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)     

An assessment of granular urea/ammonium sulphate and urea/potassium nitrate fertilizers on nitrogen recovery by ryegrass

The comparative effects of ammonium sulphate (AS), potassium nitrate (KNO₃), urea (U) or combined 1:1 (w/w) U/KNO₃, U/AS granular products were investigated on dry matter (DM) yield and¹⁵N utilisation by perennial ryegrass grown under controlled environmental conditions.The DM yield and apparent N recovery with the single N sources was in the order KNO₃ > AS > U. The¹⁵N budget in shoots, roots and soil indicated that only 55% of the urea N was recovered at the end of the experiment compared with 87% and 86% for AS and KNO₃ respectively. The DM yield and apparent N recovery from the combined U/AS source was significantly higher than would be expected (P < 0.01) based on the proportions of each N source in the mixture. Differentially labelling the U and AS with¹⁵N indicated that AS enhanced the shoot % utilisation of urea by 38% (P < 0.001) whereas urea reduced the shoot % utilisation of AS by 14% (P < 0.01). These results indicate that an interaction occurred between U and AS when combined in a 1:1 (w/w) ratio in the same pellet.     

Plant recovery of 15N-labelled nitrogen applied to reed canary grass grown for biomass

Reed canary grass (Phalaris arundinacea L.) is apotential crop for production of bioenergy and biomass in northern Europe. In this study labelled ¹⁵N was used to follow the fate of applied N in roots and shoots of reed canary grass during a year. Two rates of¹⁵N fertiliser were applied in spring 1995 and 1996 to a clay (50 kg ha⁻¹ and 100 kg ha⁻¹) and an organic soil (30 kg ha⁻¹ and 60 kg ha⁻¹). The data did not indicate significant differences between recoveries of nitrogen following application of fertiliser at recommended and half of the recommended rates. The recovery of added N in shoots was highest at midsummer. The median values were 68% and 58% inorganic soil and 42% and 65% in clay soil, in 1995 and 1996respectively. Some of the N utilised by shoots was remobilised to the roots during autumn. The highest median recovery of applied N in roots was 19%in clay soil in October 1996, corresponding to a 13 percentage unit increase in recovery during autumn. In contrast, the lowest remobilisation was recorded after a rainy spring in clay soil, being only 3 percentage units. During winter the loss of N and fertiliser N from the shoots continued, and consequently the total N content in shoots was about half of that for autumn. In spring, one year after N application, the shoots contained 9–20% of applied N. The data suggest both intensive uptake and remobilisation of fertiliser N during over a year, following delayed harvest, and indicate the importance of the rhizome system in N turnover. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fate of fertilizer nitrogen applied to wheat under simulated mediterranean environmental conditions

Triticum aestivumThe fate of fertilizer nitrogen applied to dryland wheat was studied in the greenhouse under simulated Mediterranian-type climatic conditions. Wheat, L., was grown in 76-cm-deep pots, each containing 50–70 kg of soil, and subjected to different watering regimes. Two calcareous clay soils were used in the experiments, Uvalde clay (Aridic Calciustoll) and Vernon clay (Typic Ustochrept). Fertilizer nitrogen balance studies were conducted using various¹⁵N-labeled nitrogen sources, including ammonium nitrate, urea, and urea amended with urea phosphate, phenyl phosphorodiamidate (a urease inhibitor), and dicyandiamide (a nitrification inhibitor). Wheat yields were most significantly affected by available water. With additional water during the growing period, the recovery of fertilizer nitrogen by wheat increased and the fraction of fertilizer nitrogen remaining in the soil decreased. In the driest regimes, from 40 to 65% of the fertilizer nitrogen remained in the soils. In most experiments the gaseous loss of fertilizer nitrogen, as estimated from unaccounted for¹⁵N, was not significantly affected by water regime. The¹⁵N not accounted for in the plant and the soil at harvest ranged from 12 to 25% for ammonium nitrate and from 12 to 38% for regular urea. Direct measurement of labeled ammonia loss from soil indicated that ammonia volatilization probably was the main N loss mechanism. Low unaccounted-for¹⁵N from nitrate-labeled ammonium nitrate, 4 to 10%, indicated that N losses due to denitrification, gaseous loss from plants, or shedding of anthers and pollen were small or negligible. Amendment of urea with urea phosphate to form a 36% N and 7.3% P product was ineffective in reducing N loss. Dicyandiamide did not reduce N loss from urea presumably because N was not leached from the sealed pots and denitrification was insignificant. Amendment of urea with 2% phenyl phosphorodiamidate reduced N loss significantly. However, band placement of urea at as 2-cm soil depth was more effective in reducing N loss than was amendment of broadcast urea with phenyl phosphorodiamidate.     

Fate of nitrogen applied in different fertilizers to the surface of a calcareous soil in Syria

¹⁵N-labelled ammonium sulphate or¹⁵N-labelled urea were each applied in solutionat a rate of 30 kg N ha^-1 to the surface of 20soil cores (52 mm internal diameter × 100 mm deep)located on a field experiment at the ICARDA station,Tel Hadya, Syria. Recovery of ¹⁵N-label in theammonium, nitrate, organic and/or urea-N pools in thesoil was measured on days 0, 1, 2, 5 and 13 afterapplication. Total recovery of ¹⁵N was initially100%, but by day 13 after application it had declinedto 51% with urea and 73% with ammonium sulphate.Ammonium nitrate labelled either as ammonium or asnitrate was also applied to the soil surface of 8other cores at the same time. ¹⁵N recovery in thefour soil N pools was measured only on day 12 afterapplication. Total recovery of ¹⁵N-label was 75%with labelled ammonium and 57% with labelled nitrate.Volatilization of ammonia from this calcareous soil(pH 8.1) is one probable mechanism of N loss fromammonium and urea fertilizers: with nitrate bothleaching beyond the base of the core (i.e. 100 mm) and denitrification were responsible for Nlosses. These large losses of N immediately afterapplication have implications for fertilizermanagement practices.     

Initial and residual effects of nitrogen fertilizers on grain yield of a maize/bean intercrop grown on a Humic Nitosol and the fate and efficiency of the applied nitrogen

Initial and residual effects of nitrogen (N) fertilizers on grain yield of a maize/bean intercrop grown on a deep, well-drained Humic Nitosol (66% clay, 3% organic carbon) were evaluated. Enriched (¹⁵N) N fertilizer was used to study the fate of applied N in two seasons: using urea (banded) at 50 kg N ha^–1 in one season, and¹⁵N-enriched urea (banded), calcium ammonium nitrate (CAN, banded), and urea supergranules (USG, point placement) were applied in the other season (different field) at 100 kg N ha^–1. Nitrogen fertilizer significantly (P = 0.05) increased equivalent maize grain yield in each season of application with no significant differences between N sources, i.e., urea, CAN, and USG. Profitmaximizing rates ranged from 75 to 97 kg N ha^–1 and value: cost ratios ranged from 3.0 to 4.8. Urea gave the highest value: cost ratio in each season. Most (lowest measurement 81%) of the applied N was accounted for by analyzing the soil (to 150 cm depth) and plant material. Measurements for urea, CAN, and USG were not significantly different. The high N measurements suggest low losses of applied N fertilizer under the conditions of the study. Maize plant recovery ranged from 35 to 55%; most of this N (51–65%) was in the grain. Bean plant recovery ranged from 8 to 20%. About 34–43% of the applied N fertilizer remained in the soil, and most of it (about 70%) was within the top soil layer (0–30 cm). However, there were no significant equivalent maize grain increases in seasons following N application indicating no beneficial residual effect of the applied fertilizers.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. II. Punjab,India

Two modified urea products (urea supergranules [USG] and sulfur-coated urea [SCU]) were compared with conventional urea and ammonium sulfate as sources of nitrogen (N), applied at 58 kg N ha^–1 and 116 kg N ha^–1, for lowland rice grown in an alkaline soil of low organic matter and light texture (Typic Ustipsamment) having a water percolation rate of 109 mm day^–1. The SCU and USG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled; the SCU was prepared in the laboratory and was not completely representative of commercial SCU. The SCU was broadcast and incorporated, whereas the USG was point-placed at a depth of 7–8 cm. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation. All fertilizers except the last one-third of the urea and ammonium sulfate were labeled with¹⁵N so that a fertilizer-N balance at flowering and maturity stages of the crop could be constructed and the magnitude of N loss assessed.At all harvests and N rates, rice recovered more¹⁵N from SCU than from the other sources. At maturity, the crop recovered 38 to 42% of the¹⁵N from SCU and only 23 to 31% of the¹⁵N from the conventional fertilizers, urea and ammonium sulfate, whose recovery rates were not significantly different. In contrast, less than 9% of the USG-N was utilized. Fertilizer nitrogen uptake was directly related to the yield response from the different sources. Most of the fertilizer N was taken up by the time the plants were flowering although recovery did increase up to maturity in some treatments.Analysis of the soil plus roots revealed that less than 1% of the added¹⁵N was in the mineral form. Between 20 and 30% of the¹⁵N applied as urea, SCU, and ammonium sulfate was recovered in the soil plus roots, mainly in the 0–15 cm soil layer. Only 16% of the¹⁵N applied as USG was recovered in the soil, and this¹⁵N was distributed throughout the soil profile to a depth of 70 cm, which was the lowest depth of sampling.Calculations of the¹⁵N balance showed that 46 to 50% of the urea and ammonium sulfate was unaccounted for and considered lost from the system. Only 27 to 38% of the¹⁵N applied as SCU was not recovered at maturity, but 78% of the USG application was unaccounted for. The extensive losses and poor plant recovery of USG at this site are discussed in relation to the high percolation rate, which is atypical of many ricegrowing areas.     

Leaching loss of calcium,magnesium, potassium and nitrate derived from soil,lime and fertilizers as influenced by urea applied to undisturbed lysimeters in south-east Nigeria

Calcium hydroxide was applied to monolith lysimeters at Onne in south-east Nigeria. Eight lysimeters were cropped with maize followed by upland rice and four were uncropped. The cropped and two uncropped lysimeters received Mg, K and urea in the first season. Two uncropped lysimeters received no fertilizers. Drainage water was collected during the two growing seasons and analyzed for calcium, magnesium, potassium, sodium, nitrate and chloride. The fertilizer applied in the second season was not leached during the year of application.The cropped lysimeters lost 27 percent of the sum of the exchangeable Ca in the soil profile and the calcium added, and 29 percent of the corresponding sum for Mg. With no crop, the losses increased to 34 and 37 percent, respectively, but with no crop or fertilizer, the losses were similar to those from the cropped lysimeters. The loss of potassium ranged from 6 percent from the unfertilized lysimeters to 10 percent in the cropped lysimeters. The amounts of sodium leached ranged from 29 to 35 kg Na ha^–1. The bulk of the calcium and magnesium leached from calcium hydroxide and fertilizers occurred in the second season when the loss was in good agreement with the amount of nitrate lost giving (Ca + Mg)/NO₃ charge ratios of approximately one. Urea increased the amount of nitrate leached and led to a corresponding increase in the amounts of calcium and magnesium lost in the drainage water. The charge ratio remained unchanged when the cations were leached only with nitrate derived from the mineralization of soil organic matter. In the cropped lysimeters, this source accounted for about four times more nitrate in the drainage water than the fertilizer.     

Recovery of nitrogen by spring barley from ammonium nitrate,urea and sulphur-coated urea as affected by time and method of application

The objective of this study was to increase the efficiency of fall-applied N either by placement in bands or by using a slow-release fertilizer. Four field experiments were conducted in north-central Alberta to determine the influence of N source, time of application and method of placement on the recovery of fall-applied N as soil mineral N in May, and on yield and recovery of N in grain of spring-sown barley. The recovery in soil of mineral N by May from the fall-applied fertilizers varied among treatments. More specifically, the recovery was lowest with topdressed application, highest with banding, and tended to be less with incorporation application as compared to banding. Recovery of mineral N was least for sulphur-coated urea (SCU) compared with A.N. and urea, regardless of method of application. The loss of fall-applied N was substantial, but leaching did not go beyond 60 cm deep.Yield and recovery of N in barley grain were much greater with spring application than with fall application at the 4 sites for ammonium nitrate (A.N.) and at 3 sites for urea. The SCU treatments were inferior. The A.N. and urea had greatest yield and N recovery with banding, followed by incorporation and then with topdressing for both fall- and spring-applied N. Method of application had little effect on yield and N uptake with SCU. In all, the greatest yield or crop N uptake was obtained with spring banding of A.N. or urea, while SCU did not function well as a fall- or spring-applied N fertilizer.(Contribution No. 680)     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.     

Influence of delaying flood and preflood nitrogen application on dry-seeded rice

In the southern U.S. rice belt it is recommended that rice (Oryza sativa L.) grown in the dry-seeded, delayed flood cultural system have the preflood N fertilizer applied and the field flooded at the fourth to fifth leaf stage of plant development. The objective of this field study was to determine if delaying the flood and preflood N application past the fifth leaf stage was detrimental to rice total N and fertilizer¹⁵N uptake, total dry matter, and grain yield. This study was conducted on a Crowley silt loam (Typic Albaqualfs) and a Perry clay (Vertic Haplaquepts). The preflood N fertilizer and flood were delayed 0, 7, 14, or 21 d past the fourth to fifth leaf stage, after which time a permanent flood was established and maintained until maturity. All treatments received 20.5 g N m^–2 as¹⁵N-labeled urea in three topdress applications. All plant and soil samples were taken at maturity. Harvest index increased as the preflood N and flood were delayed past the 4 to 5 leaf stage. Total N in the grain + straw either decreased or showed a decreasing trend as the N and flood were delayed. Similarly, uptake of native soil N decreased as flood was delayed. Conversely, percent recovery of fertilizer N in the rice plant and the plant-soil system increased as the preflood N and flood were delayed. Rice grain yield was not significantly affected by delaying the preflood N and flood up to 21 d.Received....... . Published with permission of the Director of the Arkansas Agric. Exp. Stn. Project ARK01386. Supported in part by the Tennessee Valley Authority National Fertilizer and Environmental Research Center and the Arkansas Rice Research and Promotion Board.     

A conceptual assessment of the importance of denitrification as a source of soil nitrogen loss in tropical agro-ecosystems

This paper attempts to answer the question: is denitrification a major route of N loss from tropical agro-ecosystems? This question turns out to be very difficult to answer due to a severe shortage of data on this process for tropical agro-ecosystems other than rice. Given this lack of data, I approach this question by analyzing data on denitrification and nitrous oxide flux in tropical native forest and pasture soils and attempt to make some conclusions and pose some hypotheses about the significance of denitrification in tropical agricultural soils. I also briefly review methods for measuring denitrification. The data analysis suggests that denitrification in tropical forest soils is strongly influenced by the nature and amount of soil C and N turnover. Studies to examine differences in denitrification in different tropical agricultural systems should focus on the effects of system management on C and N turnover. The data analysis also suggests that, just as in temperate regions, denitrification may not be a significant route of N loss from most tropical agricultural systems. However, field studies are necessary to determine if this is actually the case.     

Uptake of fertilizer and soil nitrogen by ryegrass swards during spring and mid-season

Double-labelled¹⁵N ammonium nitrate was used to determine the uptake of fertilizer and soil N by ryegrass swards during spring and mid-season. The effects of water stress (40% of mean rainfall v 25 mm irrigation per 25 mm soil water deficit) and the rate of application of N in the spring (40 v 130 kg ha^–1) on the recovery of 130 kg N ha^–1 applied in mid-season were also evaluated. Apparent recovery of fertilizer N (uptake of N in the fertilized plot minus that in the control expressed as a percentage of the N applied) was 95 and 79% for fertilizer N applied in the spring at rates of 40 and 130 kg ha^–1, respectively. Actual recovery of the fertilizer N assessed from the uptake of¹⁵N was only 31 and 48%, respectively. The uptake of soil N by the fertilized swards was substantially greater than that by the control. However, the increased uptake of soil N was always less than the amount of fertilizer N retained in or lost from the soil. Broadly similar patterns for the uptake of fertilizer and soil N were observed during mid-season. Uptake of N in mid-season was highest for swards which received 40 kg N ha^–1 in the spring and suffered minimal water stress during this period. Application of 130 kg N ha^–1 in spring reduced the uptake of N in mid-season to an extent similar to that arising from water stress. Only 1.8 to 4.2 kg ha^–1 (3 to 10%) of the N residual from fertilizer applied in the spring was recovered during mid-season. Laboratory incubation studies suggested that only a small part of the increased uptake of soil N by fertilized swards could be attributed to increased mineralisation of soil N induced by addition of fertilizer. It is considered that the increased uptake of soil N is partly real but mostly apparent, the latter arising from microbially mediated exchange of inorganic¹⁵N in the soil.     

Nitrogen losses from fertilizers applied to maize, wheat and rice in the North China Plain

Ammonia volatilization, denitrification loss and total nitrogen (N) loss (unaccounted-for N) have been investigated from N fertilizer applied to a calcareous sandy loam fluvo-aquic soil at Fengqiu in the North China Plain. Ammonia volatilization was measured by the micrometeorological mass balance method, denitrification by the acetylene inhibition – soil core incubation technique, and total N loss by ¹⁵N-balance technique. Ammonia loss was an important pathway of N loss from N fertilizer applied to rice (30–39% of the applied N) and maize (11–48%), but less so for wheat (1–20%). The amounts of unaccounted-for fertilizer N were in the order of rice > maize > wheat. Deep placement greatly reduced ammonia volatilization and total N loss. Temperature, wind speed, and solar radiation (particular for rice), and source of N fertilizer also affect extent and pattern of ammonia loss. Denitrification (its major gas products are N₂ and N₂O) usually was not a significant pathway of N loss from N fertilizer applied to maize and wheat. The amount of N₂O emission (N₂O is an intermediate product from both nitrification and denitrification) was comparable to denitrification loss for maize and wheat, and it was not significant in the economy of fertilizer N in agronomical terms, but it is of great concern for the environment.     

Significance of timing and method of N fertilizer application for the N-use efficiency in flooded tropical rice

N-use efficiency in flooded tropical rice is usually low. Fertilizer N losses result mainly from losses of volatile NH₃ after broadcast application of urea into floodwater between transplanting and early tillering which is a common practice of farmers. Losses appear predominantly during the first week after urea application. With broadcast and incorporation of N into soil at transplanting losses may be reduced but are still substantial. Deep placement of urea supergranules (USG) has not been adopted by farmers because it is very laborious. A new application technique, namely injection of dissolved urea into the upper soil layer, was developed by which fertilizer N losses were effectively minimized while at the same time allowing flexible timing of application independent of crop stage and water management. It provides N-use efficiency equal to that achieved by USG point placement but is less labor-intensive.     

The comparative effects of ammonium nitrate,urea or a combined ammonium nitrate/urea granular fertilizer on the efficiency of nitrogen recovery by perennial ryegrass

The comparative effects of ammonium nitrate (AN), urea or a combined 1:1 (w/w) AN/urea granular fertilizer with two different fillers (CaCO₃ or silica) were investigated on the efficiency of dry matter production and¹⁵N recovery by perennial ryegrass grown in pots under controlled environmental conditions.There was no significant difference between CaCO₃ and silica as the filler and therefore no indication that the presence of CaCO₃ in the pellet enhanced N loss from urea. Ammonium nitrate was the most efficient N source and urea the least efficient in terms of all the parameters studied. The¹⁵N budget in shoots, roots and soil indicated that only 60% of the nitrogen from urea was recovered at the end of the experiment compared with 95% for AN. However, the % recovery of¹⁵N from urea was increased by 17% in the presence of AN whereas the % recovery of AN was decreased by 19% in the presence of urea. The combined 1:1 (w/w) AN/urea source therefore gave intermediate yields between AN and urea alone. The results indicate that an interaction occurred between AN and urea in the granule.     

Rate,source and time of N application for meadow bromegrass in central Alberta,Canada

Field experiments were conducted from 1988 to 1991 or 1992 at two sites (Lacombe-Black Chernozem and Eckville-Gray Luvisol) in central Alberta, Canada to determine the effect of rate (0 to 300 kg N ha^–1), source [urea and ammonium nitrate (AN)] and time (early fall, late fall, early winter, early spring and late spring) of N application on dry matter yield (DMY), protein yield (PY), protein concentration (PC), N-use efficiency (NUE), % N recovery (% NR) and nitrate-N (NO₃–N) concentration in meadow bromegrass (Bromus bibersteinii Roem and Shult. cv. Regar). The DMY, PY and PC increased with increasing applied N, but the NUE and % NR decreased at high N rates. The increases in PY from fertilizer N were proportionately greater than DMY due to increase in PC at high N rates. Potentially toxic NO₃–N levels (>2.3 g kg^–1) were not found in the forage. Urea generally produced lower DMY, PY, PC, NUE and % NR than AN, regardless of time of application and cut. Early spring application had the highest and early winter application had the lowest DMY and PY. In conclusion, urea was less effective than AN as a forage fertilizer and early spring application was most effective.     

Effect of lime nitrogen on the efficiency of urea and other ammonium nitrogen fertilizers

Five pot experiments were conducted with wheat and rice in a net house to study the effect of lime nitrogen (LN, contains about 55% calcium cyanamide) amendment rates on the efficiency of urea, the recovery urea-¹⁵N, the efficiency of the three nitrogen fertilizers(NF), on the efficiency of urea in the three soils, and on NO ₃ ^- -N leaching from a flooded soil. A rate of LN-N of 5–8% of applied fertilizer N increased the recovery of labeled urea-N by 9.42%. The effect of LN on the efficiency of NF was urea > ammonium sulfate > ammonium chloride. Under flooded conditions, LN decreased NO ₃ ^- formation and leaching.Responses of several crops to LN amended fertilizers were also studied in field experiments. At equal NPK applications, the efficiency of basal applications to rice, wheat, corn, potatoes, soybean, peanut, grapes, peaches, melon and watermelon were bette r with LN than without. Efficiency with a basal fertilizer for rice or wheat with LN were the same as with the same fertilizer without LN applied in split applications.     

Effect of climate on the recovery in crop and soil of15N-labelled fertilizer applied to wheat

Data was assembled from experiments on the fate of¹⁵N-labelled fertilizer applied to wheat (Triticum spp.) grown in different parts of the world. These data were then ranked according to the annual precipitation-evaporation quotient for each experimental location calculated from the average long-term values of precipitation and potential evaporation. Percentage recovery of¹⁵N fertilizer in crop and soil varied with location in accordance with the precipitation-evaporation quotient. In humid environments more¹⁵N fertilizer was recovered in the crop than in the soil, while in dry environments more¹⁵N fertilizer was recovered in the soil than in the crop. Irrespective of climatic differences between locations 20% (on average) of the¹⁵N fertilizer applied to wheat crops was unaccounted for at harvest. Most of the¹⁵N fertilizer remaining in the soil was found in the 0–30 cm layer. The most likely explanation of these differences is that wheat grown in dry environments has a greater root:shoot ratio than wheat grown in humid environments and, further, that the residue of dryland crops have higher C/N ratios. Both factors could contribute to the greater recovery of¹⁵N fertilizer in the soil in dry environments than in humid ones.