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a 141 Experiment Station Road, P.O. Box 345, Stoneville, MS 38776 (Previously at: Department of Agronomy and Range Science, University of California, One Shields Ave., Davis, CA 95616)
b Department of Agronomy and Range Science, University of California, One Shields Ave., Davis, CA 95616
c University of California Cooperative Extension, 680 N. Campus Drive, Ste. A, Hanford, CA 93230
d University of California, Shafter Research and Extension Center, 17053 N. Shafter Ave., Shafter CA 93263
* Corresponding author (ffritschi{at}ars.usda.gov).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: LAD, leaf area duration • LAI, leaf area index
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In the Cotton Production Manual for California, Weir et al. (1996) recommended that fertilizer N application be based on residual soil nitrate levels and yield goal, and to use petiole nitrate levels as in-season measure of the adequacy of N supply. For example, their recommendations for a yield goal of about 1600 kg lint ha–1 and residual soil nitrate-N between 56 and 100 kg ha–1 call for an application of 224 kg N ha–1. In the mid-1990s, California cotton growers applied an average of around 200 kg N ha–1 (USDA Econ. and Stat. Syst., 2001). Considering the importance of N for cotton development and the relatively inexpensive nature of N fertilizer, it may be tempting for some growers to apply high rates to ensure adequate supply, but actual crop demand. Although N fertilizer generally represents only a small fraction of the total costs to produce a crop of cotton in the San Joaquin Valley, farmers should maximize N fertilizer use efficiency in order to optimize return.
Many N response studies conducted to adapt fertilization to specific environments have examined yield response but not N uptake and partitioning dynamics. Once we understand plant needs, including uptake rates and partitioning, and if we know what is available in the soil (reserves, mineralization, etc.), we can more accurately assess fertilizer needs. The information on plant N uptake, allocation pattern, and tissue N concentrations presented here provides reference points for the reevaluation of fertilization guidelines, and, when combined with data on soil N dynamics, will allow the design of fertilization strategies that maximize lint yield and minimize N losses in a particular environment.
This study was conducted (i) to determine the effects of different N fertility levels on seasonal N uptake and partitioning in irrigated Acala and Pima cotton and (ii) to examine the N treatment effects on Acala cotton grown on a sandy loam and a clay loam.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Nitrogen treatments with target rates of 56, 112, 168, and 224 kg N ha–1 were established at each location with individual treatments imposed on the same plots throughout the 2 (Pima) or 3 (Acala) yr. Every year the amount of fertilizer N applied (Table 1) was determined by subtracting prefertilization soil NO–3 levels in the top 0.6 m of soil (sampled about 1 wk after seedling emergence) from the target rate of each of the four treatments. Fertilizer N was applied when cotton plants had developed three to five true leaves by sidedressing the appropriate amounts of urea approximately 0.15 m deep in bands 0.2 m away from the row on both sides of the plants. Acala and Pima experiments were set up as randomized complete block designs with four (Acala) or three (Pima) replications and main plots six to 12 rows wide (row spacing: 0.96 or 1.01 m; average plant density: 10.2 plants m–2) and 80 to 170 m long. Within these fertilized plots randomly located areas, 12 m long and spanning the entire width of each plot, were maintained as unfertilized control plots. Commercial four and six-row farm equipment was used for all management operations. All treatments were irrigated uniformly within a location and across the two species. Potassium and phosphorus fertilizer application rates were based on soil test results. Management was consistent with typical agronomic practices of the region and uniform across all treatments within each location.
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Physiological N use efficiency was calculated by dividing lint yield (kg ha–1) by total aboveground plant N (kg ha–1) (Singh et al., 1998) in plants sampled just before defoliation, and, since hand harvested samples were not ginned, was only calculated for machine harvested plots.
Leaf area was measured on all samples in 1998 and 1999 with a LI-3050A leaf area meter (LI-COR Inc., Lincoln, NE), and, together with plant density information, was used to compute leaf area index. Total leaf area duration (LAD) was calculated by multiplying the number of days between two consecutive samplings by the mean leaf area index (LAI), and then summing the LAD of the individual samplings (n) for the season:
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The SAS software package was used for mixed model analysis of variance for repeated measures (years and developmental stages) (SAS Institute, 1999). For each year and location, an analysis to predict LAD, LAI at early bloom, and lint yield as a linear function of leaf nitrogen concentration was performed with Proc Mixed (SAS Institute, 1999). Additional analyses were performed to combine years and locations. Slopes for year and location were compared by an F test for homogeneity of slopes. If slopes were not significantly different, a regression fitting a common slope was used. Treatment means were compared by least square means by Fisher's Protected LSD Test at a significance level of 5%.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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When analyzed across both locations and all three years, N concentrations in leaves, stems, fruit, seed, bur (all P < 0.001), and fiber (P < 0.01) fractions were affected by N treatment. Although combined analysis of N concentration in the final tap root sampling from 1998 and 2000 did not reveal significant N treatment effects, repeated measures ANOVA revealed a response of root N concentration to N treatment in 2000, when tap root samples were regularly collected throughout the growing season. With exception of the tap root fraction in 1998, the positive response of the N concentration in all tissues to added N was observed in every year even though the N concentrations differed between years. The magnitude of the response of N concentration to N rate varied among plant fractions and cotton developmental stages. On average across all three years, the sensitivities of the leaf, stem, and fruit fractions to N were greater at early bloom than at other sampling stages. This was the case at both locations and indicates that monitoring of cotton N status at early bloom may be especially helpful in deciding whether supplemental N applications may be required.
At early bloom the average (3-yr) response of N concentration to N treatment was comparable for leaf and stem tissues on the Panoche clay loam, while on the Wasco sandy loam stem tissue was more sensitive than leaf tissue. Thus, both leaf and stem N concentrations may be useful in monitoring cotton N status at early bloom. However, sampling of leaf tissue rather than stem or total above ground tissue would be more practical. In addition, rather than the collection of leaves from the entire canopy, selection of a well defined sample (leaf age, location on main-stem, etc.) of one or more leaves from one or more particular main-stem nodes (i.e., uppermost, fully mature leaf blade was used by Bell et al., 2003) should be more sensitive and realistic. Previously, Gerik et al. (1994) reported that leaf N concentration was a better measure of cotton N status in relation to vegetative growth and boll number than petiole nitrate. Obviously, similar to petiole nitrate tests, critical ranges as bases for management recommendations would have to be established. A very simple and rapid method to monitor cotton N status, the petiole nitrate test is currently the most popular tool used to make in-season N fertilization decisions. However, the petiole nitrate test is not without its drawbacks (e.g., sensitivity to factors influencing plant transpiration) and monitoring leaf N concentration would provide an alternative view and complement results obtained by petiole analysis. Recent efforts to establish critical leaf-blade N data for cotton in the mid-south were published by Bell et al. (2003). They reported that sampling leaf-blade N at early bloom predicted seedcotton yield better than at first pin-head square or at mid-bloom and recommended a critical value of 4.3% N in the uppermost, fully mature leaf blade at early bloom.
Boquet and Breitenbeck (2000) suggested that N concentrations of carpels may be useful in a post-season evaluation of N fertilization practices. Since, at harvest, the bur fraction exhibited the greatest relative response to N treatment, our data support their findings. Unfortunately, in the present study, immature bolls were combined with the bur fraction, which may at least in part, have been responsible for the sensitivity of the bur fraction and thus precludes a final conclusion.
As a result of residual N levels in the soil only minimal or no N additions (Table 1) were necessary to achieve the target rate of 56 kg N ha–1. Because of that, N concentrations in the control and N-56 treatments were essentially the same throughout the experiment (Tables 2 and 3). However, differences in N concentrations were observed between N-168 and the control and N-56 treatments.
Generally, N concentrations observed in the different plant fractions were similar to those reported for cotton grown in the San Joaquin Valley in 1958 and 1959 (Bassett et al., 1970). However, late season N concentrations reported by Bassett et al. (1970) (averages of six sites) in leaves (about 30 vs. 20 g kg–1) and burs (about 10 vs. 6 g kg–1) tended to be greater than the 3-yr overall averages observed in the present study. Stem N concentrations were higher at the Wasco site (13 g kg–1) and lower at the Panoche site (8 g kg–1) than those reported by Bassett et al. (1970)(about 10 g kg–1). For the leaf fraction, differences may in part be due to the fact that the leaf fraction as defined in this study only included leaf blades without petioles while Bassett et al. (1970) combined petioles and leaf blades. However, many other factors could have resulted in these discrepancies, including management practices, weather, and possibly changes in partitioning and translocation patterns between the varieties grown (Acala 4-42 vs. Maxxa). Wells and Meredith (1984a)(1984b, 1984c) reported that for cotton cultivars released between 1905 and 1978, the more recent cultivars made the transition from vegetative to reproductive growth at a smaller cumulative heat sum and developed a greater reproductive/vegetative ratio. If this trend continued with subsequent cultivars, lower leaf N concentrations toward the end of the season for Maxxa (present study) than Acala 4-42 (Bassett et al., 1970) seem plausible, and underscore the importance of periodic reevaluations of N management practices as cotton cultivars change.
Leaf N concentrations reported by Boquet and Breitenbeck (2000) for cotton grown in the southeastern USA were generally considerably higher, especially early in the season, than those found in the present study. Mullins and Burmester (1990) determined leaf N concentrations only slightly higher than those reported here. Their data indicate sharper drops in leaf N concentration from early to late season on a Decatur silt loam than a Norfolk sandy loam. This was probably due to differences in fertilizer N applied at the two sites (78 vs. 112 kg N ha–1 on Decatur vs. Norfolk, respectively) but may in part have been due to a modulating effect of soil type similar to the one observed in the present study. Such differences in the N concentration levels and pattern become particularly important when leaf N concentration is intended as a criterion for management decisions and demonstrate the need to determine critical values (minimum leaf-blade N concentrations required to achieve maximum yield) adapted to specific production conditions and, as implied above, modern cultivars.
Equations from trend analyses between leaf N concentration at early bloom and leaf area index, leaf area duration, and lint yield, are presented in Table 4. Block effects were included in this analysis as a random classification effect. A simple linear model was chosen; other curilinear trends were evaluated and did not significantly improve the regression results. In some cases, significant relationships between the selected variables and leaf N concentration could be detected at early square or near peak bloom, but they were generally weaker than at early bloom. Similarly, leaf-blade N concentration at early bloom was reported by Bell et al. (2003) as a more accurate predictor of lint yield than leaf-blade N concentration at first pin-head square midbloom or cut-out. In addition, in a pot study Gerik et al. (1994) found that leaf N concentration was well correlated with boll number in the early stages of flowering but not before flowering or in the late stages of flowering and boll development. Thus, it appears that leaf N concentration at early bloom is an important characteristic in defining cotton yield. In this study, linear equations predicting lint yield based on early bloom leaf N concentration were significant on the Panoche clay loam in 1999 and 2000 and on the Wasco sandy loam in 1998 (Table 4). While slopes differed significantly among years on the Panoche clay loam, this was not the case on the Wasco sandy loam. The relationships between early bloom leaf N concentration and early bloom leaf area index and leaf area duration were significant in each year at both locations (Table 4). The relationship observed between early bloom leaf N concentration and leaf area duration suggests that the effect of leaf N concentration on yield may in part be due to its importance in maintaining leaf area. It appears that leaf N concentration at early bloom would lend itself to guage cotton N status.
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Pima
Nitrogen concentration in Pima plant fractions essentially developed in the same manner as for Acala as the seasons progressed (Tables 2 and 3). Leaf N concentration decreased on average across years and treatments by over 60% from early square to near defoliation. Stem and root N concentrations decreased from early square to a low near peak bloom and defoliation. As for Acala, observed increases between defoliation and final harvest suggest that some translocation from leaves into roots and stems occurred after treatment with defoliant. Nitrogen treatment affected N concentration in all tissues (P < 0.05). While the control and N-56 treatments were not significantly different from each other, differences between N-168 and N-56 and control treatments were generally significant. In addition, as indicated by interaction effects (P < 0.05) between N level and sampling time, treatment effects differed as cotton development progressed, underlining the differences in N concentration responsiveness to N treatment with plant development. This was not surprising and may largely be the result of the fertilization practices (i.e., a single N fertilization at the beginning of the seasons). The largest response of leaf and stem N concentration to N treatment was observed at early bloom in each of the two years. The percentage response at early bloom was similar for the leaf and stem fractions and greater than for other plant fractions. This observation is consistent with results obtained for Acala and reinforces the potential value of leaf and/or stem analyses at early bloom for in-season N management adjustments. Differences in fiber N concentrations in response to N level were minimal and only significant when data were analyzed across both years. As seen above for Acala, and suggested by Boquet and Breitenbeck (2000), Pima bur fraction N concentration at harvest time may lend itself for post-season evaluation of N management practices.
Pima leaf N concentration at various stages of cotton development was related to leaf area duration, leaf area index, and lint yield. As for Acala, leaf N concentration at early bloom was correlated more closely to these parameters than at early square, near peak bloom, or later (analyses not shown). Early bloom leaf N concentration was more closely correlated with leaf area duration than leaf area index in 1999 (Table 4). As for Acala, the results of the trend analyses indicate the importance of early bloom leaf N concentration for lint yield.
The N concentrations in leaves were initially higher and later in the season lower than the 30 and 23 g N kg–1 reported by Unruh and Silvertooth (1996) for Pima grown in Arizona. On the other hand, stem N concentrations appeared to be in a range similar to that reported by Unruh and Silvertooth (1996), while seed N concentrations were lower than their 33.8 g N kg–1. Since in the present study, Acala and Pima were not grown in the same trial no statistical comparisons can be made between the two species; however, a couple of observations may be noteworthy since the two experiments were conducted side by side in the same field. Interestingly, Unruh and Silvertooth (1996) found greater N concentrations in Pima than in Upland seeds while in the present study average (1999 and 2000, all N levels, at final harvest) seed N concentration was 33.4 g N kg–1 for Acala (Panoche clay loam site only) and 28.6 g N kg–1 for Pima. However, consistent with Unruh and Silvertooth (1996), a pattern toward greater bur fraction N concentrations in Pima than Acala (Panoche clay loam only) was observed throughout most of the 1999 and the entire 2000 growing seasons.
Nitrogen Uptake and Partitioning
Acala
Nitrogen uptake differed between locations (P < 0.001), treatments (P < 0.001), and years (P < 0.001), and was strongly affected by developmental stage (P < 0.001; Fig. 1 and 2)
. Maximum N assimilated in aboveground plants parts was on average across years and treatments 27% greater on the Wasco sandy loam than the Panoche clay loam. Differences in total N uptake between the two locations tended to be smaller with increased N application. In 1998, total N taken up by early square was greater on the Panoche clay loam than the Wasco sandy loam. By early bloom it was essentially the same at the two locations and by peak bloom the amount of N in the plant was greater at the Wasco than the Panoche site. Greater total N uptake on the Wasco sandy loam became evident earlier in the growing season with every year, and was evident by early square in 2000.
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As expected, application of N above the N-56 target rate increased total N uptake (Fig. 1 and 2). Maximum N accumulation in the N-168 was about 1.5 (Wasco sandy loam) and 1.8 (Panoche clay loam) times that of the control and N-56 treatments (3-yr averages).
Not surprisingly, partitioning of N among the different aboveground plant components was dominated by the effect of cotton developmental stage, in particular the switch from vegetative to reproductive growth. The location and treatment effects on N partitioning were minimal and, when present, were inconsistent across years and sampling dates (statistics not shown). At the time of the defoliation treatment, accumulated N was distributed as follows: 21.0% in leaves, 11.1% in stems, 8.8% in burs, 55.4% in seed, and 3.7% in fiber (averages of both locations, all years, and all treatments). The proportion of N removed from the field was high compared to reports by Mullins and Burmester (1990) and Boquet and Breitenbeck (2000) who found that N removed in seed cotton represented 39 to 43% of the total assimilated N. However, removal of over 50% of the assimilated plant N from the field at harvest was reported before (Bassett et al., 1970). In addition, leaves and fruits shed prior to defoliation were considered by Boquet and Breitenbeck (2000) but not in this study. Removal of N from the field at harvest in seed and lint ranged from 36 kg to 122 kg ha–1 on the Panoche clay loam, and from 81 to 150 kg ha–1 on the Wasco sandy loam. Thus, maximum amounts of N removed from the field at harvest exceeded 109 kg ha–1 reported by Halvey (1976), 100 kg ha–1 by Boquet and Breitenbeck (2000), and 83 kg ha–1 by Bassett et al. (1970).
Pima
Nitrogen uptake and partitioning as a function of cotton developmental stage, N treatment, and year are illustrated in Fig. 3
. Total N uptake increased with increasing N fertilization (P < 0.001). In 1999, N uptake was nearly completed by early bloom with over 90% of the maximum amount of N assimilated by that time. In 2000, N uptake continued until later in the season. While N assimilation had reached over 93% in treatments N-168, and N-224 at peak bloom, N uptake in the lower N treatments was noteworthy even later in the season. However, maximum uptake rates peaked between early square and early bloom averaging 1.0 kg ha–1 d–1 for the N-56 treatment and 2.7 kg ha–1 d–1 for the N-168 treatment over the two years. Maximum amounts of N accumulated ranged from 72 (2-yr average, control treatment) to 149 kg ha–1 (2000, N-224).
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The distribution of accumulated N in aboveground plant parts at defoliation time was 18.8% in leaves, 8.6% in stems, 15.2% in burs, 52.0% in seed, and 5.4% in fiber (averages across all treatment years). The relatively large amount of N in the burs was due to immature bolls being grouped with this fraction. The comparatively large percentage of N in the bur fraction in Pima vs. Acala (8.8%) was likely the result of a greater number of bolls in Pima. Removal of N from the field at harvest in Pima seed and lint ranged from 47 kg (2000: check and 56 kg N ha–1) to 93 kg N ha–1 (1999: 168 kg N ha–1).
Physiological Nitrogen Use Efficiency
Physiological N use efficiency of Acala was greater on the Panoche clay loam than on the Wasco sandy loam (Table 5). On average across all three years the difference was 17% and was most pronounced in 2000. While differences between N treatments were generally not significant in individual years, repeated measures analysis revealed a significant (P < 0.01) trend for decreased physiological N use efficiency of Acala with increased N application. Physiological N use efficiency of Pima decreased with increasing N rate in 2000 but not in 1999 (Table 5).
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Tissue N concentrations were strongly influenced by cotton developmental stage, N treatment, and location. High-N treatments resulted in greater N concentrations in most tissues throughout much of each growing season as compared to low-N treatments. In general, tissue N concentration varied more throughout the growing season for cotton grown on the Panoche clay loam than on Wasco sandy loam. At the Wasco site tissue N concentrations tended to decrease less as the season progressed and were generally higher than at the Panoche site. On the whole, leaf N concentration at early bloom better predicted lint yield than leaf N concentration earlier or later in cotton development, suggesting that leaf N is particularly important around the period of early bloom. Thus, for maximum benefit, N should be applied before early bloom if there is an indication of N shortage. Combining information on soil mineral N supply and crop N-status monitoring using leaf-blade N concentration of one or multiple leaves (with attention to leaf position and age) may allow growers to anticipate N shortage and use corrective N applications effectively.
Nitrogen uptake increased with increasing N application at both locations. While N uptake continued until defoliation, maximum uptake rates were observed between early square and early bloom. In the 56 and the 168 kg N ha–1 treatments of Acala the greatest uptake rates (3-yr averages) ranged from 1.3 to 2.9 kg N ha–1 d–1 on the Panoche clay loam and from 1.7 to 3.6 kg N ha–1 d–1 on the Wasco sandy loam. Pima uptake rates (2-yr averages) peaked at 1.0 (N-56) and 2.7 (N-168) kg N ha–1 d–1.
Total accumulation in aboveground plant parts tended to be greater on the Wasco sandy loam than the Panoche clay loam. Before the defoliation treatment, Acala plants grown in the 56 and the 168 kg N ha–1 treatments had accumulated 132 and 214 kg N ha–1 on the Panoche clay loam and 145 and 219 kg N ha–1 on the Wasco sandy loam (3-yr averages). The 2-yr averages for Pima plants were 79 kg N ha–1 for the N-56 treatment and 124 kg N ha–1 for the N-168 treatment. The amount of N removed from the field ranged from 36 to 122 kg N ha–1 on the Panoche clay loam and from 81 to 150 kg N ha–1 on the Wasco sandy loam. Effects of N treatment and location on partitioning of N into the different plant fractions were minimal and, when present, inconsistent across years and sampling.
Physiological N use efficiency of Acala was greater on the Panoche clay loam than on the Wasco sandy loam. A tendency for physiological N use efficiency to decrease with increasing N application was observed.
Leaf N concentration at early bloom was more closely related to lint yield than at other developmental stages, possibly making it a useful tool in judging the need for in-season N management adjustments. Under current production conditions in California it appears likely that 100 to 150 kg N ha–1 for Acala and 80 to 100 kg N ha–1 for Pima would have to be applied to replace the N removed from the field in harvested seedcotton.
The differences in N uptake and N concentration between Acala cotton grown on Panoche clay loam vs. Wasco sandy loam underline the importance of soil type to cotton N management. These differences were observed even though soil NO3–N concentrations near planting were accounted for in the establishment of the treatments. Therefore, accounting for N mineralization throughout the growing season can be important in the design of N management strategies to optimize yield on a particular soil type. Mineralization rates define the extent and timing at which additional soil N will become available to the production system. In addition, results of our 5-yr study (Hutmacher et al., 2004) indicate excessive N buildup in many California cotton soils. This results from high N application rates for rotation crops (corn, tomatoes) and previous cotton crops. An important aspect of N-management should be the reduction of that component. Reducing application rates for cotton, when appropriate, will aid in reducing soil excesses. This, in turn, will decrease the movement of N into ground and surface waters.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication December 6, 2002.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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