Assessment of biological nitrogen fixation

The four commonly used methods for measuring biological nitrogen fixation (BNF) in plants are: the total nitrogen difference (TND) method, acetylene reduction assay (ARA) technique, xylem-solute (or ureide production) method and the use of¹⁵N labelled compounds.The TND method relies on a control non-N₂-fixing plant to estimate the amount of N absorbed by the fixing plant from soil. It is one of the simplest and least expensive methods, but works best under low soil N conditions. The ARA technique measures the rate of acetylene conversion to ethylene by the N₂-fixing enzyme, nitrogenase. The ethylene produced can then be converted into N₂ fixed, using a conversion ratio, originally recommended as 3. Although the method is inexpensive and highly sensitive, its major disadvantages are, the short-term nature of the assays, the doubtful validity of always using a conversion ratio of 3 and the auto-inhibition of acetylene conversion to ethylene. The ARA technique is therefore not a method of choice for measuring BNF.The xylem-solute technique can be used to measure BNF for those species that produce significant quantities of ureide as product of BNF. Although simple and relatively inexpensive, it is an instantaneous assay and also needs to be calibrated against a known method. The most serious limitation is, that only a small proportion of N₂-fixing plants examined are ureide exporters, and the method is therefore not widely applicable.The¹⁵N methods, classified into the isotope dilution and A-value methods, appear to be the most accurate, but also the most expensive. They involve labelling soil with¹⁵N fertilizer and using a non-N₂-fixing reference plant to measure the¹⁵N/¹⁴N ratio in the soil. The¹⁵N isotope dilution approach is both operationally and mathematically simpler than the A-value approach. To limit potential errors in the selection of reference crops, it is recommended to use¹⁵N labelled compounds or soil labelling methods that result in the slow release of¹⁵N or the slow decline of¹⁵N/¹⁴N ratio in the soil. Additionally, the use of several reference plants rather than a single one can improve the accuracy of the results.     

15N isotope dilution techniques to study soil nitrogen transformations and plant uptake

The use of¹⁵N as a tracer in soil/plant research is examined. The limitations of the so-called Ndff approach are discussed to show the need to consider not just the fate of the added label but also the path that was followed and the rate of the transformation. The development of¹⁵N isotope dilution techniques to determine gross rates of nitrogen transformation in soil is reviewed with some indications as to the further development of the approach.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. I. The Philippines

Urea is the main form of fertilizer nitrogen applied to wetland rice. As part of an effort to evaluate the efficiency of nitrogen fertilizers, conventional urea and modified urea products such as sulfur-coated urea (SCU), urea supergranules (USG), and sulfur-coated urea supergranules (SCUSG) were compared with ammonium sulfate on an Aquic Tropudalf at the experimental farm of the International Rice Research Institute (IRRI) in the Philippines. The sulfur-coated materials were prepared in the laboratory and were not completely representative of commercial SCU. Two experiments were conducted in the wet season (1978, 1979) and one in the dry season (1979). All fertilizers were labeled with 5% or 10% excess¹⁵N so that the fertilizer-N balance at two or three sampling times during the growing season could be constructed and the magnitude of N loss assessed. The SCU, USG, and SCUSG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation; only the initial dose was¹⁵N-labeled.Deep-point placement (10 cm) of urea supergranules (USG) between the rice hills consistently provided the highest plant recovery of¹⁵N in all experiments and at all harvest times; recoveries ranged from 48% to 75% with an average of approximately 58% at maturity. Among the fertilizers broadcast and incorporated before transplanting, average plant recoveries of¹⁵N were only approximately 34% and 26% from urea and ammonium sulfate, respectively. Plant recovery of¹⁵N from the broadcast and incorporated SCU (37%) was far inferior to that from USG. Sulfur coating of supergranules did not improve plant recovery over USG alone although sulfur coating delayed the plant uptake of¹⁵N from the USG.The¹⁵N not accounted for in the plant and soil was presumed lost. Loss of N from urea and ammonium sulfate was high (63%) in the dry season. Coating with sulfur gave a slight improvement, and deep placement of USG and SCUSG greatly reduced the losses. Losses of N were substantially lower in the wet season than in the dry season for broadcast and incorporated urea, SCU, and ammonium sulfate (9%–30%), whereas losses from deep-placed urea remained more or less the same as in the dry season. Net immobilization of¹⁵N from the broadcast fertilizers in the wet season ranged from 49% to 53% in the first experiment and from 16% to 32% in the second experiment, presumably because of aquatic weeds and green algae; immobilization was proportionally less at higher rates of fertilizer application. Deep placement reduced the extent of¹⁵N immobilization in the soil plus roots to less than 21% in all experiments.     

Nitrogen fertilization of dry-seeded rice in south-east Australia

Dry-seeded rice is often fertilized by broadcasting prilled urea on the soil surface shortly before permanent flood commences. Although this method is more efficient than nitrogen application at other stages, nitrogen loss is still substantial (up to 40%). Therefore we compared the efficiency of broadcast urea prills with various forms of urea banded below the soil surface shortly before permanent flood.Banding urea prills increased plant recovery of applied¹⁵N from 37% (broadcast) to 46% (banded), while recovery with point-placed urea supergranules was even higher (49%). Plant recovery from banded urea treated with DCD (dicyandiamide) or PPD (phenylphosphorodiamidate) was not higher than from banded urea prills. Loss of applied¹⁵N averaged 25%, with no significant differences between treatments. The increased plant nitrogen recovery with banding was associated with reduced immobilization of¹⁵N in the soil, rather than reduced losses. The increase in plant nitrogen recovery was relatively small compared with total nitrogen uptake by the plants, and consequently there was no significant improvement in yield or agronomic efficiency.There are practical difficulties associated with banding before permanent flood. When the soil is too wet, unacceptable amounts of soil disturbance and plant damage occur. On the other hand, when the soil is dry and cracked, the depth of banding is no greater than the depth to which broadcast urea prills are transported by the irrigation water at the commencement of permanent flood.     

Balance sheet of15N labelled urea applied to rice in three Australian vertisols differing in soil organic carbon

A glasshouse experiment was conducted to study the balance sheet of¹⁵N labelled urea at three rates (zero, 31.48 and 62.97 mmol N pot^–1) applied to rice under flooded conditions with two moisture regimes (continuous and alternate flooding) using three Australian vertisols differing in organic carbon level. Walkley-Black organic carbon values for the three soils were 0.65, 2.13 and 3.76 for the low carbon (LC), medium carbon (MC) and high carbon (HC) soils respectively.Rice dry weight and nitrogen uptake was significantly affected by N fertilizer rates, water regimes and soils. Alternate flooding gave much lower dry weight and nitrogen uptake than continuous flooding and the LC soil gave lower dry weight and nitrogen uptake than for the MC and HC soils.Recovery of¹⁵N labelled urea fertilizer in the rice plant was low (15.4 to 38.4%) and the¹⁵N urea not accounted for in the plant or soil and presumed lost was high (36.2 to 76.0%). Recovery was lower and loss higher under alternate flooding and for the LC soil. There was no effect of fertilizer rate. The results obtained stress the need for careful management to reduce losses of nitrogen fertilizer, particularly for soils low in organic carbon.     

Determination of dinitrogen emission and retention in floodwater and porewater of a lowland rice field fertilized with15N-urea

This paper reports a study on the distribution of dinitrogen between the atmosphere, floodwater and porewater of the soil in a flooded rice field after addition of¹⁵N-labelled urea into the floodwater.Microplots (0.086 m²) were established in a rice field near Griffith, N.S.W., and labelled urea (80 kg N ha^–1 containing 79.25 atoms %¹⁵N) was added to the floodwater when the rice was at the panicle initiation stage. Emission of nitrous oxide and dinitrogen was measured directly during the day and overnight, using a cover collection method and gas chromatographic and mass spectrometric analytical methods. Ammonia volatilization was calculated with a bulk aerodynamic method from measurements of wind speed and floodwater pH, temperature and ammoniacal nitrogen concentration. Seven days after urea application the¹⁵N₂ content of the floodwater and soil porewater was determined and total fertilizer nitrogen loss was calculated from an isotopic balance.Throughout the experimental period gas fluxes were low; nitrous oxide, ammonia and dinitrogen flux densities were less than 5, 170 and 720 g N ha^–1 d^–1, respectively. The greatest dinitrogen flux density was observed two days after urea addition and this declined to ~ 100 g ha^–1 d^–1 after seven days.The data indicate that, of the urea nitrogen added, 0.02% was lost to the atmosphere as nitrous oxide, 0.9% was lost by ammonia volatilization, and 3.6% was lost as dinitrogen gas during the 7 days of measurement. At the end of this period 0.028% and 0.002% of the added nitrogen was retained as dinitrogen gas in the floodwater and soil porewater respectively. Recovery of the¹⁵N applied as nitrogen gases, plant uptake, and soil and floodwater constituents totaled about 94% of the nitrogen added.     

Compound-Specific 14N/15N Analysis of Amino Acid Trimethylsilylated Derivatives from Plant Seed Proteins

Isotopic analyses of plant samples are now of considerable importance for food certification and plant physiology. In fact, the natural nitrogen isotope composition (δ¹⁵N) is extremely useful to examine metabolic pathways of N nutrition involving isotope fractionations. However, δ¹⁵N analysis of amino acids is not straightforward and involves specific derivatization procedures to yield volatile derivatives that can be analysed by gas chromatography coupled to isotope ratio mass spectrometry (GC-C-IRMS). Derivatizations other than trimethylsilylation are commonly used since they are believed to be more reliable and accurate. Their major drawback is that they are not associated with metabolite databases allowing identification of derivatives and by-products. Here, we revisit the potential of trimethylsilylated derivatives via concurrent analysis of δ¹⁵N and exact mass GC-MS of plant seed protein samples, allowing facile identification of derivatives using a database used for metabolomics. When multiple silylated derivatives of several amino acids are accounted for, there is a good agreement between theoretical and observed N mole fractions, and δ¹⁵N values are satisfactory, with little fractionation during derivatization. Overall, this technique may be suitable for compound-specific δ¹⁵N analysis, with pros and cons.     

The measurement of15N in soil and plant material

A complete procedure for analysing soil and plant samples for total N and atom % excess¹⁵N is described. The salicylic acid version of the Kjeldahl method for measuring total N was modified for use in a digestion block, giving quantitative reduction of nitrate in both soil and plant material. Procedures for minimising cross-contamination between samples are specified, including a double-distillation procedure that eliminates memory effects when distilling NH₃ from Kjeldahl digests. A simple and robust apparatus for converting (NH₄)₂SO₄ to N₂ gas for mass spectrometric determination of atom % excess¹⁵N is described. The coefficient of variation for replicate measurements of total N in soil and plant material over the range 0.1–2.2% N was 1.0%. The coefficient of variation for measurements of¹⁵N in plant material over the range 0.4–2.9 atom % excess¹⁵N was 0.2%.     

The role ofAzolla in curbing ammonia volatilization from flooded rice systems

By the year 2020, an additional 300 million tons of rice are needed annually to meet the demands of a growing population. If our natural resource base is to be preserved, intensification strategies should rely on integrated nutrient management, making full use of biological nitrogen fixation. TheAzolla-Anabaena complex is amongst the most effective systems of fixing nitrogen. In this paper we present evidence from greenhouse studies on the potential ofAzolla to curb the volatilization of NH₃ following the application of urea to a mixedAzolla-rice culture, providing a new incentive for developing ways of integratingAzolla in intensive rice cultivation systems.The results of a series of short term greenhouse experiments show that a full cover ofAzolla can significantly reduce losses of applied urea-N from 45 and 50% to 20 and 13% for the 30 and 60 kg N ha^–1 treatments, respectively. About one-quarter of the applied N was tied up in theAzolla biomass. The applied N inhibitedAzolla growth as well as the amount of N fixed. Inoculation with smaller quantities ofAzolla allowing for more vigorousAzolla multiplication was equally effective in reducing NH₃ volatilization and doubled the amount of¹⁵N tied-up byAzolla. The reduction in NH₃ volatilization is largely related to the depression byAzolla of the floodwater pH, which in its absence may reach values between 9 and 10 as a result of algal activity.Early rice growth responded positively to urea as well as the large quantities of appliedAzolla and increased the yield potential of the crop. Smaller quantities ofAzolla alone were not effective in this regard. The conservation of fertilizer N byAzolla, particularly when it fully covered the water, was reflected in a synergistic effect on rice dry matter production, amounting to 9% at the 30 kg N rate and 16% at the 60 kg N rate. In all likelihood this interaction is attributable to the higher efficiency of the applied N. The benefits ofAzolla in conserving basal urea-N even in small quantities (200-500 kg fresh material ha^–1), outweighed competition for the applied N and may be as important as its BNF. The most promising integratedAzolla/rice management systems emerging from our studies should be given further attention under field conditions.     

Recovery of15N-labelled urea: Influence of zero tillage,and time and method of application

The availability of N fertilizer to the crops under zero tillage versus conventional tillage may be affected by position of applied N, N immobilization and N loss from soil. The objectives of this study was to determine the influence of tillage, time of application and method of placement on the recovery of¹⁵N-labelled urea in barley (Hordeum vulgare L.) plants and in soil. Field experiments were conducted during 1984–85 at two locations (Rimbey and Ellerslie) in north-central Alberta. The lowest N recovery in barley plants occurred with surface broadcasting on zero tillage or with incorporation on conventional tillage. Placing urea in bands (23 or 46 cm lateral spacing) or nests (at poits 23 or 46 cm apart) increased the plant N recovery substantially. The plant N recovery was markedly lower with fall application than spring-applied N. For spring broadcast application, the N recovery in the plant was lower under zero tillage than conventional tillage. The¹⁵N recovery in soil (immobilized N) at harvest was greater with broadcast compared to bands or nests, and immobilized N was much greater with fall rather than spring application. The ratios of recoveries of¹⁵N in plant:soil with banding or nesting tended to be higher on zero tillage compared to conventional tillage. In all, placing urea in bands or nests increased the recovery of applied N in plants and decreased the amount of immobilized N under both zero and conventional tillage. The plant N recovery was inferior with fall application, but less so with bands or nests on zero tillage.(Scientific Paper No. 647)     

Fate of the 15N-labelled faeces fraction of dairy farm effluent (DFE) irrigated onto soils under different water regimes

Irrigation of dairy farm effluent (DFE) onto pasture is the preferred treatment method in New Zealand for this very dilute organic effluent. Whereas the dynamics of the urine fraction is comparatively well understood, there is a lack of data on the fate of the mainly organic faecal fraction. To improve our understanding of the complex turnover processes, we labelled both the inorganic and organic N compounds of the faecal fraction of DFE with ¹⁵N. We then measured the ¹⁵N dynamics in various soil and plant fractions in a laboratory experiment at two water contents for up to 254 days. Feeding a dairy cow with ¹⁵N-labelled pasture yielded faeces that had a mean ¹⁵N abundance of 2.95 atom%. Unlabelled urine and water were added to the labelled faeces to construct the DFE, which contained 90.4% of the ¹⁵N in the organic nitrogen fraction (2.87 atom%) and 9.6% in the ammonium fraction (1.23 atom%). As N turnover and losses depend on the method of application and the soil structure, we simulated field conditions by surface-applying our DFE onto intact soil cores with pasture. Two soil water treatments were imposed; dry (30% water content) and wet (water table at 17 cm below the soil surface). The surface application resulted in filtration of the DFE, with a high proportion of the ¹⁵N remaining on the soil surface, where it was relatively unavailable for plant uptake but prone to gaseous and physical losses. Of the applied ¹⁵N, 9.9% in the dry and 13.5% in the wet treatment were still recovered as DFE on the soil surface at day 254. Plant uptake of faecal ¹⁵N accounted for 9.3% and 13.0% in the dry and wet treatments, respectively. The bulk of the ¹⁵N was recovered in the soil organic nitrogen fraction (35.1% in dry, 42.5% in wet), whereas ¹⁵N in inorganic and microbial nitrogen accounted for only very small amounts (< 2%). Total recoveries of the applied ¹⁵N in plant, soil and DFE remaining on the surface at day 254 were 58.4% in the dry, and 71.5% in the wet treatment. Separate analysis of the total and ammonium nitrogen contents and ¹⁵N enrichments of the constructed DFE and filtered subsamples (0.5 mm, 0.2 m) showed that the faecal fraction was not labelled homogeneously. Due to this heterogeneity, which was exacerbated by the filtration of DFE on the soil surface, it was difficult to calculate the turnover of the total faecal fraction based on ¹⁵N results.     

Estimation of the contribution of biological N2 fixation to twoPhaseolus vulgaris genotypes using simulation of plant nitrogen uptake from15N-labelled soil

A technique for the application of the¹⁵N isotope dilution technique for the quantification of plant associated biological nitrogen fixation (BNF) was tested and applied to quantify the BNF contribution to two genotypes ofPhaseolus vulgaris. The technique makes use of sequential measurements of the¹⁵N enrichment of soil mineral N, and the uptake of labelled N by the N₂-fixing plant, to simulate its uptake of soil N (the soil to plant simulation technique). The test was made with two non-N₂-fixing crops (non-nodulating beans and wheat) and two bean genotypes (PR 923450 and Puebla 152), at two levels of N fertilizer addition (10 and 40 kg N ha^–1), to compare the actual N uptake with that simulated from the soil and crop¹⁵N data. The simulation of the soil N uptake by the non-nod bean crop using this soil to plant simulation technique underestimated by 20 to 30% the true N uptake, suggesting that the mineral N extracted from soil samples taken from the 0–15cm layer had a higher¹⁵N enrichment than that N sampled by the roots of this crop. In the case of the wheat crop the simulation resulted in a much greater underestimation of actual N uptake. In general the results using this technique suggested that BNF inputs to the bean cultivars was higher than would be expected from the nodulation and acetylene reduction data, except for the early PR beans in the 40 kg N ha^–1 treatment. In this case the total N and simulated soil N accumulation were well matched suggesting no BNF inputs. An allied technique (the plant to plant simulation technique) was proposed where the¹⁵N enrichrnent of soil mineral N was simulated from the data for total N and labelled N accumulation taken from sequential harvests of either of the non-N₂ -fixing control crops. This was then utilized in combination with the labelled N uptake data of the other crop to simulate its soil N uptake. However, the results using either technique indicated that the wheat and non-nod or nodulating beans exploited pools of N in the soil with completely different¹⁵N enrichments probably due to differences in exploitation of the soil N with depth.     

Fallow replacement and alternative nitrogen management for reducing nitrate leaching in a semiarid region

A suite of ¹⁵N-based methodologies have been applied to quantify the transfer of symbiotically-fixed or legume N to succeeding non-legumes in crop sequences. The structure of these methods, their comparative advantages and efficacy are scrutinized in the present review. Methods are either direct or indirect, the former involving labelling of the legume with ¹⁵N₂, the substrate for symbiotic N₂ fixation, while in the indirect methods the legume is either enriched or depleted in ¹⁵N through addition of labelled fertilizer or is otherwise unamended at ¹⁵N natural abundance. The methods can be classified further according to their yield dependency, and whether they involve the relocation of residues or are located in situ as green manures or harvest residues. The greatest number of studies has involved the relocation of ¹⁵N-labelled plant materials produced artificially at locations remote from the actual crop sequence in the field. Depending on the physico-chemical properties of the residues and the environmental and edaphic conditions pertaining at the experimental site, the proportions of non-legume N in phase 2 of the rotation derived from the phase 1 legume have been quite variable. Nevertheless, there are many recorded instances where green manures and harvest residues represent significant sources of N in legume–non-legume crop sequences. The application of ¹⁵N-based methods has resulted in a better understanding of the relative contribution of below- and above-ground legume N in N transfer, and the efficacy of management practices such as incorporation of green manures or use of surface mulches.     

Nitrogen uptake efficiency in maize production using irrigation water high in nitrate

In order to achieve efficient use of nitrogen (N) and minimize pollution potentials, producers of irrigated maize (Zea mays L.) must make the best use of N from all sources. This study was conducted to evaluate crop utilization of nitrate in irrigation water and the effect N fertilizer has on N use efficiencies of this nitrate under irrigated maize production. The study site is representative of a large portion of the Central Platte Valley of Nebraska where ground water nitrate-N (NO₃-N) concentrations over 10 mg L^–1 are common. Microplots were established to accommodate four fertilizer N rates (0, 50, 100, and 150 kg ha^–1) receiving irrigation water containing three levels of NO₃-N (0, 10, 20 mg L^–1). Stable isotope¹⁵N was applied as a tracer in the irrigation water for treatments containing 10 and 20 mg L^–1 NO₃-N. Plots that did not receive nitrate in the irrigation water where tagged with¹⁵N fertilizer as a sidedress treatment. Sidedressed N fertilizer significantly reduced irrigation-N uptake efficiencies. When residual N uptake is added to first year plant usage, total irrigation NO₃-N uptake efficiencies are similar to total sidedress N fertilizer uptake efficiencies for our cropping system over the two year period. Efficiency of irrigation-N use depends on crop needs and availability of N from other sources during the irrigation season.     

The effect of plant residues on plant uptake and leaching of soil and fertilizer nitrogen in a tropical red earth soil

Two field experiments, in which differing amounts and types of plant residues were incorporated into a red earth soil, were conducted at Katherine, N.T., Australia. The aim of the work was to evaluate the effect of the residues on uptake of soil and fertilizer N by a subsequent sorghum crop, on the accumulation and leaching of nitrate, and on losses of N.Stubble of grain sorghum applied at an exceptionally high rate (~ 18 000 kg ha^–1) reduced uptake of N by sorghum by 13% and depressed the accumulation of nitrate under a crop and particularly under a fallow.Loss of fertilizer N, movement of nitrate down the profile, and uptake by the crop was studied in another experiment after application of N as¹⁵NH₄ ¹⁵NO₃ to field microplots. By four weeks after fertilizer application 14% had been lost from the soil-plant system and by crop maturity 36 per cent had been lost. The pattern of¹⁵N distribution in the profile suggested that losses below 150 cm had occurred during crop growth. The recovery of¹⁵N by the crop alone ranged from 16 to 32 per cent. There was an apparent loss of N from the crop between anthesis and maturity. Residue levels common to sorghum crops in the region (~ 2000 kg ha^–1) did not significantly affect uptake by a subsequent sorghum crop, N losses, or distribution of nitrate in the profile.     

NO-HN03 Exchange System for Production of 14N Highly Depleted of 15N

Abstract

The isotopic exchange reaction between NO and HNO₃ was used to prepare ¹⁴N highly depleted ¹⁵N. This required an efficient reflux system of low holdup to convert oxides of nitrogen to nitric acid by reaction with oxygen and water. A study of the characteristics of the one constructed showed that less than 1 part in 32,000 of the nitric acid was lost from the refluxer as oxides of nitrogen. ¹⁴N with an atom fraction of less than 0.00004 of ¹⁵N was prepared at the rate of 46.0 g/day. The application of the refluxer to a system for producing ¹⁵N is discussed.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

Effect of phenyl phosphorodiamidate on the fate of urea applied to wetland rice fields

The effect of phenyl phosphorodiamidate (PPD) on floodwater properties, N uptake,¹⁵N recovery, and grain yield of wetland rice (Oryza sativa L.) was evaluated in a series of field studies conducted at Muñoz and Los Baños, Philippines. Prilled urea and PPD-amended urea were applied to soil and incorporated immediately prior to transplanting or applied to floodwater after transplanting. Urea was also deep-placed or added in a coated form in two studies.The addition of PPD with urea retarded urea hydrolysis by 1–3 days, depending on the time and method of application. Significant reductions in the concentration of ammoniacal-N in floodwater resulted when PPD-amended urea was applied between 18 and 26 days after transplanting (DT). In contrast, PPD did not appreciably affect the concentration of ammoniacal-N in floodwater when applied with urea either immediately before or after transplanting of the seedlings.Plant N uptake and grain yield were not significantly affected by the addition of PPD with urea in three of the four experiments conducted, even though PPD substantially reduced the concentration of ammoniacal-N in the floodwater in several treatments in these studies. The¹⁵N balance studies conducted at both field locations showed PPD to increase total¹⁵N recovery by between 10% and 14% of the¹⁵N applied, 14 days after the application of urea. No further loss of¹⁵N occurred between the initial sampling (40 DT) and grain harvest at Los Baños. An increase in¹⁵N recovery occurred at grain harvest at Muñoz because¹⁵N-labeled urea was applied at 50 DT in the study. PPD increased the amount of¹⁵N in the plant and nonexchangeable soil N fraction at all harvests at Los Baños. In contrast, at Muñoz, PPD increased the quantity of¹⁵N in the KCL-extractable pool 14 days after urea was applied. Reasons for the discrepancies in results between experiments and the overall failure of PPD to increase grain yield are discussed.     

Assessing nitrogen mineralization from soil organic matter using anion exchange membranes

A simple method to assess differences in potential contribution of organic nitrogen mineralization to plant available N among soils may be useful in fertility research as well as routine soil testing. We deployed a method to assess mineralizable soil organic N using anion exchange membrane (AEM) burial. The method is based on a simple closed incubation system in which strips of AEM are buried directly in soil to adsorb NO ₃ ^- released from organic matter. An index of mineralization was obtained using the amount of NO ₃ ^- adsorbed on an AEM strip removed at the end of each incubation. The same incubation system but using 0.001M CaCl₂ solution to extract NO_3-N was used as the reference method. The mineralization indices obtained from both methods were compared with each other and with plant uptake. A total of 74 soils from across Saskatchewan were used in the study to provide a range of soil properties. Correlations between test values and N uptake by plants in two separate experiments showed the 2 week AEM incubation to be more closely correlated with plant N uptake (r² = 0.86^**** and 0.57^****, respectively) than the reference method (r² = 0.60^**** and 0.48^****, respectively).With this method, we were able to determine the influence of different tillage systems and landscape positions on mineralizable N. The results showed that the NO ₃ ^- released from soil organic matter and accumulated on the AEM reflected the expected effect of three different tillage systems and two landscape positions on mineralizable N. Cropping systems with continuous alfalfa (Medicago sativa) showed higher N release from soil organic matter than a canola (Brassica napus)-lentil (Lens culinaris)-barley (Hordeum vulgare) rotation did. Higher N mineralization was found in the lower slope positions of the landscape where organic matter contents are highest. Direct burial of AEM appears to be a simple and effective method of including a measure of N mineralization in a soil test.     

Methylene urea as a slow-release nitrogen source for processing tomatoes

The potential for improved fertilizer N use efficiency was tested using a slow release N fertilizer, methylene urea (MU), on processing tomato (Lycopersicon esculentum Mill.) in a 2-year field study in the Sacramento Valley, California. Fertilizer N use efficiency of urea and a (50:50, w:w) mixture of urea and MU (uMU) was determined in direct-seeded and transplanted tomato plots with winter cover crop (CC) or winter fallow (F) using ¹⁵N labeled fertilizers. Residual MU-N was estimated from tomato N uptake in the ¹⁵N microplots, and from residual ¹⁵N uptake of wheat grown after two tomato crops. No significant differences were found in the quantity and quality of tomato yields among fertilizer and management treatments during the first year. Total yields in transplanted FuMU plots were significantly lower in the second test year, suggesting slow mineralization of MU-N in the F treatment. On average, about 40% of added fertilizer N was taken up in both fertilizer treatments, and the recovery of ¹⁵N in plant biomass and soil was 75–96 and 50–74% in seeded and transplanted blocks, respectively. In the laboratory, mineralization of MU started faster in soils with past MU use, but the enhanced mineralization did not affect the plant N uptake in the field. MU is potentially an environmentally attractive fertilizer, but without an immediate increase in yield and N use efficiency compared to conventional fertilizers, its use on row crops may not be economically feasible unless the positive environmental factors like decreased leaching of N are considered.