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Published online 5 June 2006
Published in Agron J 98:890-898 (2006)
DOI: 10.2134/agronj2005.0084
© 2006 American Society of Agronomy
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Alfalfa

Nitrogen Fixation and Transfer in a Mixed Stand of Alfalfa and Bermudagrass

Vincent A. Habya,*, Stephen A. Stoutb, Frank M. Honsc and Allen T. Leonarda

a Texas Agric. Exp. Stn., Texas A&M Univ., P.O. Box 200, Overton, TX 75684-0200
b Los Alamos National Lab., Los Alamos, NM
c Texas Agric. Exp. Stn., Texas A&M Univ., College Station, TX 77843

* Corresponding author (v-haby{at}tamu.edu)

Received for publication March 21, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Information about dinitrogen (N2) fixation and transfer is needed to determine potential benefits from growing binary mixtures of alfalfa (Medicago sativa L.) and hybrid bermudagrass [Cynodon dactylon (L.) Pers.] on Coastal Plain soils of the USA. Our objectives were to quantify N2 fixed by alfalfa and transferred to bermudagrass. ‘Alfagraze’ alfalfa was grown at row spacings (RS) of 23, 46, 69, and 91 cm in a mixed stand with ‘Coastal’ bermudagrass on a Darco loamy fine sand (loamy siliceous, semiactive thermic Grossarenic Paleudult). The mixed stand was fertilized with 0, 28, 56, 84, and 112 kg N ha–1 applied for each bermudagrass regrowth. Isotope dilution was used to estimate N2 fixation and subsequent transfer to bermudagrass in the zero-applied N plots. Wider alfalfa RS significantly increased bermudagrass yields. Higher N rate (NR) significantly improved bermudagrass yield during cool temperature and drought stress periods in 1994 and 1996. Narrower RS and higher NR significantly increased alfalfa production. Legume N derived from the atmosphere ranged from 42 to 91% and the fixed N yield ranged from 80 to 222 kg N ha–1 yr–1. Bermudagrass N derived from atmospheric N2 fixation ranged from 1.0 to 77%, and the transfer N yield (TNY) was ≤18 kg N ha–1 yr–1. Results varied depending on harvest date, year, treatment, and grass yield. Alfalfa RS had little effect on TNY. The low bermudagrass yield and subsequent TNY in this binary forage production system suggest that monoculture alfalfa may be the best approach for managing alfalfa on Coastal Plain soils.

Abbreviations: %Ndfa, percentage N derived from the atmosphere • %Ngdfa, percentage N in grass derived from the atmosphere • DMY, dry matter yield • FNY, fixed-N yield • NR, nitrogen rate • RS, row spacing • TNY, transferred-N yield


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
COASTAL BERMUDAGRASS is an important hay and pasture forage on the Coastal Plain in the southern and southeastern USA because it grows well on the acidic, highly weathered soils of this region. Coastal bermudagrass production is frequently limited by low N fertility (Jackson et al., 1959). Scientists indicate that alfalfa may improve nutritive value of bermudagrass when grown as a mixed forage system (Burton, 1976; Brown and Byrd, 1990; Heichel and Henjum, 1991; Stringer et al., 1994, 1996; Haby et al., 1999).

Growing alfalfa, a legume well known for its ability to fix N2, with bermudagrass may reduce the N fertilizer requirement of this grass. Isotopic methods have often been used to quantify the actual amount of atmospheric N2 fixed by legumes and the amount of symbiotically fixed N transferred to companion grasses (McAuliffe et al., 1958; LaRue and Patterson, 1981; Chalk, 1985) despite possible problems (Rennie, 1986; Witty, 1983).

Isotope-based studies of N2 fixation by alfalfa grown with warm-season grasses are few. The majority of this work has been done with cool-season grasses. Alfalfa derived 93 to 95% of its N symbiotically when grown with reed canarygrass (Phalaris arundinacea L.) (Brophy et al., 1987; Heichel and Henjum, 1991); 80 to 85% of its N from the atmosphere when grown in a mixed stand with annual ryegrass (Lolium multiflorum Lam.) (Danso et al., 1988; Hardarson et al., 1988); and 91% of its N from atmospheric N2 fixation when grown in mixed stand with orchardgrass (Dactylis glomerata L.) (West and Wedin, 1985).

Isotope dilution techniques have also been used to measure the amount of N transferred from legumes to nonlegumes. Reed canarygrass received 68% of its N from alfalfa and 79% of its N from birdsfoot trefoil (Lotus corniculatus L.) (Brophy et al., 1987). Orchardgrass received 8 to 46% of its N from birdsfoot trefoil (Farnham and George, 1994); ryegrass received 79% of its N from white clover (Trifolium repens L.) (Broadbent et al., 1982); and Morris et al. (1990) found that <13% of the N contained in annual ryegrass was transferred from arrowleaf clover (Trifolium vesiculosum Savi). Roberts and Olson (1942) found increased N concentration in grasses grown in mixture with legumes, but they reasoned that this could be due either to N transfer or to suppression of grass growth by the legumes.

While many studies have been done on mixtures of legumes and nonlegumes, few studies show the effect of legume proportion on N2 fixation and transfer. By using 15N-isotope dilution techniques, it should be possible to determine the optimum alfalfa proportion or RS to achieve maximum N2 fixation and transfer. In a field study, West and Wedin (1985) found that as the proportion of alfalfa in an alfalfa–orchardgrass mixture increased, the amount of N2 fixed by the alfalfa decreased. These authors concluded that as the amount of grass in the mixture decreases, more N is available in the soil nutrient pool for the legume. This additional N may inhibit the production of new N2-fixing nodules or nitrogenase activity in existing nodules. West and Wedin (1985) also reported that as the proportion of legume in the mixture increases, N transfer tends to decline. Brophy et al. (1987), from studies on N transfer from alfalfa and birdsfoot trefoil to reed canarygrass, indicated that N transfer occurred over a distance of 20 cm with maximum N transfer in areas of a high legume/grass ratio.

Our research was initiated to determine the amounts of N2 fixed from the atmosphere by rhizobia on alfalfa roots as influenced by alfalfa RS in a mixed stand with bermudagrass. The amount of symbiotically fixed N transferred from alfalfa to bermudagrass as affected by alfalfa RS was also investigated.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This experiment was conducted for 3 yr on a Coastal Plain Darco loamy fine sand at 94°59' W and 32°18' N. Recent history of this site included removal of pine forest, liming the 0- to 15-cm depth to {cong}pH 7.0, and sprigging Coastal bermudagrass in spring 1988. In 1990, alfalfa was no-till seeded into the Coastal bermudagrass meadow at 23-, 46-, 69-, and 91-cm row spacings. On-site treatments and soil analyses for 1991 through 1993 were reported by Haby et al. (1999). Extractable P in the 0- to 15-cm soil depth was considered high by soil test rating, while K remained at a low level. Hot-water soluble B averaged 0.34 mg kg–1 at the beginning of this 3-yr study. Initial median soil pH in the 0- to 15-cm depth was above 7.0 with main effects ranging from 7.3 to 6.6 caused by previous N treatments ranging from 0 to 112 kg ha–1, respectively, for each grass harvest.

The experimental design was a split plot within four randomized complete blocks with alfalfa row spacings as main plots. Nitrogen rates of 0, 28, 56, 84, and 112 kg ha–1 as ammonium nitrate were applied to split plots at initiation of spring regrowth of bermudagrass and after each harvest except the last in 1994 and 1996 for a total of 15 applications. Five applications of these NR were made each year. In addition to the split-plot N treatments, all plots were uniformly fertilized with 35 kg P ha–1 in 1994 and 1996. Potassium was applied as KCl at rates of 280 kg ha–1 the first year and 140 kg ha–1 in following years. Magnesium and S were annually applied at rates of 21 and 37 or more kg ha–1, respectively. Annual applications of B were 3.6 kg ha–1 in 1994 and ≥1.4 kg ha–1 in 1995 and 1996. Zinc and Cu were applied in 1994 and 1996 at rates below 1.0 kg ha–1 and Mo was applied once in 1994 at 94 g ha–1. No additional limestone was applied during this study.

A 1.0-m2 micro plot located in each zero-N split plot in each RS main plot was used to evaluate N fixation and transfer. Dinitrogen fixation was not evaluated in plots receiving fertilizer N. Four 1.0-m2 reference plots of Coastal bermudagrass without alfalfa were located immediately adjacent to the main plots. All micro plots were relocated within the 3.1 by 6.1 m zero-N split plots each year of the study to prevent the uptake of residual 15N from previous treatments. Double-labeled 15NH415NO3 (10.1 atom % 15N excess- specifies the level of isotopic abundance above a given background reading that is considered zero) was applied in solution to each of the micro plots at a rate of 11.2 kg 15N ha–1 when Coastal bermudagrass initiated regrowth in spring. To prevent potential ammonia volatilization, the 15N solution was injected at 10-cm intervals in a grid pattern using a needle and syringe the first year of the study. In subsequent years, the solution was uniformly sprinkled onto each micro plot using a garden watering container with an attached nozzle to disperse the flow. Three liters of additional water were immediately applied to each micro plot using the same container.

In March 1994 and 1995, and February 1996, carbofuran (2, 3-dihydro-2, 2-dimethylbenzofuran-7-yl methylcarbamate) was applied to the experiment at rates of 2.4, 1.2, and 1.2 L ha–1, respectively, to control alfalfa weevil [Hypera postica (Gyllenhal)]. In June 1996, chlorpyrifas (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate) was applied at a rate of 1.2 L ha–1 to control three-cornered alfalfa hopper [Spissistilus festinus (Say)]. Imazethapyr [(±)-2-4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-y1]-5-ethyl-3-pyridinecarboxylic acid] was applied in March 1994 at 0.45 L ha–1 and in 1995 at 1.2 L ha–1 to control broadleaf weeds.

Plant samples from 1-m2 micro plots were hand clipped to a 5-cm stubble height at about 4-wk intervals from April into September each year when the alfalfa was about 10% bloom. The bermudagrass and alfalfa components were separated, dried in a forced-draft oven at 60°C for 48 h, weighed to determine botanical composition, ground to <0.85-mm, and transferred to a ball mill where they were pulverized to a fine powder and not re-sieved.

About a 3-m length of each main plot was harvested using a 1.5-m-wide sickle-bar equipped Hege 211-B forage harvester (Wintersteiger, Salt Lake City, UT). The actual harvested strip length was measured. Cut forage was weighed and sampled for moisture content and chemical analysis. Forage samples were sorted to determine botanical composition and dried in a forced-draft oven for 48 h at 60°C. Dry matter yields (DMY) were calculated based on harvested fresh weight of alfalfa and grass with adjustment for varying moisture content and harvested area. Alfalfa and bermudagrass DMY percentages in the mixed stands were calculated.

Plant tissue was analyzed for total N and 15N concentrations using continuous-flow, isotope-ratio mass spectrometers. In 1994, samples were analyzed using an Europa 20-20 IRMS with an Europa ANCA sample combustion system (Sercon, Ltd., Crewe, Cheshire, UK) at Michigan State Univ. In 1995 and 1996, samples were analyzed at the Los Alamos National Laboratory using an automated mass spectrometer Model VG-Isomass (VG, Winsford, Cheshire, UK) with a Carlo-Erba NA 1500 gas chromatograph (ThermoElectron Corp., Waltham, MA). Reference standards and multiple replications were used to ensure precision and accuracy. The isotopic composition was used to calculate the percentage of alfalfa N derived from the atmosphere (%Ndfa) and the legume fixed-N yield (FNY). The FNY is the quantity of legume N derived from the atmosphere expressed on an area basis. The amount of fixed N transferred to the associated bermudagrass was expressed either as the percentage of grass N derived from the atmosphere (%Ngdfa) or as grass transferred-N yield (TNY) (McAuliffe et al., 1958; Farnham and George, 1994). Formulas used for these calculations follow:

Formula

Formula

Formula

Formula
Soil samples were collected from depths of 0 to 15, 15 to 30.5, 30.5 to 61, 61 to 91.5, and 91.5 to 122 cm at termination of this study. Samples were dried at 60°C for 48 h, screened to ≤0.85 mm, and analyzed for pH in 1:2 (v/v) soil:0.01 M CaCl2 suspension (Thomas, 1996), and for NH4+ and NO3– using 2 M KCl as the extractant at a soil:solution ratio of 1:4 (w/v) and shaking for 30 min on a reciprocating shaker (Bremner and Keeney, 1965). Filtered extracts were analyzed using colorimetric auto analyzer techniques (Technicon Industrial Systems, 1977a, 1977b).

Statistical analyses were conducted using Proc. ANOVA and Proc. GLM from SAS Institute (1990). Response variables were evaluated by RS and NR. The mean square (MS) for Block x RS was used for error A and the MS for Block x NR plus Block x NR x RS was used for error B. Tests for linear and quadratic responses were included in the ANOV. Total alfalfa and bermudagrass DMY were evaluated for differences caused by years, RS, and NR. The FNY was analyzed by Proc. ANOVA and Proc. GLM on a yearly basis and at each harvest. Means from significant effects were separated by Student Newman-Keuls comparisons at P ≤ 0.05 for alfalfa and bermudagrass dry matter yields and at P ≤ 0.10 for %Ndfa and %Ngdfa.

Weather data reported in Fig. 1 include monthly rainfall totals compared to the average rainfall received, and maximum and minimum air temperatures. These data were collected using a maximum/minimum thermometer and a manually recorded rain gauge.


Figure 1
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  		Fig. 1. Monthly rainfall (bars), maximum (Tmax) and minimum (Tmin) temperatures, and the average annual rainfall pattern at 94° W and 32° N. White bars (No. 1, 13, and 25) represent January.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Bermudagrass Yield
Yields of Coastal bermudagrass overseeded with Alfagraze alfalfa were much lower than adequately-fertilized monoculture bermudagrass yields reported by Burton and Hanna (1995). When evaluated over years, alfalfa RS had significant quadratic and linear effects on bermudagrass production (Table 1). The quadratic regression equation for the annual means of the 3-yr total bermudagrass yield was DMY = –558.87 + 67.90 x RS – 0.47 x RS2, with an R2 of 0.14 and the probability of a greater F < 0.0001. Computations using this equation indicate maximum bermudagrass DMY occurred near the 69-cm alfalfa RS. A quadratic response to RS occurred for total bermudagrass yield in 1995 and 1996 and in at least one harvest each year (Table 2). Alfalfa RS had significant linear effects on Coastal bermudagrass yields in most harvests in 1994 and 1995 but only in one harvest in 1996. The significant increase in Coastal bermudagrass DMY at the 46-cm and wider row spacings (Table 3) supports results reported by Stringer et al. (1994) and Haby et al. (1999). Stringer et al. (1994) reported that wider alfalfa row spacings to 60 cm aided in retaining bermudagrass in the mixture, probably because of reduced shading of the bermudagrass by alfalfa at the wider RS. Wider alfalfa row spacings as reported by Stringer et al. (1994) and Haby et al. (1999) gave the bermudagrass greater access to soil water and to sunlight until increasing growth and transpiration requirement of the alfalfa plants stressed availability of soil water and shaded the inter-row spaces. In earlier research, Brown and Byrd (1990) reported that narrower alfalfa RS of 15 and 30 cm in bermudagrass had no effect on yield or botanical composition of the mixture. As in the research reported by Haby et al. (1999), we observed that alfalfa initiated regrowth more rapidly after harvest than bermudagrass and at the early-bud growth stage alfalfa plants shaded the inter-row spaces even at the 91-cm alfalfa RS. Alfalfa in narrower spaced rows is expected to be more competitive for soil water and to restrict sunlight from the grass earlier during vegetative regrowth compared to wider-spaced rows.


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  		Table 1. Analysis of variance for the effects of year, alfalfa row spacings (RS), and N rate (NR) on total alfalfa and bermudagrass dry matter yield.

 

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  		Table 2. Analysis of variance for response of Coastal bermudagrass dry matter yield to row spacing (RS) and N rate (NR) over 3 yr.

 

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  		Table 3. Coastal bermudagrass dry matter yield response to N rate (NR) and alfalfa row spacing (RS) by harvest date over 3-yr.

 
The ANOV for total bermudagrass yield across years indicated a significant linear DMY response to applied N (Table 1). When evaluated by years, increasing NR produced significant quadratic and linear responses for total bermudagrass yield in 1996 (Table 2). In May 1996, in the first harvest after the grass broke dormancy in highly completive stands of alfalfa, and in the July–August period with above-normal rainfall (Fig. 1), increasing NR significantly increased bermudagrass yields. A year-long drought began in June 1995 that significantly lowered bermudagrass DMY and limited regrowth following the August harvest. Response to applied N was negligible during this dry period (Table 3). Regression analysis for total bermudagrass production averaged over 3 yr (DMY = 1250.70 + 0.907 x NR; R2 = 0.02) showed there was 0.9 kg bermudagrass dry matter produced kg–1 of N ha–1 applied. Although statistical analysis indicated a significant linear yield response occurred with increasing NR, this response was minimal and a total yield increase of 508 kg ha–1 with 560 kg of applied N ha–1 is not economically sustainable when the total amount of N applied over one or more years is considered.

Alfalfa Yield
Total alfalfa DMY was different across years with significant quadratic and linear responses to NR, but there was no NR x year interaction (Table 1). Yield response to NR occurred in most harvests in 1994 but was limited to early- and mid-season harvests in 1995 (Table 4). Response to applied N in 1996 occurred only in the first and fourth harvests. The alfalfa DMY response to applied N that occurred in the first harvest each year apparently was affected by residual plant-available N (Table 5). This N remained in the soil during winter from fertilizer N treatments in previous growing seasons since no N was applied to the dormant bermudagrass. Regression analysis (DMY = 8153.72 + 5.042 x NR – 0.0051 x NR2; R2 = 0.03) estimated total annual alfalfa DMY for the 3 yr at 8154 kg ha–1 with no N applied and projected an increase to about 9378 kg ha–1 at the annual NR of 560 kg ha–1. Despite this significant regression effect, NR accounted for only 3% of the variability in yield. A total annual yield increase of 606 kg ha–1 with the initial NR increase over the check, and 1224 kg ha–1 when a total of 560 kg of N ha–1 was applied is unsustainable and economically insignificant. Except for the first cutting in 1996, alfalfa was less responsive to applied N in 1995 and 1996 due to the drought that occurred from June 1995 through June 1996 (Fig. 1). Lack of normal rainfall limited alfalfa to only five harvests in 1995, and the fifth cutting was greatly reduced.


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  		Table 4. Analysis of variance for response of alfalfa to row spacing (RS) and N rate (NR) over 3-yr.

 

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  		Table 5. Alfalfa dry matter yield response to alfalfa row spacing (RS) and N rate (NR) by harvest date over 3-yr.

 
The effect of alfalfa RS on alfalfa DMY was significant in the early harvests each year (Table 4). The drought that began in 1995 lowered DMY in July and August and eliminated a sixth harvest that year (Table 5). Even with below-normal winter rainfall, accumulated soil water was sufficient to produce excellent first-harvest alfalfa DMY in 1996 but later cuttings continued to have reduced DMY due to low rainfall through June. Widening the alfalfa RS significantly lowered alfalfa DMY in the first two cuttings in 1994 and 1995. Alfalfa DMY response to RS was inconsistent in the first harvest of 1996. Alfalfa RS had no significant linear or quadratic effects on total alfalfa DMY in individual years (Table 5). Individual plants die as alfalfa stands age, but stand density compensates as crowns of surviving plants expand laterally by increasing the number of stems per plant (Sheaffer et al., 1995). In winter-hardy cultivars such as Alfagraze, crown size can reach 15- to 30-cm across in 3- to 4-yr-old plants (Barnes and Sheaffer, 1995). Increased crown size narrows the original distance between rows. Even at the 91-cm row spacing, alfalfa at flower bud stage shaded the space between rows.

The effect on alfalfa DMY of pH decline to 5.8 in the surface 15 cm of plots treated with the highest NR could not be determined in this study. Rice (1975) reported rhizobium survival at pH <6 can restrict effective symbiosis, thereby possibly restricting alfalfa yield. However, in our study, applied N was the main cause of the pH decline and alfalfa DMY was significantly greater at higher NR.

Dinitrogen Fixation by Alfalfa
In 1994, %Ndfa varied from 75 to 91 over harvest dates and was not affected by RS (data not shown). In the last 2 yr, %Ndfa ranged from 42 to 91 depending on harvest date and RS (Table 6). The effect of RS on %Ndfa was inconsistent in 1995, but in 1996, widening the alfalfa row spacing consistently increased %Ndfa in alfalfa at all harvest dates. The largest increase in %Ndfa caused by widening RS occurred in the August 1996 harvest and the two smallest increases occurred in the first and last harvests. Heichel et al. (1984) reported that the percentage of alfalfa N that was obtained through N2 fixation in a pure stand was 33 to 80%. Ta and Faris (1987) reported that alfalfa fixed 70 to 84% of its N when grown in a mixture with timothy (Phleum pratense L.). Heichel and Henjum (1991) reported that alfalfa grown in a mixed stand with reed canarygrass received, on average, >93% of its N from symbiotic N2 fixation, but they also observed that the amount of N fixed increased each year of a 3-yr study. These results may differ because of the greater stand age in our experiment and different climatic conditions and cultivars. When alfalfa RS significantly affected %Ndfa, as in 1996, greater amounts of N2 were fixed when alfalfa was grown at row spacings >23 cm, or when a greater proportion of grass was present in the mixture (Table 6). When years were included in the ANOV, a statistically significant decrease in %Ndfa occurred with increasing stand age (data not shown), but this result may be related to the prolonged drought as indicated in Fig. 1.


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  		Table 6. Row spacing (RS) and harvest date effects on percentage N derived from the atmosphere (%Ndfa){dagger} for alfalfa in 1995 and 1996.

 
Alfalfa FNY was significantly different across years but was not affected by RS. Median total FNY declined from 208 kg N ha–1 yr–1 in 1994 to 136 kg N ha–1 yr–1 in 1995, and to 92 kg N ha–1 yr–1 in 1996 (data not presented). Across harvests and years, FNY ranged from 2 kg N ha–1 at the 23-cm RS in the drought-affected harvest of 1 July 1996 to 69 kg N ha–1 at the 46-cm RS in the first harvest of 1994. These results agree with the findings of Heichel et al. (1984) who reported FNY of 224 kg N ha–1 yr–1 for a pure stand of alfalfa. Our data indicate alfalfa was the dominant forage as bermudagrass DMY in the 23-cm RS alfalfa was only 272 kg ha–1 in 1994 (Table 3) when FNY achieved 222 kg ha–1. Ta and Faris (1987) observed similar FNY yields of 192 and 215 kg N ha–1yr–1 in a mixed stand of alfalfa and timothy. In our study, total FNY was similar in the first harvests of 1994 and 1995. In succeeding cuttings, FNY declined in 1995 even though %Ndfa remained relatively constant in 1994 and 1995. Fixed N yield and %Ndfa declined in 1996 compared to 1994.

Nitrogen Transfer to Grass
The %Ngdfa ranged from 1.0 to 77% and varied with year, harvest, and RS (Table 7). The %Ngdfa appeared to decline each year of the study, and as a general trend, reached a maximum during the middle of the growing season (Table 7), whereas the %Ndfa remained relatively constant throughout the growing season for 1994 and 1995 (Table 6). However, during 1996, the %Ndfa appeared to decline during the middle of the growing season (Table 6). The drought from May 1995 through June 1996 (Fig. 1) may have reduced symbiotic fixation of N by alfalfa. The establishment and activity of the legume rhizobium symbiosis is extremely sensitive to drought stress (Sprent, 1972; Zablotowicz et al., 1981; Kirda et al., 1989; Serraj et al., 1999). If the alfalfa was previously receiving the N required for maximum growth, then became desiccated because of lack of water, some alfalfa root tissue and nodules may have died and sloughed. Mineralization of this organic plant material releases fixed N2 into the soil N pool making it available for succeeding bermudagrass growth. Thus, the %Ndfa in a single harvest is not expected to correlate with %Ngdfa in that same harvest.


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  		Table 7. Response of percentage of bermudagrass N derived from the atmosphere (%Ngdfa) over 3-yr. to alfalfa row spacing (RS).

 
The percentage Ngdfa was significantly affected by RS in at least two cuttings each year (Table 7). The effect of RS appeared to be inconsistent relative to time of year. The greatest %Ngdfa occurred at the narrower row spacings where alfalfa was most competitive with bermudagrass and decreased as RS increased. It is likely that a significant percentage of the available soil N pool was derived from atmospherically fixed N at all alfalfa row spacings but increased bermudagrass production at the wider row spacings diluted this source of N in the grass. Although shading by the alfalfa reduced growth of bermudagrass, the bermudagrass still used fixed N.

The greatest annual TNY was ≤18 kg N ha–1 (data not shown). Total TNY was not significantly affected by alfalfa RS during 1994 or 1995. However, during 1996, TNY responded to alfalfa RS with the widest alfalfa row widths having the lowest TNY. This agrees with results from Brophy et al. (1987) who indicated that a similar decline may be related to the legume's inability to transfer N over increasing distances. The difference in bermudagrass TNY between 1994 and 1996 may be related to changes in the composition of the forage mixture and climatic conditions, although TNY was low in all years. These results are supported by West and Wedin (1985) who reported a trend indicating that as the proportion of legume in the mixture increases, N transfer tends to decline. At individual harvests, alfalfa RS had little effect on bermudagrass TNY and these effects did not exhibit a clear trend as to which alfalfa RS allowed the greatest TNY.

Residual Soil Nitrogen and pH
Ammonium-N concentrations ranging from 9 to 12 mg kg–1 in the 0- to 15-cm depth and from 5 to 6 mg kg–1 in the 91- to 122-cm depth were unaffected by NR or block. The lack of NR effect on soil ammonium may be caused by rapid conversion of ammonium to nitrate, uptake of ammonium by plants, and/or fixation in soil minerals. Extractable nitrate-N (NO3–N) concentrations at the end of this 3-yr study were significantly increased at all depths by increasing NR. Nitrate-N concentrations were highest in the surface 15-cm depth, ranging from 13 to 48 mg kg–1 as NR increased from zero to 112 kg ha–1, respectively. Total NO3–N levels in the 0- to 122-cm depth ranged from 27 to 186 mg kg–1 as the N-rate applied for each bermudagrass regrowth period increased from 0 to 112 kg ha–1, respectively. Nitrate levels at all NR for our experimental site were much lower than levels reported by Heichel and Vance (1979) to inhibit nodule growth on alfalfa roots.

Initial soil pH in the 0- to 15-cm depth ranged from 7.3 to 6.6 as the previous NR (Haby et al., 1999) applied for each regrowth of bermudagrass was increased from 0 to 112 kg ha–1. After continuing similar N treatments for an additional three seasons, pH in the 0- to 15-cm soil depth declined to 6.4 at the zero NR, and to 5.8 at the 112 kg N ha–1 rate when evaluated across RS that had no significant effect on pH (data not presented). Because the alfalfa had access to high levels of applied N and residual soil nitrate, alfalfa DMY was not affected by soil factors related to pH 5.8, even though decreased rhizobium activity at pH <6.0 can restrict effective symbiosis (Rice, 1975). However, in our study pH 5.8 was measured in 0.01 M CaCl2 (pHs), a dilute salt suspension, while the pH discussed by Rice (1975) was determined in 1:2.5 soil/water (pHw) and the Haby et al. (1999) work involved pHw determined in 1:2 soil/water. Haby and Leonard (2002) showed that pHs in 1:2 soil/0.01 M CaCl2 can vary in low buffer capacity soils from about 0.3 to 1.0 unit lower than pHw in 1:2 soil/water depending on the time of year the soil is sampled. Thus, the pHs 5.8 reported in our study would have been above the pHw 6.0 mentioned by Rice (1975) as restricting effective symbiosis.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
When alfalfa was grown in mixture with bermudagrass, the proportion of N that alfalfa derived from atmospheric fixation varied from 42 to 91% of total plant N. These values were affected by the alfalfa RS at most harvests throughout the study, with alfalfa at the 23-cm RS fixing the least N. The %Ndfa remained relatively constant across all harvests within each year of the study. The FNY ranged from 80 to 222 kg N ha–1 yr–1 and varied from 2 to 69 kg N ha–1 in individual harvests. The TNY ranged from 2 to 17 kg N ha–1 yr–1 and varied from 0 to 12 kg N ha–1 in individual harvests. These results verify that while alfalfa receives a large percentage of its N from atmospheric fixation, little of this N was transferred to the bermudagrass, likely because of low bermudagrass yield caused by competition from alfalfa for water and the effect of shading of the bermudagrass by alfalfa. On this Darco soil, Coastal bermudagrass was unable to compete with alfalfa that initiates regrowth in winter as early as 2 mo before spring regrowth initiation of the bermudagrass. Even at alfalfa row spacings as wide as 91 cm, competition from alfalfa greatly lowered inter-row bermudagrass production. At the 23-cm alfalfa row spacing, only a few sprigs of bermudagrass were able to survive. This lack of bermudagrass growth in alfalfa on this soil was a major factor in the low %Ngdfa transferred from mineralization of alfalfa plant material. Because of these competitive effects, we suggest that alfalfa not be overseeded into a stand of bermudagrass if bermudagrass is intended for use as a long-term mixture.

With proper site/soil selection, sufficient limestone to raise pH to about 7, and adequate P, K, B and other plant nutrients as indicated by soil analysis, rain-fed alfalfa production on selected Coastal Plain soils appears to be a viable alternative to Coastal bermudagrass. The expense of additional limestone to elevate soil pH into the range of 6.8 to 7.0 for alfalfa is rapidly offset by the savings in fertilizer N normally recommended for hybrid bermudagrasses but that is not needed for alfalfa. Nitrogen for bermudagrass production is the highest-cost nutrient input. The ability of alfalfa to use symbiotically fixed atmospheric N, the poor response of alfalfa to N applied as fertilizer demonstrated in our study, and the increased value of alfalfa hay compared to bermudagrass hay on the open market appear to give this perennial legume an edge in production efficiency and economics.


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





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