Soil Science Society of America Journal 65:1823-1828 (2001)
© 2001 Soil Science Society of America
DIVISION S-8 - NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS
Nitrogen and Water Stress Interact to Influence Carbon-13 Discrimination in Wheat
D. E. Clay*,a,
R. E. Engelb,
D. S. Longc and
Z. Liua
a Plant, Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
b Land Resources and Environmental Sciences, Montana State Univ., Bozeman, MT
c Northern Agricultural Research Center, Montana State Univ., Havre, MT 59501
* Corresponding author (david_clay{at}sdstate.edu)
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ABSTRACT
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The impact of interactions between water and N stress on 13C isotopic discrimination (
) is not well understood. The objective of this study was to determine the impact of N on in wheat (Triticum aestivum L.) grown under low, moderate, and high water stress. In a field study located near Havre, Montana, USA (48° 30' N lat. and 109° 22' W long.), wheat grown under three different water stress environments (low, moderate, and high) was fertilized with three different N rates (none, moderate, and high). Each treatment was replicated four times. The grain N fertilizer use efficiency increased as water stress decreased. A differential response of to N was observed. In general, if plants were grown under high water stress and N increased yield, then adding N to N-deficient plants reduced (-0.01
for every kg of N added); and if plants were grown under low water stress and N increased yield, then adding N had little or no impact on . The break point between N impacting or not impacting was 
17.45. Under non-N limiting (moderate and high N) conditions the equation relating to yield was, yield (kg ha-1) = -11000 + 884 , r = 0.92**. Wheat grown under N-deficient conditions (0N treatment) did not fit this curve. By accounting for the impact of water and N stress on , this variation could be explained. Results from this study suggest that can be used to characterize N and water stress at different landscape positions in watershed studies.
Abbreviations: , 13C isotopic discrimination • FUE, fertilizer use efficiency • OY, optimum yield • YLND, yield loss due to N deficiency • 0N, N-deficient conditions • **Siginificant at the 0.01 probability level
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INTRODUCTION
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WATER STRESS limits yields in many areas of the world (Batchelor and Paz, 1999; Braga, 2000; Gan et al., 2000; Clay et al., 2001). In spite of the importance of water stress, many field experiments do not evaluate interactions between water stress, N deficiencies, and crop yield. Several reasons are responsible for this lack of effort. First, many traditional diagnostic tools for measuring water use (stomata conductance and plant transpiration) are point measurements that contain large spatial and temporal variability. Second, experimental approaches that directly measure water use, i.e., weighing lysimeters, are expensive to build, operate, and maintain. Third, many scientists assume that climatic conditions are unpredictable, and therefore management recommendations should not be dependent on unpredictable climatic conditions.
In site-specific farming, management recommendations that do not account for water stress can result in large errors. For example, Clay et al. (2001) and Batchelor and Paz (1999) showed that within a single field, yields in summit areas can be limited by too little water, while yields in footslope areas can be limited by too much water. Rockström et al. (1999) and Clay et al. (2001) reported that summit areas may contain less plant-available water than footslope areas. Zollinger and Kells (1993) reported that perennial sowthistle (Sonchus arvensis L.) reduce soybean (Glycine max L. Merr.) yields more under drought than nondrought conditions and Nolan et al. (1998) reported that simple landscape classification delimited areas that responded differently to N. This research suggests that management recommendations can be improved by accounting for the predictable impacts of topography on water stress. To test this hypothesis, a relatively inexpensive technique that integrate the net effect of water stress on plant growth over whole seasons are needed. Prior research suggests that 13C isotopic discrimination () provides such a measure (Farquhar and Richards, 1984; Farquhar et al., 1988; Boutton, 1991; Araus et al., 1993; Farquhar and Lloyd, 1993; O'Leary, 1993; Araus et al., 1997; Saranga et al., 1998).
Three definitions are needed prior to discussing why 13C discrimination during C3 photosynthesis is related to water stress. The first two definitions are that: (i) the ratio between 13C and 12C is the R value (O'Leary, 1993), and (ii) the R value is used to calculate 
13C using the equation:
| [1] |
where, R(sample) is the 13C/12C ratio of the sample and R(standard) is the 13C/12C ratio of PDB, limestone from the Pee Dee formation in South Carolina (Farquhar and Lloyd, 1993; O'Leary, 1993). Typically, 13C values for air, C3, and C4 plants are -8, -27, and -13, respectively. A negative sign indicates that the sample has a lower 13C/12C ratio than PDB. The third definition is that 13C discrimination () is calculated using the equation:
 | [2] |
where, 13Ca is the 13C value of air (-8) and 13Cp is the measured value of the plant.
In C3 plants, 13C-isotopic discrimination can be used as an estimator of water stress because under conditions where the plant is not water stressed, the stomata are open, stomatal conductance is high, and CO2 diffusion in and out of the leaf is relatively free. Under these conditions, RuBisCO preferentially fixes 12CO2 and the fixed CO2 becomes depleted in 13C. As water stress increases, plants reduce water loss by closing stomata, which reduces CO2 diffusion between the pore and the atmosphere. The net result of stomatal closure is increased 13CO2 fixation by RuBisCo and decreased of fixed C (Boutton, 1991; Farquhar and Lloyd, 1993; O'Leary, 1993). Based on these relationships, has been related to the CO2 intercellular (Ci) and atmospheric (Ca) partial pressures by the equation:
 | [3] |
where, a is the 13C discrimination because of CO2 diffusion in air (4.4), and b is 13C discrimination caused by carboxylation (30, when corrected for the equilibrium effect of CO2 dissolution) (Farquhar and Lloyd, 1993; O'Leary, 1993).
Equation [3] predicts that decreases with increasing water stress.
Carbon isotope discrimination in C3 plants has been used to evaluate drought stress and water use efficiency in different crop cultivars and seasonal water stress (Hubick, 1990; White et al., 1990; Condon et al., 1992; Knight et al., 1994; Bettarini et al., 1995; Ngugi et al., 1996; Jefferies and Mackerron, 1997; Pate and Arthur, 1998; Saranga et al., 1998). However, the general adoption of as a water stress index has been hindered by the fact that any factor (nutrients, diseases, and soil compaction) that influences the Ci/Ca ratio has the potential to influence . For example, Yin and Raven (1998) and Betterini et al. (1995) reported that N stress has the potential to reduce the photosynthetic capacity, which in turn increases . The objective of this study was to determine the influence of N on in wheat grown under low, moderate, and high water stress. This research should provide some insight into how can be used to characterize N nutritional adequacy and water stress in wheat grown at various landscape positions within fields.
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MATERIALS AND METHODS
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The experimental procedures, previously described in Engel et al. (1999) are summarized below. The 3-yr study (1996–1998) was conducted in a 4-ha field at Montana State University Northern Agricultural Research Center (Havre, Montana) at latitude and longitude coordinates of 48° 30' N and 109° 22' W. The soil association at the site was a Telstad (fine-loamy, mixed, superactive, frigid Aridic Argiustolls)-Joplin (fine-loamy, mixed, superactive, frigid Aridic Arguistolls) Loam. The study was moved to a new site within the field each year. Characteristics of the site were that water stress gradually increased during the growing season, and soil NO3-N levels in the surface 60 cm prior to spring planting averaged 8.5 kg N ha-1 in 1996, 45.0 kg N ha-1 in 1997, and 45.5 kg N ha-1 in 1998.
A solid-set sprinkler irrigation system was used to create high, moderate, and low water stress environments (Table 1). These environments were located in three different areas of the field. High water stress was induced by applying a single irrigation of between 6.2 to 6.8 cm for stand establishment after wheat emergence. In this treatment, wheat was grown under water stress during vegetative, reproductive, and grain-fill periods. Moderate water stress was produced by irrigating at three dates (establishment, late-tillering, and heading). In this treatment, wheat was grown under a minimal water stress during the vegetative and reproductive stages and was water stressed during grain fill. Low water stress was created by irrigating at four dates (establishment, later-tillering, heading, and grain fill). Each water regime was split into four blocks containing plots with the dimension of 1.8 by 6.1 m. Each plot was treated with one of three N rates (Table 1). To account for yield potential differences in the different water stress environment, the moderate and high N rates were modified.
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Table 1. The influence of water stress environment and year on irrigation, rainfall, and the amount of N applied.
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The spring wheat cultivar ‘McNeal’ was seeded on 25 Apr. 1996, 2 May 1997, and 10 Apr. 1998, at a row spacing of 30.5 cm and a density of 215 plants m-2. Urea fertilizer was applied in a band about 10 cm to the side of the seed row. Sufficient triple superphosphate was applied to ensure adequate P nutrition according to Olsen soil test P recommendations. Irrigation amounts were measured with catch cans placed inside each water regime. Rainfall was measured by a gauge placed next to the study area. Subsamples of harvested grain were collected, ground, and analyzed for total N and 13C on an Europa 20–20 isotope ratio mass spectrometer (Europa Scientific Ltd., Cheshire, UK). Approximately 30% of the samples were either standards or duplicates.
Grain N fertilizer use efficiency was computed by dividing the difference between the grain yields in fertilized and 0N treatments by the N rate. The change in resulting from N fertilizer ( kg N-1), was computed by dividing the difference between in 0N and fertilized treatments by the N rate. Measured yield losses from N deficiencies in 0N rate treatments were calculated by subtracting yields in moderate or high N rate treatments from yields in 0N rate treatments. The moderate N rate, in the above calculations, was used in the high and moderate water stress environments, and the high N rate was used in the low water stress environment.
The steps to calculate yield losses because of N stress include: (i) determine the relationship between optimum yield (OY) and under non-N limiting conditions (OY = f(fertilized); (ii) determine the relationship between grain N fertilizer use efficiency (FUE) and (FUE = f(N deficient); (iii) define the relationship between N and (fertilized = f(N deficient, N fertilizer)); (iv) determine the relationship between yield loss because of N deficiency (YLND) and measured yields of the N deficient plant [YLND = f(N deficient, OY of fertilized plants)]; and (v) develop a relationship between the amount of N needed to achieve optimum yield (for a given amount of water) (N recommendation = f(YLND, FUE). Once these relationships are derived, then yield loss due to N stress for a plant with a given yield and N deficient value can be calculated by: (a) using the N deficent value to estimate the OY (step i); (b) using the estimated OY to estimate YLND (step iv above); and (c) using YLND to calculate the N recommendation (step v above) and the fertilized value (step iii above). If the new calculated fertilized is not equal to the value used in step a, then a new value for fertilized is input into step a. Analysis of variance was used to determine N treatments differences within a water stress environment.
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RESULTS AND DISCUSSION
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In the distinctly different water and N stressed environments produced over the 3-yr study, grain yields ranged from 1120 to 5260 kg ha-1 (Table 2). Grain yields generally increased with decreasing water stress. Overall, the grain yield response to N fertilizer improved as water stress diminished. Additional information on the relationships between yield, protein, and N are available in Engel et al. (1999).
Increasing N from the N-deficient level to the moderate N level, decreased in the high and moderate water stress environments (Table 3). The kg N-1 appeared to be a function of water and N (Fig. 1)
. In general, if plants were grown under high water stress and N increased yield, then adding N to N-deficient plants reduced (-0.01 for every kg of N added); and if plants were grown under low water stress and N increased yield, then adding N had little or no impact on . The break point between N impacting or not impacting was 17.45l (Fig. 1). Negative kg N-1 values resulted from a combination of factors. First, adding N to N deficient plants increases the machinery needed for photosynthesis (chlorophyll) which in turn, increases carboxylation and reduces [Eq. 3]. Second, adding N to N-deficient plants tends to increase biomass production which increases water use, stomatal closure, and water stress. These results were slightly different from Bettarini et al. (1995) and Syvertsen et al. (1997) who reported that adding N to N-deficient plants reduced . Differences may have resulted from the limited number of treatments included in Bettarini et al. (1995) and Syvertsen et al. (1997) studies.
Plants growing under high water stress had smaller grain N FUE than plants growing under low water stress (Fig. 2)
. These findings have implications in precision agriculture because plants grown in summit areas often have less available water than plants grown in foot and toeslope areas (Malo and Worchester, 1975; Halvorson and Doll, 1991; Rockström et al., 1999; Clay et al., 2001). Landscape induced differences in water availability can be caused by water redistribution following rainfall or more capillary movement of water from the groundwater to surface soil in footslope than summit areas.
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Fig. 2. The relationship between grain fertilizer use efficiency and in wheat harvested from plots with three water stress levels.
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Under non-N limiting conditions (moderate and high N treatments), 13C discrimination was correlated positively to yield (Fig. 3a) . Plants growing under high water stress had lower than plants growing under low water stress. Similar results relating to grass (Laundré, 1999), durum wheat (Triticum durum Desf.) (log10 yield[Mg ha-1] = -2.77 + 0.1745 ), and barley (Hordeum vulgare L.) (log10 yield[Mg ha-1] = -1.415 + 0.11156 ) yields have been reported (Araus et al., 1999). The observed yield reduction due to water stress is well known, and in Montana plant available water is used to calculate fertilizer recommendations (Brown and Carlson, 1990). The relationship between yield, , and water stress are also being used in archaeology to estimate historic climatic conditions and yields. For example, Araus et al. (1999) combined values in grain samples obtained from archaeological sites with estimates on how changes in genetics and atmospheric CO2 concentrations influenced photosynthesis efficiency, to estimate that durum wheat yields averaged 1.61 Mg ha-1 during the Neolithic (7500–5000 BP), Chalcolithic-Bronze (5000–3000 BP), Iron (3000–2200 BP), and Middle ages (800 BP) in the northeast Iberian Peninsula. It is interesting to note that current yields in the northeast Iberian Peninsula were almost three times higher than the value reported above.
The relationship between and yields in all plots (Fig. 3b), was much weaker than the relationship between wheat yield and in non-N limited wheat (Fig. 3a). This variation could be explained by considering the relationships kg N-1, water stress, and yield. For example, yield loss because of N stress (YLNS) for a field with a yield of 2000 kg ha-1 and a value of 16, was determined by solving the following equations:
 | [4] |
 | [5] |
 | [8] |
 | [9] |
where, OY was optimum yield, FUE was grain fertilizer use efficiency, YLND was yield loss because of N stress. For this field, the calculated OY and YLND were 2863 kg and 863 kg grain ha-1. Graphic representations of these values are shown in Fig. 4
. Yield losses due to N stress in the 0N plots were highly correlated to measured yield losses (Fig. 5)
. These findings suggest that if the interactions among N, yield, water stress, and are known, then it may be possible to quantify the impact of both water and N stress on yield. Using nonisotopic approaches similar results have been reported in the literature. For example, Bauer et al. (1965) in North Dakota reported that if the stored water was <5.1 cm then wheat did not respond to N and if the soil contained >15.2 cm of stored water then the grain unit fertilizer response was 10 kg grain kg-1 N. In Wisconsin, Bundy and Andraski (1995) indirectly accounted for this interaction by characterizing fields into medium and high yield potentials. Fields with high yield potentials had a higher N response than fields with a medium yield potential. Gan et al. (2000) reported that over 60% of cereal yield variability resulted from water use differences. Crops with the lowest evapotranspiration had the lowest yield. Gorny (2001) reported that for barley, the tolerance to less favorable nutrition increased with decreasing water use.
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Fig. 4. Hypothetical impact of N and water stress on grain yield, , and the yield reduction because of N deficiency (YLND).
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Fig. 5. A comparison between calculated and measured yield losses because of N stress in wheat grown in low, moderate, and high water stress.
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This paper presents an approach for quantifying the interaction between N and water stress in soil fertility studies. Many previous studies have minimized this interaction by converting actual yields to relative yields (Dahnke and Olsen, 1990; Bundy and Andraski, 1995; Engel et al., 1999). Converting actual yields to relative yields can provide very useful information. For example, Engel et al. (1999) used relative yields to calculate a critical protein content for wheat grown in Montana. A problem with relative yield is that information about the factor causing much of the yield variability, i.e., water stress, is lost. Bundy and Andraski (1995) indirectly solved this problem by converting actual yields to relative yields and then determining fertilizer response functions for both medium and high yield potential fields. Field characterization was based on root zone depth, water holding capacity, and length of growing season. We hypothesize that by directly accounting for water stress, site-specific as well as traditional fertilizer recommendations can be improved.
In summary, grain yields as expected were largest in low water stress environments and smallest in high water stress environments. Generally, the fertilizer use increased as water stress decreased. In the 0N high water stress treatment, adding N decreased (-0.01 for every kg of N added). However, in the 0N low water stress treatment, N had a mixed impact on . Under non-N limiting (moderate and high N) conditions, the equation relating to yield was yield (kg ha-1) = -11000 + 884 , r = 0.92**. Wheat grown under N deficient conditions (0N treatment) did not fit this curve. This variation could be explained by considering the impact of N on water use, plant growth, and . Findings from this study show that when yield variability is caused by both water and N stress, then the benefit from the N fertilizer decreases with increasing water stress, and that can be used to characterize both water and N stress.
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ACKNOWLEDGMENTS
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Support for this project was provided by USDA–CSREES–NRI, South Dakota and Montana Wheat Commissions, Montana Agricultural Experiment Station, and South Dakota Agricultural Experiment Station.
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NOTES
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South Dakota State Univ. Experimental Station Journal Series No. 3227.
Received for publication January 31, 2001.
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TOP
ABSTRACT
INTRODUCTION
