The effect of pellet size on the ammonia loss from urea applied to forest soil

Summary Using diluted phosphoric acid- and glycerol-treated polyurethane plastic foam discs as static ammonia absorbers, the extent of ammonia loss from applied urea was determined on forest soil under field conditions. The investigation, which primarily involved a comparison of urea materials of two different pellet sizes (2–4 mg and 2060 mg urea per pellet, respectively), extended over a period of 28 days. The urea was applied to the soil surface at a rate of 200 kg N per hectare.It was found that the gaseous ammonia loss from the large-pellet urea (tablets) was markedly lower than that from the small-pellet urea during the first two weeks of exposure. On extending the observation period to four weeks, this difference in cumulative ammonia loss decreased successively. After 28 days' exposure, at a daily mean temperature of 13°C, the volatile loss amounted to 20 and 22 per cent, respectively. An addition of 5% (w/w) of a solution of concentrated orthophosphoric acid, or a fine-crystalline ortho-boric acid to the large-pellet urea, resulted in a reduction of the loss to half the value. The possibilities of controlling the ammonia loss from urea by combining the large pellet size with the addition of urease inhibitors are discussed. re]19721012     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.     

施用坡缕石对黄绵土中尿素氮的挥发和淋溶损失的影响

通过室内模拟试验,采用吸收法和土柱淋溶法研究了施用坡缕石对黄绵土中尿素氮的挥发和淋溶的影响.结果表明：施用坡缕石+尿素处理能降低尿素氨挥发高峰期的挥发速率,比单施尿素处理的氨挥发损失减少了13.6%～15.0%.坡缕石施用量为0.3和0.6 g·kg^-1时,降低了NH₄⁺-N和NO₃^--N的淋溶速率,无机氮淋溶损失比单施尿素处理分别减少13.7%和13.6%;而坡缕石施用量为0.9 g·kg^-1时,加快了NH₄⁺-N和NO₃^--N的淋溶速率,无机氮淋溶损失比单施尿素处理增加了6.1%.施用低量（0.3 g·kg^-1）坡缕石+尿素处理土壤的NH₄⁺-N含量比单施尿素处理提高了0.20 mg·kg^-1,而施用高量（0.9 g·kg^-1）坡缕石+尿素处理土壤的NH₄⁺-N含量比单施尿素处理降低了0.42 mg·kg^-1;施用坡缕石+尿素处理土壤的NO₃^--N含量比单施尿素处理增加1.24～2.52 mg·kg^-1.表明施用坡缕石能减少土壤中尿素氨的挥发损失,在一定用量范围内能降低NH₄⁺-N和NO₃^--N的淋失,提高土壤NH₄⁺-N和NO₃^--N含量.     

几种控释氮肥减少氨挥发的效果及影响因素研究

采用“静态吸收法”和“土柱淋溶法”、室内模拟试验,研究几种控释氮肥施入土壤后的氨挥发损失情况、N溶出速率、土壤脲酶活性及pH值变化的关系．结果表明,施氮450mg·kg^-1土时,3种控释氮肥氨挥发损失氮总量分别比普通尿素减少49．7％、28．0％和71．2％;施氮600mg·kg^-1土时,3种控释氮肥氨挥发损失氮总量分别比普通尿素减少34．6％、12．3％和69．9％．控释氮肥能显著降低土壤氨挥发量,减少因施肥而引起的大气环境污染．控释氮肥氨挥发量与不同氮肥引起的土壤脲酶活性、pH值、土壤中氮溶出速率密切相关．土壤的氨挥发总量与肥料在土壤中溶出总量的相关系数达到0．9533,在肥料施入的前期土壤氨挥发量同土壤脲酶活性、pH值的相关系数达到0．9533和0．9908。     

Lysimeter studies on recovery of15N-labeled urea in wetland rice

Summary Results of a two year study on the fate on¹⁵N-labelled urea (9.95 atoms percent excess¹⁵N) applied @ 180 kg N/ha to flooded rice in monolith lysimeters at the Punjab Agricultural University Farm, Ludhiana are reported. The soil of the experimental field was sandy clay loam in texture (Typic Ustochrept), had pH 7.9, organic carbon 0.36 percent, available N 187 kg/ha and total N 0.08 percent. The results revealed that 18.1 to 53.0 per cent of the fertilizer N was utilized by the rice plant, 25.1 to 41.1 percent was immobilized in the soil and 4.8 to 7.2 percent was lost by denitrification. The losses due to ammonia volatilization and leaching were negligible. The data on vertical distribution of labelled N in the soil profile reflected a higher concentration (38.3 to 39.5 per cent) in the surface (0–30 cm) soil. The content sharply decreased (1.8 to 2.4, percent) in lower soil layers (30–150 cm). A balance sheet of the various pathways of applied N showed that 58.8 to 72.2 and 66.2 to 83.0 percent N was recovered in 1976 and 1977, respectively and 17 to 41.2 per cent of labelled N still remained unaccounted for. Utilization of fertilizer N by rice was increased and losses decreased when N was applied in three equal splits as compared to the single N application at transplanting.Availability of fertilizer N immobilized in the soil was investigated in the succeeding crops of wheat and rice. The results showed that 2.1 tot 3.4 per cent of the N applied to the preceding rice was utilized by the second rice crop grown in succession. This may look small but cannot be neglected on a long term basis. But there is need to initiate long term studies to investigate the, turnover of residual N and to determine the fate of applied N in varying soil and cropping systems by using improved techniques.     

Ammonia volatilization losses from prilled urea,urea supergranules (USG) and coated USG in rice fields

Summary About 8.4 per cent of applied nitrogen was lost as ammonia during a week after application when prilled urea was broadcast or banded and incorporated in soil 20 days after sowing of rice. Ammonia volatilization was reduced to 3.3 per cent when urea supergranules (USG) were used. Coating of USG with DCD or neem cake showed no advantage. Ammonia volatilization was only 0.7 to 1.6 per cent when fertilizer was applied at panicle initiation stage of rice; highest values were again obtained with prilled urea. The experiments were carried out in closed cages.     

The fate of fresh and stored 15N-labelled sheep urine and urea applied to a sandy and a sandy loam soil using different application strategies

The fate of nitrogen from ¹⁵N-labelled sheep urine and urea applied to two soils was studied under field conditions. Labelled and stored urine equivalent to 204 kg N ha^–1 was either incorporated in soil or applied to the soil surface prior to sowing of Italian ryegrass (Lolium multiflorum L.), or it was applied to ryegrass one month after sowing. In a sandy loam soil, 62% of the incorporated urine N and 78% of the incorporated urea N was recovered in three cuts of herbage after 5 months. In a sandy soil, 51–53% of the labelled N was recovered in the herbage and the distribution of labelled N in plant and soil was not significantly different for incorporated urine and urea. Almost all the supplied labelled N was accounted for in soil and herbage in the sandy loam soil, whereas 33–34% of the labelled N was unaccounted for in the sandy soil. When the stored urine was applied to the soil surface, 20–24% less labelled N was recovered in herbage plus soil compared to the treatments where urine or urea were incorporated, irrespective of soil type. After a simulated urination on grass, 69% of the labelled urine N was recovered in herbage and 15% of the labelled N was unaccounted for. The labelled N unaccounted for was probably mainly lost by ammonia volatilization.Significantly more urine- than urea-derived N (36 and 19%, respectively) was immobilized in the sandy loam soil, whereas the immobilization of N from urea and urine was similar in the sandy soil (13–16%). The distribution of urine N, whether incorporated or applied to the soil surface prior to sowing, did not influence the immobilization of labelled urine N in soil. The immobilization of urine-derived N was also similar whether the urine was applied alone or in an animal slurry consisting of labelled urine and unlabelled faecal N. When urine was applied to growing ryegrass at the sandy loam soil, the immobilization of urine-derived N was significantly reduced compared to application prior to sowing. The results indicated that the net mineralization of urine N was similar to that of urea in the sandy soil, but only about 75% of the urine N was net mineralized in the sandy loam soil, when urine was applied prior to sowing. Thus, the fertilizer effect of urine N may be significantly lower than that of urea N on fine-textured soils, even when gaseous losses of urine N are negligible.     

南京郊区番茄地中氮肥的效应与去向

采用田间小区和微区试验,研究了施用化学氮肥对南京郊区菜地番茄产量、氮肥去向及氮素损失的影响.结果表明, 由于土壤和有机肥供氮充分,氮肥施用未增加番茄产量.用差值法计算得到的氮肥利用率在14.5%～22.5%之间.¹⁵N标记尿素微区试验表明,施入氮量的16.6%～28.8%被作物吸收,氮素总损失为34.2%～46.0%.施用化学氮肥增加了土壤剖面中的硝酸盐含量,番茄收获时,10%～10.2%的标记尿素被淋洗到40 cm以下土层.增施化学氮肥也显著增加了菜地土壤的反硝化损失和N₂O排放,其中反硝化损失占施入氮量的5.50%～6.01%;N₂O排放量占施入氮量的2.62%～4.92%.但番茄生长期间未检测到氨挥发.减少氮肥用量或施用包衣尿素可降低菜地施用氮肥的环境风险,特别是减少硝酸盐淋洗和硝化反硝化损失.     

Patterns of ammonia volatilization from a forest soil

Summary Ammonia volatilization from urea-treated soils was estimated under field and laboratory conditions. Acid-washed filter papers were hung in the air in a spruce stand treated with N and P fertilizers in a factorial design. In the laboratory, moss sods were incubated to quantify ammonia volatilization.Ammonia volatilization increased with the level of N applied and more ammonia was absorbed by filter papers at 0.6 m above the ground than those at 1.2 m. Maximum rates of ammonia volatilization in urea-treated plots were observed between the third and fourth day after fertilizer application and similar absorption patterns were observed in areas not treated with urea. It is, therefore, suggested that ammonia volatilized from urea-treated plots can move to untreated areas. Addition of P along with urea significantly reduced ammonia volatilization under field conditions.Laboratory experiments showed that addition of urea to moss sods increased the pH of the organic layer from about 3.6 to 8.8. Sphagnum moss sods volatilized more ammonia (about 1.7 per cent of the added material) than feather moss sods (about 0.8 per cent). At higher incubation temperatures, however, the rate of ammonia volatilization decreased in sphagnum moss sods but increased in feather moss sods.     

15N标记水稻控释氮肥对提高氮素利用效率的研究

本文应用^15N示踪技术研究了水稻对空控释氮肥和尿素氮吸收利用效率的影响以及氮的去向,结果表明：施肥后11天内,水稻控释氮肥和尿素的NH3挥发损失分别占施入氮量的0.69%和1.81%,NH3的挥发损失在施肥后第5天时达到最大值,此后逐渐降低。水稻控释氮肥和尿素氮的淋溶损失分别占施入氮量的0.95%和1.02%,水稻控释氮肥氮的淋溶损失在水稻整个生长期间均比较平缓,施肥后40天时略有上升,此后又缓慢降低。用氮素平衡帐中的亏缺量和缺量扣除氨的损失量后计为硝化-反硝化损失量的结果表明,水稻控制氮肥氮的硝化-反硝化损失量占施氮量的3.46%,而尿素氮在硝化-反硝化损失量却高达37.75%,肥料氮在土壤中的残留主要集中在0～35cm的土层中,达91.4%-91.5%,残留在35cm以下土层中的氮甚微,水稻控制氮肥残留在土壤中的氮量略高于尿素处理。水稻控释氮肥利用率高达73.8%,比尿素高出34.9%,水稻控释氮肥氮利用率高的原因是因氮从颗粒中缓慢释放、受淋溶、氨挥发、尤其受硝化-反硝化途径损失的氮较少。在施等氮量的条件下,施用水稻控制氮肥的稻谷产量比尿素的增产25.5%,达到p=0.05的显著水平。     

不同缓控释肥对鲜食玉米产量、品质及氨挥发的影响

采用田间试验,设置不施氮对照(CK)、常规施肥(U)、增效尿素(DU)、包膜尿素(CU)、缓控释掺混尿素(CDU)共5个处理,研究了常规施肥(240 kg N·hm^-2)和不同缓控释肥料一次性减量施用(180 kg N·hm^-2)对鲜食玉米产量、品质与土壤无机氮变化和氨挥发的影响。结果表明: U处理氨挥发总量最高,追肥是产生氨挥发损失的重要因素;与U处理相比,DU、CU、CDU处理氨挥发减排78%～81%。收获后U处理80～100 cm土层硝态氮浓度最高,为51.6 mg·kg^-1,氮淋溶风险较高,而DU、CU、CDU处理同土层硝态氮含量均较低,降低了淋溶风险。与U处理相比,减氮25%的3个缓控释肥处理没有减产,并增加了籽粒维生素C、可溶性糖和蛋白质含量;缓控释肥处理之间,DU处理的氮肥农学效率和经济效益最高。综上,减量施用新型缓控释肥可以实现鲜食玉米稳产提质,显著降低氨挥发损失和硝态氮淋失风险。与成本较高的树脂包膜控释肥相比,双效抑制剂增效肥(DU)成本低、制作便捷,是鲜食玉米专用肥的较好选择。     

Urea-triazone N characteristics and uses

Urea-triazone nitrogen (N) is a stable solution resulting from a controlled reaction in aqueous medium of urea, formaldehyde, and ammonia which contains at least 25% total N. This N source contains no more than 40%, nor less than 5%, of total N from unreacted urea and not less that 40% from triazone. All other N shall be derived from water-soluble dissolved reaction products of the above reactants. It is a source of slowly available N. The rate of mineralization of urea-triazone is about 66% that of urea after 8 days when incorporated in a Munjor sandy loam. Ammonia volatilization losses of N applied as urea-triazone were about 41% of those from urea on a Cecil sandy loam in the first week after application. N leaching losses through saturated Yolo loam columns of urea-triazone were about two thirds that of urea or nitrate N. This N source has proven to be a safer and more effective material for direct application on plant foliage. Tomato growth was enhanced with foliar application of urea-triazone relative to that obtained from ammonium nitrate or urea. The stability of this N source from potential losses via ammonia volatilization and nitrate leaching when soil applied is also documented by results from university trials.     

不同包膜控释尿素对农田土壤氨挥发的影响

为了探索包膜控释尿素土壤氨挥发损失规律特征和提高肥料氮素利用率,采用小麦玉米轮作田间试验,通过与普通尿素进行对比,运用土壤氨挥发原位测定方法——通气法系统研究了硫包膜和树脂包膜控释尿素的施用对小麦玉米轮作农田土壤氨挥发的影响.研究结果表明:在两种施氮量水平下(210 kg/hm2和300 kg/hm2),与普通尿素相比,硫包膜和树脂包膜控释尿素在小麦基肥期、小麦追肥期和玉米施肥期的施用均减少了土壤氨挥发的累积损失量,分别达35.1％-54.3％、59.6％-75.2％、65.6％-98.1％;有效降低了土壤氨挥发通量峰值且延迟其出现时间3-8 d,并能延缓土壤氨挥发主要阶段的时间分别为4-12 d、5-12 d.在小麦玉米轮作周年中,控释尿素土壤氨挥发累积损失量为28.39-43.35 kg/hm2,土壤氨挥发损失率为4.48％-5.63％,控释尿素时段土壤氨挥发通量比普通尿素降低了51.0％-70.8％;且树脂包膜控释尿素的施用降低小麦玉米轮作农田土壤氨挥发的效果优于硫包膜控释尿素.     

脲酶抑制剂和硝化抑制剂的协同作用对尿素氮转化和N2O排放的影响

根据培养试验,论述了脲酶抑制剂氢醌和硝化抑制剂双氰胺和碳化钙的不同组合在土壤正常水分和渍水的条件下对于土中尿素的水解及其释出的氨的吸附、氧化和挥发以及Ｎ２Ｏ生成的影响．文章指出,配合使用氢醌和双氰胺既能延缓土中尿素的水解并使水解后释出的氨在土中得以更多和更长时间的保持,还能减少土中硝酸盐的累积、氨挥发的损失及Ｎ２Ｏ的生成．这表明在脲酶抑制剂和硝化抑制剂间可能存在一定的协同作用．很好利用这一作用,将有益于提高尿素肥效和减少其Ｎ损失与环境污染．     

Agronomic assessment and 15N recovery of urea amended with the urease inhibitor nBTPT (N-(n-butyl) thiophosphoric triamide) for temperate grassland

Three field experiments were undertaken concurrently at one site to evaluate a range of surface-applied nBTPT-amended urea products (0.01, 0.05, 0.1, 0.25 and 0.5% nBTPT w/w) on NH₃ volatilization, grass yield and ¹⁵N recovery in the plant-soil system. Each experiment was repeated on five separate occasions over the 1992 growing season to cover a range of weather conditions. Total NH₃ loss from unamended-urea ranged from 5.5% in early May to 20.8% in June. The inhibitor was highly effective in reducing ammonia volatilization and delaying the time at which maximum rate of NH₃ loss occurred. Over all time periods the % inhibition was 50.4, 82.8, 89.0, 96.5 and 97.0% at the 0.01, 0.05, 0.1, 0.25 and 0.5% nBTPT levels respectively. There was no significant difference in the overall % inhibition in ammonia loss at different times suggesting that the effectiveness of the inhibitor was not dependent on climatic conditions.Over all times incorporation of nBTPT at the 0.05% level increased dry-matter yield by 9% compared to urea alone and increased the shoot recovery of N from 66.7% to 80.9%. Nitrogen saved from volatilization was taken up by the plant, however, the subsequent translation into dry-matter yield appeared to be adversely affected at the high inhibitor rates.There was no significant effect of inhibitor on ¹⁵N recovery in soil at any depth down to 15 cms. nBTPT significantly increased (p < 0.001) the % N derived from fertilizer (% N dff) in the shoot compared to unamended-urea and increased (p < 0.01) the shoot recovery of ¹⁵N from 32% up to 39%. Total ¹⁵N recovery in the soil-plant system was increased by up to 17% by amending urea with nBTPT. This urease inhibitor has been shown to improve the efficiency of urea however, its potential for the European market will be dependent on economic factors.Faculty of Agriculture and Food Science, The Queen's University of Belfast     

Effect of phenylphosphorodiamidate on urea hydrolysis,ammonia volatilization and rice growth in an alkali soil

Summary In order to improve nitrogen recovery by rice, the effect of a urease inhibitor phenylphosphorodiamidate (PPD) on the efficiency of fertilizer urea was studied in laboratory and greenhouse. Addition of PPD to urea (5% w/w) delayed urea hydrolysis by 3 to 4 days and reduced ammonia volatilization from 45% (without PPD) to 8.5% (with PPD). Ammonia volatilization obeyed first order kinetics. Urea hydrolysis was sufficiently strongly inhibited to match the nitrification potential of the soil. N application to rice by three different modes showed that a delayed mode (4 splits) was superior to two conventional modes (3 splits) in nitrogen recovery and fertilizer efficiency since it met nitrogen requirement of plants at reproductive stage. In 2 out of 3 modes of application, there was a 14% increase (relative) in grain yields and dry matter, and 6.8% increase in N uptake efficiency on application of PPD along with urea. The results indicate that urease inhibitors like PPD can be effectively used to block urea hydrolysis, reduce ammonia volatilization losses and improve N use efficiency by rice.     

控释肥料与普通氮肥混施对春白菜产量、品质和氮素损失的影响

在京郊露地生产条件下,研究了控释肥料与速效化肥混配施用对春白菜产量、品质、氨挥发、土壤硝态氮累积和淋失的影响.结果表明：与习惯施肥处理（施N 300 kg·hm^-2）相比,控释肥料与普通化肥按纯氮比2∶1混配施用（共施N 150 kg·hm^-2）没有造成白菜减产,并显著降低了菜叶中硝酸盐和有机酸含量;与半量施肥处理(施N 150 kg·hm^-2)相比,控释肥与化肥混施处理产量和叶片硝酸盐含量无显著差异.控释肥与化肥混施处理提高了白菜氮肥利用率,减少了N₃-N淋失量和氨挥发总量.白菜收获后,控释肥与化肥混施处理在20～40、60～80、80～100 cm土层的NO₃^--N含量显著低于习惯施肥处理.     

Tracer studies on nitrogen immobilization-mineralization relationships in forest raw humus

Summary An investigation was conducted to study the effect of fertilizer-N, application rate, and incubation temperature on the immobilization-mineralization relationships of N in forest raw humus. Urea, ammonium chloride, and potassium nitrate enriched with N¹⁵ were used at rates of 0, 50, 100, 200, and 400 ppm N. During a 60 days' experimental period the soil was incubated at 12° and 20°C. The interchanges of N added to this highly acid humus material are to a great extent found to be governed by the N-carrier itself. The total recovery in the N¹⁵ inorganic pool at the end of the 60 days' period, increased with increasing fertilizer application rate regardless of the source of N added. With one exception only, the recovery of KCl-extractable N¹⁵ was larger in the ammonium treatment than in the urea-treated humus. The more striking differences between the interchanges of N¹⁵ in the urea and the ammonium treatments, as compared with the nitrate-treated humus, are revealed in the extremely high recovery of KCl-extractable N, and a concomitant low recovery of the added N in the non-extractable pool in the nitrate treatment. The immobilization and re-mineralization of N are found to be positively correlated with temperature. The total recovery of N¹⁵ was high in the nitrate treatment, and even higher in the ammonium-treated soil (96 to 104 per cent). However, the unaccounted-for losses reached a maximum of about 48 per cent of the added N at 20°C in the 400 ppm urea-N treatment. The results are consistent in showing a statistically highly significant effect of N-fertilizer, application rate, and temperature on the net mineralization of humus N. The release of soil N is discussed in relation to the highly significant two- and three-way interactions between the above-mentioned main variables. Contribution from the Forest Soil Fertilization Research Group, Vollebekk, Norway. This work was supported by the Agricultural Research Council of Norway.     

A model to predict transformations and losses of nitrogen in UK pastures grazed by beef cattle

The model simulates the cycling of N in grassland systems grazed by beef cattle and predicts the annual amount of N in liveweight gain, and the amounts lost through ammonia volatilization, denitrification and leaching, on the basis of fertilizer application and soil and site characteristics. It aims to provide a better understanding of the way in which these various factors interact in their influence on N transformations. The model has been programmed to run on IBM-compatible personal computers and responds rapidly to changes in input parameters. The model has been constructed from the average annual amounts of N passing through various components of the N cycle in ten field systems grazed by beef cattle. The amounts were either measured directly or were calculated from empirical sub-models, assuming a balance between inputs to, and outputs from the soil inorganic N pool. The model is given wide applicability through the inclusion of a mineralization sub-model which is sensitive to soil texture, sward age, previous cropping history, and climatic zone. Another important sub-model determines the partitioning of soil inorganic N to either plant uptake or the processes of loss: the proportion partitioned to plant uptake decreases as the total amount of soil inorganic N increases. Outputs from the model indicate that fertilizer N has a strong influence on ammonia volatilization, denitrification and leaching at a given site but that, over a range of sites with a given rate of fertilizer N, total loss and the proportions lost by the three processes are greatly influenced by the amount of N mineralized by the soil. The model indicates how fertilizer N should be matched with mineralization to limit gaseous and leaching losses and to achieve optimum efficiency of N use in grazing systems.     

Is the high 15N natural abundance of trees in N-loaded forests caused by an internal ecosystem N isotope redistribution or a change in the ecosystem N isotope mass balance?

High δ¹⁵N of tree foliage in forests subject to high N supply has been attributed to ¹⁵N enrichment of plant available soil N pools after losses of N through processes involving N isotope fractionation (ammonia volatilization, nitrification followed by leaching and denitrification, and denitrification in itself). However, in a long-term experiment with high annual additions of NH₄NO₃, we found no change in the weighted average δ¹⁵N of the soil, but attributed the high δ¹⁵N of trees to loss of ectomycorrhizal fungi and their function in tree N uptake, which involves redistribution of N isotopes in the ecosystem (Högberg et al. New Phytol 189:515–525, 2011), rather than a loss of isotopically light N. Here, we compare the effects of additions of urea and NH₄NO₃ on the δ¹⁵N of trees and the soil profile, because we have previously found higher δ¹⁵N in tree foliage in trees in the urea plots. Doing this, we found no differences between the NH₄NO₃ and urea treatments in the concentration of N in the foliage, or the amounts of N in the organic mor-layer of the soil. However, the foliage of trees receiving the highest N loads in the urea treatment were more enriched in ¹⁵N than the corresponding NH₄NO₃ plots, and, importantly, the weighted average δ¹⁵N of the soil showed that N losses had been associated with fractionation against ¹⁵N in the urea plots. Thus, our results in combination with those of Högberg et al. (New Phytol 189:515–525, 2011) show that high δ¹⁵N of the vegetation after high N load may be caused by both an internal redistribution of the N isotopes (as a result of change of the function of ectomycorrhiza) and by losses of isotopically light N through processes fractionating against ¹⁵N (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).