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Crop Science 40:1685-1693 (2000)
© 2000 Crop Science Society of America

CROP ECOLOGY, PRODUCTION & MANAGEMENT

Nitrogen Rate Effect on Partitioning of Nitrogen and Dry Matter by Cotton

Donald J. Boqueta and Gary A. Breitenbeckb

a Louisiana Agric. Exp. Stn., Northeast Research Station, Winnsboro, LA 71295 USA
b Dep. of Agronomy, Louisiana State University, Baton Rouge, LA 70803-2110 USA

dboquet{at}agctr.lsu.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Summary
 REFERENCES
 
Increased understanding of the fate of N in cotton (Gossypium hirsutum L.) fields will improve N efficiency, optimize crop development and yield, and may help to avoid excessive N fertilization. This study quantified the effects of N fertilization rate (0, 84, and 168 kg ha-1) on seasonal uptake and partitioning of N and dry matter in field-grown cotton during 1989 and 1990. The N rates studied were part of a larger experiment initiated in 1987 where plots received annual preplant applications of 0, 28, 56, 84, 112, 140, or 168 kg N ha-1. Experiments were conducted on Commerce silt loam (fine-silty, mixed, thermic, nonacid, aeric Fluvaquent) on the Louisiana State University Agricultural Center Northeast Research Station near St. Joseph, LA. Plants were collected at five dates in each year at about 20-d intervals. Shed plant debris was also collected. The N content and aerial biomass of plant components were determined. Maximum N uptake occurred between 49 and 71 d after planting and was 2.9 and 4.3 kg ha-1 d-1 for cotton receiving 84 and 168 kg N ha-1, respectively. At maturity, plants receiving 84 kg N ha-1 contained 160 kg N ha-1 in aerial biomass and an additional 50 kg N ha-1 in abscised plant debris. The total amount of N assimilated by plants receiving 84 kg fertilizer N ha-1 averaged 111 kg N ha-1 more than plants receiving no fertilizer N for an apparent fertilizer efficiency greater than 100%. By the end of effective bloom, plants receiving 168 kg N ha-1 assimilated 15 to 40% more N, primarily in leaves and lower bolls, than plants receiving 84 kg N ha-1. This excess assimilated N was recovered in surface litter and in N-enriched plant components. Of the plant components studied, leaf-blades most consistently reflected the amounts of fertilizer N applied.

Abbreviations: DAP, days after planting • EEB, end of effective bloom • MAT, crop maturity • CV, coefficient of variation


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Summary
 REFERENCES
 
IMPROVEMENTS IN COTTON cultivars and pest control have stimulated interest in improving N management to achieve optimum economic yields concurrent with the need to minimize the environmental impact of fertilizer N. It is widely recognized that N supply exerts a marked influence on vegetative and reproductive growth, and there is a tendency for some producers to attempt to increase maximum yield potentials by applying higher than recommended N rates. The effects of N deficiency are readily evident in reduced vegetative growth, early cutout, and reduced fruiting index (Guinn, 1982; Radin and Mauney, 1986), but the effects of excessive N are often less readily apparent. Application of N in excess of that required for optimum crop performance can reduce yield or fiber quality (Gardner and Tucker, 1967; Gerik et al., 1989) and may contribute to ground and surface water pollution. Excessive N, especially in combination with high late-season moisture availability, can delay maturity, reduce harvesting and ginning percentages, and promote boll shedding, disease, and insect damage (Harris and Smith, 1980; Hodgson and MacLeod, 1988).

A number of practices have been employed to improve N fertilization management for cotton production, including use of split applications (Maples and Frizzell, 1985; McConnell et al., 1993), deep placement (Eberhart and Tucker, 1988), various N formulations (Rickerl, 1989), nitrification inhibitors (Gordon, 1990), foliar applications (Hodgson and MacLeod, 1988; Miley and Maples, 1988), and plant growth regulators (McConnell et al., 1992). These practices typically are more effective in some cotton-growing regions than in others because of differences in soils, climate, cropping history, and other management practices. Because of these differences, development of optimum strategies depend upon regional estimates of the N required by the crop to achieve maximum potential yield, the efficiency of N fertilization practices, and the availability of residual NO-3 and mineralizable soil organic N (Olson and Kurtz, 1982).

Several studies have addressed the relationship between N uptake and dry matter production in cotton. Bassett et al. (1970) reported total dry matter production up to 8900 kg ha-1 and total N uptake of 142 kg ha-1 for irrigated cotton in California fertilized with 134 kg N ha-1. Dry matter production and N uptake of irrigated Acala cultivars in Israel were studied by Halevy (1976) and Marani and Aharonov (1964). For two cultivars fertilized with 100 kg N ha-1, they reported a maximum rate of dry matter production of 280 kg ha-1 d-1 between 72 and 112 d after planting with total seasonal dry matter production between 12 200 and 13 480 kg ha-1. The maximum daily N uptake was 4.5 kg ha-1, which occurred between 98 and 112 d after planting. The total seasonal N uptake was 235 kg ha-1, of which 42 to 49% was removed from the field at harvest.

Most information concerning N requirements of dryland cotton grown in southeastern USA was obtained prior to 1942 when yield potentials were lower than present day (Fraps, 1919; McHargue, 1926; Olson and Bledsoe, 1942). Mullins and Burmester (1990) monitored nutrient uptake of four cotton cultivars grown on two Paleudults in Alabama. They reported that mature plants contained 127-155 kg N ha-1 in standing biomass. No significant genotypic effect was found, and differences in uptake were attributed primarily to the amounts of N applied at different sites. None of the above studies accounted for the assimilated N that was shed as leaves and fruiting structures during the growing season nor did the studies attempt to determine the efficiency of the applied fertilizer N.

Comparisons of N uptake by fertilized crops to that of unfertilized controls have been used extensively to approximate the efficiency of N fertilization of corn (Zea mays, L.) and other crops (Bock, 1984; Harmsen and Moraghan, 1988; Rao et al., 1992). Surprisingly, few such studies have been performed to estimate the efficiency of cotton N fertilization. Constable and Rochester (1988), in Australia, reported an average apparent recovery of only 30% of the N applied to cotton grown on irrigated clay soils. Other experiments have identified a number of factors influencing N fertilizer recovery by cotton, including prior crop (Hearn, 1986) and water-related stress (Hearn and Constable, 1984; Hodgson and MacLeod, 1988).

The principal objectives of the work reported were to determine (i) the influence of deficient, adequate, and excessive N fertility on biomass production and seasonal N uptake and partitioning by cotton in the lower Mississippi Delta, and (ii) the apparent fertilizer N efficiency for each of the included fertilizer rates.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Summary
 REFERENCES
 
Field experiments were performed during 1989 and 1990 at the Louisiana State University Agricultural Center Northeast Research Station 5 km northwest of St. Joseph, LA, on a field containing Commerce silt loam. The three fertilizer treatments of 0, 84, and 168 kg N ha-1 presented in this paper were part of a fertilizer N study that ran from 1987 through 1993 and included N rates of 0, 28, 56, 84, 112, 140, and 168 kg ha-1. The 0, 84, and 168 kg N ha-1 rates were selected for intensive study because they clearly were deficient, adequate, and excessive rates, respectively (Boquet et al., 1994). Each fertilizer treatment received preplant applications of the appropriate N rate annually beginning in 1987. Treatments were replicated four times in a randomized complete block design. Plots were 19 m in length and consisted of eight rows of cotton planted in raised beds on 1-m-row spacings. Application rates were maintained on the same plots each year by broadcasting NH4NO3 with a Gandy (Owatonna, MN) fertilizer spreader and incorporation with a rolling cultivator and harrow. One day after fertilization, plots were planted to `Deltapine 41' cotton (5 May 1989 and 27 Apr 1990) at a rate of 130 000 seed ha-1. Management was consistent with typical agronomic practices used for dry land production in the region and have been reported previously (Boquet et al., 1994).

Soil samples (0–15 cm) collected 20 d prior to planting indicated no significant effects of prior N treatment on soil pH (avg. 6.0), organic matter (avg. 7.9 g kg-1), total N (avg. 680 mg kg-1), or the amounts of P (avg. 250 mg kg-1), K (avg. 160 mg kg-1), Ca (avg. 1844 mg kg-1), or Mg (avg. 363 mg kg-1). Although available soil P and Mg levels were high, and K and Ca levels were adequate (Peevy, 1972), 20 kg P ha-1 and 38 kg K ha-1 were applied prior to planting in 1989. Samples for residual soil N were collected 20 d prior to planting to a depth of 150 cm using a tractor-drawn Guiddings probe (Fort Collins, CO) fitted with a 5-cm coring tube. Five cores were collected from each plot, partitioned into various depths, and the corresponding increments combined prior to N analysis (Table 1) . Exchangeable NH4+ and NO3-were determined in 2 M KCl soil extracts by the NH4 diffusion technique described by Carlson (1978). Monthly rainfall during the 1989 and 1990 growing seasons and 50-yr averages are shown in Table 2 . The 1989 growing season was characterized by above average rainfall in June and July, whereas in 1990, heavy rainfall occurred immediately following fertilization and was below average the remainder of the growing season.


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  		Table 1 Amounts of exchangeable soil NH+4 and NO-3 in field plots receiving 0, 84, or 168 kg N ha-1 in 1987, 1988, and 1989. Soil samples were taken 20 d prior to planting cotton in 1989 and 1990

 

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  		Table 2 Rainfall and average air temperature during 1989 and 1990 growing seasons and 50-yr averages

 
Plant samples were collected 28, 48, and 71 d after planting (DAP), at the end of effective blooming (EEB) and at crop maturity (MAT). For the purposes of this work, EEB was defined as 28 d after the daily bloom rate reached an average of one white bloom per 3 m2, and MAT was when 60% of the bolls were open. The sampling date for EEB in 1989 was 96 d after planting for all N rates. In 1990, EEB sampling was 89, 93 and 98 d after planting for the 0, 84 and 168 kg ha-1 N rates, respectively. The MAT sampling in 1989 was 133 DAP for the 0 N rate, 143 DAP for the 84 kg N rate, and 154 DAP for the 168 kg N rate. In 1990, MAT sampling was 112 DAP for the 0 N rate, 124 DAP for the 84 kg N rate, and 154 DAP for the 168 kg N rate. Ten plants were randomly collected 28 DAP from each plot, dried (65°C; 24 h), weighed, and total N determined by the salicylic acid-thiosulfate modification of the Kjeldahl procedure to quantitatively recover nitrate (Bremner and Mulvaney, 1982). On three subsequent sampling dates, five plants were collected at random and partitioned into stems, branches, leaf blades, petioles, squares, and bolls for determination of aerial biomass and N content. At MAT, bolls were separated into carpels, lint, and seed. The amounts of assimilated N shed as plant debris between EEB and MAT were estimated by collecting all recognizable plant material at weekly intervals contained within 1-m2 frames placed between rows. Plant populations determined at 48 DAP and at EEB were used to calculate N assimilated per hectare.

Statistical analyses of the data were accomplished by the GLM analysis of variance procedures of SAS (1989) to determine the effects of N treatments on N uptake and partitioning. All data were initially analyzed within years and then combined across years. Mean separations were accomplished by applying the Fisher protected LSD test, P = 0.05.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Summary
 REFERENCES
 
Daily N Assimilation Rates
The initial N uptake rate from planting to 28 DAP was low for all three fertilizer N rates, ranging from 0.18 kg ha-1 d-1 in the 0 N rate up to 0.29 kg ha-1 d-1 in the 168 kg ha-1 N rate (Fig. 1) . The low demand for N up to 28 DAP, combined with the high capacity of alluvial soils to supply residual and other forms of N, suggests that fertilizer N application can be delayed in cotton until 28 DAP. This should have a minimal effect on early-season plant growth and no detrimental effect on lint yield.



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  		Fig. 1 Assimilation rate of N in cotton receiving deficient (0), adequate (84 kg ha-1), or excessive (168 kg ha-1) N fertilization, averaged across 2 yr

 
Between 28 and 71 DAP, the rate of N uptake increased with time and fertilizer N rate, although, the assimilation rates of cotton receiving 84 and 168 kg N ha-1 were similar up to 48 DAP (Fig. 1). The maximum assimilation rate for each N-rate treatment was attained between 49 and 71 DAP, coinciding with blooming and early boll set. Maximum assimilation rate was attained 20 to 30 d earlier in this study than in previous studies conducted in the southeast USA (Olsen and Bledsoe, 1942; Mullins and Burmester, 1990) and with Acala cultivars in Israel (Halevy, 1976). Where adequate N of 84 kg N ha-1 was applied, the maximum accumulation rate was 2.9 kg N ha-1 d-1, and where an excessive amount of N was applied (168 kg N ha-1), the maximum assimilation rate was 4.3 kg ha-1 d-1. The assimilation rate of 2.9 kg ha-1 d-1 fell within the range of 2.5 to 3.6 kg ha-1 d-1 reported by Mullins and Burmester (1990) for cotton grown with optimum N fertilization on two Pauleudult soils in Alabama but was higher than the 1.5 to 2.0 kg ha-1 d-1 found by Bassett et al. (1970) in California. There was a decline in the rate of N uptake between 71 DAP and EEB for all N rates coinciding with the onset of rapid reproductive growth, possibly related to reduced root efficiency. Although plants continued to assimilate N until MAT, the loss of N through abscission of plant structures after EEB resulted in net loss of N from the standing crop.

Apart from plant growth stage and fertilizer N rate, it was obvious that N uptake rate was influenced by rainfall. The significant decline in the rate of N uptake between 71 DAP and EEB coincided with the absence of rainfall (Table 2). Dry conditions limited mineralization processes and the N available for assimilation, and thus there was a more precipitous drop in N uptake in the 0 N-rate treatment than in fertilized treatments. The N uptake rate did not decrease as much in cotton supplied with fertilizer N, possibly because N fertilization stimulated the development of larger plants (Fig. 2) and a more extensive root system capable of supplying the increased water and nutrient demand of the larger plants. The fertilized cotton therefore drew from a larger pool of both added and indigenous N, which influenced the efficiency of fertilizer N (recovery vs. applied) as well overall N efficiency (Table 3) .



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  		Fig. 2 Total seasonal dry matter accumulation in cotton receiving deficient (0), adequate (84 kg ha-1) or excessive (168 kg ha-1) N fertilization, averaged across 2 yr. EEB, end of effective bloom; MAT, crop maturity. LSD (P = 0.05) for mean comparisons among N rates: 28 DAP, vegetative = 14; 48 DAP, vegetative = 44, reproductive = 2.8; 71 DAP, vegetative = 168, reproductive = 38; EEB; vegetative = 221, reproductive = 178, plant debris = 85; MAT, vegetative = 217, reproductive = 348, plant debris = 239

 

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  		Table 3 Nitrogen uptake and apparent efficiency of fertilizer N in cotton receiving deficient, adequate and excessive fertilizer N

 
Between EEB and MAT, the daily rate of uptake in the 84 and 168 kg N rates continued to decrease but uptake rate in the 0 N rate plots increased in both years (Fig. 1). This resulted in similar uptake rates for fertilized and unfertilized cotton at MAT. The reduced uptake rate for fertilized treatments suggested that most of the applied fertilizer N had been taken up by the crop or was no longer available for plant uptake in plots receiving either optimum or excessive application rates. Crowther (1934) proposed that, during boll fill, the bolls become the primary sink for photosynthates, which deprives roots of sustenance, limiting root efficiency and decreasing nutrient uptake for a time during boll fill. This suggestion is consistent with the data shown in Fig. 3 . The probable reason for the increase in the daily uptake rate at MAT in the 0 N rate, which occurred in both years, was related to sampling time relative to growth stage. This sampling coincided with completion of boll fill, and some N uptake had occurred because of the resumption of vegetative and root growth. As shown in Fig. 3a, however, there was actually very little N accumulation in the standing crop of the 0 N rate cotton after 71 DAP.



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  		Fig. 3 Total seasonal N uptake in cotton receiving deficient (0), adequate (84 kg ha-1) or excessive (168 kg ha-1) N fertilization, averaged across 2 yr. EEB, end of effective bloom; MAT, crop maturity. LSD (P = 0.05) for mean comparisons among N rates: 28 DAP, vegetative = 1.8; 48 DAP, vegetative = 3.0, reproductive = 0.2; 71 DAP, vegetative = 7.8, reproductive = 1.6; EEB; vegetative = 8.7, reproductive = 6.6, plant debris = 2.3; MAT, vegetative = 3.1, reproductive = 6.3, plant debris = 8.0

 
Early to Mid-Season Biomass Accumulation and N Uptake
The total aerial biomass 48 DAP in the 84 and 168 kg ha-1 N treatments was similar and 17% higher than that of the 0 N rate treatment (Fig. 2). There was little difference among the three N rate treatments in the relative ratios of the plant fractions. Stems and branches accounted for 61 to 65% of the total aerial biomass and leaves accounted for 24 to 27% of the total aerial biomass in each of the treatments.

By 71 DAP, application of 84 kg N ha-1 had significantly increased the aerial biomass of all plant fractions compared with the 0 N rate. Increasing the N rate from 84 to 168 kg ha-1 further increased the vegetative biomass. Although an increase in N rate increased vegetative growth, the relative proportions of the plant vegetative fractions remained similar among treatments. The 71 DAP proportions, however, were different from those in the corresponding treatments 48 DAP. At 71 DAP, stems and branches comprised 57 to 59% and leaves comprised 14 to 15% of the plant biomass in each of the N treatments. With the initiation of reproductive growth, squares represented 12% of the standing crop.

Allocation of assimilated N to plant structures changed throughout the growing season as the plants progressed from vegetative through reproductive growth. At 48 DAP, the total N uptake by plants fertilized with 84 or 168 kg N ha-1 was 60% greater than in the 0 N rate treatment (Fig. 3). For all treatments, 95 to 96% of the assimilated N accumulated in vegetative structures. At this early date in the growing season, allocation of N to branches, stems, leaves, petioles, and squares was proportionally similar among all treatments but the concentration of N in most plant components increased in response to fertilization. In leaf blades, for example, the N concentration was 51 g kg-1 at the 0 N rate, 69 g kg-1 at the 84 kg N rate, and 62 g kg-1 at the 168 kg N rate.

During the short time between 48 and 71 DAP, the accumulated N more than doubled in all N rate treatments (Fig. 3), paralleling the increase in aerial biomass (Fig. 2). With the onset of reproductive growth, 6% of the accumulated N was found in squares. For all N rates, the largest percentage of assimilated N was in the leaf blades. Leaf N accounted for an average 66% of the total N in all treatments as the crop entered the boll development stage.

Late-Season Aboveground Biomass and N Uptake
Aerial biomass at EEB was substantially increased by N fertilization. Cotton plants receiving 84 kg N ha-1 accumulated 30% more dry weight than those receiving 0 N (Fig. 2). Applying 168 kg N ha-1 led to 18% greater dry weight than applying 84 kg N ha-1. There was little difference among N-rate treatments in relative proportion of vegetative plant fractions. Branches and stems represented 49% of the plant biomass in the 0 N rate, 51% in the 84 kg N rate and 53% in the 168 kg N rate. Leaves represented 22% of the plant biomass in the 0 N rate and 19% in the 84 and 168 kg N rate treatments. Reproductive structures (squares and bolls) accounted for 23, 25, and 21% of the plant dry weight for the 0, 84, and 168 kg N ha-1 treatments, respectively. A small amount of plant debris had accumulated on the soil surface but was not collected until one week later and is not included in these data.

The final partitioning of dry matter at MAT is shown in Table 4 . Increase in N rate from 0 to 84 kg ha-1 increased the dry weight of each plant component. Increase in N rate to 168 kg N ha-1 increased stem and branch weight and plant debris, but decreased the dry weight of leaves, petioles, and lint. The reduction in leaf dry weight was actually due to increased leaf abscission at the high N rate, probably because of shading within the plant canopy. At all N rates, the plant components that contributed the greatest dry weight at MAT were the stem and branches, which represented 25 to 28% of the total dry matter. This was partly because other vegetative plant components were subject to abscission.


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  		Table 4 Final dry matter and N partitioning in cotton grown with deficient, adequate, and excessive fertilizer N rates, averaged across 2 yr

 
Bassett et al. (1970) reported that stems represented 23.1% and leaves 17.4% of total dry matter. Mullins and Burmester (1990) found that partitioning of dry matter differed with soil type. Cotton on Decatur soil (clayey, thermic, kaolinitic Rhodic Paleudults) had 35% partitioned to stems and 18.5% to leaves, and cotton on Norfolk soil (fine loamy, thermic siliceous Typic Paleudults) had 16% partitioned to stems and 14% to leaves. The percentage of dry matter proportioned to leaves at MAT in our study was lower (4.8–8.6%) than in either of the earlier studies, regardless of the fertilizer N rate. The low dry matter percentage of leaves compared with the earlier reports was related to several factors including sampling time relative to shedding of plant material. Previous reports did not include the dry matter of plant structures shed during the growing season.

Plant debris (plant structures abscised prior to MAT) in this study accounted for a large part of the total dry matter production (Fig. 2). In the 0 and 84 kg N rate treatments, 18% of the total dry matter produced was abscised prior to MAT. In cotton fertilized with 168 kg N ha-1, plant debris was the largest aerial plant fraction, representing 24% of the total dry matter production (Table 4).

As expected, the total assimilated N increased with increase in N rate (Table 3, Fig. 3 and 4) . Within an N rate, the total quantity of assimilated N was similar in both years. As a result of the large quantity of plant debris at MAT, a large percentage of the assimilated N was found in abscised plant debris. As a percentage of total N uptake, the highest percentage of N shed as plant debris (26%) was in cotton receiving 0 N. As the N rate increased, the quantity of N in plant debris increased but the proportion of the total assimilated N shed as plant debris decreased to 23% at the 84 kg N rate and to 20% at the 168 kg N rate. The additional N assimilated in fertilized cotton and not shed as plant debris was partitioned to the stem and branches to support a larger plant architecture and to enrich the N concentration of carpels and seed.



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  		Fig. 4 Total N assimilated by cotton receiving deficient (0), adequate (84 kg ha-1) or excessive (168 kg ha-1) N fertilization. EEB, end of effective bloom; MAT, crop maturity

 
Previous work (Mullins and Burmester, 1990) has associated larger plants with a decrease in the harvest index (seedcotton/total aerial dry matter). In this study, the harvest index of cotton receiving either 0 or 84 kg N ha-1 was the same at 32%. Thus the vegetative to reproductive ratio did not change with application of sufficient N to produce optimum seedcotton yields. With application of excessive fertilizer N (168 kg ha-1), however, the harvest index decreased to 26% because of luxurious vegetative growth and lower seedcotton yield. These observations indicate that application of excessive N rates has a negative impact on production efficiency. Basset, 1970; Halevy, 1976; and Mullins and Burmester, 1990; reported higher harvest indexes of 33 to 52%, but did not include shed plant debris as part of the plant biomass production.

Quantity of Assimilated N vs Quantity Removed in Harvested Seedcotton
At the 0 N rate, 39 kg N ha-1 (39% of the total assimilated N) was contained in the seedcotton and subject to removal by harvest. To produce one kg of seedcotton in the unfertilized cotton, the crop assimilated 48 g of N of which 19 g was eventually contained in the seedcotton. With application of 84 kg N ha-1, the N content of seedcotton increased to 89 kg ha-1; more than double that in the 0 N rate treatment. This represented 43% of the total assimilated N. For each kg of seedcotton produced, the crop assimilated 52 g of N of which 22 g was partitioned to harvested seedcotton. With application of 168 kg N ha-1, the N content of harvested seedcotton increased to 100 kg N ha-1 and represented 41% of the total assimilated N. At this excessive N rate, the cotton crop assimilated 65 g of N for each kg of seedcotton produced, of which 27 g accumulated in the seedcotton. In previous studies where cotton received optimum fertilizer N of 72 or 112 kg ha-1, Mullins and Burmester (1990) found that seedcotton contained an average 26.8 g N kg-1, which was 42% of the total N uptake. Halevy (1976) and Oosterhuis et al. (1983) also reported that 42% of assimilated N was found in the seedcotton. It is interesting that the percentage of total assimilated N in the harvested seedcotton in our study was about the same as that recorded in the Alabama, Israel and Zimbabwe studies. Our values for total N uptake and seedcotton N content for fertilized cotton were higher than that reported in the earlier studies by Mullins and Burmester (1990) and Oosterhuis et al. (1983), and about the same as reported by Halevy (1976).

Small quantities of additional N were removed at harvest in the plant debris contained in the seedcotton. Using the known N concentration of plant debris collected from the soil surface and assuming the same concentration for debris in seedcotton and 5 to 10% total trash in the seedcotton, the additional N harvested with seedcotton represented only 1.5 to 2.5 kg ha-1.

Nitrogen Concentration in Plant Components
The response of various plant components to N availability during crop development is useful for identifying plant components suitable for tissue analysis to establish critical N levels and identify N deficiency, sufficiency and excess. Increase in the N rate from 0 to 84 kg N ha-1 significantly increased the N content of all plant components sampled 48 and 71 DAP with the exception of stems (Table 5) . At these sampling dates, increasing the amount of applied N from 84 to 168 kg ha-1 significantly increased N concentration only in reproductive structures. Application of the higher amount of N increased total N uptake in vegetative tissues but also increased biomass production of these tissues, which tended to dilute N concentrations to levels similar to those observed in vegetative tissues of cotton receiving 84 kg N ha-1.


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  		Table 5 Nitrogen concentration in cotton plants with deficient, adequate and excessive applied fertilizer N, averaged across 2 yr

 
From EEB to MAT, the N concentration of all plant components except stems and branches continued to increase in response to increase in N fertilization rate. The tissue N concentrations in Table 5 reflect those of cotton grown under N deficiency (0 rate), sufficiency (84 kg N rate), and excess (168 kg N rate). Sufficiency levels for total leaf blade N have not been clearly established, but recent studies suggest a concentration of 35 to 52 g kg-1 represents a critical level for maximum yield (Bell et al., 1998; Mitchell and Baker, 1997). These levels depend upon growth stage at time of sampling and may also depend upon leaf position. On the basis of these critical values, and discounting the importance of leaf position, the leaf-blade N concentration of 51 g kg-1 in the 0 rate at 48 DAP was sufficient for crop development. In fact, the capacity of this soil to supply N appeared to be adequate for cotton growth until 71 DAP (Fig. 2). These findings are consistent with earlier work indicating that Mississippi River alluvial silt loam soils typically supply sufficient mineralizable and residual soil N for optimum early-season cotton growth (Boquet et al., 1995).

Another goal of this study was to characterize the magnitude and variability of N concentrations of various plant components to reflect N status, and thereby identify those most valuable for establishing reliable critical levels at various stages of crop development. While most plant components responded to N fertilization with increase in N concentration, some components displayed two to three times the response of others when 84 kg N ha-1 was applied (Table 5). The total N content of petioles showed a marked response to fertilizer N at all sample dates and appeared to be the most sensitive component for indicating excessive N fertilization prior to EEB. The response of leaf-blade N was about half that of petioles at 48 and 71 DAP, but later in the growing season (EEB and MAT) leaf blades showed response similar to petioles. Branches also displayed a marked response to fertilizer N, especially later in the growing season. At MAT, the N content of carpels showed the largest percentage response to applied N. Carpels were effective in indicating excessive N fertilization as well as sufficiency, and therefore their analysis may have potential as a post-season evaluation of N fertilization practices. Harvested seed showed less response than carpels, but seed N contents were among the least variable of the various components at MAT (Table 6) . The N contents of lint and plant debris were inadequate indicators of N status.


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  		Table 6 Coefficients of variation associated with plant component sampling for total N concentration in cotton plants with deficient, adequate and excessive applied fertilizer N, averaged across 2 yr

 
The ability to resolve differences in the N content of plant components is a function not only of the magnitude of the response but also of the variability associated with sample analysis. Variability in the N concentrations of plants is the result of many factors that affect N availability and uptake as well as the microenvironmental effects on individual plants and their components. Analytical errors resulting from mixing, drying and analysis also contribute to variability. The coefficients of variability in this study ranged from 6% to as high as 30% and varied by plant component and sampling date (Table 6). In general, the N concentration of vegetative plant components was greater and the associated CVs higher at 48 and 71 DAP than later in the growing season. When tissue testing is used as a diagnostic tool for midseason N adjustments, however, the crop must be sampled during early to midseason despite higher CVs associated with samples collected at these times.

No single plant component displayed the ideal combination of a large response to available N and a low sampling error at all sample dates (Table 6). At 48 DAP, the N concentration of stems was the least variable component and showed a >62% increase in response to application of 84 kg N ha-1. However, stem concentrations did not reflect excessive N availability and declined markedly between 48 and 71 DAP, suggesting that variations to time of sampling would inflate overall variability. The total N contents of petioles offered the largest response and lowest CVs at 48 DAP. These data are insufficient, however, to determine if use of petiole total N contents is subject to the same limitations as use of petiole nitrate where large seasonal and year-to-year variations prevent the establishment of reliable critical values in humid regions (Phillips et al., 1987). In contrast to stem and petiole N levels, the similarity in leaf-blade concentrations at 48 and 71 DAP suggest that leaf blades are less subject to seasonal variation. Although the magnitude of leaf blade response to various N rates was less than that of petioles, the CVs of leaf blades were similar to or less than that of petioles. These findings support the conclusion that leaf-blade N analysis offers an attractive basis for an early- to midseason test for cotton N status (Gerik et al., 1994; Bell et al., 1998).


   Summary
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary
REFERENCES
 
The N uptake quantities and patterns of cotton grown on Mississippi River alluvial silt loam was similar across 2 yr despite large differences between years in rainfall amounts and distribution. Nitrogen uptake was a season-long process that fluctuated with rainfall, crop growth stage and fertilizer N rate. The maximum daily uptake of N occurred between 49 and 71 DAP. With application of an optimal fertilizer N rate of 84 kg ha-1, the maximum uptake rate was 2.9 kg ha-1 d-1 and with an excessive fertilizer N rate of 168 kg ha-1, the maximum uptake rate was 4.3 kg ha-1 d-1. The total N uptake with optimal fertilizer N of 84 kg ha-1 was 209 kg ha-1, which was 111 kg ha-1 more than the unfertilized cotton, or an apparent fertilizer efficiency of 132%. Increasing the fertilizer N rate to an above-optimal rate of 168 kg ha-1 increased total N uptake to 242 kg ha-1 and decreased lint yield and fertilizer efficiency. The excess assimilated N was not used efficiently because it either accumulated in plant structures that abscised or increased the plant components N concentration above the sufficiency level.

With application of the adequate fertilizer N rate of 84 kg ha-1, the quantity of N removed from the field in harvested seedcotton was 89 kg N ha-1. This, and the soil NO3 data in Table 3, suggests that N fertilization of cotton for optimal yield is exceptionally efficient and probably does not result in loss of N to nearby surface and ground waters. With application of excessive N (168 kg ha-1), however, the removal of N from the field in harvested seedcotton was increased minimally to a total 100 kg ha-1. The remaining 68 kg fertilizer N ha-1 accumulated in plant residue and in the soil and may eventually be subject to transport from the cotton field to surface and ground waters.

Of the plant components studied, leaf-blades most consistently reflected the amounts of fertilizer N applied. Leaf blades had a large N-concentration response to fertilizer N rate and usually lower sampling error than other components.Armstrong Albert 1931; Elberhar Tupper 1988; Gene 1982; Maples Mark Frizzell. 1985; SAS Institute 1989


   ACKNOWLEDGMENTS
 
The authors thank A.B. Coco and D.A. Schellinger for their valuable assistance in plant and soil sampling and laboratory analyses.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary
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Approved for publication by the Director of the Louisiana Agric. Exp. Stn. as manuscript no. 99-76-0324.

Received for publication August 23, 1999.


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