Added Nitrogen Interaction Research Articles

Occurrence of added nitrogen interaction affected by nitrogen stabilizer and glucose additions in an Alfisol

Nitrogen (N) fertilizer application provides available N and affects indigenous soil N availability, inducing added N interaction (ANI). However, limited information is available on the impacts of inhibitors combined with organic substances on ANI. A microcosm experiment was conducted to examine the effects of the additions of nitrification inhibitor (dicyandiamide [DCD]), urease inhibitor (hydroquinone [HQ]), and glucose on ANI. In non-glucose treatments, positive ANI was observed at the initial stage of incubation. DCD addition generally increased ANI. Conversely, HQ addition decreased ANI. Glucose application led to a significantly negative ANI caused by net microbial immobilization at the beginning. ANI recovered because of the remineralization of immobilized N. Glucose combined with DCD and HQ further decreased ANI up to 24.4% and 30.7% and retarded the appearance of positive ANI by 2.6 and 6.0 days, respectively, compared with the corresponding treatment without inhibitor. ANI was primarily driven by biotic processes, and the contribution of abiotic processes, such as isotope displacement reaction in the fixed ammonium (NH4+) pool, to ANI ranged from 15.6% to 31.0%. This contribution increased with DCD and glucose additions, but decreased with HQ application. Generally, DCD addition tended to hold urea-derived N in soil and enhanced the availability of indigenous soil N, whereas HQ addition resulted in a reverse tendency. Glucose amendment remarkably induced the negative ANI and reduced the availability of N derived from either fertilizer or indigenous soil. These results can provide helpful implications for optimizing N management practices and remaining N availability.

Effect of Nitrogen Fertilizer on Wheat Uptake of Soil N in a Pot Experiment

A pot experiment was conducted to determine the effect of nitrogen fertilizer on the uptake of soil native N and fertilizer N by spring wheat using15N isotopic dilution method. The results showed that fertilizer N addition restrained the uptake of soil N via a negative added nitrogen interaction (ANI), and the effect being more at higher rates of application. Additionly, fertilizer N increased the uptake of fertilizer N by wheat, in which grain uptook most of the nitrogen, followed by straw and root. The nitrogen use efficiency (NUE) ranged from 27%~51%, while 36%~63% of the fertilizer-N remained in soil ready for successive crop growth.

Manipulating the N release from 15N-labelled celery residues by using straw and vinasses in Flanders (Belgium)

The effect of straw and vinasses on the N mineralization–immobilization turnover of celery residues was investigated in a field experiment in Flanders (Belgium). The field was laid out in raised beds on a loamy sand soil. At the start of the experiment, 15N-labelled celery residues (4 t dry matter (DM) ha −1) were mixed with straw (12 t DM ha −1) in order to immobilize the released celery-N followed by an incorporation of vinasses (4 t DM ha −1) after 200 days aiming to remineralize the immobilized N. Total N, mineral N and their 15N enrichments as well as microbial biomass N were determined at regular time intervals. During the first 62 days after the incorporation, straw immobilized the celery derived 15N in the microbial biomass and reduced the total celery- 15N losses from the top 25 cm by 38%. However, after 62 days, the N immobilization ceased due to low temperatures (<10 °C) in the raised beds, and was followed by natural remineralization of immobilized celery- 15N (without addition of vinasses) at a moment when the risk of nitrate leaching was still high. Hence, straw was not able to reduce the celery-N losses during the complete winter period. The addition of vinasses in spring caused no real positive priming effect, although it did increase the amount of remineralized celery- 15N (7.6% of celery derived 15N) compared to the straw treatment without vinasses (1.7% of celery derived 15N) probably due to an apparent added nitrogen interaction caused by displacement reactions with the soil microbial biomass. In conclusion, in raised beds, it is not possible to reduce nitrate leaching and to achieve a synchronization between the N release from crop residues and N demand by the following crop by using straw and vinasses to manipulate the N release from crop residues.

Comparative effects of applying leguminous and non-leguminous green manures and inorganic N on biomass yield and nitrogen uptake in flooded rice ( Oryza sativa L.)

The beneficial role of green manures in rice production is generally ascribed to their potential of supplying plant nutrients, particularly nitrogen (N). However, the mechanisms through which green manures enhance the crop productivity are poorly understood. Pot experiments were conducted using a 15N-tracer technique: (1) to compare the biomass production potential of sesbania (Sesbania aculeata Pers.) and maize (Zea mays L.) as green manuring crops for lowland rice and (2) to compare the effect of the two types of green manure and inorganic N on the dry matter accumulation and N uptake by two rice (Oryza sativa L.) cultivars, viz. IR-6 and Bas-370. Although maize produced three times higher shoot biomass compared with sesbania, the latter showed higher N concentration; and thus the total N yield was similar in the two types of plants. Applying the shoot material of the two plants to flooded rice significantly enhanced the dry matter yield and N uptake by the two rice cultivars, the positive effects generally being more pronounced with sesbania than with maize amendment. The difference in the growth-promoting potential of the two plant residues was related more to an increased uptake of the native soil N rather than to their direct role as a source of plant-available N. A positive added nitrogen interaction (ANI) was observed due to both plant residues, the effect was much more pronounced with the application of sesbania than with maize residues. In both rice cultivars, inorganic N also caused a substantial ANI, particularly at higher application rate. Losses from the applied N were 2–3 times lower from sesbania, compared with maize treatment. Green manuring with sesbania also caused much lower N losses than the inorganic N applied at equivalent or higher rates. The overall benefit of green manuring to rice plants was higher than inorganic N applied at comparable rates. The two rice cultivars differed in their response to green manuring, IR-6 generally being more responsive than Bas-370.

Added nitrogen interaction as affected by soil nitrogen pool size and fertilization – significance of displacement of fixed ammonium

AbstractDisplacement of NH4+ fixed in clay minerals by fertilizer 15NH4+ is seen as one mechanism of apparent added nitrogen interactions (ANI), which may cause errors in 15N tracer studies. Pot and incubation experiments were carried out for a study of displacement of fixed NH4+ by 15N‐labeled fertilizer (ammonium sulfate and urea). A typical ANI was observed when 15N‐labeled urea was applied to wheat grown on soils with different N reserves that resulted from their long‐term fertilization history: Plants took up more soil N when receiving fertilizer. Furthermore, an increased uptake of 15N‐labeled fertilizer, induced by increasing unlabeled soil nitrogen supply, was found. This ANI‐like effect was in the same order of magnitude as the observed ANI. All causes of apparent or real ANI can be excluded as explanation for this effect. Plant N uptake‐related processes beyond current concepts of ANI may be responsible. NH4+ fixation of fertilizer 15NH4+ in sterilized or non‐sterile, moist soil was immediate and strongly dependent on the rate of fertilizer added. But for the tested range of 20 to 160 mg 15NH4+‐N kg–1, the NH4+ fixation rate was low, accounting for only up to 1.3 % of fertilizer N added. For sterilized soil, no re‐mobilization of fixed 15NH4+ was observed, while in non‐sterile, biologically active soil, 50 % of the initially fixed 15NH4+ was released up to day 35. Re‐mobilization of 15NH4+ from the pool of fixed NH4+ started after complete nitrification of all extractable NH4+. Our results indicate that in most cases, experimental error from apparent ANI caused by displacement of fixed NH4+ in clay is unlikely. In addition to the low percentage of only 1.3 % of applied 15N, present in the pool of fixed NH4+ after 35 days, there were no indications for a real exchange (displacement) of fixed NH4+ by 15N.

Response of flooded rice (Oryza sativa L.) to nitrogen application at two root-zone temperature regimes in a pot experiment

In a greenhouse experiment, the response of three cultivars of rice to N application was studied at two root-zone moisture regimes. The response was measured in terms of dry matter yield, uptake of N from applied fertilizer and soil organic matter, loss of fertilizer N, and interaction of applied N with the native soil N. Nitrogen was applied as 15N-labelled ammonium sulphate and two temperature regimes in the root-zone were established by digging one set of pots into the soil, while keeping the other set on the surface. This setup created and maintained a temperature difference of 5–7°C in the rhizosphere, temperature being lower in the buried pots throughout the growth period. The rice cultivars included a coarse-grain and high-yielding cultivar (IR-6) and two aromatic fine-grain cultivars (Bas-198 and Bas-Pak). IR-6 showed a negative response to higher root-zone temperature with resultant reduced efficiency of soil N and fertilizer N. The other two cultivars were relatively insensitive. The effect of temperature was more pronounced for N yield than dry matter accumulation. Loss of applied N occurred in all cases, but it was more pronounced in IR-6 at the higher temperature regime. Application of labelled fertilizer N led to an increase in the uptake of unlabelled N due to a real "added nitrogen interaction" which was attributed mainly to an increase in root biomass.

Added Nitrogen Interaction in the Soil-Plant System-A Review

Added Nitrogen Interaction in the Soil-Plant System-A Review

Soil textural effects on mineralization of nitrogen from crop residues and the added nitrogen interaction

A laboratory incubation was made to compare the amount of inorganic nitrogen (N) released from crop residues incorporated in three soils of different texture and to determine the effect of soil texture on the mineralization of soil organic N. Residues of 15N‐labeled maize (Zea mays L.), soybean (Glycine max [L.] Merr), or alfalfa (Medicago sativa L.) were mixed with sand, loam, or clay soils at rates of 0 or 220 mg N kg‐1 of soil. Nitrogen mineralization was measured after 2, 4, and 6 weeks of incubation at 30°C. After 6 weeks, from 6 to 41% of plant residue N was mineralized. During the incubation, alfalfa residue N was mineralized more in the sand soil than in the loam or clay soils. In contrast, maize and soybean residue N was mineralized more in the fine‐textured soils. Native soil organic N was more susceptible to decomposition in the clay and loam soils than in the sand soil. In all soils, alfalfa residue released the most inorganic N followed by maize and soybean according to their C:N ratios and insoluble fiber contents. The amounts of N mineralized estimated with the isotopic method (IM) and by the difference method (DM) were higher when alfalfa residues were incorporated in the sand soil than in the loam or clay soils. In contrast, soil textural effects were not evident when maize and soybean residue N mineralization were estimated with either method. The isotopic method generally gave higher values than the difference method when used to estimate N mineralization from labeled crop residues. However, both methods were highly correlated (r=0.93; p<0.001), suggesting that the DM is valid for estimating N mineralization from organic residues. Throughout the entire incubation, alfalfa residue N stimulated native soil N mineralization in the sand soil (positive added nitrogen interaction, ANI), whereas soybean and maize residue N led to negative ANI in all soils. These negative ANI values were attributed to the immobilization of soil native mineral N as well as to N mineralized from soil organic matter and crop residues.

Fate and Interaction with Soil N of Fertilizer 5N Applied to Wheat at Different Growth Stages

A greenhouse study was conducted to compare three wheat cultivars for fertilizer-N (applied as 15N-labelled ammonium sulphate) and soil-N uptake at different growth stages. Differences were observed for bread wheat and durum wheat for different plant parameters. Contribution of fertilizer-N to the total plant-N of the three wheat cultivars ranged between 4.1 and 23.7%. Applied fertilizer N had a significant bearing on the plant-uptake of soil-N resulting in substantial added nitrogen interaction (ANI); the interaction of the two N sources was variable for the three wheat cultivars. Although the three wheat cultivars differed in uptake of fertilizer N applied at different growth stages, durum was least efficient in making use of N applied at anthesis. Losses of N applied at different growth stages varied (22–56%), but were maximum from that applied at anthesis. Post-anthesis dry matter accumulation (PADMA) and post-anthesis N uptake (PANU) were more for bread wheat than durum wheat and seemed to contribute significantly to higher dry matter and N yield in the former.

Fate of 15N-labelled fertilizer in a long-term field trial at Ropsley, UK

In 1992 and 1993, eight rates of 15N-labelled fertilizer (0–245 kg ha−1) were applied to winter wheat growing on the Ropsley long term field trial where eight different N amounts had been applied annually since 1978. The fate of the labelled N in the crop and topsoil (0–23 cm) was determined at harvest in the year of 15N application and in the first and second residual years.By harvest in the second residual year, 60–77% of the original labelled application had been recovered in the crop and topsoil with 23–40% lost. These losses virtually all occurred within the first two growing seasons; there was no significant loss during the second residual year. Significant changes in the 15N balance were observed at N applications in excess of the range 140–175 kg ha−1 which suggested a marked decrease in the efficiency of N use and an increase in residual labile N in the soil at harvest. At low N applications (&lt;175 kg ha−1), a positive added nitrogen interaction (ANI) was observed: 40–50% of this was a residual ANI due to the short or long term effect of applying N fertilizer, and the remainder was probably an apparent ANI due to pool substitution in the immobilization process. At large N applications (&gt;175 kg ha−1) a negative ANI was observed: large N applications resulted in a net suppression of soil N uptake due to substitution by fertilizer for a limited plant N demand.

Effect of fertilizer rate and form on the recovery of 15N-labelled fertilizer applied to wheat in Syria

15N-labelled fertilizer was applied at different rates (0, 30, 60, 90 kg N ha−1) and in different forms (urea or ammonium sulphate) to wheat grown in Syria in three seasons (1991/92, 1992/93 and 1994/95).Recovery of 15N-labelled fertilizer in the above-ground crop at harvest was low (8–22%), with the amount of 15N-labelled fertilizer recovered in the crop increasing as the rate of application increased. Fertilizer application caused a significant increase in the amount of unlabelled soil N in the crop, suggesting that the application of N fertilizer caused a ‘real’ added nitrogen interaction. Recovery of 15N-labelled fertilizer in the crop was unaffected by the form of the fertilizer.On average 31% (14–54%) of the 15N-labelled fertilizer remained in the soil at harvest, mostly in the 0–20 cm layer. At the lowest application rate (30 kg N ha−1) most of the residual fertilizer was as organic N, but at the higher application rates (60 and 90 kg N ha−1), a greater proportion of the 15N-labelled fertilizer was recovered as inorganic N, presumably as the result of top-dressing N in dry conditions in the spring. The amount of 15N-labelled fertilizer remaining in the soil increased as the fertilizer rate increased, but was unaffected by the form of fertilizer applied.Losses of 15N-labelled fertilizer were large (&gt;35%), probably caused by gaseous losses, either through volatilization of N from the calcareous soil, or through denitrification from wet soils rich in organic residues.N fertilization strategies in the West Asia/North Africa (WANA) region should take note of the low recovery of N fertilizer by the crop in the season of application, and the resultant large quantities of residual fertilizer.

Influence of Split Ammonium Sulfate on Nitrogen Availability from 15N-Labeled Matured Soybean as Green Manure for Buckwheat(Fagopyrum esculentum Moench).

A pot experiment was conducted at the Experimental Farm of Gifu University to study the effects of 15N-labeled matured soybean as green manure, with and without splitted ammonium sulfate, on the growth and nitrogen (N) nutrition of buckwheat. Crop yield ranged between 0.7 to 2.8g/pot. Applying green manure together with ammonium sulfate split in a ratio of 2:1:1 enhanced higher dry matter yield and seed nitrogen content. The crop derived 5.3% N from green manure when applied alone, and between 7.4 to 13.4% N when applied in combination with split ammonium sulfate and basal P2O5 and K2O. Nitrogen from green manure was most efficiently utilizcd by the crop when green manure was applied together with split ammonium sulphate at the ratio of 2:1:1. The "added nitrogen interaction" (ANI) was positive, indicating that the green manure did not furnish the crop with only N but also caused the effective utilization of soil resources as well.

Fate of 15N-labelled fertilizer applied to spring barley grown on soils of contrasting nutrient status

An experiment with 15N-labelled fertilizer was superimposed on the Rothamsted Hoosfield Spring Barley Experiment, started in 1852. Labelled 15NH4 15NO3 was applied in spring at (nominal) rates of 0, 48, 96 and 144 kg N ha-1. The labelled fertilizer was applied to microplots located within four treatments of the original experiment: that receiving farmyard manure (FYM) annually, that receiving inorganic nutrients (PK) annually and to two that were deficient in nutrients: applications were made in two successive years, but to different areas within these original treatments. Maximum yields in 1986 (7.1 t grain ha-1) were a little greater than in 1987. In 1987, microplots on the FYM and PK treatments gave similar yields, provided enough fertilizer N was applied, but in 1986 yields on the PK treatment were always less than those on the FYM treatment, no matter how much fertilizer N was applied. In plots with adequate crop nutrients, about 51% of the labelled N was present in above-ground crop and weed at harvest, about 30% remained in the top 70 cm of soil (mostly in the 0–23 cm layer) and about 19% was unaccounted for, all irrespective of the rate of N application and of the quantity of inorganic N in the soil at the time of application. Less than 4% of the added fertilizer N was present in inorganic form in the soil at harvest, confirming results from comparable experiments with autumn-sown cereals in south-east England. Thus, in this experiment there is no evidence that a spring-sown cereal is more likely to leave unused fertilizer in the soil than an autumn-sown one. With trace applications (ca. 2 kg N ha-1) more labelled N was retained in the soil and less was in the above-ground crop. Where P and K were deficient, yields were depressed, a smaller proportion of the labelled fertilizer N was present in the above-ground crop at harvest and more remained in the soil. Although the percentage uptake of labelled N was similar across the range of fertilizer N applications, the uptake of total N fell off at the higher N rates, particularly on the FYM treatment. This was reflected in the appearance of a negative Added Nitrogen Interaction (ANI) at the highest rate of application. Fertilizer N blocked the uptake of soil N, particularly from below 23 cm, once the capacity of the crop to take up N was exceeded. Denitrification and leaching were almost certainly insufficient to account for the 19% loss of spring-added N across the whole range of N applications and other loss processes must also have contributed.

Growth and nitrogen nutrition of maize (Zea mays L.) in soil treated with the nitrification-inhibiting insecticide Baythroid

A pot experiment was conducted to compare the uptake and dry matter production potential of NH inf4 sup+ and NO inf3 sup- and to study the effect of Baythroid, a contact poison for several insect pests of agricultural crops, on growth and N uptake of maize (Zea mays L.). Nitrogen was applied as (15NH4)2SO4, K15NO3, or 15NH4NO3 and in one treatment Baythroid was combined with 15NH4NO3. Source of N had, in general, a nonsignificant effect on dry matter and N yield, but uptake of NO inf3 sup- was significantly higher than that of NH inf4 sup+ when both N sources were applied together. Substantial loss of N occurred from both the sources, with NH inf4 sup+ showing greater losses. Baythroid was found to have a significant positive effect on dry matter yield of both root and shoot; N yield also increased significantly. Uptake of N from both the applied and native sources increased significantly in the presence of Baythroid and a substantial added nitrogen interaction (ANI) was determined. The positive effect of Baythroid was attributed to: (1) a prolonged availability of NH inf4 sup+ due to inhibition of nitrification, (2) an increased availability of native soil N through enhanced mineralization, and (3) an enhanced root proliferation.

Evaluating the “added nitrogen interaction”; effect in forage grasses

Abstract In northeastern United States, intensive grazing systems utilizing cool‐season grasses are being increasingly adopted by dairy fanners as a way of reducing operating costs. Since nitrogen (N) fertilization is a major factor in cool‐season grass production in northeastern United States, the accurate determination of N‐use efficiency is necessary in determining N fluxes in intensive grazing systems. When using isotopic N, the added N interaction (ANI) or ‘priming’ effect can greatly affect the calculation of N‐use efficiency of crops and forages. The objective of this paper is to evaluate the magnitude of ANI as it relates to determination of N‐use efficiency of forage grasses in northeastern United States. Data from experiments on the effects of soils, environment, and N fertilization on the yield and N‐use efficiency of various forage grasses was used in this study. In these experiments multiple rates of isotopic ammonium nitrate and ammonium sulfate were applied to cool‐and warm‐season grasses. Nitrogen‐use efficiency was calculated using the isotopic and difference methods, and ANI was determined as the difference in N uptake as determined by these two methods. In experiments in which the N source was ammonium sulfate, ANI averaged 24.2 and 38.5% of the applied N on cool‐and warm‐season grasses, respectively. The ANI averaged 7.4% of the applied N on cool‐season grass where the N source was ammonium nitrate. The ANI was always greater in the spring growth of cool‐season grasses than it was in the fall growth. However, the seasonal difference was greater in the case where the N source was ammonium sulfate than it was where the N source was ammonium nitrate.

Effect of ammonium fixation and displacement on the added nitrogen interaction in incubation experiments

A laboratory incubation experiment was conducted to study the effect of NH 4 + fixation/defixation on the added N interaction (ANI) in three Illinois Mollisols fertilized with 100 or 200 mg N kg-1 soil. A positive ANI was observed in all three soils, which was greater at the higher rate of applied N. However, very little exchange was observed between applied 15NH 4 + and the native clay-fixed NH 4 + , and the ANI observed were attributed largely to microbial immobilization-mineralization. The results suggested that variations in the NH 4 + fixation capacity of soils will not have a significant bearing on the interpretation of data obtained from studies of the ANI.

The effect of inorganic nitrogen on the added nitrogen interaction of soils in incubation experiments

A laboratory incubation experiment was conducted to study the effect of indigenous inorganic N on the immobilization of applied N and on the occurrence of an added N interaction (ANI). Samples of six Mollisols from Illinois were incubated with 15N-labelled (NH4)2SO4 (100 or 200 mg N kg-1 soil), with or without the use of 0.01 M CaCl2 to extract inorganic N (mainly NOinf3sup-) before incubation. From 6 to 49% of the N applied was immobilized, higher percentages being obtained with unextracted soils than with the extracted soils and with the higher rate of N addition. Net mineralization of native N occurred in both the unextracted and extracted soils, but was more extensive in the unextracted soil and increased with the addition of N. The increases were accompanied by a positive ANI, which usually exceeded the amount of applied N immobilized and increased with the rate of addition. The ANI values observed with extracted soils were attributed to increased mineralization of native organic N.

Nitrogen-15 recovery fraction in flooded tropical rice as affected by Added Nitrogen Interaction

The application of N fertilizer has been shown to cause an apparent increase in the uptake of native soil N via an effect termed ‘Added nitrogen interaction’ (ANI). This ANI caused by pool-substitution, can affect the 'IN-recovery fraction (NRF) by plants as calculated by the isotope-dilution method. The ANI effect was studied in a field experiment with transplanted and direct seeded flooded rice, comparing three methods of N-application (broadcast and incorporation of prilled urea ; band placement of urea solution ; and point placement of urea supergranules). ANI's for broadcast and incorporation treatments were generally greater than those for band and point placement treatments. The values for NRF calculated by the isotope-dilution method were lower than those of the apparent N-recovery fracton (ARF) as calculated by the difference method. Most of the discrepancy between plant nitrogen recoveries estimated by the isotope-dilution and the difference method could be explained by fertilizer losses and by pool-substitution, which means that fertilizer N stands proxy for soil N.

Mineralization of N from plant residues and its interaction with native soil N

A laboratory incubation experiment was conducted to compare the mineralization of N from 15N-labelled soybean ( Glycine max L.) tops, vetch ( Vicia villosa Roth.) tops and corn ( Zea mays L.) stover in six Mollisols from Illinois, and to study the interaction of the applied residue N with the native soil N. From 13 to 41% of the applied N was mineralized during the 4-week incubation (25°C), the proportions decreasing in the order, vetch > corn > soybean, in accordance with their total N content. Mineralization of N from soybean and corn was most rapid in the soil having the highest content of mineral and potentially mineralizable N, whereas mineralization of N from vetch was slowest in this soil. Higher values for the amount of N mineralized were obtained by the difference method than by the isotopic method. However, the values were highly correlated ( r = 0.92∗∗), which suggests that the difference method is a satisfactory alternative to the isotopic method in assessing the mineralization of N from applied organic materials. The three plant materials had different effects on the mineralization of soil N, resulting in either a positive or negative added nitrogen interaction (ANI). Negative ANIs were more common with soybean and corn than with vetch, and were attributed to greater immobilization of native mineral N during early stages of decomposition, and to enhanced denitrification of native NO − 3 following organic amendment.

Urea Nitrogen Budget for a Subarctic Agricultural Soil

AbstractUrea is the predominant form of fertilizer N used in Alaska. However, little information is available on the N‐recovery efficiency (NRE) or the ultimate fate of urea‐N in subarctic soils. This study was conducted to determine the NRE and the ultimate fate of urea‐N applied in the spring for barley (Hordeum vulgare L.) production in interior Alaska. Urea and Ca(NO3)2 were applied at 100 kg N ha−1 and either incorporated into or left on the soil surface in each of three cropping years. Also, 15N‐labeled urea was applied to small plots. Ammonia traps and soil and plant samples were used to track the form and movement of the applied N. Ammonia volatilization losses were negligible. Deep soil samplings indicated negligible leaching of N. The NRE, as determined by the difference method, was estimated to be about 60% for urea and 73% for Ca(NO3)2. By the isotope dilution method, NRE for urea was calculated to be about 41%. An apparent added nitrogen interaction (ANI) accounted for an additional 16.5%. Approximately 43% of the applied urea‐N was recovered in the soil organic matter at the end of the season, 1% remained in the soil as inorganic N, and 15% was unaccounted for and assumed lost through denitrification. Grain yields did not differ due to N source or placement, but total plant N uptake was greater from the Ca(NO3)2 than from the urea source. Nitrogen placement did not affect N uptake. Fertilizer N cycling in this subarctic soil did not differ greatly from fertilizer N cycling in more temperate regions.