Effects of 15N-labelled ammonium nitrate and urea on soil nitrogen during growth of wheat (Triticum aestivum L.) under field conditions

We studied the effects of ¹⁵N-labelled ammonium nitrate and urea on the yield and uptake of labelled and unlabelled N by wheat (Triticum aestivum L., cv. Mexi-Pak-65) in a field experiment. The dry matter and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The wheat crop used 33.6–51.5 and 30.5–40.9% of the N from ammonium nitrate and urea, respectively. Splitting the fertilizer N application had a significant effect on the uptake of fertilizer N by the wheat. The fertilizer N uptake showed that ammonium nitrate was a more available source of N for wheat than urea. The effective use of fertilizer N (ratio of fertilizer N in grain to fertilizer N in whole plant) was statistically similar for the two N fertilizers. The application of fertilizer N increased the uptake of unlabelled soil N by wheat, a result attributed to a positive added N interaction, which varied according to the fertilizer N split; six split applications gave the highest added N interaction compared to a single application or two split applications for both fertilizers. Ammonium nitrate gave 90.5, 33.5, and 48.5% more added N interaction than urea with one, two, and six split N applications. A values were not significantly correlated with the added N interaction (r=0.557). The observed added N interaction may have been the result of pool substitution, whereby added labelled fertilizer N replaced unlabelled soil N.     

Priming effects of 15N-labelled ammonium nitrate on uptake of soil N by wheat (Triticum aestivum L.) under field conditions

Summary The uptake of labelled and unlabelled N by wheat was measured in a field experiment using ¹⁵N-labelled ammonium nitrate fertilizer. The dry matter yield and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The uptake of applied N by wheat ranged between 25 and 34%. Fertilizer N application increased the uptake of unlabelled soil N which was attributed to a positive priming effect or added N interaction. The added N interaction observed by applying 20, 60, and 120 kg fertilizer N was 11.4, 19.1, and 27.9 kg, corresponding to 26, 44 and 64%, respectively of the N taken up from unfertilized soil. The A values did not alter with the increase in fertilizer N application. The observed added N interaction may have been the result of pool substitution whereby added labelled fertilizer N stood proxy for unlabelled soil N. A significant correlation coefficient (r=0.996^**) between the uptake of soil N and the dry matter yield showed that soil N was more important than fertilizer N in wheat production.     

Fate and interaction with native soil N of ammonium N applied to wetland rice (Oryza sativa L.) grown under saline and non-saline conditions

Summary The effect of salts on the balance of fertilizer N applied as ¹⁵N-labelled ammonium sulphate and its interaction with native soil N was studied in a pot experiment using rice (Oryza sativa L.) as a test crop. The rice crop used 26%–40% of the applied N, the level of applied N and salts showing no significant bearing on the uptake of fertilizer N. Losses of fertilizer N ranged between 54% and 68% and only 5%–8% of the N was immobilized in soil organic matter. Neither the salts nor the rate of N application had any significant effect on fertilizer N immobilization. The effective use of fertilizer N (fertilizer N in grain/fertilizer N in whole plant) was, however, better in the non-saline soil. The uptake of unlabelled N (N mineralized from soil organic matter and that originating from biological N₂ fixation in thes rhizosphere) was inhibited in the presence of the salts. However, in fertilized soil, the uptake of unlabelled N was significantly enhanced, leading to increased A values [(1-% Ndff/% Ndff)x N fertilizer applied, where Ndff is N derived from fertilizer], an index of interaction with the added N. This added N interaction increased with increasing levels of added N. Since the extra unlabelled N taken up by fertilized plants was greater than the fertilizer N immobilized, and the root biomass increased with increasing levels of added N, a greater part of the added N interaction was considered to be real, any contribution by an apparent N interaction (pool substitution or isotopic displacement) to the total calculated N interaction being fairly small. Under saline conditions, for the same level of fertilizer N addition, the added N interaction was lower, and this was attributed to a lower level of microbial activity, including mineralization of native soil N, rootdriven immobilization of applied N, and N₂ fixation.     

Fate of Azolla spp. and urea nitrogen applied to wetland rice (Oryza sativa L.)

Summary Using ¹⁵N, the fate of N applied to wetland rice either as Azolla or urea was studied in a field at the International Rice Research Institute (IRRI). In bigger plots nearby, yield response and N uptake were also determined with unlabelled N sources. Azolla microphylla was labelled by repeated application of labelled ammonium sulfate. Labelled and unlabelled N were used alternately in applications of Azolla or urea 0 and 42 days after transplanting, in order to determine the effect of the time of application on the availability of Azolla N. The quantities of Azolla N incorporated were 23% more than those of urea N (30 kg N ha^–1) in the isotope plots or 7% less in the yield response plots. Grain yield and total N uptake by the rice plants in the yield-response plots were higher in the urea-treated plots than in the Azolla-treated plots, but the physiological effect of Azolla N (grain yield response/increase in N uptake) was higher than that of rea. The labelled N balance was studied after the first and second crops of rice. Losses of labelled N after the first crop were higher from urea (30%–32%) than from Azolla (0%–11 %). Losses in N applied as a side dressing 42 days after transplanting were less than those of N applied basally. No further losses of ¹⁵N occurred after the first crop. The recovery of Azolla ¹⁵N in the first crop of rice was 39% from the basal application and 63% from the side dressing. The recovery of urea ¹⁵N was 27% from the basal application and 48% from the side dressing. Recoveries of residual N from both Azolla and urea during the second rice crop were similar. Laboratory incubation of the Azolla used and the changes in labelled exchangeable N in the soil showed that at least 65% of Azolla N (4.7% N content) was mineralized within 10 days.     

Availability of soil and fertilizer nitrogen to wetland rice following wheat straw amendment

Summary A pot experiment was conducted to study the availability of soil and fertilizer N to wetland rice as influenced by wheat straw amendment (organic amendment) and to establish the relative significance of the two sources in affecting crop yield. Straw was incorporated in soil at 0.1, 0.2, and 0.3% before transplanting rice. Inorganic N as ¹⁵N-ammonium sulphate was applied at 30, 60, and 90 g g^-1 soil either alone or together with wheat straw in different combinations. After harvesting the rice, the plant and soil samples were analyzed for total N and ¹⁵N. Straw incorporation significantly decreased the dry matter and N yield of rice, the decrease being greater with higher rates of straw. The reduction in crop yield following the straw incorporation was attributed mainly to a decrease in the uptake of soil N rather than fertilizer N. The harmful effects of organic matter amendment were mitigated by higher levels of mineral N addition. The uptake of applied N increased and its losses decreased due to the straw incorporation. Mineral N applied alone or together with organic amendment substantially increased the uptake of unlabelled soil N. The increase was attributed to a real added N interaction.     

Relative significance of soil and nitrogenous fertilizer in nitrogen nutrition and growth of wetland rice (Oryza sativa L.)

Summary A pot experiment was conducted to compare the yields from five commercially cultivated varieties (Bas-198, Bas-370, Bas-Pak, Bas-385, and IR-6) of rice (Oryza sativa L.) and to establish the relative significance of soil N and fertilizer N (¹⁵N-labelled ammonium sulphate) in affecting crop performance. Another aim was to study the interaction of fertilizer N and soil N as influenced by different rice varieties. Among the five varieties tested, Bas-Pak gave the maximum dry matter and N yield. The N-use efficiency (percentage of applied N taken up by the plants) of different varieties ranged between 33.7 and 43.7%, Bas-Pak being the most efficient. Significant losses of fertilizer N occurred from the soil-plant system. The maximum N loss (52.1% of applied N) was observed with IR-6 and the minimum loss (39.2%) with Bas-Pak. A substantial increase in the uptake of soil N following the application of fertilizer and an interaction between the two N sources were observed with all varieties except Bas-385. The interaction was attributed to greater root proliferation following the application of fertilizer. It was concluded that a varietey with greater potential to use soil N is likely to give a better yield and that, of the two N sources, the availability of soil N was more important in determining the yield performance of different varieties of rice.     

Dynamics of 15N-labeled ammonium sulfate in various inorganic and organic soil fractions of wetland rice soils

Summary The dynamics of basally applied ¹⁵N-labeled ammonium sulfate in inorganic and organic soil fractions of five wetland rice soils of the Philippines was studied in a greenhouse experiment. Soil and plant samples were collected and analyzed for ¹⁵N at various growth stages. Exchangeable NH₄ ⁺ depletion continued after 40 days after transplanting (DAT) and corresponded with increased nitrogen uptake by rice plants. Part of the applied fertilizer was fixed by 2:1 clay minerals, especially in Maligaya silty clay loam, which contained beidellite as the dominant clay mineral. After the initial fixation, nonexchangeable ¹⁵N was released from 20 DAT in Maligaya silty clay loam, but fixation delayed fertilizer N uptake from the soil. Part of the applied N was immobilized into the organic fraction. In Guadalupe clay and Maligaya silty clay loam, immobilization increased with time while the three other soils showed significant release of fertilizer N from the organic fraction during crop growth. Most of the immobilized fertilizer N was recovered in the nondistillable acid soluble (alpha-amino acid + hydrolyzable unknown-N) fraction at crop maturity. Between 61% and 66% of applied N was recovered from the plant in four soils while 52% of fertilizer N was recovered from the plant in Maligaya silty loam. Only 20% – 30% of the total N uptake at maturity was derived from fertilizer N. Nmin (mineral N) content of the soil before transplanting significantly correlated with N uptake. Twenty-two to 34% of applied N was unaccounted for possibly due to denitrification and ammonia volatilization.     

Availability of soil and fertilizer nitrogen to wheat (Triticum aestivum L.) following rice-straw amendment

Summary A pot experiment was conducted to study the N availability to wheat and the loss of ¹⁵N-labelled fertilizer N as affected by the rate of rice-straw applied. The availability of soil N was also studied. The straw was incorporated in the soil 2 or 4 weeks before a sowing of wheat and allowed to decompose at a moisture content of 60% or 200% of the water-holding capacity. The wheat plants were harvested at maturity and the roots, straw, and grains were analysed for total N and ¹⁵N. The soil was analysed for total N and ¹⁵N after the harvest to determine the recovery of fertilizer N in the soil-plant system and assess its loss. The dry matter and N yields of wheat were significantly retarded in the soil amended with rice straw. The availability of soil N to wheat was significantly reduced due to the straw application, particularly at high moisture levels during pre-incubation, and was assumed to cause a reduction in the dry matter and N yields of wheat. A significant correlation (r=0.89) was observed between the uptake of soil N and the dry matter yield of wheat with different treatments. In unamended soil 31.44% of the fertilizer N was taken up by the wheat plants while 41.08% of fertilizer N was lost. The plant recovery of fertilizer N from the amended soil averaged 30.78% and the losses averaged 45.55%     

Effect of acetylene provided by wax-coated calcium carbide on transformations of urea nitrogen applied to an irrigated wheat crop

The effect of acetylene (provided by wax-coated calcium carbide, CaC₂) on N transformations in a red-brown earth was measured in a field experiment with irrigated wheat by determining the change in the concentration and ¹⁵N enrichment of the organic N and mineral N pools with time. The study was conducted in the Goulburn-Murray Irrigation region of south-eastern Australia using 0.3 m by 0.3 m microplots fertilized with ¹⁵N-labelled urea (10 g N m^-2; 5 atom% ¹⁵N). Acetylene was effective in slowing the nitrification of both unlabelled and labelled N. Nitrate derived from the added fertilizer reached a maximum 19 days after sowing in the treatment without CaC₂, whereas little nitrate accumulated in the 8 g CaC₂ m^-2 treatment. There was significant immobilization of the urea N by 19 days after sowing in all treatments, but the extent of immobilization was not affected by the acetylene. The addition of acetylene slowed net mineralization of labelled and unlabelled N from the organic N pool, and resulted in increased accumulation of both unlabelled and labelled N in wheat tops.     

Excessive nitrogen application decreases grain yield and increases nitrogen loss in a wheat–soil system

Abstract

Excessive use of nitrogen (N) fertilizers in wheat fields has led to elevated NO₃-N concentrations in groundwater and reduced N use efficiency. Three-year field and ¹⁵N tracing experiments were conducted to investigate the effects of N application rates on N uptake from basal and topdressing ¹⁵N, N use efficiency, and grain yield in winter wheat plants; and determine the dynamics of N derived from both basal and topdressing ¹⁵N in soil in high-yielding fields. The results showed that 69.5–84.5% of N accumulated in wheat plants derived from soil, while 6.0–12.5%and 9.2–18.1% derived from basal ¹⁵N and top ¹⁵N fertilizer, respectively. The basal N fertilizer recovery averaged 33.9% in plants, residual averaged 59.2% in 0–200 cm depth soil; the topdressing N fertilizer recovery averaged 50.5% in plants, residual averaged 48.2% in 0–200 cm soil. More top ¹⁵N was accumulated in plants and more remained in 0–100 cm soil rather than in 100–200 cm soil at maturity, compared with the basal ¹⁵N. However, during the period from pre-sowing to pre-wintering, the soil nitrate moved down to deeper layers, and most accumulated in the layers below 140 cm. With an increase of N fertilizer rate, the proportion of the N derived from soil in plants decreased, but that derived from basal and topdressing fertilizer increased; the proportion of basal and top ¹⁵N recovery in plants decreased, and that of residual in soil increased. A moderate application rate of 96–168 kg N ha^?1 led to increases in nitrate content in 0–60 cm soil layer, N uptake amount, grain yield and apparent recovery fraction of applied fertilizer N in wheat. Applying above 240 kg N ha^?1 promoted the downward movement of basal and top ¹⁵N and soil nitrate, but had no significant effect on N uptake amount; the excessive N application also obviously decreased the grain yield, N uptake efficiency, apparent recovery fraction of applied fertilizer N, physiological efficiency and internal N use efficiency. It is suggested that the appropriate application rate of nitrogen on a high-yielding wheat field was 96–168 kg N ha^?1.     

Assessment of slow release fertilizers and nitrification inhibitors in flooded rice

This work evaluates the effect of different slow-release fertilizers and nitrification inhibitors (NI) on N-use efficiency, grain yield and N₂ fixation in rice fields of Valencia (Spain) during three consecutive crop seasons (1998–2000). Eight N sources [ammonium sulphate, urea, polymer-coated urea (PCU 32% and 40% N), sulphur coated urea, isobutylidene diurea (IBDU), ammonium sulphate nitrate (ASN) plus dicyandiamide and ASN plus dimethyl pyrazole phosphate, were applied at 120 kg N ha^–1 and at 2 or 15 days before flooding (DBF) during 1998. Another experiment was based on the use of urea blended with PCU (40% N) at four ratios (1:0, 3:1, 1:1, 1:3) and applied at 15 DBF and at four rates (70, 95, 120 and 145 kg N ha-1) during 1999 and at only one rate (120 kg N ha^–1) during 2000. Both experiments also included a control (no N). The results showed that, when applied shortly before flooding, PCU (32% and 40% N) and IBDU application improved biological N₂ fixation compared to the conventional fertilizer application, with or without NI, reaching similar values to those observed in the absence of N fertilizer. Slow release fertilizers, particularly PCU 40% N applied basally before flooding, were the best N source for grain yield and improved recovery efficiency. Differences among N sources were only significant when the flooding was delayed for 15 days after fertilizer application.     

Fate of residual 15N-labeled fertilizer in dryland farming systems on soils of contrasting fertility

Abstract

Up to 50% of nitrogen (N) fertilizer can remain in soil after crop harvest in dryland farming. Understanding the fate of this residual fertilizer N in soil is important for evaluating its overall use efficiency and environmental effect. Nitrogen-15 (¹⁵N)-labeled urea (165 kg N ha^?1) was applied to winter wheat (Triticum aestivum L.) growing in three different fertilized soils (no fertilizer, No-F; inorganic nitrogen, phosphorus and potassium fertilization, NPK; and manure plus inorganic NPK fertilization, MNPK) from a long-term trial (19 years) on the south of the Loess Plateau, China. The fate of residual fertilizer N in soils over summer fallow and the second winter wheat growing season was examined. The amount of the residual fertilizer N was highest in the No-F soil (116 kg ha^?1), and next was NPK soil (60 kg ha^?1), then the MNPK soil (43 kg ha^?1) after the first winter wheat harvest. The residual fertilizer N in the No-F soil was mainly in mineral form (43% of the residual ¹⁵N), and for the NPK and MNPK soils, it was mainly in organic form. The loss rate of residual ¹⁵N in No-F soil over summer fallow was as high as 48%, and significantly (P < 0.05) higher than that in the NPK soil (22%) and MNPK soil (19%). The residual ¹⁵N use efficiency (RNUE) by the second winter wheat was 13% in the No-F soil, 6% in the NPK soil and 8% in the MNPK soil. These were equivalent to 9.0, 2.0 and 2.2% of applied ¹⁵N. The total ¹⁵N recovery (¹⁵N uptake by crops and residual in 0–100 cm soil layer) in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. The ¹⁵N uptake by wheat in two growing seasons was higher in the MNPK soil than in NPK soil. Therefore, we conclude that a high proportion of the residual ¹⁵N was lost during the summer fallow under different land management in dryland farming, and that long-term combined application of manure with inorganic fertilizer could increase the fertilizer N uptake and decrease N loss.     

Fate of Fertilizer-15N to a Maize Crop Grown in Northern Zambia

Abstract

A field study with maize (Zea mays L.) was conducted in the 1988/89 cropping season to investigate the fate of ¹⁵NO₃-N-labelled NH₄ ¹⁵NO₃ applied at 40, 80 and 120 kg N ha^?1 (unlabelled N applied at 0, 80, 160 and 240 N ha^?1) with and without lime. The investigations were conducted in northern Zambia at Misamfu Regional Research Centre, Kasama on a Misamfu red sandy loam soil. The experimental design was a split plot arrangement with four replications with main plots receiving 0 and 2 Mg ha^?1 dolomitic limestone, while subplots received fertilizer N at various rates. Significant (p < 0.001) grain and DM yield responses to applied N up to 160 kg ha^?1 were observed. At higher rates little or no crop responses were observed and fertilizer use efficiency declined. Partitioning of amounts of total N and ¹⁵N in plants was in the order of seed = tassel > leaf> cob = earleaf> stem. Fertilizer N rates showed a highly significant (p < 0.001) effect on plant uptake of labelled N. Lime and its interaction with N rates had no effect on all measured parameters. Leaching of NO₃-N fertilizer to lower soil depths was in proportion to the rate of N applied, with highly significant (p < 0.001) differences among soil depths. Although higher concentrations of fertilizer-¹⁵N were recovered in the 0–20 cm depth the recovered portion at lower soil depths was still significant. Total recovery of labelled N by plant and by soil after crop harvest averaged 75, 55 and 54% of originally applied fertilizer-¹⁵N at 40, 80 and 120 kg N ha^?1, respectively. Corresponding unaccounted for ¹⁵N was 25, 45 and 46%. The most probable loss mechanism could have been by leaching to depths greater than 60 cm, gaseous losses to the atmosphere and root assimilation.     

Green manure application improves rice growth and urea nitrogen use efficiency assessed using 15N labeling

ABSTRACT

Green manure is an efficient nitrogen (N) source when used as an alternative to chemical fertilizer. However, the N taken up by rice derived from green manure, chemical fertilizers or soil native N in complex nutrient systems is unclear. A pot experiment with partial substitution of urea with Chinese milk vetch (a green manure) implemented with ¹⁵N-labeled urea and Chinese milk vetch was set up to study the sources of N in rice and the fate of the fertilizers. The dry weights, N contents, N uptake, and urea N use efficiency were notably higher (by 15–16%, 4–13%, 22–30% and 182%-203%, respectively) in the Chinese milk vetch applied with urea treatment than in the urea alone treatment. The uptake of N from Chinese milk vetch and the use efficiency of Chinese milk vetch N were increased with reductions in the urea input amount. The application of Chinese milk vetch substantially changed the fate of urea: higher amounts of urea N were taken up by rice (approximately 29%) and remained as residue in the soil (approximately 15%) in the related treatments than in the treatment with urea alone (10% and 9%). More urea N than Chinese milk vetch N was taken up by rice (29% vs 20%, respectively) and lost (56% vs 14%, respectively), but less urea N than Chinese milk vetch N remained as residue in the soil (15% vs 66%, respectively). The partial substitution of chemical fertilizer with green manure is an effective method of promoting rice growth by supplying N for rice uptake and promoting more efficient N use.     

冬小麦对铵态氮和硝态氮的响应

在陕西省永寿县和河南省洛阳市分别设置了11和7处大田试验,分5层采集0~100 cm土壤样品并测定其起始硝态氮含量。永寿试验设7个处理,分别为不施氮,硝态氮、铵态氮品种、硝态氮与铵态氮2∶1组合各2个处理;洛阳试验设6个处理(硝态氮肥只有1个品种),施氮处理均施N 150 kg hm-2,研究小麦对铵态氮和硝态氮肥响应的差异及其与不同深度土层硝态氮累积量的关系。试验表明,同一形态不同氮肥品种之间的增产差异显著低于不同形态之间的差异。比较不同形态氮肥的小麦产量、增产量和增产率的平均值,硝态氮肥最高,硝态氮、铵态氮组合次之,铵态氮最低。氮肥增产量和增产率随土壤累积硝态氮量增加而显著下降;累积量越低,氮肥增产效果越突出,硝态氮的效果也越显著。由此可见,土壤累积的硝态氮量是决定氮肥肥效的主要因子,也是决定不同形态氮素效果的主要因子。只有在硝态氮累积量低的土壤上,氮肥才能充分发挥作用,硝态氮也才能表现出明显的优势。     

The Leaching Behaviour and Balances of Nitrogen and Other Elements under Spring Wheat in Lysimeter Experiment 1985–92

Abstract

In a lysimeter study it was found that moderate rates of ammonium nitrate increased utilization percentages in spring wheat, and the leaching was 10% or less of added N. Over-optimal rates reduced utilization percentages and increased leaching to almost 50% of the highest doses. Late split application of calcium nitrate increased the percentage of N in grain. Furthermore, leaching of N was not reduced, but occurred somewhat later in the fall and winter seasons. Leaching of Cl^? was more rapid and that of SO₄ ^2- was delayed relative to the leaching of NO₃ ^?. Rather large negative N balances were obtained, also after over-optimal application rates, and total N content of the soil was reduced. Compared with the N₀ treatment, differences in soil N residues amounted to 15–25% of added N in seven years. Gaseous losses had apparently taken place both from the added N and from soil N according to the total-N analysis.     

15N studies on the transfer of legume-fixed nitrogen to associated cereals in intercropping systems

Summary An attempt has been made to estimate quantitatively the amount of N fixed by legume and transferred to the cereal in association in intercropping systems of wheat (Triticum aestivum L.) — gram (Cicer arietinum L.) and maize (Zea mays L.) —cowpea (Vigna unguiculate L.) by labelling soil and fertilizer nitrogen with ¹⁵N. The intercropped legumes have been found to fix significantly higher amounts of N as compared with legumes in sole cropping if the intercropped cereal-legume received the same dose of fertilizer N as the sole cereal crop. But when half of the dose of the fertilizer N applied to sole cereal crop was received by intercropped plants, the amount of N fixed by legumes in association with cereals was significantly less than that fixed by sole legumes. Under field conditions 28% of the total N uptake by maize (21.2 kg N ha^–1) was of atmospheric origin and was obtained by transfer of fixed N by cowpea grown in association with maize. Under greenhouse conditions gram and summer and monsoon season cowpea have been found to contribute 14%–20%, 16% and 32% of the total N uptake by associated wheat and summer and monsoon maize crops, respectively. Inoculation of cowpea seeds with Rhizobium increased both the amount of N fixed by cowpea and transferred to maize in intercropping system.     

Ammonia volatilization from nitrogen fertilizers surface-applied to corn (Zea mays) and grass pasture (Dactylis glomerata)

Summary The major agronomic concern with NH₃ loss from urea-containing fertilizers is the effect of these losses on crop yields and N fertilizer efficiency. In this 2-year study, NH₃ volatilization from surface-applied N fertilizers was measured in the field, and the effects of the NH₃ losses detected on corn (Zea mays L.) and orchardgrass (Dactylis glomerata L.) yield and N uptake were determined. For corn, NH₄NO₃ (AN), a urea-AN solution (UAN), or urea, were surface-broadcast at rates of 0, 56 and 112 kg N ha^–1 on a Plano silt loam (Typic Argiudoll) and on a Fayette silt loam (Typic Hapludalf). Urea and AN (0 and 67 kg N ha^–1) were surface-applied to grass pasture on the Fayette silt loam. Significant NH₃ losses from urea-containing N sources were detected in one of four corn experiments (12%–16% of applied N) and in both experiments with grass pasture (9%–19% of applied N). When these losses occurred, corn grain yields with UAN and urea were 1.0 and 1.5 Mg ha^–1, respectively, lower than yields with AN, and orchardgrass dry matter yields with urea were 0.27 to 0.74 Mg ha^–1 lower than with AN. Significant differences in crop N uptake between N sources were detected, but apparent NH₃ loss based on N uptake differences was not equal to field measurements of NH₃ loss. Rainfall following N application markedly influenced NH₃ volatilization. In corn experiments, NH₃ loss was low and yields with all N sources were similar when at least 2.5 mm of rainfall occurred within 4 days after N application. Rainfall within 3 days after N application did not prevent significant yield reductions due to NH₃ loss from urea in grass pasture experiments.     

Effects of Irrigation Water Quality and Nitrogen Rate on the Recovery of 15N Fertilizer by Sorghum in Field Study

The effects of ¹⁵N-labeled ammonium nitrate on yield, uptake of nitrogen (N) by sorghum (Sorghum sudanense, Piper), and on N remaining in the soil were studied in a field experiment with different N rates (0, 50, and 100 kg N ha^?1) and with two irrigation water qualities, well water (WW) and treated wastewater (TWW). Treated wastewater irrigation increased dry matter and N yield compared to WW. At equal N rates, recovery of ¹⁵N-labeled fertilizer by plants increased with TWW irrigation compared to WW (36% versus 23%). Neither fertilizer rate nor water quality had an effect on the ¹⁵N-labeled fertilizer remaining in the 0- to 60-cm layer of soil. On average 41% in the TWW treatment (49–33%) and 38% in WW treatment was mostly present in the surface 20-cm layer. Losses of ¹⁵N-labeled fertilizer were unaffected by irrigation water quality (35%) and increased with N application rate in TWW (4% versus 31%).     

Alder-poplar associations: Determination of plant nitrogen sources by isotope techniques

Summary Three¹⁵N isotopic dilution methods (¹⁵N natural abundance, labelled mineral fertilizer, and organic matter) were used to determine the proportion of N derived from different available sources in seedlines ofAlnus glutinosa andPopulus nigra planted together or in monoculture under natural climatic conditions. The proportion of N derived from N₂ fixation in associated alders was appreciably higher than that determined in monoculture. The reduction of soil N uptake by associated alders contributed to an increase in total plant N and biomass production in associated poplars. When slightly N-labelled organic matter (alder leaf litter) was incorporated into the soil, 10–15% of its initial N content was recovered in poplar tissues, showing that this N source makes an important contribution to the N yield of associated non-fixing plants. There were no significant differences between the results obtained by¹⁵N natural abundance and those obtained by labelled fertilizer methods, suggesting that the ¹⁵N method could be used to evaluate annual N budgets in natural ecosystems.