air temperature) were pulse labelled by the addition of ¹³CO₂. The elevated [CO₂] treatments were continuously labelled by the addition of fossil-fuel derived CO₂ (^{13 C} of -40 to -50 ‰). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the soil macro-organic matter (‘old’ C) was at −32‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was at −14 and −54‰ in the ambient and elevated [CO₂] treatments, respectively. During the experiment, the ¹³C isotopic mass balance method was used to calculate, for the top soil (0–15 cm), the carbon turnover in the stubble and roots and in the soil macro-organic matter above 200 μ (MOM). Elevated [CO₂] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N−. At the high N supply, the mean replacement time of ‘old’ C by ‘new’ C declined in elevated, compared to ambient [CO₂], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates of ‘new’ C accumulation and of ‘old’ C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation of ‘new’ C, the soil C pools did not turnover faster in elevated [CO₂], as the rate of ‘old’ C decomposition was reduced. A 3 °C temperature increase in elevated [CO₂] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the ‘old’ C decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble and in the MOM, in ambient but not in elevated [CO₂]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic N availability and of a possible priming effect between C-substrates are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

Labelled urea or ammonium nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha^–1 at tillering (early March) and 110 kg N ha^–1 at the beginning of stem elongation (mid-April). The kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occured at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha¹) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha¹). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net¹⁵N remineralization was found during the growth cycle.The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.     

Adaptation of methods for evaluating N2 fixation in food legumes and legume cover crops

Methods for partitioning the nitrogen assimilated by nodulated legumes, between nitrogen derived from soil sources and from N₂ fixation, are described as applied in peninsular Malaysia. The analysis of nitrogenous components translocated from the roots to the shoots of nodulated plants in the xylem sap is outlined, with some precautions to be observed for applications in the tropics. Some examples of the use of the technique in surverying apparent N₂ fixation by tropical legumes, in studying interrow cropping in plantation systems and in assessing effects of experimental treatments on N₂ fixation by food legumes, are described. Techniques for assesing N₂ fixation by means of¹⁵N abundance have been used to show that applications of nitrogenous fertilizers commonly used in Malaysia for soybeans depress N₂ fixation, that similar results are obtained with natural abundance and¹⁵N-enrichment methods and that, in at least two locations in Malaysia, differences between the natural abundance of¹⁵N in plant-available soil nitrogen and in atmospheric N₂ are great enough to permit application to measurement of N₂ fixation by leguminous crops.     

Isotopic fractionation accompanying fertilizer nitrogen transformations in soil and trees of a Scots spine ecosystem

Foliage from a mature stand of Scots pine (Pinus sylvestris L.) receiving increasing doses of ammonium nitrate and urea nitrogen was assayed during the five subsequent growing seasons for total N concentration and ¹⁵N abundance. The aim of the study was to examine the potential of the ¹⁵N technique to provide estimates on fertilizer N recovery and its fate in the ecosystem. The ¹⁵N abundance in the foliage increased in proportion to the dose of fertilizer application. This was generally owing to the fact that the ¹⁵N of the fertilizer N was significantly higher than that in the soil inorganic-N pool, as well as in the needle biomass of the Scots pine trees on the nonfertilized plots. Due to ¹⁵N isotope discrimination occurring during N transformations in soil the relationship was however not very close. Calculations based on the principle of isotope dilution yielded only rough and, in some cases, even misleading estimates of the fraction of the fertilizer-derived nitrogen (N_dff) in the needles. This was especially the case for the urea-N, which undergoes significant isotopic fractionation during the process of ammonia volatilization and possibly microbial NH₄ ⁺ assimilation in soil. Over five growing seasons, foliar total N concentration peaked at the end of the second season while the ¹⁵N abundance continued to increase. Although large methodological errors may be involved when interpreting natural ¹⁵N abundance, the measurement of ¹⁵N seems to provide semi-quantitative information about fertilizer N accumulation and transformation processes in coniferous ecosystems. A better understanding of the tree and soil processes causing isotopic fractionation is a prerequisite for correct interpretation of ¹⁵N data.     

Comparative effects of organic and inorganic nitrogen sources applied to a flooded soil on rice yield and availability of N

A pot experiment was conducted to study the effect of organic and inorganic nitrogen (N) sources on the yield and N uptake of rice from applied and native soil-N. The residual effect of these N sources on a succeeding wheat crop was also studied. Organic N was applied in the form of ¹⁵N-labelled Sesbania aculeata L., a legume, and inorganic N in the form of ¹⁵N-labelled ammonium sulphate. The two sources were applied to the soil separately or together at the time of transplanting rice. Recovery of N by rice from both the applied sources was quite low but both sources caused significant increases in biomass and N yield of rice. Maximum increase was recorded in soil treated with organic N. The residual value of the two materials as source of N for wheat was not significant; the wheat took up only a small fraction of the N initially applied. Loss of N occurred from both applied N sources, the losses being more from inorganic N. Both applied N sources caused a substantial increase in the availability of soil-N to rice and wheat; most of this increase was due to organic N and was attributed to the so-called ‘priming’ effect or ANI (added nitrogen interaction) of the applied material.     

Measurement of N2-fixation in field-grown pigeonpea [Cajanus cajan (L.) Millsp.] using15N-labelled fertilizer

Two experiments were carried out from 1981 to 1983 in Vertisol field at ICRISAT Center, Patancheru, India to measure N₂-fixation of pigeonpea [Cajanus cajan (L.) Millsp.] using the¹⁵N isotope dilution technique. One experiment examined the effect of control of a nodule-eating insect on fixation while another in vestigated the effect of intercroping with cereals on fixation and the residual effect of pigeonpea on a succeeding cereal crop. Although both experiments indicated that at least 88% of the N in pigeonpea was fixed from the atmosphere, one result is considered fortuitous in view of the differential rates of growth of the legume and the control, sorghum [Sorghum bicolor (L.) Moench]. The difference method of calculation in dieated negative fixation and the results emphasized the problem of finding a suitable nonfixing control. In a second experiment, when all plants were confined to a known volume of soil to which¹⁵N fertilizer was added in the field, these problems were overcome, and isotope dilution and difference methods gave similar results of N₂-fixation of about 90%. In intercropped pigeonpea 96% of the total N was derived from the atmosphere. This estimate might be an artifact. There was no evidence of benefit from N fixed by pigeonpea to intercropped sorghum plants. Plant tissue¹⁵N enrichments of cereal crops grown after pigeonpea indicated that the cereal derived some N fixed by the previous pigeonpea. Thus residual benefits to cereals are not only an effect of ‘sparing’ of soil N.     

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Nitrogen cycling in leucaena (Leucaena leucocephala) alley cropping in semi-arid tropics

In an alley cropping system, prunings from the hedgerow legume are expected to supply nitrogen (N) to the associated cereal. However, this may not be sufficient to achieve maximum crop yield. Three field experiments with alley-cropped maize were conducted in a semi-arid environment in northern Australia to determine: (1) the effect of N fertilizer on maize growth in the presence of fresh leucaena prunings; (2) the effect of incorporation of leucaena and maize residues on maize yield and the fate of plant residue¹⁵N in the alley cropping system; and (3) the¹⁵N recovery by maize from¹⁵N-labelled leucaena, maize residues and ammonium sulphate fertilizer.Leucaena residues increased maize crop yield and N uptake although they did not entirely satisfy the N requirement of the alley crop. Additional N fertilizer further increased the maize yield and N uptake in the presence of leucaena residues. Placement of leucaena residues had little effect on the availability of N to maize plants over a 2 month period. The incorporation of leucaena residues in the soil did not increase the recovery of leucaena¹⁵N by maize compared with placement of the residues on the soil surface. After 2 months, similar proportions of the residue¹⁵N were recovered by maize from mulched leucaena (6.3%), incorporated leucaena (6.1%) and incorporated maize (7.6%). By the end of one cropping season (3 months after application) about 9% of the added¹⁵N was taken up by maize from either¹⁵N-labelled leucaena as mulch or¹⁵N-labelled maize residues applied together with unlabelled fresh leucaena prunings as mulch. The recovery of the added¹⁵N was much higher (42.7%) from the¹⁵N-labelled ammonium sulphate fertilizer at 40 kg N ha^-1 in the presence of unlabelled leucaena prunings. Most of the added¹⁵N recovered in the 200 cm soil profile was distributed in the top 25 cm soil with little leached below that. About 27–41% of the leucaena¹⁵N was apparently lost, largely through denitrification from the soil and plant system, in one cropping season. This compared with 35% of the fertilizer¹⁵N lost when the N fertilizer was applied in the presence of prunings. ei]H Lambers