HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (14) Citing Articles via Google Scholar Google Scholar Articles by Bronson, K.F. Articles by Ladha, J.K. Search for Related Content PubMed Articles by Bronson, K.F. Articles by Ladha, J.K. Agricola Articles by Bronson, K.F. Articles by Ladha, J.K.

DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Use of ¹⁵N-Labeled Soil in Measuring Nitrogen Fertilizer Recovery Efficiency in Transplanted Rice

^a Texas A&M Univ., Texas Agric. Exp. St., Route 3, Box 219, Lubbock, TX 79401 USA
^b National Agricultural Research Center, Land Resources Inst., P.O. NIH, Islamabad, Pakistan
^c Soil and Water Sci. Div., International Rice Research Inst., P.O. Box 933, 1099 Manila, Philippines

k-bronson{at}tamu.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
In transplanted flooded rice (Oryza sativa L.) culture in Asia, N fertilizer recovery efficiency (RE) estimates are usually greater with the difference method than with the ¹⁵N method. These reported differences are greater and more common for transplanted irrigated rice than for upland crops and might be associated with basal N applications when plant demand is low. Added N interactions (ANI), or greater soil N accumulation in fertilized plants than in unfertilized plants, are often cited as the reason for these differences. The causes of ANI can be stimulation of mineralization of soil organic matter by fertilizer or greater root exploration in fertilized plants. The objectives of this pot study were (i) to compare the ¹⁵N dilution method using enriched ¹⁵N-urea (5.0 atom %), and ¹⁴N-urea (0.366 atom %) plus ¹⁵N-labeled (0.447 atom %, Maahas clay [isohyperthermic Andaqueptic Haplaquoll]) soil, with the difference method of measuring RE of N, as affected by timing of application, and (ii) to use ¹⁵N-labeled (0.447 atom %) soil to directly measure ANI as ¹⁵N accumulation in ¹⁴N fertilized pots minus ¹⁵N accumulation in unfertilized pots. Recovery efficiency measures at physiological maturity were higher with the difference method (54%) than with the two isotope dilution methods (44%), which were in turn similar. The real ANI measured was negligible, as the root biomass in this clay soil was not greatly affected by fertilization. Therefore, the ANI was apparent and due to isotope substitution. The RE estimate by difference (54.4%) was therefore more accurate than the RE measure by ¹⁵N methods (44.4%). The 33.2% of added ¹⁵N not accounted for in the plant–soil system was a measure of N fertilizer losses (NH₃ volatilization from floodwater, denitrification, NH₃ loss through the plant during grain fill) that were not affected by isotope substitution. Recovery efficiency estimated with ¹⁵N-enriched soil plus ordinary urea was identical to the RE estimate using enriched ¹⁵N-urea, although both were underestimations because of isotope substitution.

Abbreviations: ANCA-MS, automated N–C analyzer mass spectrometry • ANI, added N interaction • Ndff, N derived from fertilizer • Ndfs, N derived from soil • RE, recovery efficiency

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
MANY STUDIES have been conducted with transplanted rice in Asia using ¹⁵N-labeled fertilizers to measure RE of added N and to construct N balances. Most of these studies have concluded that RE in rice is poor, that is, in the range of 20 to 40% (De Datta et al., 1987, 1988; Schnier et al., 1990). The losses of fertilizer-N from flooded rice systems are usually attributed to NH₃ volatilization and denitrification (Freney et al., 1990). In most of the earlier ¹⁵N balance work where low REs were reported, a two-thirds basal N–one-third N at panicle initiation timing scheme was used, which was recommended in the 1980s (DeDatta et al., 1987, 1988). The use of four equal splits of N (i.e., one-fourth each basally, at mid-tillering, at panicle inititation, and at flowering) has since been reported to give much higher RE values of 70 to 80% (Peng et al., 1996a).

In upland crops, numerous workers have compared ¹⁵N methods and the difference method for estimating RE (Westerman and Kurtz, 1974; Olson, 1980; Olson and Swallow, 1984; Rao et al., 1991; review on topic by Yamaguchi, 1991). The estimate of RE is often higher using the difference method than using the ¹⁵N method. The mechanism cited as being involved in these conflicting results is ANI. Added N interaction, previously known as the priming effect, originally described a real stimulation of N mineralization in the presence of fertilizer (Westerman and Kurtz, 1973). An ANI can also be real if root exploration is greater in fertilized plots than in unfertilized plots. Apparent ANIs can result from pool substitution, in which added labeled N stands proxy for native unlabeled N (Jenkinson et al., 1985). However, few detailed studies have been conducted of ANI processes in flooded rice where the discrepancy in RE estimates between the difference and ¹⁵N methods is large (Cassman et al., 1993; Schnier, 1994). We hypothesized that by using stable, well-mixed, ¹⁵N-labeled soil we could directly measure ANI as the difference between total ¹⁵N accumulation in plants fertilized with ordinary ¹⁴N (99.6337 atom % ¹⁴N or 0.3663 atom % ¹⁵N) urea and ¹⁵N accumulation in unfertilized rice plants. Using ¹⁵N-labeled soil would be difficult in field conditions, so we decided to conduct pot studies, where thorough mixing and uniform distribution could be assured.

The objectives of this study were (i) to compare the ¹⁵N dilution method using enriched ¹⁵N-urea, and ¹⁴N-urea plus ¹⁵N-labeled soil with the difference method in measuring RE of N fertilizer, as affected by timing of application, and (ii) to use ¹⁵N-labeled soil to directly measure ANI as ¹⁵N accumulation in ¹⁴N fertilized pots minus ¹⁵N accumulation in unfertilized pots.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Maahas clay used in this pot study in the greenhouse was labeled with ¹⁵N 5 yr earlier by applying 6.3 kg N ha^-1 of 99.9 atom % ¹⁵N urea to soil in a cement container 6.5 m long by 2 m wide by 0.25 m deep and mixing well (Shrestha and Ladha, 1996). The soil had the following chemical soil properties at the time our pot study began: total N of 1.1 g kg^-1 with an atom % ¹⁵N of 0.447%, 20.8 mg NH⁺₄–N kg^-1 with an atom % ¹⁵N of 0.502%, 12.5 mg Olsen-extractable P kg^-1, pH (1:1 H₂O) of 7.1, and cation-exchange capacity of 41.9 cmol(+) kg^-1. Three-kilogram portions of soil were placed in polyvinyl chloride pots 10 cm in diameter and 20 cm high. The experimental design of the study was completely randomized with six replications. The five treatments were: (i) no N; (ii) ¹⁴N urea—two-thirds basal and one-third at panicle initiation; (iii) ¹⁵N urea—two-thirds basal and one-third at panicle initiation; (iv) ¹⁴N urea—one-fourth basal, one-fourth at mid-tillering, one-fourth at panicle initiation, and one-fourth at flowering; and (v) ¹⁵N urea—one-fourth basal, one-fourth at mid-tillering, one-fourth at panicle initiation, and one-fourth at flowering.

The ¹⁵N-urea treatment was 5 atom % ¹⁵N and the ¹⁴N-urea treatment was ordinary urea at natural abundance (0.366 atom % ¹⁵N). All N was applied at a total rate of 235.2 mg N pot^-1. Urea-N was added to pots in solution. The basal applications were prior to puddling (wet tillage) of wet soil by hand with a large spatula. Phosphorus and K were applied each at 50 mg pot^-1 during puddling. Two 3-wk-old seedlings of rice cultivar IR64 were soaked overnight in a 100 g kg^-1 solution of ZnO and then transplanted into each pot on 20 Jan. 1997 in the dry season of the Philippines. The crop was grown to physiological maturity under flooded conditions and plants were harvested at soil level on 25 Apr. 1997. The number of productive tillers was counted prior to hand-threshing the grain. Straw and grain from each pot were weighed after overnight oven drying at 70°C. Root and algae dry matter were processed similarly. The plant tissue samples were fine-ground to <0.15 mm for atom % ¹⁵N and N percentage analysis with an automated C–N analyzer-mass spectrometer (ANCA-MS) similar to that described by Bronson and Fillery (1998). All fresh soil was excavated from the pots and well-mixed. A 10-g sample of soil was extracted with 50 mL of 2 M KCl by shaking for 2 h and then centrifuged and filtered. Exchangeable NH⁺₄ in the extract was measured by spectrophotometer (Kemper and Zweers, 1986). A modified diffusion method of Brooks et al. (1989) was used to prepare acidified glass fiber disks for NH⁺₄–N and ¹⁵N analysis of the soil extract by ANCA-MS. The soil that was extracted with 2 M KCl was shaken and centrifuged three more times with 50 mL of distilled water and air dried and fine-ground to <0.15 mm prior to N percentage and atom % ¹⁵N analysis of organic N by ANCA-MS (Bronson and Fillery, 1998).

Percentage of recovery of ¹⁵N in plant or soil was calculated using the formula of Hauck and Bremner (1976):

(1)

where TN is the total N in the plant part or soil (mg N pot^-1); F is the rate of ¹⁵N fertilizer applied (235.2 mg N pot^-1); and A_u, A_uf, and A_f are the atom % ¹⁵N in the labeled urea fertilizer (5%), plant part or soil receiving no ¹⁵N, and plant part or soil receiving ¹⁵N , respectively.

The amount of ¹⁴N accumulated in the grain or straw was calculated using the isotope dilution formula (Hauck, 1982):

(2)

Where x is the amount of fertilizer-N in the plant part; and A_u, A_uf, and A_m are the values of atom % ¹⁵N in the plant part derived from urea fertilizer (0.3663 %), atom % ¹⁵N of available soil N (same as in unfertilized plants, 0.502 %), and atom % ¹⁵N in the plant (mixture of fertilizer- and soil-N), respectively; y is the amount of total N in the plant part derived from soil (total N in plant part - x). Recovery of added ¹⁴N was calculated as:

(3)

Recovery of both labeled and unlabeled urea-N was also referred to as N derived from fertilizer (Ndff). Subtracting Ndff from total N accumulation gave N derived from soil (Ndfs). Agronomic and physiological efficiencies of added urea-N were calculated as suggested by Peng et al. (1996a). These were made at physiological maturity and were relative to the unfertilized control.

Analysis of variance was performed on measured and calculated variables and the least significant difference test was used to test differences between treatments. (SAS Institute, 1996).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Yield and Yield Parameters
Grain yield was higher in pots receiving the two-thirds–one-third split than in pots receiving four equal splits (Table 1) . This is the opposite of what is usually observed in the field, where three or four equal splits give higher N accumulation, RE, and yields than the two-thirds–one-third split (Peng et al., 1996a). In greenhouse conditions, the lack of competition between hills of rice probably favored the higher N supply early in the season in the two-thirds–one-third split. Productive tiller numbers were also higher with two vs. four splits and were positively correlated with grain yield ( ).

Nitrogen Recovery Efficiency
Recovery efficiency of added urea-N averaged 54.4% with the difference method, which was significantly higher than the 44.4% recovery by ¹⁵N methods (P < 0.05, Table 1). These measures of RE by the respective methods were similar to the earlier ¹⁵N recovery studies on this Maahas clay soil for transplanted rice with either 120 kg N ha^-1 applied prior to transplanting or in a two-thirds basal–one-third at panicle initiation treatment, as summarized by Cassman et al. (1993). However, the recovery efficiencies by ¹⁵N dilution were lower than those reported for pre-flood and pre-flood plus split applications of N in drill-seeded rice in the USA (Wilson et al., 1989; Guindo et al., 1994). Our measures of RE by all methods may have been affected by the occurrence of N losses through the plant during grain fill, but in the reported work on this site, declining N accumulation after flowering was more common in direct wet–seeded rice than in transplanted rice (Schnier et al., 1990; Peng et al., 1996b). Cassman et al. (1997) reported that recovery of ¹⁵N applied at flowering in the Philippines continued between mid grain filling and maturity. Our RE measures were made at only one stage, physiological maturity, to compare and give insight to the earlier ¹⁵N-RE measures, which were all made at maturity. Although we observed no effect of timing of fertilizer application on RE estimates, the similarity of the REs to the earlier work in the Philippines allows us to make reasonable comparisons with the earlier studies.

There were no differences in the RE estimates made by adding either enriched ¹⁵N-urea or ¹⁴N-urea to the ¹⁵N-enriched soil (Table 1). Using ¹⁵N-enriched soil and ordinary urea is similar to the use of depleted ¹⁵N sources since the enrichment of the soil was higher than the urea used in the ¹⁴N-urea treatment (0.447 vs. 0.366 atom % ¹⁵N urea) (Starr et al., 1974). The atom % ¹⁵N contents of grain and straw were significantly lower in the ¹⁴N treatments than in the control, indicating that the level of ¹⁵N enrichment of our soil of 0.447 atom % ¹⁵N was high enough to measure accumulation of ¹⁴N (Table 2) . However, in our study we could not measure immobilization of added ¹⁴N in the soil. The dilution of the large sink of total N, which was 0.447 atom % ¹⁵N, by ¹⁴N-urea, which was 0.3663 atom % ¹⁵N, was not detected even though the amount of immobilization (assuming equal immobilization to the ¹⁵N treatment) was 17.0 % of the added N (Tables 2 and 3) . Immobilization of added N is such a large sink that without it we could not construct a balance of added ¹⁴N.

Timing of N applications also had no effect on RE estimates, but timing did affect atom % ¹⁵N and N contents of grain and straw (Tables 1 and 2). With ¹⁵N-enriched urea, four splits resulted in higher atom % ¹⁵N in the grain than with the two-thirds–one-third split, but atom % ¹⁵N of straw had the opposite trend (Table 2). This may indicate that added ¹⁵N was more available than soil-derived N for straw with earlier applications and for grain with late applications. However, higher N contents but lower yields of both grain and straw with four splits of N than with the two-thirds–one-third split resulted in similar total N accumulations and RE among all fertilized treatments.

Although not strongly demonstrated in this pot study, matching the timing of N applications with periods of high plant demand is extremely important in achieving high RE. The discrepancy between the ¹⁵N and difference methods of measuring RE are also minimized when no basal N is applied. Recovery efficiencies by the difference method of 74 to 78% and ¹⁵N recoveries of 69 to 75% were achieved by Cassman et al. (1997) with urea top-dressings at panicle initiation.

Root biomass in our study was not greatly affected by added N (Table 1). In field conditions of clayey, puddled rice soils there is usually no difference between the root biomass of fertilized and unfertilized rice plants (Schnier, 1994). Cassman et al. (1997) reported that 92% of the root biomass is in the top 20 cm of heavy rice soils. This means that the underlying assumption of the difference method that equal amounts of soil N are accumulated by fertilized and unfertilized plants is probably acceptable in this study; however, we did not account for fine roots apart from the total soil organic matter determinations. In coarser-textured rice soils, and in rice soils that do not form plowpans, deeper rooting and thus higher soil N accumulation may occur with fertilized plants than with unfertilized plants, resulting in overestimations of RE by the difference method.

Added Nitrogen Interaction
Added N interaction, calculated in this study as the amount of ¹⁵N pot^-1 accumulated by the ¹⁴N-fertilized rice minus the amount of ¹⁵N pot^-1 accumulated by the unfertilized rice, averaged 0.027 mg ¹⁵N pot^-1 (Table 4) . This direct measure of ANI represents a real ANI, possibly a small stimulation of mineralization of soil organic N, that would not be affected by isotope substitution. However, although this amount is statistically different from zero, if we add 0.027 mg ¹⁵N pot^-1 back to the amounts of ¹⁵N pot^-1 accumulated by the ¹⁵N-treatments (minus the background of 0.447 atom % ¹⁵N) and recalculate the RE estimates in Table 1, the additional recovery is a negligible 0.3 to 0.4%.

In ¹⁵N studies in a rice–wheat (Triticum aestivum L.) rotation, Bronson et al. (1997) reported that a discrepancy between the difference and the isotope methods of measuring RE was observed in rice but not in wheat. The fact that heterotrophic bacteria involved in the mineralization–immobilization turnover reactions between inorganic and organic pools of N prefer NH⁺₄ (Jansson, 1958; Jenkinson et al., 1985), the dominant form of inorganic N in flooded soils, to NO^-₃ may explain large apparent ANIs in rice soils.

Nitrogen-15 Balance
Immobilization of ¹⁵N in soil organic matter plus fine roots (18.1%) was not affected by timing of N, nor was total ¹⁵N recovery (66.8%) or losses (33.2%) (Table 3). Only in algae and in roots was ¹⁵N recovery influenced by timing of N. The low recovery of ¹⁵N as exchangeable NH⁺₄ (1.1%) indicates that the N fertilizer rate matched plant N accumulation, or that NH⁺₄ in excess of plant needs was lost as gases.

The loss pathways of added ¹⁵N in this study were probably NH₃ volatilization from the floodwater, and nitrification–denitrification, but it is not apparent which loss pathway was more important (Freney et al., 1990). No work, unfortunately, has been conducted in transplanted rice in Asia on loss of NH₃ through the plant during the grain-filling period. Guindo et al. (1994) and Wetselaar and Farquhar (1980) reported that this loss pathway in rice can be important, especially with high N rates. However, our loss estimate of 33.2% from the soil–plant system would not be affected by isotope substitution and therefore this measure remains a robust and important use of ¹⁵N in rice and other crops.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
In pot studies of transplanted rice, we measured greater RE of urea-N by the difference method than by ¹⁵N isotope dilution methods. This was similar to several earlier works with transplanted rice in Asia, in which most of the N was basally applied. Using a direct measure of ANI with ¹⁵N-labeled soil, we were able to confirm the report of Schnier (1994) that ANI in transplanted rice in Asia is mostly apparent. The real ANI we measured using ¹⁵N-labeled soil was negligible. We can conclude therefore that most of the discrepancy between the two methods of estimating RE was due to isotope substitution. We also conclude that the difference method is probably more accurate than ¹⁵N methods of measuring RE in flooded transplanted rice when the difference estimates are higher than the ¹⁵N estimates. This supports the assertion of Cassman et al. (1993, 1997) that N fertilizer RE in irrigated rice in heavy clay soils is not as low as previous ¹⁵N work suggested. However, on rice soils that do not restrict rooting, deeper root exploration may develop with fertilized rice plants compared with unfertilized plants, making the difference method less reliable. Our results in transplanted rice could probably not be applied to drill-seeded rice culture in the USA, where high REs are measured, especially when N applications are delayed until the permanent flood is imposed at the four- or five-leaf stage. We additionally showed that soil with a stable ¹⁵N enrichment can be used with naturally abundant urea to obtain an RE estimate equivalent to one measured by using enriched ¹⁵N-urea, although in this case it was an underestimation due to isotope substitution.

	ACKNOWLEDGMENTS

 
The authors would like to thank Ruby Jimenez and Ruben Chavez for their capable analytical services.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Contribution from National Agricultural Research Center, Land Resources Institute and International Rice Research Institute.

Received for publication December 8, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

• Bronson K.F., Fillery I.R.P. Fate of nitrogen-15-labelled urea applied to wheat on a waterlogged texture-contrast soil. Nutr. Cycl. Agroecosyst. 1998;51(2):175-183.
• Bronson, K.F., Y. Singh, Bijay-Singh, Yadvinder-Singh, and U. Singh. 1997. Nitrogen-15 balances in the rice–wheat systems of North India. p. 225. In 1997 Agronomy Abstracts. ASA, Madison, WI.
• Brooks P.D., Stark J.M., McInteer B.B., Preston T. Diffusion methods to prepare soil extracts for automated nitrogen-15 analysis. Soil Sci. Soc. Am. J. 1989;53:1707-1711.[Abstract/Free Full Text]
• Cassman K.G., Kropff M.J., Gaunt J., Peng S. Nitrogen use efficiency of rice reconsidered: What are the key constraints?. Plant Soil 1993;155(156):359-362.
• Cassman K.G., Peng S., Dobermann A. Nutritional physiology of the rice plant and productivity decline in irrigated rice systems in the tropics. Soil Sci. Plant Nutr. 1997;43:1101-1106.
• De Datta S.K., Obcemea W.N., Chen R.Y., Calabio J.C., Evangelista R.C. Effect of water depth on nitrogen use efficiency and nitrogen-15 balance in lowland rice. Agron. J. 1987;79:210-216.[Abstract/Free Full Text]
• De Datta S.K., Buresh R.J., Samson M.I., Kai-Rong W. Nitrogen use efficiency and nitrogen-15 balances in broadcast-seeded flooded and transplanted rice. Soil Sci. Soc. Am. J. 1988;52:849-855.[Abstract/Free Full Text]
• Freney J.R., Trevitt A.C.F., De Datta S.K., Obcemea W.N., Real J.G. The interdependence of ammonia volatilization and denitrification as nitrogen loss processes in flooded rice fields in the Philippines. Biol. Fertil. Soils 1990;9:31-36.
• Guindo D., Norman R.J., Wells B.R. Accumulation of fertilizer nitrogen-15 by rice at different stages of development. Soil Sci. Soc. Am. J. 1994;58:410-415.[Abstract/Free Full Text]
• Hauck R.D., Bremner J.M. Use of tracers for soil and fertilizer nitrogen research. Adv. Agron. 1976;28:219-266.
• Hauck R.D. Nitrogen isotope ratio analysis. In: Page A.L., et al. , ed. Methods of soil analysis. Part 2, 2nd ed Madison, WI: ASA and SSSA, 1982:735-780.
• Jansson S.L. Tracer studies on nitrogen transformations in soil with special attention to mineralisation–immobilisation relationships. Ann. R. Agric. Coll. Sweden. 1958;24:101-361.
• Jenkinson D.S., Fox R.H., Rayner J.H. Interaction between fertilizer nitrogen and soil nitrogen: The so-called priming effect. J. Soil Sci. 1985;36:425-444.
• Kemper A., Zweers A. Ammonium determination in soil extracts by the salicylate method. Commun. Soil Sci. Plant Anal. 1986;17:715-723.
• Olson R.V. Fate of tagged nitrogen fertilizer applied to irrigated corn. Soil Sci. Soc. Am. J. 1980;44:514-517.[Abstract/Free Full Text]
• Olson R.V., Swallow C.W. Fate of labeled nitrogen applied to winter wheat for five years. Soil Sci. Soc. Am. J. 1984;48:583-586.[Abstract/Free Full Text]
• Peng S., Garcia F.V., Laza R.C., Sanico A.L., Visperas R.M., Cassman K.G. Increased nitrogen use efficiency using a chlorophyll meter in high yielding irrigated rice. Field Crops Res. 1996;47:243-252 a.
• Peng S., Garcia F.V., Gines H.C., Laza R.C., Samson M.I., Sanico A.L., Visperas R.M., Cassman K.G. Nitrogen use efficiency of irrigated tropical rice establised by broadcast and transplanting. Fert. Res. 1996;45:123-143 b.
• Rao A.C.S., Smith J.L., Papendick R.I., Parr J.F. Influence of added nitrogen interactions in estimating recovery efficiency of labeled nitrogen. Soil Sci. Soc. Am. J. 1991;55:1616-1621.[Abstract/Free Full Text]
• SAS Institute. SAS system for Windows release 6.12. Cary, NC: SAS Inst, 1996.
• Schnier H.F., Dingkuhn M., De Datta S.K., Marqueses E.P., Faronilo J.E. Nitrogen-15 balance in transplanted and direct seeded flood rice as affected by different methods of urea application. Biol. Fertil. Soils 1990;10:89-96.
• Schnier H.F. Nitrogen-15 recovery fraction in flooded tropical rice as affected by added nitrogen interaction. Eur. J. Agron. 1994;3:161-167.
• Shrestha R.K., Ladha J.K. Genotypic variation in promotion of rice dinitrogen fixation as determined by nitrogen-15 dilution. Soil Sci. Soc. Am. J. 1996;60:1815-1821.[Abstract/Free Full Text]
• Starr J.L., Broadbent F.E., Stout P.R. A comparison of ¹⁵N-depleted and ¹⁵N-enriched fertilizers as tracers. Soil Sci. Soc. Am. Proc. 1974;38:266-267.
• Stevenson F.J. Origin and distribution of nitrogen in the soil. In W.V. Bartholomew and F.E. Clark (ed.) Soil nitrogen. Agronomy 1965;10:1-42.
• Westerman R.L., Kurtz L.T. Priming effect of ¹⁵N-labeled fertilizers on soil nitrogen in the field experiment. Soil Sci. Am. Proc. 1973;37:725-727.
• Westerman R.L., Kurtz L.T. Isotopic and non-isotopic estimations of fertilizer and nitrogen uptake by sudan grass in field experiments. Soil Sci. Soc. Am. Proc. 1974;38:107-109.
• Wetselaar R., Farquhar G.D. Nitrogen losses from tops of plants. Adv. Agron. 1980;33:263-302.
• Wilson C.E., Jr., Norman R.J., Wells B.R. Seasonal uptake patterns of fertilizer nitrogen applied in split application to rice. Soil Sci. Soc. Am. J. 1989;53:1884-1887.[Abstract/Free Full Text]
• Yamaguchi J. Fertilizer-nitrogen absorption determined by the ¹⁵N isotopic and difference methods. Jpn. Agric. Res. Q. 1991;25:93-100.

This article has been cited by other articles:

				F. Zvomuya, C. J. Rosen, M. P. Russelle, and S. C. Gupta Nitrate Leaching and Nitrogen Recovery Following Application of Polyolefin-Coated Urea to Potato J. Environ. Qual., March 1, 2003; 32(2): 480 - 489. [Abstract] [Full Text] [PDF]

				A. J. Eagle, J. A. Bird, J. E. Hill, W. R. Horwath, and C. van Kessel Nitrogen Dynamics and Fertilizer Use Efficiency in Rice following Straw Incorporation and Winter Flooding Agron. J., November 1, 2001; 93(6): 1346 - 1354. [Abstract] [Full Text] [PDF]

				J. A. Bird, W. R. Horwath, A. J. Eagle, and C. van Kessel Immobilization of Fertilizer Nitrogen in Rice: Effects of Straw Management Practices Soil Sci. Soc. Am. J., July 1, 2001; 65(4): 1143 - 1152. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (14) Citing Articles via Google Scholar Google Scholar Articles by Bronson, K.F. Articles by Ladha, J.K. Search for Related Content PubMed Articles by Bronson, K.F. Articles by Ladha, J.K. Agricola Articles by Bronson, K.F. Articles by Ladha, J.K.