Published online 2 March 2006
Published in Agron J 98:382-387 (2006)
DOI: 10.2134/agronj2005.0188
© 2006 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Agroclimatology
Nitrogen Fertilization Affects Bahiagrass Responses to Elevated Atmospheric Carbon Dioxide
Yoana C. Newmana,
Lynn E. Sollenbergerb,*,
Kenneth J. Booteb,
L. Hartwell Allen, Jr.c,
Jean M. Thomasb and
Ramon C. Littelld
a Texas A&M Univ. Research and Extension Center, 1229 North U.S. Hwy. 281, Stephenville, TX 76401
b Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0300
c USDA-ARS, P.O. Box 110965, Gainesville, FL 32611-0965
d Statistics Dep., P.O. Box 110339, Gainesville, FL 32611-0339
* Corresponding author (les{at}ifas.ufl.edu)
Received for publication June 23, 2005.
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ABSTRACT
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Increased atmospheric CO2 and temperature typically lead to greater DM (dry matter) production of grassland plants; however, limited plant N may reduce this response. A 2-yr study (1998–1999) was conducted to evaluate the effects and interactions among atmospheric CO2, temperature, and N fertilization rate on yield and nutritive value of ‘Pensacola’ bahiagrass (Paspalum notatum Flügge). Bahiagrass was field grown in Millhopper fine sand (loamy, siliceous Grossarenic Paleudult) under greenhouses with controlled atmospheric CO2 and temperature. Atmospheric CO2 levels were 360 and 700 µmol mol–1, and temperatures were B (baseline, corresponding to ambient in the greenhouse), B + 1.5°C, B + 3.0°C, and B + 4.5°C. Bahiagrass was fertilized at 80 kg N ha–1 (BG-80) or 320 kg N ha–1 (BG-320). Dry matter yield for BG-80 remained the same regardless of CO2 level (7.5 and 6.3 Mg ha–1 in Years 1 and 2), but BG-320 DM yield increased with increasing CO2 concentration in three of four harvests in Year 1 and from 14.8 to 17.3 Mg ha–1 in Year 2. Total N harvested response followed a similar trend as DM yield. Increasing temperature from B to B + 4.5°C had a positive effect on DM yield of BG-80 (23%) and a lesser positive effect on BG-320 (9% increase). Bahiagrass nutritive value increased due to greater N fertilization, but elevated CO2 concentration and temperature had no effect. Nitrogen fertilization affected bahiagrass DM yield response to CO2, but not the nutritive value response to elevated atmospheric CO2 or temperature.
Abbreviations: B, baseline temperature • BG-80, bahiagrass fertilized at 80 kg N ha–1 yr–1 • BG-320, bahiagrass fertilized at 320 kg N ha–1 yr–1 • DM, dry matter • IVDOM, in vitro digestible organic matter • TGG, temperature gradient greenhouse
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INTRODUCTION
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CLIMATE CHANGE, or the increase in atmospheric CO2 and associated global warming, has fueled studies of plant response to CO2 and temperature. There is widespread evidence that elevated CO2 increases plant growth (Kimball, 1983; Morgan et al., 2001; Newman et al., 2001), but the effects of increasing temperature on biomass yield generally have not been as pronounced as those of CO2 (Cure and Acock, 1986; Fritschi et al., 1999). Greater increases in growth under elevated CO2 are expected for plants of the C3 carbon fixation pathway (30–40%, Kimball, 1983) than for C4 species (10%, Newton, 1991).
Most grassland ecosystems are N limited, resulting in reduced productivity and contributing to lower herbage nutritive value (Wedin, 2004). The relationship of grass N status and response to increasing atmospheric CO2 and temperature is of interest because reduced nutrient availability could potentially limit plant responses to environmental changes that normally stimulate growth (Jones, 1997). The C4 bahiagrass, with leaf N concentration of <10 g kg–1, showed only a trend toward increasing yield with increasing atmospheric CO2 (Newman et al., 2001). It is uncertain whether the limited response to CO2 simply was typical for a C4 grass, or if the lack of response to CO2 was due in part to N deficiency. In the same study, the C3 legume rhizoma peanut (Arachis glabrata Benth.) increased yield 25% as CO2 increased from 360 to 700 µmol mol–1. It is not clear to what extent this greater yield response of the legume than the grass to elevated CO2 was due to the C3 pathway of the legume and to what extent it was due to greater N concentration of the legume relative to bahiagrass. Other studies, however, have shown greater response of legumes than C3 grasses to elevated CO2 in N-limiting soils (Hebeisen et al., 1997), suggesting that plant N status plays a critical role in CO2 response.
More information is needed to determine if C4 grass DM production and nutritive value response to climate change factors interact with N fertilization rate. The hypothesis for the current study was that bahiagrass yield response to elevated CO2 concentration is greater under high than under low N fertilization. The specific objectives were to quantify the effects of N fertilization rate on bahiagrass yield and nutritive value responses to increasing temperature and atmospheric CO2 concentration.
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MATERIALS AND METHODS
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This experiment was conducted during 1998 and 1999 at the Irrigation Research and Education Park, University of Florida, Gainesville (29°38' N, 82°22' W) and was part of a long-term study (Boote et al., 1999) of bahiagrass and rhizoma peanut growth in TGGs (temperature gradient greenhouses). Four TGGs (27 m long by 4.4 m wide) were constructed over field soils. A detailed description of the construction and design is given in Sinclair et al. (1995) and Fritschi et al. (1999). Lengthwise, half of each TGG was planted to bahiagrass and the other half to rhizoma peanut, separated by a 0.7-m center alley. Stands were planted in 1995 in undisturbed natural Millhopper fine sand at a rate of 30 kg ha–1 seed (bahiagrass) and 1.6 Mg ha–1 fresh weight of rhizomes (rhizoma peanut). Soil pH was 6.2 to 6.7, and soil organic matter ranged from 12 to 15 g kg–1. Mehlich I extractable P, K, and Mg at the site were 79 to 97, 6 to 11, and 98 to 115 mg kg–1, respectively.
For this study, the N rates were imposed only on ‘Pensacola’ bahiagrass and only bahiagrass data will be reported. Treatments were a factorial arrangement of two CO2 levels (360 and 700 µmol mol–1, with two TGGs per level of CO2), four temperatures (B, B + 1.5°C, B + 3.0°C, and B + 4.5°C, created in strips across the width of each greenhouse), and two levels of N fertilizer (80 and 320 kg N ha–1, randomly split within each temperature strip of bahiagrass; Fig. 1
). Plot size was 5 m2 (2.5 by 2 m).
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Fig. 1. Example of treatment layout in a temperature-gradient greenhouse (not to scale). One greenhouse is shown and represents one replicate of one CO2 main plot. Unidirectional arrows indicate the direction of airflow. BG-80 and BG-320 are bahiagrass fertilized at 80 and 320 kg N ha–1 yr–1, respectively; RP = rhizoma peanut stands on which data were not collected for this study.
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Temperature and CO2 treatments were continuously imposed in TGGs year-round. Carbon dioxide was injected in two of the four TGGs, using a distribution system located at the inlet end of the TGG, and regulated through an algorithm using inputs from ventilation fan speed inside the TGG, current measurements, and feedback of recent past values of atmospheric CO2 concentrations. The four temperature zones were created in a gradient at 5.5-m increments, along their length, from the inlet to the outlet end of the TGGs. Solar radiation reaching the TGGs was the primary heat source during the day. Temperature gradients were produced by regulating the rate of unidirectional ventilation relative to heat input from incident solar radiation. At night and during low solar irradiance periods, hot air (120°C) was infused at the beginning of the three 5.5-m elevated-temperature zones. Each zone was represented by temperature measurements recorded at 0.6 m above soil surface. A controller and data logger system based on readings from aerial thermocouples placed at 0.9 m above ground level over plots adjacent to inlet and outlet sections was used to adjust the exhaust fan speed (increased or decreased) to maintain the target temperature difference (4.5°C) between inlet and outlet ends of the TGGs.
The desired temperature gradient was achieved in each of the TGGs as illustrated for 1998 (Fig. 2
). Detailed descriptions of establishment procedures in 1995, maintenance of the forages from 1995 to 1998, setting and controlling of artificial temperature gradient and environmental conditions, and experimental procedures for that period have been described (Fritschi et al., 1999; Newman et al., 2001). Refer to these sources for additional information.
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Fig. 2. Monthly averages of average daily temperature in Greenhouse 1 for baseline (B) through B + 4.5°C temperature treatments during 1998.
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Fertilization and Irrigation
Bahiagrass is a rhizomatous warm-season perennial forage that starts growth in March of most years in North Florida. Fertilizer was split applied starting in March and continuing until shortly before the last harvest of each year. Total annual applications to all plots were 36 kg P and 152 kg K ha–1. Annual N applications were according to treatment. For each plot, the annual total of all nutrients was divided into 16 equal applications; there were four applications during the growth period leading up to each of the four harvests per year. Thus each application for BG-80 included 5 kg N ha–1 and for BG-320 included 20 kg N ha–1.
Each TGG was equipped with a double-overlapping microjet sprinkler irrigation system designed to apply 7 to 8 mm d–1 during the 8- to 9-mo growing season. During November through February, the irrigation amount was reduced to 3 to 4 mm d–1.
Weed and Pest Control
Plots were hand weeded as needed. During winter, the major weed was narrow leaf cudweed (Gnaphalium falcatum Lam) of the Compositae family; during summer it was long-stalked phyllanthus (Phyllanthus tenellus Roxb.) of the Euphorbiaceae family, a C4 plant. Phyllanthus was very prolific at the warmest temperatures and enriched CO2.
The major pest of bahiagrass was mole cricket (Scapteriscus borelli Giglio-Tos). Labeled sprays and baits were applied on an as-needed basis for control.
Plant Sampling and Nutritive Value Analysis
Plots were harvested on 18 May, 6 July, 24 Aug., and 9 Nov. 1998; and 27 May, 8 July, 30 Aug., and 16 Nov. 1999. These dates correspond with Day of the Year 138, 187, 236, and 313 in 1998; and Day of the Year 147, 189, 232, and 310 in 1999. Herbage mass was measured by clipping two randomly placed 0.25-m2 quadrats per plot to a 5-cm stubble, compositing herbage from the two quadrats within a plot, and drying the herbage for 48 h at 55°C. An additional sample of approximately 400 g fresh weight was clipped to a 5-cm stubble from each plot to assess nutritive value. Samples for nutritive value were oven dried for 48 h at 55°C and ground to pass through a 1-mm mesh screen in a Wiley mill. Ground forage samples were analyzed for DM, IVDOM (in vitro digestible organic matter), and total N concentration. A 1-g aliquot was used for determining absolute DM (drying at 105°C for 15 h; AOAC International, 1990). Total organic matter was determined by ashing for 15 h at 550°C (AOAC International, 1990). The two-stage technique of Moore and Mott (1974) was used to determine IVDOM. Total N was determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by Gallaher et al. (1975). Crude protein was calculated as N x 6.25 (AOAC International, 1990).
Data Analyses
Data were analyzed using mixed-model methods (SAS Institute, 1996). In all models, effects of CO2 concentration, temperature, N rate, year, and their interactions, were considered fixed effects. Greenhouses nested within CO2 levels were modeled as random effects. There were several treatment x year interactions; thus, data are presented by year. Orthogonal polynomial contrasts (linear, quadratic, and cubic) were used to determine the nature of responses to temperature. All means reported in the text are least squares means. Treatments were considered different if P 
0.10.
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RESULTS AND DISCUSSION
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Total Dry Matter Yield and Nitrogen Harvested
Nitrogen x Carbon Dioxide Interaction Effects
Probability values are presented in Table 1 for the effects of year, N rate, CO2, temperature, and their interactions on DM yield and N harvested. There was a CO2 x N rate x year interaction for total annual DM yield and N harvested (P < 0.01 and P = 0.09, respectively), therefore data were analyzed by year. When analyzed by year, there was a CO2 x N rate interaction (P < 0.01) for total DM yield in 1999 and a trend (P = 0.22) toward interaction in 1998 (Table 2).
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Table 1. Levels of probability (P) for the effects of year, CO2, N rate, temperature (Temp), and their interactions on total dry matter (DM) harvested, N harvested, crude protein (CP), and in vitro digestible organic matter (IVDOM).
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Table 2. Nitrogen rate x CO2 interaction means for total annual dry matter harvested. Data are means averaged across four temperatures and two replicates (n = 8).
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The CO2 x N rate interaction effect on DM yield (Table 2) occurred because total seasonal DM yield increased or tended to increase under enriched CO2 conditions only for BG-320, while for BG-80 there was no response to elevated CO2. In 1999, yield for BG-320 was greater at elevated CO2 than at ambient, and in 1998 there was a trend (P = 0.12) toward greater yield at higher atmospheric CO2 concentration. Presence of only a trend for the total-season response in 1998 can be attributed to the first harvest, at which there was no effect of CO2 level on yield of BG-320 plots (Fig. 3
). In three subsequent harvests that year, yield was greater for elevated than ambient CO2. Absence of a first-harvest CO2 effect probably reflected the low-N status of the BG-320 plots at initiation of the experiment, a hypothesis that is supported by markedly lower Harvest 1 (18 May) yields (2.7 Mg ha–1) in 1998 compared with any subsequent harvest (
4.0 Mg ha–1) in 1998 or 1999 (Fig. 3). During the 2 yr preceding the current research, the BG-320 plots had received 80 kg N ha–1 yr–1 and yield decreased from year to year. In addition, herbage N concentration during those 2 yr was <10 g kg–1 (Newman et al., 2001).
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Fig. 3. Dry matter (DM) yield by harvest of bahiagrass fertilized at 80 (BG-80) or 320 kg N ha–1 yr–1 (BG-320) under ambient (360 µmol mol–1) and enriched (700 µmol mol–1) CO2 levels for 1998 and 1999. There were no CO2 effects at any harvest for BG-80 in 1998 (P > 0.16) or 1999 (P > 0.50). For BG-320, P values for Harvests 1 through 4 were 0.26, 0.02, 0.09 and 0.05, respectively, in 1998, and 0.09, <0.01, 0.42, and 0.18, respectively, in 1999.
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The magnitude of the yield difference between elevated and ambient CO2 for BG-320 plots was consistent for the final two harvests of 1998 and all harvests in 1999 (Fig. 3), although somewhat greater standard errors made it impossible to detect differences between CO2 levels in two harvests of 1999. For BG-80, DM yields were not affected by atmospheric CO2 concentration, and they averaged 7.5 and 6.3 Mg ha–1 yr–1 in 1998 and 1999, respectively. In addition, there was no effect of CO2 on DM yield for BG-80 at any of the eight harvest dates (Fig. 3).
Nitrogen deficiency has been thought to limit yield of grasses under enriched atmospheric CO2 conditions (Wong, 1979; Grant et al., 2001). Similarly, greater N fertilization has been associated with greater herbage accumulation responses to CO2 enrichment in C4 grasses (Ghannoum and Conroy, 1998), but this response has not been totally consistent. Lack of response to CO2 by N-fertilized treatments was attributed to a CO2-induced greater N demand (Morgan, 2000), to reduced N remobilization to shoot regrowth (Skinner et al., 1999), and in other cases, the lack of response was associated with differences in above- and belowground DM partitioning (Morgan et al., 2001) or with allocation of the additional N to the root system for exploitation of a larger soil volume (Hebeisen et al., 1997). Data from the current study support the conclusions that responsiveness to a CO2-enriched environment may be mediated by the N status of the plant (Greenwood et al., 1990; Stitt and Krapp, 1999), and N deficiency may not allow the expected yield response to elevated CO2 (Zanetti et al., 1997).
Total seasonal DM yields were greater for the high than low N rate in both years and at both levels of CO2 (Table 2), with BG-320 annual yields two to almost three times greater than BG-80. This magnitude of bahiagrass response to N has been observed by others (Burton et al., 1997; Twidwell et al., 1998), despite the reputation of bahiagrass as being adapted to low-input systems (Gates et al., 2004).
Total N harvested (Table 3) followed a similar pattern to that of total DM harvested. The CO2 x N rate interaction for total N harvested approached significance in 1998 (P = 0.20) and was significant in 1999 (P < 0.01). Total annual N harvested did not respond to enriched CO2 conditions at BG-80 in either year, and across CO2 levels averaged 64.5 and 62.5 kg ha–1 yr–1 in 1998 and 1999, respectively. For BG-320, total N harvested increased from 192 to 224 kg ha–1 yr–1 with increasing CO2 concentration. Total N harvested was greater for BG-320 than BG-80 in both years and at both levels of CO2 (Table 3).
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Table 3. Nitrogen rate x CO2 interaction effect for total annual N harvested. Data are means averaged across four temperatures and two replicates (n = 8).
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Temperature Effects
There was a temperature x CO2 interaction (Table 4, P < 0.01) for total annual DM yield. Interaction occurred because the response to temperature at ambient CO2 was greater than at elevated CO2 concentration. At ambient CO2, total DM harvested increased from 9.9 Mg ha–1 yr–1 at B to 12.2 Mg ha–1 yr–1 at B + 4.5°C (linear and cubic effects, P < 0.01). This is a 23% increase, compared with only a 9% increase for total DM harvested at the enriched CO2 concentration. The lesser response at elevated CO2 levels suggests that there may be some acclimation to greater temperatures in the CO2-enriched environment (Sage and Kubien, 2003). Dry matter harvested at elevated CO2 increased from B (11.0 Mg ha–1) to B + 1.5°C (12.4 Mg ha–1), but did not change significantly thereafter.
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Table 4. Carbon dioxide x temperature interaction means for total annual DM harvested. Data are means across two N rates, 2 yr, and two replicates (n = 8).
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For total annual N harvested, there were temperature effects but no temperature x CO2 interaction. Total N harvested increased with increasing temperature from 126 kg ha–1 yr–1 at B to 139 kg ha–1 yr–1 at B + 4.5°C (linear effect, P = 0.07). The overall increase in N harvested across the range of increasing temperature was 10%.
Nutritive Value
Probability values are presented in Table 1 for the effects of year, N rate, CO2, temperature, and their interactions on IVDOM and crude protein. Bahiagrass IVDOM and crude protein were affected only by year and N rate. In vitro organic matter digestibility was consistently greater for BG-320 than BG-80 in both years (Table 5). Crude protein followed the same trend as IVDOM and, overall, the N-rate effect was greater on crude protein than on IVDOM.
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Table 5. Year and N-rate means for herbage in vitro digestible organic matter (IVDOM) and crude protein (CP) concentrations. Data are means across four temperatures and two replicates (n = 8). There was no N rate x year interaction for IVDOM (P = 0.14) or CP (P = 0.14).
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The crude protein response to N rate is well established in the literature, while the positive effect on IVDOM is less consistent. There are, however, several recent studies with limpograss [Hemarthria altissima (Poir.) Stapf & Hubb; da Lima et al., 1999], stargrass (Cynodon nlemfuensis Vanderyst; Hernández Garay et al., 2004), and bahiagrass (Stewart et al., 2003) that show a positive response of herbage IVDOM to increasing N rates. Other studies have shown decreasing nutritive value of several warm-season forages to increasing temperature, including rhizoma peanut (Newman et al., 2005), ‘Coastal’ and common bermudagrass [Cynodon dactylon (L.) Pers.], bahiagrass, and dallisgrass (P. dilatatum Poir.; Henderson and Robinson, 1982). The nutritive value response to CO2 has been inconsistent and generally small (Seligman and Sinclair, 1995).
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CONCLUSIONS
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The results of this study support the conclusion that low bahiagrass N status may limit the response of this C4 species to elevated CO2. The yield response of well-fertilized bahiagrass to CO2 enrichment was from 7 to 17%, within the range expected for C4 plants, while bahiagrass that received inadequate N did not respond to elevated CO2. This result implies that to make effective use of a potentially greater supply of C provided by elevated atmospheric CO2 concentrations, plants growing in marginal soil-N environments probably will require additional N fertilizer.
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ACKNOWLEDGMENTS
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Appreciation is expressed to Wayne Wynn and Andrew Frenock, engineering technicians, for assistance in maintaining the operational system in the greenhouses.
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NOTES
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This research was sponsored in part by National Institute for Global Environmental Change, Southeastern Region, Grant 94UOF006CR3. Florida Agric. Exp. Stn. Journal Series no. R-10421.
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ABSTRACT
INTRODUCTION
