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Published in Agron. J. 96:646-655 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

COTTON

Fate of Nitrogen-15 Applied to Irrigated Acala and Pima Cotton

Felix B. Fritschi*,a, Bruce A. Robertsb, D. William Rainsc, Robert L. Travisc and Robert B. Hutmacherd

a 141 Experiment Station Rd., P.O. Box 345, Stoneville, MS 38776 (previously at: Dep. of Agron. and Range Sci., Univ. of California, One Shields Ave., Davis, CA 95616)
b Univ. of California Coop. Ext., 680 N. Campus Drive, Suite A, Hanford, CA 93230
c Dep. of Agron. and Range Sci., Univ. of California, One Shields Ave., Davis, CA 95616
d Univ. of California, Shafter Res. and Ext. Cent., 17053 N. Shafter Ave., Shafter CA 93263

* Corresponding author (ffritschi{at}ars.usda.gov).

Received for publication February 10, 2003.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Better knowledge on the fate of fertilizer N will aid in developing effective N management strategies balancing plant requirements for optimal lint yield with environmental concerns. Field studies were initiated to determine the fate of 15N fertilizer applied to Acala (Gossypium hirsutum L.) and American Pima (G. barbadense L.) cotton. Four N regimes (56, 112, 168, and 224 kg N ha–1 corrected for residual soil nitrate N levels) were established for Acala cotton grown on a Panoche clay loam [fine-loamy, mixed (calcareous) thermic Typic Torriorthents] and on a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents) in 1998, 1999, and 2000. Pima cotton was evaluated on the Panoche clay loam for the same N regimes in 1999 and 2000. To trace the fate of fertilizer N, 15N-labeled urea was applied to microplots in selected treatments and years. Acala fertilizer use efficiency by 15N dilution (FUE–15N) averaged 49% on Panoche clay loam and 43% on Wasco sandy loam. Pima FUE–15N on the Panoche clay loam averaged 48% and was not affected by N treatment. Recovery of fertilizer N in the soil was not different between the Panoche clay loam and the Wasco sandy loam in the Acala experiments and combined across both Acala and Pima trials averaged 42%. Averaged across all experiments, more than 75% of the 15N recovered in the soil was found in the top 0.9-m layer. The total recovery of 15N fertilizer in plant and soil averaged 89% (76–98%) across all treatments, suggesting that production practices employed in this study resulted in only small losses of fertilizer N during the season of application.

Abbreviations: ANI, added nitrogen interaction • FUE-ND, fertilizer use efficiency by nitrogen difference • FUE–15N, fertilizer use efficiency by nitrogen-15 dilution • 15NFR-S, nitrogen fertilizer recovery in the soil by nitrogen-15 dilution


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
FERTILIZER N APPLIED to crops is subject to many fates. A portion of the N will remain in the soil and undergo numerous transformations. Plants will take up a fraction of soil N, and some may be lost to the air, surface water, and/or ground water. In recent years, NO3– contamination of ground and surface water has received considerable interest in the popular press, drawing more attention to agricultural N management practices. Although planted area varies annually, cotton remains one of the major row crops grown in the San Joaquin Valley, California. Hutmacher et al. (2004) estimated that, over the past decade, N applied to cotton accounted for almost 18% of agricultural N use in California. Based on the premise that quantitative data concerning the fate of fertilizer N applied to cotton can be used to improve fertilizer use efficiency while maintaining or increasing yield and minimizing negative environmental impacts, this type of information can be of agronomic, economic, and environmental interest.

Nitrogen fertilizer use efficiency is commonly estimated by either the N difference method (FUE-ND) or the 15N isotope dilution method (FUE–15N), both of which have been employed to examine a number of cotton production systems. Most recent studies examining N fertilizer recovery and the relationship between N accumulation and yield in cotton have been conducted in the southeastern USA and Australia. Australian researchers reported FUE–15N ranging from 14.2 to 46.6%, with even greater recoveries in some cases when nitrification inhibitors were added (Freney et al., 1993; Rochester et al., 1993, 1994). Apparent fertilizer N recoveries (N difference method) reported by these researchers were commonly under 50%, with higher recoveries only found at very low N application rate or as a result of the addition of nitrification inhibitors (Constable and Rochester, 1988; Freney et al., 1993). These authors suggested that, in the heavy soils on which their studies were conducted, the majority of N not recovered by the crop was lost from the system by denitrification and that the addition of nitrification inhibitors appeared to improve recovery through curtailing denitrification by substrate limitation. Torbert and Reeves (1994) and Karlen et al. (1996) examined the fate of 15N-labeled NH4NO3 applied to cotton grown in the southeastern USA and reported FUE–15N ranging from 25 to 35% and losses from the plant-soil system of 40 to 60%. Recently Boquet and Breitenbeck (2000) determined FUE-ND of up to 132% for cotton grown with 84 kg ha–1 fertilizer N. Navarro et al. (1997) reported a general trend to decreasing FUE-ND (ranging between 0 and 100%) with increasing fertilizer N applications (50–400 kg N ha–1) for irrigated cotton grown in the southwestern USA.

Studies on the fate of fertilizer N in cotton production practices common in the San Joaquin Valley have been lacking. In addition, no data are available for Pima, an extra-long staple cotton that generally yields less than Upland cotton (Unruh and Silvertooth, 1996). Compared with Upland cotton, Pima is more sensitive to delayed planting and excessive N fertility and has a more pronounced indeterminate growth habit, which limits production to regions with long growing seasons (Tewolde et al., 1995; Kittock et al., 1981, Silvertooth et al., 1995). This research was conducted as a complement to a 5-yr, multilocation re-evaluation of N fertilizer management guidelines for Acala cotton production in the San Joaquin Valley (Hutmacher et al., 2004). The principal objective of this study was to determine the recovery of soil-applied 15N-labeled fertilizer in Acala and Pima cotton in response to different N fertility levels. In addition to seasonal dynamics of 15N uptake and partitioning in the plant, recovery in the soil was also addressed. In the case of Acala, these parameters were examined for two contrasting soil types.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field experiments were conducted from 1998 through 2000 at the University of California West Side Research and Extension Center in Fresno County and in on-farm trials in Kings County, California. At the Fresno County location, a Panoche clay loam [fine-loamy, mixed (calcareous), thermic Typic Torriorthent] was cropped with tomato (Lycopersicon esculentum Mill.) before the initiation of this study. The Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthent) at the Kings County site was planted with corn (Zea mays L.) in the previous season. The soil organic matter content in the uppermost 0.3 m was 6.6 g kg–1 for the Panoche clay loam and 7.8 g kg–1 for the Wasco sandy loam. Additional information on selected physical and chemical properties of the two soils is reported elsewhere (Fritschi et al., 2003). Both ‘Maxxa’ (Acala) and ‘S-7’ (Pima) were grown at the Fresno County site (in the same field but as two separate experiments). At the Kings County site, only Acala cotton was planted. Acala trials were conducted in all 3 yr; Pima cotton was only included in 1999 and 2000. However, in the Acala trial conducted in 1999, no 15N-labeled fertilizer was applied.

Approximately 1 wk after seedling emergence, soil was sampled in each treatment (six subsamples per plot) from the upper 0.6 m of the profile and analyzed for NO3–N. Nitrogen treatments were established by subtracting the amount of residual soil NO3–N from the target rates of 56, 112, 168, and 224 kg N ha–1 and then applying the difference as urea N in a single application before the first in-season irrigation. Since no yield responses to two applications of N (before first and second irrigations) were detected in 1996, no treatments with split applications were established (Hutmacher et al., 2004). The appropriate amount of urea N was shanked into the soil approximately 0.15 m deep in bands about 0.2 m on either side of each plant row at the three to five true leaf stage. Treatments with main plots 6 to 12 rows wide (row spacing: 0.96 or 1.01 m) and 80 to 170 m long (length of irrigation run) were arranged in a randomized complete block design with four (Acala) or three (Pima) replications. Particular treatments were maintained on the same plots for the duration of this study. When N fertilizer was applied in the main plots (for Acala in 1998 and for Pima in 1999), randomly located plots, 24 to 150 m2 in size, were dedicated as zero-fertilizer-N plots and maintained within the main plots through 2000. In addition, the equipment used for the injection of N fertilizer was lifted from the soil and fertilizer flow turned off for 12-m-long plot segments to create areas designated for the establishment of 15N-labeled microplots. The same procedure was used to establish the microplot locations in 2000. In these unfertilized areas, microplots were established and fertilized with 15N-enriched urea (amount based on the soil sample analyses from the main plots) within 1 to 7 d of the N application in the main plots. Equations and soil NO3–N data from the main plots were used to determine the amount of labeled N fertilizer to be applied. The microplots varied in length and width: 7 m by seven rows for those established in 1998, 4 m by six rows for those established in Pima in 1999, and 4 m by five rows for all those established in 2000. The exact location of the microplots was marked for the duration of the study by burying a metal marker in one corner of each microplot at the time of establishment. Metal markers were located using triangulation measurements and a metal detector. Applications of labeled fertilizer were made in 1998 and 2000 to Acala and in 1999 and 2000 to Pima cotton. In 1998 and 1999, microplots were established only in the 56 kg N ha–1 (N-56) and the 168 kg N ha–1 (N-168) treatments. In 2000, microplots fertilized with 15N-labeled urea were established in all four treatments. To match application rates within the microplots to those of their associated main plots, fertilizer solutions were prepared using either 10 atom% 15N-labeled urea by itself or in combination with nonenriched urea. Simulating the placement by the large-scale application, the fertilizer solution was injected about 0.2 m on each side of the cotton rows to a depth of about 0.15 m. The injection system consisted of a self-refilling repetitive syringe (10 mL) connected through tubing to a 1.25 m long injection rod made of stainless steel. At 0.02 m from the closed, sharpened tip of the injection rod, four holes (0.8 mm diam.) were evenly spaced around the circumference, allowing for the injection of the fertilizer solution. To maintain constant depth of injection, a rubber stopper was attached to the rod 0.15 m behind the ejection holes. Using a template made of aluminum flashing, fertilizer solution was injected in 0.1-m intervals along the cotton rows.

Except for the injection of 15N fertilizer, commercial four- or six-row farm equipment was used for all management operations. Irrigation was uniform across both species and for all treatments within a location. Cotton was irrigated three times on the Panoche clay loam (approximately 0.8 m including preirrigation) and six times on the Wasco sandy loam (approximately 1.0 m including preirrigation) in every growing season. Water was provided by furrow (every furrow) or flood irrigation. Applications of P and K were made as needed based on soil test results (Weir et al., 1996; Miller et al., 1997). Management was consistent with typical agronomic practices of the region and uniform across all treatments within each location. Further particulars concerning N applications in the main plots, the prevailing weather conditions, and soil parameters were described previously (Fritschi et al., 2003, 2004).

In 1998, five biomass samples were collected based on cotton developmental stages (early square, early bloom, peak bloom, just before defoliation corresponding to >60% open bolls, and physiological maturity). Using plant mapping data and heat unit accumulation (Zalom et al., 1983) since planting, cotton developmental stages for 1999 and 2000 samplings were matched as closely as possible to those in 1998. In 1998 and 1999, samplings were conducted in the control, N-56, and N-168 treatments only. However, with the exception of the early bloom samplings in the N-112 and N-224 treatments, plants were collected from all treatments in 2000. Plant samples were harvested from the microplots leaving one row or at least 0.5 m (0.5 m at the first sampling; 0.75 to 1.0 m later in the season) at the microplot edge as buffer. Plants from row sections 1 m long (0.5 m long in 2000 for Samplings 2, 3, and 4) were cut 25 mm below the cotyledonary node and removed from the field. To avoid or minimize effects of previous samplings within a microplot, at least one row or a 0.5-m row segment was maintained between each sampled row section and the next sampling area. Aboveground plants were separated into stems (branches, petioles, squares, and flowers), leaves, and bolls. As cotton development progressed, bolls at or near maturity were separated into burs (carpel walls), seeds, and lint, and immature bolls were pooled with the bur fraction. Samples were dried at 65°C, weighed, ground in a Wiley mill, and then pulverized in a ball mill.

Soil samples were collected after cotton harvest from each microplot to a depth of 2.4 m. Samples were either obtained in eight 0.3-m increments or in six increments consisting of four 0.3-m increments in the top 1.2 m and then two 0.6-m increments using a power-driven soil core sampling device fitted with a 0.05-m tube. Triplicate samples per depth increment were collected at about 0.25-m distance from the rows, air-dried at ambient temperature, ground, sieved through a 2-mm screen, and pulverized in a ball mill. Plant and soil samples were analyzed for total N and 15N content using a Europa Scientific Integra Mass Spectrometer (PDZ Europa Ltd., Crewe, UK).

Fertilizer use efficiency by the difference method was calculated as follows:

where PlantNfert is total aboveground plant N (kg ha–1) for N-fertilized plots, PlantNnonfert is total aboveground plant N (kg ha–1) for nonfertilized plots, and FertilizerN is the amount of fertilizer N (kg ha–1) applied (Rao et al., 1991). Associated with this method is the assumption that plant uptake of soil N is the same in fertilized and unfertilized plots. Since processes including soil N transformations (i.e., mineralization, immobilization etc.) and root development have been observed to differ between fertilized and control plots, this assumption is often questioned (Westerman and Kurtz, 1974; Olson and Swallow, 1984; Rao et al., 1991). To calculate fertilizer use efficiency according to the isotope dilution method and fertilizer recovery in the soil (15NFR-S), the following equation was employed:

where a is the atom% excess (above background) in plant tissue or soil, f is the atom% excess in fertilizer, TN is the total amount of N (kg ha–1) in the aboveground plant or in the soil, and FertilizerN is the amount of fertilizer N applied (kg ha–1) (Hauck and Bremner, 1976).

Fertilizer N contribution to total plant N uptake (N derived from fertilizer) was calculated by dividing the 15N atom% excess in aboveground biomass by the 15N atom% excess of the applied fertilizer.

Added N interaction (ANI) was calculated as the difference between the amount of soil N found in plants sampled in 15N-fertilized plots and the amount of N in plants from control plots (Rao et al., 1991).

The SAS software package was used for mixed-model analysis of variance across years and sites and then for each site-year (SAS Inst., 1999). In light of the differences in the number of treatments and weather conditions among years, most data are presented by site and year although differences between the sites and/or years were not always significant. For data collected in 2000, an F test for a linear trend of N treatment effects was used (SAS Inst., 1999).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Fertilizer Nitrogen Recovered in the Crop
Many of the parameters analyzed were significantly affected by the growing seasons. Due to considerable differences in weather conditions among the 3 yr, significant effects of growing season were not unexpected (Fritschi et al., 2003). Unusually high rainfall early in the year and a cool spring characterized the first year of the study. In the two subsequent years, weather conditions were closer to normal. Although growing seasons differed, the effects of N treatments were generally similar in all 3 yr.

Fertilizer use efficiency was calculated by the isotope dilution method and the difference method (Fig. 1) . Even though some estimates of FUE-ND and FUE–15N diverged considerably for plants sampled near defoliation, the differences between the two methods were not statistically significant. Discrepancies between the two methods are common, with the difference method usually resulting in higher estimates of fertilizer use efficiency than the isotope dilution method (Hauck and Bremner, 1976; Harmsen and Moraghan, 1988; Torbert et al., 1992; Glendining et al., 1997). However, in circumstances when available soil N levels are high or low fertilizer rates are applied, FUE-ND can also be lower than that obtained with the isotope dilution method (Allison, 1966; Moraghan et al., 1984; Schindler and Knighton, 1999).



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  		Fig. 1. Fertilizer use efficiency of cotton at defoliation as affected by N treatment. Vertical bars represent standard errors. FUE–15N, fertilizer use efficiency by 15N dilution; FUE-ND, fertilizer use efficiency by N difference.

 
In 1998, fertilizer recoveries estimated by the difference method for the N-56 treatments varied considerably around negative averages (not shown in Fig. 1). Since fertilizer applications to the N-56 treatments were less than 12 kg N ha–1 (Table 1), the detection of a significant response of cotton growth to applied N was not expected. Acala FUE-ND was not significantly different between the two locations. Excluding N-56 treatments from the calculations, the average FUE-ND was 52% on the Panoche clay loam and 64% on the Wasco sandy loam (2-yr averages). Average FUE–15N of the Acala crop at defoliation was 49% on the Panoche clay loam and 43% on the Wasco sandy loam, with significant (P < 0.05) effects for location x treatment and location (Table 1). Although differences in the total aboveground FUE–15N between treatments were not significant, recovery on the Panoche clay loam tended to increase as N application increased (F test for linear trend in 2000: P = 0.13). However, total aboveground FUE–15N for cotton grown on the Wasco sandy loam in 2000 tended to decrease (F test for linear trend: P = 0.052) as N application increased (Fig. 1 and Table 1). In fact, in 2000 these trends were significant (P < 0.05) for the seed and fiber pools at the two locations (Table 1).


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  		Table 1. Percentage recovery of fertilizer N in plants near defoliation and soil after harvest as determined by 15N for Acala grown on Panoche clay loam and on Wasco sandy loam and Pima grown on Panoche clay loam.

 
Information on lint yield and physiological N use efficiency is reported elsewhere (Fritschi et al., 2003, 2004) and is only summarized below. Three-year average lint yields of Acala were 1454 kg lint ha–1 on the Panoche clay loam and 1548 kg lint ha–1 on the Wasco sandy loam (Fritschi et al., 2003). The significant location x N rate interaction reported previously (Fritschi et al., 2003) was the result of a greater lint yield response to additional N on the Panoche clay loam than on the Wasco sandy loam. As opposed to Acala lint yields for which a linear response to N treatment was observed in five out of six site-years, the response of Pima lint yields to N rate was quadratic (Fritschi et al., 2003). The physiological N use efficiency (final lint yield divided by total aboveground plant N just before defoliation) averaged 13.2 kg lint kg–1 plant N for Pima and 11.4 kg kg–1 for Acala grown on the Panoche clay loam and 9.4 kg kg–1 for Acala grown on Wasco sandy loam (averages across years and treatments; Fritschi et al., 2004). The combination of greater FUE (as indicated by FUE–15N) and physiological N use efficiency translated into the more pronounced yield response on the Panoche clay loam than on the Wasco sandy loam.

Pima N fertilizer recovery in the shoot was not affected by N treatment, averaging 55% FUE-ND and 48% FUE–15N at defoliation across both years and all N rates (Table 1 and Fig. 1). However, in 1999, fertilizer recovery in the seed and fiber pool was greater (P < 0.10) in the N-168 treatment than the N-56 treatment while the opposite (P < 0.05) was true for the pool made up of leaves, stems, and burs, indicating disparity in fertilizer N partitioning between these pools (Table 1).

Previously, no significant effect of N application rate on fertilizer N recovery was observed for tobacco (Nicotiana tabacum L.) (MacKown and Sutton, 1997) and spring barley (Hordeum vulgare L.) (McTaggart and Smith, 1995) while Glendining et al. (1997) reported that percentage recoveries tended to increase as N applications increased for spring barley. The reasons for the opposing tendencies of 15N fertilizer recovery in the seed and fiber pool as well as the total aboveground biomass between the two locations in 2000 were not examined in this study. However, preliminary results from an incubation study (unpublished data, 2000) indicate greater microbial activity for the Wasco sandy loam than the Panoche clay loam, suggesting that differences in N mineralization, immobilization, and turnover may have contributed to these results.

While FUE-ND estimates obtained in this study were comparable to those observed by others (Boquet and Breitenbeck, 2000; Navarro et al., 1997), the FUE–15N tended to be higher than the reported ranges. Average FUE–15N of less than 30% was reported by Torbert and Reeves (1994) in a study examining tillage and traffic effects on cotton grown on sandy loam with an N application of 90 kg ha–1 and by Karlen et al. (1996) for cotton grown on a loamy sand fertilized with 84 kg N ha–1. Researchers in Australia found cotton FUE–15N on heavy loam of 14.2 and 46.6% (Rochester et al., 1994) or less than 30% (Rochester et al., 1993; Freney et al., 1993). Although relatively high in comparison with other cotton studies, FUE–15N levels reported in this study are similar to those of a variety of other crops. Fertilizer N recoveries by spring barley ranging from about 40 to 70% have been reported, and averages near 50% appear to be quite common (Glendining et al., 1997; McTaggart and Smith, 1995; Nielsen et al., 1988). Similar FUE–15N was also found in field studies with corn: 47 and 51% (Tran and Giroux, 1998), 40 and 45% (Schindler and Knighton, 1999), 43 to 57% (Reddy and Reddy, 1993), and 15 to 65% (Torbert et al., 1992).

There were no consistent differences in percentage fertilizer N recovery in the individual crop fractions between years and among treatments. Two-year mean 15N fertilizer recoveries in leaf, stem, seed, bur, and fiber fractions were 9.7, 5.6, 27.1, 4.8, and 2.2% for Acala grown on Panoche clay loam; 9.4, 5.1, 22.6, 5.0, and 1.4% on the Wasco sandy loam; and 10.0, 4.4, 23.2, 8.3, and 2.3% for Pima grown on Panoche clay loam. However, as mentioned above, when pooled, 2000 15N fertilizer recovery in the seed and fiber fraction of Acala cotton increased with additional fertilizer N application on the Panoche clay loam while the opposite trend was observed at the Wasco site (Table 1). On average, 29% of the fertilizer N applied was removed from the Panoche clay loam and 24% from the Wasco sandy loam with the harvested portion (i.e., seed and fiber) of Acala. The fertilizer N removed from the field at Pima harvest averaged 25% of the total fertilizer N applied. After harvest, approximately 20% of the fertilizer N was incorporated into the soil as Acala compared with 23% as Pima residues.

Contributions of soil N to total N assimilated in cotton were significantly greater on the Wasco sandy loam than on the Panoche clay loam (Table 2). Although there was a tendency toward greater amounts of soil N in the aboveground biomass with increased N in 2000, the trend was not significant for Acala; however, it was significant for Pima (F test for linear effect: P < 0.05). Greater uptake of native soil N caused by addition of fertilizer N would suggest the presence of an ANI. Averaged across N treatments, soil-derived N removed with the Acala-harvested portion amounted to 83 kg ha–1 in 1998 and 43 kg ha–1 in 2000 on the Panoche clay loam and 97 kg ha–1 in 1998 and 93 kg ha–1 in 2000 on the Wasco sandy loam. For Pima, soil N removed in the harvest averaged 57 kg ha–1 in 1999 and 49 kg ha–1 in 2000. In the case of Acala grown on Panoche clay loam, more native soil N was removed from treatments N-56 and N-112 than fertilizer remaining in the field as residue (estimated by subtracting fertilizer N removed from fertilizer N in total aboveground biomass) and in soil pools while for the N-168 and the N-224 treatments, the amount of fertilizer N recovered in the soil and crop residue exceeded that of native soil N removed from the field. On the Wasco sandy loam, fertilizer N recovered in the field after harvest was greater than native soil N removal from the field only for the N-224 treatment. For Pima, more native soil N was removed from the field than fertilizer N remained in both treatments in 1999. In 2000, the cutoff between net loss and net gain was between the N-112 and the N-168 treatments.


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  		Table 2. Nitrogen derived from native soil N and fertilizer N in different plant fractions for Acala cotton as affected by N treatment, soil type, and growing season.

 
Proportions of N derived from fertilizer increased with additional increments of fertilizer N applied (Tables 2 and 3). The contributions of fertilizer N to total aboveground plant N were greater on the Panoche clay loam than on the Wasco sandy loam. We suggest that soil N availability on the Wasco sandy loam was greater due to a more active soil microbial population. In addition, more frequent irrigations on the Wasco sandy loam may have accentuated differences in microbial dynamics.


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  		Table 3. Nitrogen derived from native soil N and fertilizer N in different plant fractions for Pima cotton as affected by N treatment and growing season.

 
Unfortunately, plant samplings in the first half of the growing season were not frequent enough to establish if the pattern of fertilizer N uptake was different from that of total N uptake during the period of greatest N assimilation. However, on average across all N treatments, more than 90% of the total amount of fertilizer N and 80% of total N taken up throughout the entire 2000 season had been assimilated by early bloom in Pima and in Acala at the two locations. In addition, relative fertilizer N uptake at the first sampling (early square) averaged 28% while relative total N uptake averaged 15% in 2000 (averages across N treatments, locations, and species; data not shown). These data suggest temporal differences in the uptake of fertilizer N compared with native soil N.

Added Nitrogen Interactions
Differences in the fertilizer N recovery between the two calculation methods (FUE–15N vs. FUE-ND) indicate the presence of ANI. In this study, the observed differences between the two calculation methods were generally not significant. Variation in total plant N uptake, particularly for the low-N treatments, likely contributed to these results. When calculating FUE-ND, relatively small differences in total plant N accumulation as a result of minimal fertilizer application rates (only 4.5 to 11.7 kg N ha–1 added to the N-56 compared with the control treatment) can cause a wide range of values. Although mostly insignificant, disparities between FUE-ND and FUE–15N observed in this study may indicate certain patterns in ANI. Examination of ANI data by analysis of variance did not reveal significant treatment or location effects, matching results reported by Norton and Silvertooth (2000) for cotton grown in the desert Southwest. However, an F test for a linear trend of N treatment effect in 2000 indicated a tendency toward increasing ANI with greater N fertility levels. Actual estimates of 2000 ANI for the N-56, N-112, N-168, and N-224 treatments were –0.9, 5.2, 5.6, and 12.8 kg N ha–1 for Acala grown on Panoche clay loam (linear trend: P = 0.10); 4.0, 13.1, 23.6, and 30.4 kg N ha–1 for Acala grown on Wasco sandy loam (linear trend: P = 0.32); and –0.7, –2.0, 2.5, and 8.6 kg N ha–1 for Pima grown on Panoche clay loam (linear trend: P = 0.06). The trend toward greater ANI with increased fertilizer N application was not unexpected. Previously, Rao et al. (1991) found ANI to increase with fertilizer rate in a pot study on spring wheat, and Westerman et al. (1972) observed greater amounts of soil N taken up at higher N fertilization rates under field conditions.

Recovery of Fertilizer Nitrogen in the Soil
The total recovery of 15N fertilizer in the soil (15NFR-S) was not different between the Panoche clay loam and the Wasco sandy loam, between years, or between treatments. However, for Acala grown on Panoche clay loam in 2000, the test for a linear trend revealed a tendency toward lower recovery in the soil at the greater compared with the smaller N rates (P < 0.10; Table 1). Mean 15NFR-S was 39% in the Panoche clay loam and 46% in the Wasco sandy loam when cropped with Acala (Table 1). When Pima was grown on Panoche clay loam, 15NFR-S averaged 42% (Table 1). These proportions of N remaining in the soil were relatively high in comparison with levels of 13 to 35% reported by others for soils planted with cotton (Torbert and Reeves, 1994; Rochester et al., 1994; Freney et al., 1993; Karlen et al., 1996). Aside from plant uptake, differences in fertilizer N recovered from the soil between the studies were likely due to sampling of the soil profile to 2.4 m (present study) compared with 0.9-m depth or less in the other studies. In addition, differences in soil processes such as denitrification may have contributed to these results (Freney et al., 1993).

Soil samples were collected in increments to a depth of 2.4 m to examine the distribution of 15N fertilizer within the profiles. When cropped with Acala, slightly more fertilizer N tended to be in the lower soil horizons of the Panoche clay loam than the Wasco sandy loam (Fig. 2) , particularly in 1998. Across all treatments, years, and cotton types (Acala and Pima), a mean of 78% of the 15N (range 61–98%) in the Panoche clay loam was recovered in the surface 0.9 m of the soil profile (Fig. 2). In comparison, more than 90% of the 15N fertilizer recovered in the Wasco sandy loam was found in this soil layer. Due to the soil texture characteristics of the two locations, it was expected that downward movement of fertilizer N would be more evident at the Wasco than at the Panoche site. However, it is possible that downward movement of fertilizer N was facilitated by extensive, deep cracks in the Panoche clay loam, which may have provided preferred pathways for fertilizer movement as well as roots. Greater variation of 15N among samples observed on this soil indicated less uniform distribution of 15N, which would be consistent with nonuniform downward movement, possibly as a result of soil cracking. Even so, only traces of fertilizer N were observed below 1.2 to 1.5 m of the profile, indicating that no or minimal leaching losses occurred. In 1998, recovery of labeled N in the 0.3- to 0.6-m layer was over twofold greater than in 2000 in the Wasco sandy loam. In the Panoche clay loam, about 1.5 times more 15N was found in the 0.3- to 0.6-m layer in 1998 than in 2000. Because of the rainy spring in 1998, and some rain within days of fertilizer application, this was not unexpected. Nonetheless, if no significant losses of leaching or denitrification occur between growing seasons, N fertilizer remaining in the upper 0.9 m of the soil profile is within the rooting zone and potentially available to subsequent crops.



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  		Fig. 2. Recovery of 15N-labeled fertilizer in Panoche clay loam and Wasco sandy loam after cotton harvest. Horizontal bars represent standard errors.

 
Total Fertilizer Recovery and Losses
Total fertilizer recovery in the Acala crop and the soil was not different between the two locations, with a mean of 88% on the Panoche clay loam and 89% on the Wasco sandy loam (Table 1). The F test for a linear effect of N treatment on total recovery was significant for Acala grown on Panoche clay loam in 2000 (P = 0.02). In 2000, total 15N fertilizer recovery in the N-56 treatment on the Wasco sandy loam was significantly greater than for the other N treatments. Linear regression analyses of percentage total recovery vs. N treatment were not significant at either location. However, results indicated a trend toward lower total recovery as N fertilizer increased. Such a trend would match with linear increases of fertilizer 15N losses with application of fertilizer N reported by Glendining et al. (1997). Unaccounted fertilizer N at the end of the season averaged 12% for Acala and 10% for Pima on Panoche clay loam and 11% for Acala on Wasco sandy loam. At the highest N fertilization rate, this would correspond to a loss of about 20 kg ha–1 of fertilizer N. However, this does not account for the fertilizer N that remained in the roots (not quantified) and was not lost from the system. Nitrogen can be lost from the plant–soil system mainly via volatilization, denitrification, nitrification, and leaching. Particularly on the Panoche clay loam, some N may have been lost by denitrification during temporary periods of oxygen deficiency as a result of irrigation. Although there was some movement of 15N into low soil horizons in the Panoche clay loam, based on recovery pattern of 15N in the soil, it does not seem likely that any significant amount of 15N was lost by leaching below 2.4-m depth. However, it is probable that N located in soil layers below 1.2 m will be lost for agronomic purposes since, under many soil conditions, cotton extracts only minimal amounts of N from below 1.2 to 1.5 m (Hutmacher et al., 1999). Hutmacher et al. (2004) postulated that small net accumulation of nitrate N in the lower soil profile as a result of high N fertilization rates could result in significant losses over years. It is not likely that cotton or other crops grown in rotation with cotton, except possibly for safflower (Carthamus tinctorius L.), would recover significant amounts of this N in the next growing season. The relative amounts of unaccounted 15N, though important, were smaller than those reported by others for cotton (Torbert and Reeves, 1994; Karlen et al., 1996; Rochester et al., 1993, 1994; Freney et al., 1993). Possible reasons besides deeper soil sampling may include effects of soil type, differences in timing and placement of the N fertilizer, and distinct effects of rainfall and irrigation.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Even though weather conditions varied considerably among the 3 yr, the effects of N treatment were similar from year to year. Losses of fertilizer N from the system were relatively small compared with other published data. Fertilizer use efficiencies were not significantly different among N treatments. However, in contrast to the Wasco site where fertilizer use efficiency in 2000 tended to decrease with increasing N application, fertilizer use efficiency tended to increase at the Panoche site, resulting in a significant location x treatment interaction effect. Most of the fertilizer N recovered in the soil was found in the top 0.9-m layer.

Relatively small losses of fertilizer N and high recovery in plants indicate that the postemergence injection of urea N as conducted in this study is an agronomically efficient way to provide cotton with fertilizer N under the given production conditions. However, although split applications of N in 1996 did not result in a significant yield response (Hutmacher et al., 2004), it is possible that multiple N applications as well as other variations in application (i.e., timing and placement) would improve fertilizer use efficiency above the levels reported in this study.


   ACKNOWLEDGMENTS
 
The authors thank Wayne and Doug Wisecarver and the staff of the West Side Research and Extension Center and the University of California Cooperative Extension at the Kings County office for their cooperation and Robert L. Nichols for his support.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This research was supported in part by grants from Cotton Incorporated, the California Department of Food and Agriculture Fertilizer Research and Education Program, and the California Crop Improvement Association.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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F. B. Fritschi, B. A. Roberts, D. W. Rains, R. L. Travis, and R. B. Hutmacher
Recovery of Residual Fertilizer-N and Cotton Residue-N by Acala and Pima Cotton
Soil Sci. Soc. Am. J., April 11, 2005; 69(3): 718 - 728.
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