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Published online 8 January 2009
Published in Agron J 101:17-24 (2009)
DOI: 10.2134/agronj2008.0078
© 2009 American Society of Agronomy
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Nitrogen Increases Evapotranspiration and Growth of a Warm-Season Turfgrass

L. Barton*, G. G. Y. Wan, R. P. Buck and T. D. Colmer

School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley 6009, Western Australia, Australia

* Corresponding author (lbarton{at}cyllene.uwa.edu.au).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The effect of N fertilizer rate on Kikuyu turfgrass [Pennisetum clandestinum (Hochst. ex Chiov)] evapotranspiration was evaluated during two summers. Evapotranspiration was measured using weighing lysimeters (205 mm in diameter by 625 mm in length) inserted in turfgrass field plots (10 m2). The experiment was a randomized plot design with three replicates. Treatments included two turfgrass ages (established from 20 wk or 20-yr-old turfgrass) and three N application rates (0, 50, or 150 kg N ha–1 yr–1). Evapotranspiration ranged from 2.8 to 7.5 mm d–1 (or 56–81% of evaporative demand), and varied with daily evaporative demand, turfgrass age, and N fertilizer rate. The older turfgrass used more water than the younger turfgrass during both summers; while increasing the N application rate also increased evapotranspiration for both turfgrass types (younger turfgrass only in the second summer). Evapotranspiration was positively correlated with turfgrass growth (r2 = 0.74–0.80) and transpiring leaf area (r2 = 0.78). Older turfgrass at all N treatments, and the younger turfgrass receiving 150 kg N ha–1 yr–1, had adequate growth, color, and leaf N concentrations. Optimizing fertilizer applications such that the minimum N required to maintain turfgrass quality is applied, is an approach for decreasing water consumption by turfgrass.

Abbreviations: ET, evapotranspiration

Received for publication March 13, 2008.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
MAINTAINING HIGH QUALITY TURFGRASS, while minimizing water use, requires good information on turfgrass water requirements; plus an understanding of how management factors may change these values. Turfgrass water use, or evapotranspiration (ET), varies depending on several factors, including: climate, quantity and quality of the water applied, and cultural practices (Kneebone et al., 1992). The main cultural practices shown to influence turfgrass ET include: choice of turfgrass species (Biran et al., 1981; Feldhake et al., 1983; Kim and Beard, 1988; Qian and Engelke, 1999; Qian et al., 1996), N fertilizer rate (Ebdon et al., 1999; Feldhake et al., 1983; Mantell, 1966; Shearman and Beard, 1973), cutting height and frequency (Biran et al., 1981; Feldhake et al., 1983; Fry and Butler, 1989), and soil water availability (Kneebone et al., 1992). For example, the previously cited research has demonstrated converting from cool-season to warm-season turfgrasses can save from 15 to 48% water, decreasing N fertilizer rates can save up to 31% water, and lowering cutting heights of cool-season grasses can decrease ET by up to 27% depending on the species and the change in height.

Promoting turfgrass management practices that decrease water use requires not only an understanding of how the proposed practices will affect ET, but also knowledge of how these strategies will impact turfgrass quality. Although manipulating N fertilizer rate, cutting height and frequency, and soil water availability have been shown to decrease turfgrass ET, these studies have not always included quantitative information on how these factors also affect turfgrass quality. As a consequence, it is often not clear to what extent water savings can be made using various turfgrass management practices, without adversely affecting turfgrass quality.

Many regions of the world, including the Mediterranean basin, western United States, southern Africa, and southern Australia, are expected to experience a decrease in water resources due to changing climate (Kundzewicz et al., 2007). For example, southwestern Australia has experienced a 20% reduction in winter rainfall since the late 1960s (Allan and Haylock, 1993). Not surprisingly, the pressure on turfgrass managers to justify water use has also increased, and the need to develop water efficient turfgrass management practices has intensified. Most studies investigating the influence of management practices on turfgrass ET have been conducted in North America, and the universal applicability of these findings is unclear. For example, although lowering turfgrass cutting heights has been shown to be a useful strategy for decreasing turfgrass ET in the United States (Biran et al., 1981; Feldhake et al., 1983; Fry and Butler, 1989; Shearman and Beard, 1973), it is unlikely to be a useful approach in southwestern Australia where cutting heights are already relatively low (viz. typically 5–15 mm) on golf courses and sports playing fields. Instead, optimizing N fertilizer rates may be a useful approach for decreasing turfgrass ET, if it can also be demonstrated that lowering N fertilizer applications does not compromise turfgrass quality. Consequently, the aim of this study was to determine the effect of N fertilizer rate on the ET and quality of a warm-season turfgrass of two ages, when grown in a Mediterranean-type climate.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Kikuyu [Pennisetum clandestinum (Holst. Ex Chiov)] turfgrass plots (10 m2) were established at The University of Western Australia's Turf Research Facility located at Shenton Park (31°56' S, 115°47' E), approximately 8 km west of Perth central business district. The turfgrass plots were used to measure growth and quality, while lysimeters housed within the plots, were used to measure turfgrass ET.

Soil and Site
Perth has a Mediterranean-type climate, and in the last 13 yr has had an annual rainfall of 746 mm, mainly falling during the winter months, a mean annual maximum temperature of 24.4°C and a mean annual minimum temperature of 12.6°C (Commonwealth Bureau of Meteorology, http://www.bom.gov.au/climate/averages). The soil at the site is known locally as Karrakatta sand (McArthur and Bettenay, 1960), or is classified as a Dystric Xeropsamments using soil taxonomy (USDA, 1992). The surface soil (0–150 mm) has a pH of 4.7 (1:5 soil: 0.01 M CaCl2 extract), electrical conductivity of 0.01 dS m–1 (1:5 soil/water extract), cation exchange capacity of 3.22 cmol (+) kg–1, C concentration of 6.5 mg g–1, and N concentration of 0.4 mg g–1. The subsurface soil ( >150–1000 mm) has on average a pH of 5.6, electrical conductivity of 0.003 dS m–1, cation exchange capacity of 1.33 cmol (+) kg–1, C concentration of 0.9 mg g–1, and N concentration of 0.2 mg g–1. The surface soil contains 92% coarse sand, 2% fine sand, 2% silt, and 4% clay (Pathan et al., 2003).

The site included a variable-speed travelling irrigator with a fixed-boom (Short and Colmer, 2007) coupled with a weather station (WeatherMaster 2000, Environdata Australia). The median daily efficiency of discharge ([actual irrigation depth/programmed irrigation depth] x 100) was 97% (data not shown). For further details of the irrigator, the reader is referred to Short and Colmer (2007). The weather station was installed to measure climatic parameters, plus calculate daily evaporative demand of the environment (also called reference ET by other authors), for use by the irrigator program. Rainfall was measured using 203 mm diameter automated tipping rain gauge with a resolution of 0.2 mm. Air temperature was measured using a semiconductor junction with an amplifier (resolution of 0.1°C), relative humidity using a capacitive humidity sensor (resolution of 0.1%), solar radiation was using a silicon pyranometer (resolution of 15 W m–2), and wind speed and direction using an anemometer (resolution of 0.1 km h–1); with sensors located at a height of 1.6 to 2.1 m above the ground. Evaporation was calculated from the appropriate weather station data with a modified Penman equation (Doorenbos and Pruitt, 1977). Pan evaporation was also measured during the 2006–2007 summer using an A class pan located near the weather station. Total evaporation, total rainfall, and average air temperature for each measurement month are presented in Table 1 .


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  		Table 1. Total evaporation, total rainfall, and average daily air temperature for each month of the study at Shenton Park, Western Australia.

 
Experimental Design and Approach
The experimental design was completely randomized, consisting of two turfgrass ages by three N fertilizer rates by three replicates. The two turfgrass ages were either turfgrass plots (10 m2) established from 20-wk-old turfgrass ("younger" turfgrass) or from 20-yr-old turfgrass ("older" turfgrass). The younger turfgrass was newly-grown sod, cut to a depth of 15 mm; while the older turfgrass was cut from a golf course fairway to a depth of 50 mm so as to include a mat layer of high organic matter content (36% by dry mass, as determined after combustion at 600°C) (Carrow et al., 1987). The N fertilizer rate was either 0, 50, or 150 kg N ha–1 yr–1 as (NH4)2SO4. Each annual N fertilizer rate was evenly split across four applications (two in spring and two in autumn: 3 Mar. 2005, 26 June 2005, 15 Sept. 2005, 10 Nov. 2005, 2 Mar. 2006, 25 June 2006, 14 Sept. 2006, 9 Nov. 2006). Turfgrass plots were planted on 19 Jan. 2005, with weighing lysimeters (described below) installed in each plot using turfgrass from the plots on 10 Mar. 2005. Evapotranspiration measurements commenced on the 6 Dec. 2005, allowing approximately 9 mo for the turfgrass in the weighing lysimeters to become established. All turfgrass plots were irrigated at a rate of 60% replacement of net evaporation, applied every second day, which is sufficient to maintain younger kikuyu turfgrass in southwestern Australia (Short, 2002).

Measuring Turfgrass Evapotranspiration
Turfgrass ET was measured on six occasions in each of two consecutive summers (Dec. 2005–Mar. 2006; Dec. 2006–Feb. 2007). Evapotranspiration was measured using weighing lysimeters inserted in the turfgrass plots. The lysimeters consisted of polyvinyl chloride (PVC) cylinders (205 mm in diameter by 625 mm in length), filled with air-dried soil. The lysimeters contained turfgrass (15 mm in depth for young turfgrass, 50 mm for older turfgrass), surface soil (100 mm) and subsurface soil (480 mm) collected from the site, a polyester filter, plus a layer of coarse quartz stones at the base. The base of each lysimeter funnelled leachate to a central exit point from which leachate was collected into a 250 mL plastic container. Each lysimeter was inserted into a metal sleeve previously dug into the field plots. This enabled the lysimeters to be lifted from the ground using a winch, and then weighed. The canopy surface for each lysimeter was flush with the canopy surface of the plots, to avoid the influence of edge effects on turfgrass ET (Aronson et al., 1987). Furthermore, by having the lysimeter turfgrass flush with the surrounding surface, both the lysimeter and plots could be mown at the same height, and the same time.

Evapotranspiration was determined by measuring the mass loss for each lysimeter over selected 24-h periods. A portable balance (Ohaus BI00S, Ohaus Corp., Pine Brook, NJ), shielded from the wind, was used to determine masses to the nearest 1 g. Turfgrass was watered (60% replacement of the previous 2 d net evaporation), weighed, and then reweighed 24 h later. Irrigation and rainfall did not occur during this 24-h period. Any leachate collected during this period was subtracted from the mass loss measured for that lysimeter.

Turfgrass Growth
Growth of each plot was assessed using the dry mass of mowing clippings. Plots were mown weekly, at a height of 15 mm, and the mass of the fresh clippings weighed. A subsample (20–25 g) of the fresh clippings was collected and weighed, and then dried (60°C) before reweighing to determine the water content (percent of dry mass). After collecting the subsample, the remaining fresh clippings were immediately redistributed across the surface of the respective plot. The dry mass of clippings from the plot was calculated from the fresh/dry mass ratio. The dry mass of the shoots, dry mass of the thatch + rhizomes, plus the green leaf area of the lysimeter turfgrass, were measured for each lysimeter approximately 2 wk following the final ET measurement (15 Mar. 2007; mowing regime had continued). The green leaf area was measured by collecting a core (70 mm in diameter) from the center of the lysimeter, removing the green leaves, and using a leaf area meter (LI-COR Portable Area Meter Model LI-3000). The dry mass of the leaves of the green leaf area sample was then determined after drying the sample (60°C). The green leaves, and thatch + rhizomes, from the remainder of the lysimeter surface were also collected for determination of dry mass.

Turfgrass Quality and Thatch Content
Turfgrass quality was assessed by measuring turfgrass color and tissue (clippings) N concentration of the turfgrass plots; plus the thatch content of the turfgrass in the weighing lysimeters at the completion of the study. Turfgrass color was measured monthly using a Chroma Meter (Minolta, CR-310, Osaka, Japan), an instrument previously shown to enable quantitative assessments of turfgrass color (Barton et al., 2006; Landschoot and Mancino, 2000). Measurements were taken at three positions along a transect in each plot by pressing a 50 mm diam. measuring cylinder firmly down onto the canopy surface to exclude external light. The Chroma Meter was calibrated after every 36 readings, using a calibration plate (CR-A44, Minolta, Osaka, Japan) and following the instructions provided by the manufacturer. Total N in the dried plot clippings was measured on three occasions each summer by fine grinding a subsample using a ball grinder, and analyzing the tissue powder using a CHN analyzer (LECO CHN 1000, St. Joseph, MI). Plant tissue nutrient concentrations were validated against plant tissue standards analyzed using the same procedures.

Industry Benchmarking
Critical values for Kikuyu turfgrass color are not established for maintained turfgrass grown in southwestern Australia. Consequently, on four occasions during the study (21 Oct. 2005, 27 Jan. 2006, 3 Nov. 2007, 25 Jan. 2007) we measured turfgrass color at six Kikuyu sports fields (3 measurements per site) managed by local government so as to benchmark findings from our experimental site with industry sites. Turfgrass color was measured using the instrument and technique described above.

Data Analyses
All data were statistically analyzed using Genstat (2007). A general ANOVA was used to determine whether turfgrass age or fertilizer N application rate affected turfgrass ET. Linear regression analysis was conducted after any large residuals, identified by Genstat as "outliers", were removed from the analysis. Repeated measures ANOVA, conducted using Genstat, was used to determine whether parameters varied with time (Webster and Payne, 2002). Post-hoc pair-wise comparisons of means were made using LSD (significance level of 0.05) calculated for each of the treatments and treatment interactions.

Evapotranspiration was calculated by converting the loss of mass during the 24-h period (minus any leachate loss) as follows:

Formula
where ET is the daily evapotranspiration (mm d–1), {Delta}Mass is the change in mass of lysimeter in 24 h (and allowing for leachate loss; g d–1), area of lysimeter is 330 cm2, and 10 is a conversion from cm to mm. Evapotranspiration was also expressed as a proportion of daily evaporative demand (calculated using weather station data as described above) and pan evaporation (2006–2007 summer only).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Turfgrass Growth and Quality
Clipping dry mass ranged from 3.5 to 480 kg ha–1 wk–1, and varied with time of collection (P < 0.001), turfgrass age (P < 0.001), and the rate of N fertilizer (P < 0.001) (Fig. 1 ). In 2005–2006, and for both turfgrass ages, growth (averaged across N fertilizer rate) tended to increase as the summer season progressed (Fig. 1); whereas in 2006–2007, growth of the turfgrass fluctuated throughout the measurement period (Fig. 1). In both years, and for each measurement period (except one), turfgrass growth was greater for the older turfgrass than the younger turfgrass plots (P < 0.05). Overall, increasing the N rate from 0 or 50 kg N ha–1 yr–1 to 150 kg N ha–1 yr–1 increased growth from the older and younger (2005–2006 only) turfgrass ages (P < 0.05) (Fig. 2 ); however increasing the N rate did not necessarily increase growth on a weekly basis (Fig. 1). The amount of thatch and rhizomes present in the lysimeters at the completion of the study ranged from 3555 to 11,665 kg ha–1 (Table 2 ). The thatch and rhizome dry mass was greater for the older turfgrass than the younger turfgrass (P < 0.001), and increased as the N fertilizer rate increased (P < 0.001; Table 2).


Figure 1
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  		Fig. 1. Influence of N rate on dry matter produced (kg dry clippings ha–1 wk–1) for (a,b) younger and (c,d) older Kikuyu turfgrass in (a,c) 2005–2006 and (b,d) 2006–2007 summers. The values are for 10 m2 plots for the weeks during which evapotranspiration (ET) measurements were also taken using lysimeters in the plots, with means (and standard errors as bars) of three values. Growth measured as dry mass of clippings, at a cutting height of 15 mm. The LSD for comparing treatments with time in 2005–2006, 38 (except when comparing means with the same turfgrass age and N rate, then use 28); LSD for comparing treatments with time in 2006–2007, 96 (except when comparing means with the same turfgrass age and N rate, then use 62). Note different vertical scale in part d.

 

Figure 2
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  		Fig. 2. Influence of N supply on dry matter production (average weekly clippings) for younger and older Kikuyu turfgrass in (a) 2005–2006 and (b) 2006–2007. The values are for 10 m2 plots for the weeks during which evapotranspiration (ET) measurements were also taken using lysimeters in the plots, with means (and standard errors as bars) of three values. Growth measured as dry mass of clippings, at a cutting height of 15 mm. Note different vertical scales.

 

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  		Table 2. Turfgrass color, tissue N concentration, and thatch + rhizome dry mass for the younger and older Kikuyu turfgrass. Values are the mean (and standard error in brackets) of three replicates. Different letters in the same column indicate significant differences at the 0.05 level.

 
Monthly turfgrass color measurements (i.e., hue angles) ranged from 96° to 120° in 2005–2006, and 99° to 114° in 2006–2007 (Table 2). The older turfgrass had more "greenness" than the younger turfgrass when compared at the same N fertilizer rates (P < 0.05; Table 2). Increasing N fertilizer application only increased the average monthly color readings of the younger turfgrass, and had no effect on the older turfgrass (P < 0.05; Table 2). Turfgrass color (i.e., hue angle) at the six industry benchmarking sites ranged from 97° to 122°, and averaged 113°. If we nominate a minimum critical value of 97°, then only the nonfertilized younger turfgrass failed to meet the minimum requirement (averaged across three replicates; Table 2).

The N concentration (% N in dry mass) of the turfgrass clippings ranged from 1.59 to 2.78% in 2005–2006, and 2.03 to 3.36% in 2006–2007 (Table 2). The older turfgrass generally had a greater N concentration than the younger turfgrass when compared at the same N fertilizer rates (P < 0.001; Table 2). Increasing N fertilizer application increased the average N concentration of the younger turfgrass in both years (P < 0.001), but only increased average N concentration of the older turfgrass in the second year (2006–2007). The younger turfgrass that did not receive N fertilizer or that received 50 kg N ha–1 yr–1 (2005–2006 only) failed to meet the minimum requirement of 2.0% (averaged across three replicates) (Johnston, 1996) in the first year (Table 2).

Turfgrass Evapotranspiration
In 2005–2006 ET ranged from 3.82 to 7.50 mm d–1, with the values influenced by turfgrass age (P < 0.001), N fertilizer rate (P < 0.001), and measurement period (P < 0.05) (Table 3 ). For example, the older turfgrass had a higher ET than the younger turfgrass while increasing the N application rate also increased ET for both turfgrass ages (P < 0.001, Table 3). In 2006–2007, ET ranged from 2.81 to 6.51 mm d–1 and varied with time (P < 0.001), with greatest average losses occurring in the first half of the summer (5 Dec. 2006 and 30 Jan. 2007, Table 4 ). In 2006–2007, the older turfgrass again had a higher ET than the younger turfgrass (P < 0.05), however increasing the N application rate only increased ET by the younger turfgrass (P < 0.05, Table 4).


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  		Table 3. Daily evapotranspiration (ET) values for Kikuyu turfgrass of two ages fertilized at three N application rates in 2005–2006. Plants were grown in field lysimeters and watered at 60% replacement of net evaporation (i.e., evaporative demand of the environment) prior to taking measurements. Evapotranspiration values are means of three values with standard errors in brackets. Different letters in the same column indicate significant differences at the 0.05 level.

 

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  		Table 4. Daily evapotranspiration (ET) values for Kikuyu turfgrass of two ages fertilized at three N application rates in 2006–2007. Plants were grown in field lysimeters and watered at 60% replacement of net evaporation (i.e., evaporative demand of the environment) prior to taking measurements. Evapotranspiration values are means of three values with standard errors in brackets. Different letters in the same column indicate significant differences at the 0.05 level.

 
On average, daily ET represented 56 to 81% of daily evaporative demand (measured using a weather station) in 2005–2006, and 56 to 76% of daily evaporative demand (weather station) in 2006–2007 (or 51–69% of pan evaporation) (Table 5 ). The average proportion of ET to evaporative demand was greater for the older turfgrass than the younger turfgrass. Increasing the N fertilizer rate increased the proportion of ET to evaporative demand for the younger turfgrass in both years, but only for the older turfgrass in the first year of study (2005–2006) (Table 5).


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  		Table 5. Average proportion of daily evapotranspiration (ET) to evaporative demand estimated using weather station data or using an A class pan for Kikuyu turfgrass of two ages and fertilized at three N application rates for two summer measurement periods. Plants were grown in field lysimeters and watered at 60% replacement of net evaporation prior to taking measurements. Values are means of three values with standard errors in brackets. Different letters in the same column indicate significant differences at the 0.05 level.

 
Relationship between Turfgrass Evapotranspiration and Growth
In 2005–2006 summer, the average ET (i.e., averaged across the six measurements) was positively correlated (r2 = 0.75) with average growth, that is, increasing growth resulted in increased ET (Fig. 3a ). As previously discussed, growth tended to be greater for the older turfgrass than the younger turfgrass; furthermore, increasing the N rate tended to increase growth for both turfgrass ages.


Figure 3
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  		Fig. 3. The relationship beUtween growth and daily evapotranspiration (ET) for Kikuyu turfgrass during (a) 2005–2006 and (b) 2006–2007 summers. Values represent the average weekly growth of turfgrass plots for the week during which ET measurements were taken, and the average daily ET for each lysimeter. Growth measured as dry mass of clippings, at a cutting height of 15 mm. Daily ET measured using weighing lysimeters installed in turfgrass plots. Note different horizontal scales.

 
In 2006–2007 summer, the relationship between average ET and growth remained positively correlated (r2 = 0.74), however instead of a linear relationship there was an exponential relationship between ET and growth (Fig. 3b). We hypothesize that this occurred because the clippings collected from the turfgrass plots also included a proportion of old, dead leaf material (Table 2); so while the dead material may have contributed to the dry mass, it would not have contributed to turfgrass ET. In 2006–2007, turfgrass ET was instead, as expected, best related to green leaf dry mass and green leaf area collected from the surface of the lysimeters (Fig. 4 ), rather than the average weekly clippings from plots (Fig. 3b). Collecting the leaf material by hand enabled the thatch and dead leaves to be removed before analysis for dry mass and leaf area.


Figure 4
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  		Fig. 4. The relationship between (a) green leaf dry mass and daily evapotranspiration (ET) and (b) green leaf area and daily ET for Kikuyu turfgrass in the final week of the 2006–2007 measurements. Leaves collected from lysimeters with a surface area of 330 cm2. Values represent the ET for each individual lysimeter. The open symbol is an outlier and not included in the regression analysis.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Manipulating the N fertilizer rate to match turfgrass requirements is one approach to minimizing turfgrass water use, but would only be a useful management strategy if the effect on turfgrass quality is understood. In the present study, not fertilizing the older turfgrass in the first year of study would have decreased water consumption by 20% without compromising turfgrass quality. By contrast, not applying N fertilizer to the younger turfgrass would have also saved water, but at the cost of turfgrass quality. Others have also reported that decreasing the N fertilizer rate can reduce turfgrass ET under nonlimiting water supply, however, these studies have not always included quantitative information on how N rate has also affected turfgrass quality (Feldhake et al., 1983; Mantell, 1966; Shearman and Beard, 1973). As a consequence, it has not necessarily been clear to what extent water savings can be made by adjusting N fertilizer rates. Notably, Ebdon et al. (1999) demonstrated that while decreasing N fertilizer rate decreased ET for Kentucky bluegrass, this was not significant within the range of N fertilizer rates required to maintain the turfgrass. Including measures of turfgrass quality is essential for assessing the extent to which adjustment of N fertilizer rates can be used as a means of decreasing turfgrass water use.

Turfgrass ET is largely influenced by turfgrass growth, which in turn was affected by turfgrass age and N application rate in the present study. Indeed, turfgrass growth and leaf area explained up to 80% of the variation in turfgrass ET among treatments. It is surprising to note that the relationship between leaf area and turfgrass ET does not appear to have been previously reported. Instead the dry mass of clippings and plant morphological characteristics (e.g., leaf width, vertical leaf extension rate), have often been used as surrogates for leaf area (Kim and Beard, 1988; Shearman, 1986; Shearman and Beard, 1973). In the present study, turfgrass growth was more strongly related to turfgrass ET in 2005–2006 than 2006–2007. This may have been because the turfgrass clippings collected from the older turfgrass plots receiving the highest rate of N fertilizer contained a proportion of dead leaves, in addition to green leaves, in 2006–2007. While the dead leaves would have contributed to the biomass, these would have contributed relatively little to ET; thereby diminishing the overall relationship between turfgrass ET and growth measured as clipping mass. We were able to improve the relationship between turfgrass ET and turfgrass growth in 2006–2007 by measuring the dry mass of the green leaves (i.e., thatch excluded) in the lysimeters at the end of the study. Furthermore, we were also able to demonstrate a good relationship between green leaf area and turfgrass ET.

Both recently established and older Kikuyu maintained to a suitable turfgrass standard used water at 69 to 75% of evaporative demand during the summer months. These ET amounts are similar to values previously reported for newly established Kikuyu turfgrass grown at the same site and receiving 180 kg N ha–1 yr–1 (Short, 2002). For Kikuyu turfgrass, Short (2002) reported ET values of 55 to 66% replacement of pan evaporation when measured using field lysimeters watered daily to field capacity. The ET values (as proportion of evaporative demand of the environment) in the present study were similar to those reported for warm-season grasses in other studies (Atkins et al., 1991; Beard et al., 1992; Garrot and Mancino, 1994; Green et al., 1991; Kneebone and Pepper, 1982), and significantly lower than that reported for cool-season grasses grown in southwestern Australia (Short, 2002). This is not unexpected as it has been well documented that warm-season grasses tend to have lower ET than cool-season grasses (Biran et al., 1981; Feldhake et al., 1983; Qian and Engelke, 1999).

In conclusion, optimizing N fertilizer applications to warm-season turfgrass should contribute to minimizing water use by turfgrass in Mediterranean-type climates. Nitrogen fertilizer applications need to be optimized to those required to maintain satisfactory turfgrass growth and quality, and rates required may vary depending on turfgrass age. The greater leaf area associated with the older turfgrass, in comparison with the younger turfgrass, promoted ET.


   ACKNOWLEDGMENTS
 
Greenacres Turf Farm is thanked for help in the design and maintenance of the irrigator. Murdoch Challenger TAFE, City of Stirling, City of Canning, City of Perth, Lovegroves and the WA Golf Course Superintendents Association for providing staff and students to assist with planting and mowing. Members of the UWA Turf Industries Research Steering Committee for their support and advice. Comments made by two anonymous reviewers improved the manuscript. This project has been facilitated by HAL in partnership with the Australian turf industry. It was funded by voluntary contributions from the Parks and Leisure Association of Australia (representing a consortium of local and state government authorities), CSBP Ltd, Organic 2000, Turf Grass Association of Australia (WA), WA Golf Course Superintendents Association, Baileys Fertilisers, Turf Master Facility Management, Turf Growers Association of Western Australia, Lawn Doctor, Micro Control Engineering, and the Water Corporation.


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





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Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome