The ability of turfgrass species and cultivars to handle high salinity can vary greatly. The relative salt tolerances of several warm-season grasses have been reported in several studies (Carrow and Duncan, 1998; Dean et al., 1996; Harivandi, 1994; Marcum, 1999a,1999b; Marcum and Murdoch, 1990). Within warm-season grasses, species can show a wide range in salinity tolerance. Marcum et al. (2009) have ranked the relative salinity tolerance of warm-season grasses in the following order (from most to least salt tolerant): inland saltgrass [Distichlis spicata (L.)], alkali sacaton [Sporobolus airoides (Torr.)], common bermudagrass, zoysiagrass [Zoysia japonica (Steud.)], and buffalograss [Buchloe dactyloides (Natt.) Englem]. In a study of shoot and growth response to salinity, Pessarakli et al. (2009) found common bermudagrass to be the most affected by salinity stress, followed by seashore paspalum [Paspalum vaginatum (Swarz.)] and inland saltgrass. Other studies have found seashore paspalum to be the most salt tolerant among warm-season grasses tested (Carrow and Duncan, 1998; Duncan and Carrow, 2000; Lee et al., 2004, 2005).
Producing a consistent ranking of salinity tolerance among turfgrasses is difficult because results vary depending on the criteria and endpoints authors used to measure salinity tolerance (Marcum, 1999a). Moreover, differences in salinity tolerance among cultivars within a given species can vary widely and can sometimes be greater than species differences. Varietal differences in salinity tolerance have been reported for zoysiagrasses (Marcum et al., 2003; Qian et al., 2000), bermudagrasses (Dudeck et al., 1983; Marcum and Pessarakli, 2006), seashore paspalum (Carrow and Duncan, 1998; Lee et al., 2005), inland saltgrasses (Marcum et al., 2005) and St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] (Dudeck et al., 1993).
Using salinity thresholds reported in the literature to select grasses for field use can be misleading because most of the aforementioned studies (Dudeck et al., 1993; Marcum, 1999b; Lee et al., 2005; Marcum and Pessarakli, 2006; Pessarakli et al., 2009) were conducted under controlled-environment greenhouse conditions. Additional environmental stresses, such as drought, cold, or heat, all of which typify arid and semiarid regions and can exacerbate the effects of salt stress, were not considered. Transitional semiarid and arid climates are characterized by extreme diurnal and/or seasonal changes in weather conditions, creating difficult conditions for growing warm-season grasses. The growing season may barely exceed 6 mo and low winter temperatures can cause damage to the grasses due to their low tolerance of cold temperatures. Exposure to the added stresses from a harsh climate might change the outcome of a salinity trial and lead to decreased survival of a turfgrass which it would otherwise tolerate.
A second strategy for reducing the amount of potable water used for irrigation purposes is to optimize irrigation efficiency. Sprinkler irrigation is the accepted practice for irrigating lawns despite a reported low efficiency in distributing water to the plant stand (Mecham, 2004). Incorrectly spaced and installed sprinkler heads, wind drift, and evaporation losses during the irrigation process all contribute to water losses that can increase overall water consumption and/or decrease plant stand quality. Subsurface drip systems that apply water laterally within the root zone from line-source or point-source irrigation tiles have been introduced as a means of supplying irrigation water more efficiently than sprinkler systems. Advantages of these subsurface irrigation systems include the uninterrupted use of the turf area during irrigation, energy savings due to a lower operating water pressure, reduced disease pressure, and potential water savings because irrigation is applied directly in the rootzone and is not affected by wind drift or evaporation (Beard, 1973; Burt and Styles, 1999, Leinauer, 1998; Duncan et al., 2009). Applications of subsurface drip irrigation have been extensively studied in agriculture (Camp et al., 1993; Bosch et al., 1992; Malash et al., 2008), but this technology only recently received attention in the field of turf management. Schiavon et al. (2011) investigated turf quality of warm-season grasses over a 4-yr period and reported neither a decline in the performance of drip irrigation systems nor in the quality of tested grasses in an arid climate. Similarly, Choi and Suarez-Rey (2003) found no drop in turf quality in bermudagrass drip irrigated with recycled water (ECw of 0.95 dS m−1).
While subirrigation systems distribute water more efficiently and uniformly (especially in times and areas of high winds) these systems may have some limitations in regard to leaching salts from the root zone. Applying water in excess of what plant growth requires is a necessary turf maintenance practice to leach salts from the rootzone and thereby avoiding salt accumulation. When turf areas are drip-irrigated with saline water, the fraction of the root zone above the emitters only receive water through capillary raise and this amount of water may not be sufficient to flush out salts. Devitt and Miller (1988) suggested that by spacing drip lines at distances which allow leaching fractions to achieve high soil water content uniformity, salt buildup in the active root zone could be reduced when drip-irrigating with saline water.
Because of the increasing pressure to conserve potable water, it is imperative that efforts be made to increase irrigation efficiency and to use recycled or other impaired water sources to sustain quality and functionality of turfgrass areas. Currently, information is lacking on the longer term sustainability of warm-season grasses in transitional arid climates when irrigated with saline water from a subsurface drip system. A study was conducted at New Mexico State University to assess the effects of water quality and type of irrigation on rootzone salinity and turf quality of several warm-season grasses in the arid southwest. Moreover, we investigated whether or not summer salinity accumulation in the rootzone can be used to predict turfgrass quality for several turfgrass species and varieties.
MATERIALS AND METHODS
The study was performed at the University's golf course in Las Cruces, NM (USDA Plant Hardiness Zone 8) from 2005 to 2007. Monthly average temperature, precipitation, and reference evapotranspiration during the research period are listed in Table 1. Grasses were established in 2004 and included bermudagrass cultivars NuMex Sahara, Princess 77, Riviera, and Transcontinental; inland saltgrass cultivars A138 and DT16; seashore paspalum cultivars SeaDwarf and Sea Spray; zoysiagrass cultivar De Anza. Plots were irrigated with potable, moderately saline, or saline water. Saline water consisted of water obtained from a saline aquifer that was pumped to the research site. Moderately saline water was prepared by mixing municipal water with saline groundwater to an EC of 2.0 dS m−1. The U.S. Salinity Laboratory (U.S. Salinity Laboratory Staff, 1954) classifies the moderately saline water as C3-S1, high in salinity and low for sodium hazard and the saline irrigation water as C4-S2, very high in salinity and medium for sodium hazard. Water samples were collected monthly to measure EC and three times during each growing season to determine all major constituents in the water. A detailed description of ion concentrations in the irrigation waters are listed in Table 2.
|——————————————————————————————— Air temperature, °C ———————————————————————————————|
|——————————————————————————————— Precipitation, mm ———————————————————————————————|
|30 yr mean||10||10||8||6||8||15||39||48||32||20||11||14|
|—————————————————————————————————— ETo, mm ——————————————————————————————————|
|Electrical conductivity, dS m−1||0.6||2.0||3.5|
|Total dissolved solids, mg L−1||400||1300||2200|
|Magnesium, mmolc L−1||0.34||0.83||1.24|
|Calcium, mmolc L−1||1.4||1.6||2.53|
|Sodium, mmolc L−1||2.9||10.01||17.4|
|Sodium adsorption ratio||1.55||6.41||8.94|
|Potassium, mmolc L−1||0.1||0.7||1.3|
|Carbonate, mmolc L−1||0.00||0.00||0.00|
|Bicarbonate, mmolc L−1||2.84||6.43||9.95|
Grasses were irrigated during the growing season from either a sprinkler or a subsurface drip system at 110% of reference evapotranspiration (ETo) (Allen et al., 2005). Irrigation was scheduled daily using irrigation software (Nimbus II Central Control System, Rainbird Corp., Tucson, AZ) that also scheduled the golf course irrigation system. Climate data used to calculate ET0 were collected at a weather station located on the golf course in close proximity to the study site. Irrigation for each sprinkler and subsurface drip main block was regulated by a separate solenoid valve and pressure regulator. The sprinkler system was comprised of eight Walla Walla MP2000 Rotators (Walla Walla Sprinkler Company, Walla Walla, WS) operated at 200 kPa and spaced 3.8 m apart to allow for uniform irrigation. Irrigation audits conducted twice during each growing period ensured a minimum distribution uniformity (DU) of 0.7 and provided data necessary to compare the irrigation systems actual water delivery rates with computer settings. The subsurface drip system consisted of porous emitterless line source pipes (Precision Porous Pipe, McKenzie, TN) with a diameter of 1.27 cm operated at 200 kPa. Each subsurface drip irrigated block had a flush valve installed to prevent sediments from potentially clogging the drip lines. The flush valve was located at the opposite corner of the water inlet and allowed for a 10 to 15 s long flush cycle at the beginning of each irrigation cycle. The pipes were installed at a soil depth of 7.5 cm and spaced 30 cm apart. The spacing of 30 cm between lines was chosen in accordance with industry recommendations (Toro, 1998). Irrigation water use on subsurface drip irrigated blocks was recorded by means of water meter (Invensys Process Systems Inc., Plano, TX) and run times were calculated based on recorded water delivery rates minus the amounts that were lost in the flush cycles. Uniform water distribution on subsurface drip irrigated blocks was monitored two times over each growing season by taking 24 volumetric soil moisture readings at depths of 0 to 6 cm with a hand held ThetaProbe soil moisture sensor (Delta-T Devices Ltd., Cambridge, England) 24 h after an irrigation cycle. Soil moisture values were subsequently analyzed for distribution uniformity, similarly to DU calculations on sprinkler irrigated blocks. Establishing DU values for both sprinkler and drip irrigated plots were considered basic maintenance steps taken to ensure that the systems were operating within normal parameters. Therefore they are not reported or discussed in this paper.
The soil at the site consisted of a sandy loam, a sandy skeletal mixed thermic Typic Torriorthent, an entisol typical for arid regions. Chemical properties of the soil before turfgrass establishment and irrigation are listed in Table 3. During the growing season (March–November) plots were mowed biweekly at a height of 5 cm and clippings were collected. Plots were fertilized at a rate of 5 gN m−2 with 15–15–15 quick release fertilizer in April, June, August, and October. A micronutrient fertilizer (Pro-Mate, Helena Chemical Company, Collierville, TN) containing Ca (1.0%), Mg (4.3%), S (18.2%), Cu (0.3%), Fe (14.3%), and Mn (2.6%) was applied in summer at a rate of 10 g m−2. The pre-emergent herbicide Pendulum (active ingredient 37.4% pendimethalin) was applied at a rate 0.65 mL m−2 in April to prevent weed germination. The systemic insecticide Merit (active ingredient 75% imidacloprid) was applied at 0.5 g m−2 in June and August to prevent grub damage.
|EC†, dS m−1||0.22||0.21||0.23|
|Mg, mmolc L−1||0.4||0.4||0.4|
|Ca, mmolc L−1||2.4||2.2||1.7|
|Na, mmolc L−1||0.69||0.6||1.2|
|Cl, mmolc L−1||0.2||0.14||0.1|
|K, mmolc L−1||0.3||0.1||0.1|
|Sodium adsorption ratio||0.59||0.53||1.17|
|Organic matter, %||0.4||0.4||0.4|
Turfgrass color and quality was assessed by means of a visual rating scale recommended by the National Turfgrass Evaluation Program (Krans and Morris, 2007). Turfgrass quality was determined monthly from March to November on a scale of 1 to 9, with 1 = dead turf and 9 = dark green, uniform turf. The monthly ratings were averaged every 3 mo (March–May, June–August, and September–November) and analyzed as three different seasons. Turfgrass color ratings were collected in March and November to evaluate the plots for spring green up and fall color retention. Plots were assessed on a scale of 1 to 9 with 1 = brown, tan colored and 9 = dark green turf. Normalized difference vegetation indices (NDVI) readings were collected by means of a Greenseeker (NTech, Ukiah, CA) monthly from April to October in 2007. The NDVI data were subsequently correlated with visual quality data.
Composite soil samples were collected bi-annually in mid-June and mid-November from depths of 0 to 10 cm, 10 to 20 cm, and 50 to 60 cm using a 4.5-cm-diam. soil auger. Exploratory soil sampling on adjacent golf course fairways with bermudagrass turf before the beginning of our study revealed that depths of 0 to 10 and 10 to 20 cm represent the areas with most of the root growth. The depth of 50 to 60 cm was chosen to also monitor changes in salinity below the rooting depth because no roots were found below a depth of 40 cm. A mid-June sampling date was deemed appropriate as it is halfway through the growing period of warm-season grasses and historically marks the beginning of the rainy season. Therefore, rootzone salt accumulation from saline irrigation is expected to be highest in June. The November sampling date was selected because it marks the end of the growing period of warm-season grasses. Chemical analysis of the soil samples for numerous parameters was conducted at a commercial soil testing laboratory. Data will be presented on EC, SAR, and Na content in the saturated soil paste extract.
The research area was 36 by 70 m in size and was designed as a randomized complete block. A combination of irrigation system and water quality served as the whole block (7.5 by 10 m) treatments and grasses (2.5 by 2.5 m) and soil depths as the subplot treatments. All treatment factors were replicated three times. To test the effects of irrigation salinity level and irrigation system on rootzone salinity, turfgrass color and quality, and NDVI, data were subjected to a repeated measures analysis using a compound symmetry covariance structure in SAS Proc Mixed (SAS, Ver. 9.2, 2002). Fisher's protected LSD test at the 0.05 probability level was used to identify significant differences among means. Proc Corr (SAS, Ver. 9.2, 2002) was used to correlate visual quality ratings with NDVI. Stepwise linear regression (Proc Reg, SAS, Ver. 9.2, 2002) was used to investigate the relationship between summer turf quality and EC, Na, and SAR in the top 10 cm of the rootzone.
Rootzone Salinity at Depths of 0 to 20 cm
The ANOVA (Table 4) for the two upper soil depths (0–10 cm and 10–20 cm) revealed significant three-way interactions between irrigation, water, and sampling date and between irrigation, depth, and sampling date for both EC and Na content. Rootzone EC and Na data were subsequently pooled over sampling depths and are displayed separately for each water quality and irrigation system at each sampling time (Fig. 1 and 2). Data were also pooled over water qualities and are shown separately for each irrigation system and depth at each sampling date (Fig. 3 and 4).
|Effect||Soil depth 0–20 cm
||Soil depth 50–60 cm
|Water quality (W)||***||***||***||***||***||***|
|Sampling date (S)||***||***||***||***||***||***|
When data for potable, moderately saline, and saline irrigated plots were pooled over both sampling depths (Fig. 1 and 2), changes in EC and Na between summer and late fall on plots irrigated with moderately saline and saline water followed the same general patterns as those of irrigation amount and precipitation. Monthly irrigation amounts increased between March and June of each year but minimal precipitation during the same time period resulted in peak EC and Na values in June of 2005 and 2006 (Table 1). When data were pooled over irrigation systems and depths, highest EC and Na levels were observed in June 2006 on plots irrigated with moderately saline and saline water averaging 3.9 and 4.7 dS m−1 for EC and 747 and 1024 mg L−1 for Na, respectively (Fig. 1 and 2). Subsequent precipitation during the rainy season (July–October) and lower irrigation amounts from July to November (compared to March through June) reduced EC and Na by November. Above average cumulative monthly precipitation from April to October 2007 (Table 1) reduced salt build-up and both EC and Na content did not differ between June and November of 2007. When data were averaged over both irrigation systems and sampling dates in 2007, salinity readings on plots irrigated with moderately saline and saline water were 1.4 and 2.8 dS m−1 and Na values were 167 and 371 mg L−1, respectively (Fig. 1 and 2). With the exception of saline irrigated plots in November 2005, EC did not differ between drip and sprinkler irrigated plots for any of the three water qualities during the research period (Fig. 1). In November 2005, plots sprinkler-irrigated with saline water exhibited greater electrical conductivity than drip-irrigated plots (Fig. 1). Type of irrigation system had no effect on Na content regardless of water quality (Fig. 2).
When EC and Na data were pooled over all three water qualities but analyzed separately for each depth and sampling date, EC was greatest in drip irrigated plots at depths of 0 to 10 cm in June of 2005 and 2006 (Fig. 3 and 4). Electrical conductivity in sprinkler irrigated plots did not differ between the two upper soil depths on any of the sampling dates. The EC in drip irrigated plots was either lower (June 2006) or similar to that measured in sprinkler irrigated plots at depths of 10 to 20 cm. These findings confirm our assumption that drip irrigation may not be as successful as sprinkler irrigation in leaching salts from the rootzone at depths above the drip lines. During the dry spring and early summer of 2005 and 2006 (Table 1) rootzone salinity under drip irrigated turfgrasses exceeded the levels found under sprinkler irrigation. However, the differences between sprinkler and drip irrigated soil EC values observed in the summer of 2005 and 2006 did not carry over into the fall and winter. Summer and fall precipitation reduced salts at both soil depths to similar levels for all treatments. Sodium content in drip irrigated plots at soil depths of 0 to 10 cm only exceeded that of sprinkler irrigated plots in June and November of 2005. For all other sampling dates Na content at soil depths of 0 to 10 cm did not differ between drip and sprinkler irrigated plots. As observed for EC, Na content did not differ between the two depths in sprinkler irrigated plots (Fig. 4). Also reflecting EC results, Na content in drip irrigated plots was either lower (June 2006) or similar to that measured in sprinkler irrigated plots at 10- to 20-cm depths. Sodium levels measured in November 2006, June 2007, and November 2007 did not differ between irrigation systems, soil depths, or sampling dates (Fig. 4).
Sodium adsorption ratio values measured at soil depths of 0 to 10 cm and 10 to 20 cm reflected the SAR of the irrigation waters used in the study, with highest SAR values measured in plots irrigated with saline water, and lowest values in plots irrigated with potable water. The SAR levels measured on plots irrigated with moderately saline water fell in between those measured on saline and potable irrigated plots (Fig. 5). However, SAR values were not affected by the irrigation system at either depth (Table 4). The SAR values on plots irrigated with saline water did not differ between summer and fall of 2005 and reached an average of 16.1 when pooled over both depths, irrigation systems, and sampling dates. The SAR on the same plots dropped from 18.6 (summer) to 14.6 (fall) in 2006 and increased from 9.4 (summer) to 12.2 (fall) in 2007 (Fig. 5). The SAR on plots irrigated with moderately saline water did not differ between summer and fall in 2005 and 2007, averaging 8.6 in 2005 and 4.9 in 2007. In 2006, SAR dropped from 13.4 (summer) to 5.0 (fall) when data were averaged over both irrigation systems and sampling depths (Fig. 5). Plots irrigated with potable water exhibited lowest SAR values reaching a maximum value of 3.4 in June 2006 and a minimum of 1.2 in June of 2007 (Fig. 5). November 2006 was the only sampling date for which SAR values in plots irrigated with moderately saline water did not differ from plots irrigated with potable water (Fig. 5). For five of the six sampling dates SAR values were similar in both sampling depths. In November 2006, SAR was greater at depths of 10 to 20 cm than at 0 to 10 cm (Fig. 6).
Rootzone Salinity at Depths of 50 to 60 cm
The ANOVA for the 50- to 60-cm soil depth (Table 4) revealed a significant two-way interaction between water quality and sampling date for EC, Na content, and SAR. Type of irrigation system had no effect on any of the three parameters regardless of water quality. The data were therefore pooled over irrigation systems and are presented separately for each water quality at each sampling date (Fig. 7).
During 2005, EC was not affected by water quality averaging 2.1 and 1.3 dS m−1 in June and November, respectively. During 2006 and 2007, plots irrigated with saline water had highest EC levels and plots irrigated with potable water exhibited lowest EC (Fig. 7). Electrical conductivity did not change between June 2006 and November 2007 on plots irrigated with potable water and averaged 0.7 dS m−1 for all four sampling dates. Electrical conductivity in plots irrigated with saline water was also similar between June and November for both 2006 and 2007 and reached 3.2 dS m−1 in 2006 and 2.2 dS m−1 in 2007. No clear trend could be established for EC in plots that received moderately saline irrigation water. In November of 2006 and November of 2007 moderately saline irrigation resulted in EC levels equal to potable irrigation, however in June of 2006 EC was equal to saline irrigation (Fig. 7).
Sodium content and SAR were highest in plots irrigated with saline water and lowest in those irrigated with potable water throughout the research period (Fig. 7). Sodium and SAR on potable irrigated plots did not change from June 2005 to November 2007 averaging 79 mg L−1 for Na and 2.4 for SAR. Sodium content on saline irrigated plots dropped from 570 mg L−1 (June) to 332 mg L−1 (November) in 2005 and from 814 mg L−1 (June) to 587 mg L−1 (November). No change between June and November was measured in 2007 with Na averaging 324 mg L−1 in 2007. The SAR on plots irrigated with saline water did not change between summer and late fall in 2005, reaching an average of 10.7. In 2006 SAR on saline irrigated plots dropped from 18.4 (June) to 14.8 (November) and increased from 8.1 (June) to 12.5 (November) in 2007. Moderately saline irrigated plots exhibited Na and SAR levels similar to saline irrigated plots in June and November of 2005. From June 2006 to the end of the research period in November 2007, SAR values for plots irrigated with moderately saline water fell between those of plots irrigated with potable and saline water (Fig. 7).
Results of the ANOVA revealed significant three-way interactions between irrigation systems, water quality, and date and between cultivar, irrigation systems, and date (Table 5). When quality data were reanalyzed separately for each sampling date and pooled over cultivars, the interactions of irrigation system by water quality were not significant for any of the nine sampling dates. Data were subsequently pooled over irrigation systems and water qualities and are displayed separately for each sampling date (Fig. 8). Turf quality data were also pooled over water qualities and are shown separately for each cultivar and irrigation system on each sampling date (Table 6).
|Effect||Quality||Spring green-up||Fall color retention||Quality 2007||NDVI†|
|Water quality (W)||ns||ns||ns||ns||ns|
|Sampling date (S)†||***||***||***||***||***|
|——————————————————————————————— Spring ———————————————————————————————|
|—————————————————————————————— Summer ——————————————————————————————|
|——————————————————————————————— Fall ———————————————————————————————|
Turfgrass quality was lowest in spring for all 3 yr, averaging 4.1, 4.0, and 3.9 in 2005, 2006, and 2007, respectively (Fig. 8). Low turfgrass quality in spring can be attributed to the winter dormancy of the warm-season grasses which resulted in loss of color and low quality ratings. Summer quality was rated at 6.1 in 2005, 6.7 in 2006, and 7.0 in 2007 when data was averaged over all cultivars, water qualities, and irrigation systems. Fall quality was greatest in 2006 averaging 7.0 and lowest in 2005 at 5.9 (Fig. 8). During the 3 yr of the investigative period summer and fall turfgrass quality reached or exceeded the minimum acceptable quality of 6.0 when ratings were pooled over water quality, irrigation systems, and cultivars (Fig. 8).
No clear trend could be observed in spring turf quality when irrigation systems were compared. In 2005, Sea Spray showed higher quality on drip irrigated plots than on sprinkler irrigated ones. In contrast, Riviera gave higher ratings on sprinkler irrigated plots than on drip irrigated plots in both 2005 and 2006 (Table 6). For all other grasses, type of irrigation system had no effect on spring quality. When quality data were averaged over all 3 yr, Sea Spray exhibited highest spring quality and De Anza lowest (Table 6) averaging 4.7 and 3.3, respectively. Whether or not drip irrigation affected summer quality appeared to depend on the grass used and on the length of drip irrigation had been installed. Summer quality ratings were consistently lower on drip irrigated De Anza and Riviera than on their sprinkler irrigated counterparts throughout the research period (Table 6). In 2007, 4 yr after the installation of the system, four of the nine drip irrigated grasses exhibited lower quality than the same grasses on sprinkler irrigated plots compared to only one cultivar at the beginning of the research period. Overall summer quality of (drip or sprinkler irrigated) Sea Spray and SeaDwarf was highest followed by Princess 77, whereas A138 exhibited lowest quality (Table 6). Fall quality on drip irrigated De Anza plots was also lower than on sprinkler irrigated plots, but all other grasses showed no difference in quality between the two irrigation systems. When data were averaged over the 3 yr, SeaDwarf exhibited highest quality and DT16 and A138 lowest, averaging 7.8, 5.3, and 5.0, respectively (Table 6).
Spring Green-Up and Fall Color Retention
Significant three-way interactions between cultivar, year, and irrigation systems, and between irrigation systems, water quality and sampling year affected spring green-up of nine warm-season cultivars during the 3-yr research period (Table 5). When green-up data were analyzed separately for each year, interactions between irrigation systems and water quality were not significant. When data were averaged over all sampling dates, water qualities, and irrigation systems, DT16 was the fastest to green up, followed by A138. Riviera, NuMex Sahara, SeaDwarf, and Transcontinental were slowest to green-up (Table 7). When data were averaged over all cultivars, drip irrigated plots were faster to green-up than sprinkler irrigated plots in 2005 and 2006. In 2007, no difference in speed of green-up between the two irrigation systems was observed (Table 7).
||Fall color retention
A significant interaction between sampling date and cultivars affected fall color retention. Data were subsequently analyzed separately for each sampling year. Sea Spray rated highest for fall color retention during all 3 yr, joined by Sea Dwarf and DT16 in 2 of the 3 yr. A138 and De Anza were the first grasses to go dormant as evidenced by the lowest fall color ratings in each of 3 yr of the investigative period (Table 7). Fall color retention was not affected by water quality or irrigation system. These findings are supported by those of Schiavon et al. (2011) who reported fastest spring green-up for inland saltgrass and greatest fall color retention for seashore paspalum when compared to several other warm-season grasses. Among bermudagrass cultivars, Princess 77, Riviera, and Transcontinental had greater fall color retention than NuMex Sahara. These results are in the agreement with findings of Rodgers and Baltensperger (2004), who also found better fall color retention in Princess 77 than in NuMex Sahara.
Correlations between Turf Quality, Normalized Difference Vegetation Indices, and Salinity
The correlation between visual turfgrass quality and NDVI was significant (p < 0.001) yielding a correlation coefficient of r = 0.51. When correlations were run separately for each cultivar, Sea Spray and SeaDwarf had the highest coefficients, with 0.64 and 0.59, respectively. The correlation between quality and NDVI was poorest for the three bermudagrasses, yielding 0.34 for Princess 77 and Riviera, and 0.35 for Transcontinental. Even though a significant relationship between NDVI and turf quality was revealed, the two variables were not always affected in the same way by the different treatments. For example, the interaction between irrigation, water quality, and sampling date had a highly significant (p < 0.001) effect on NDVI, but not on visual quality (Table 5).
Stepwise linear regression revealed that summer values of EC, Na, and SAR in the top 10 cm of the rootzone were not good predictors of summer quality. No significant relationship between any of the salinity parameters and turf quality could be established for saltgrasses A138, DT16, or seashore paspalum SeaDwarf and Sea Spray. While there was a significant regression between soil EC and quality of De Anza and Princess 77, the regression coefficients were very low (0.18 and 0.30), indicating that little of the variation in quality could be explained by variation in soil EC. There was also a significant relationship between EC and Na content and summer quality of Riviera and Transcontinental, but regression coefficients were equally low (0.33 and 0.28, respectively).
Irrigating turfgrasses with saline waters requires careful rootzone management to prevent detrimental levels of salt accumulation. Adding a leaching fraction to the required irrigation amount, blending irrigation water, or alternating sources of irrigation water have all been suggested as strategies to manage salinity accumulation (Ayers and Westcot, 1985; Dean et al., 1996; Schaan et al., 2003; Rhoades, 1989). In this study we added a leaching fraction (irrigation was applied at 110% ETo) and relied on natural precipitation to manage salinity in the top 20 cm of the rootzone. Consequently, seasonal changes in soil EC and Na content followed the irrigation and natural precipitation pattern, with higher values during the dry periods of the summer followed by lower values after the summer rains. During the 3 yr investigative period, EC and Na content were highest in summer of 2005 and 2006 when rainfall accumulated from March to the end of June amounted to only 21 and 16 mm, respectively. Electrical conductivity and Na content in the rootzone in summer of 2007 were lower due to 103 mm of precipitation during the same time period. While the frequency of cyclic irrigation in our study was less than that of Schaan et al. (2003), who reported no salt accumulation when multiple saline irrigation cycles were followed by a single cycle of potable water, the natural precipitation received by our test plots during the second half of the growing period was sufficient to leach salts from the rootzone. The highly permeable course-textured sandy soils of our test plots also contributed to the successful leaching of salts from the rootzone. Our findings are corroborated by Choi and Suarez-Rey (2003) who found for Tucson (AZ) that successful leaching can be accomplished in a desert environment through a typical rainy season despite a low overall annual precipitation. For most of the grasses tested, EC, Na content, or SAR values in the summer showed no significant relationship with turf quality during our 3-yr research period. During spring and summer of 2005 and 2006, EC and Na content were greatest for drip irrigated plots at a depth of 0 to 10 cm. Hoffman (1975) demonstrated that drip irrigation with saline water can result in a non-uniform distribution of salts with an accumulation of salts both at the surface and the periphery of the wetting front. In point source emitters, the water distribution into the soil follows a three-dimension infiltration pattern, which differs from the vertical, or one-dimensional infiltration pattern resulting from sprinkler irrigation (Bresler, 1977). In the case of very closely spaced emitters or porous pipes (used in our study) infiltration processes follow a two-dimensional distribution pattern and dissolved salts tend to also accumulate at the perimeter of the wetted zone, where the water content of the soil is lower (Bresler, 1977). Cote et al. (2003) showed that in drip irrigated highly permeable sand, the wetted depth is larger than the wetted radius, which results in more water below than above the emitter plane. In our study, soil salinity at 0- to 10-cm depths did not exceed 3.8 dS m−1 in 2005 or 4.2 dS m−1 in 2006 in plots irrigated with saline water. These relatively low salinity levels can be attributed to little upward movement of water in sandy soils resulting in little salt accumulation and values at which acceptable turfgrass quality could be maintained.
At soil depths of 50 to 60 cm irrigation type did not affect the three salinity parameters measured. Our results indicate that differences in water flow patterns between irrigation systems only affected salinity in depths immediately surrounding the drip lines. Salinity at depths well below the soil surface and drip lines was affected by the salinity of the irrigation water and by the amount of precipitation but not by the type of irrigation system. At soil depths of 50 to 60 cm, seasonal changes in salinity did not consistently follow the pattern observed at depths of 0 to 20 cm. For example, the decline in EC between summer and fall 2006 at depths of 0 to 20 cm (Fig. 1) reported for plots irrigated with saline water, was not observed at 50- to 60-cm depths. To better understand changes in salinity at depths >20 cm, additional sampling at depths between 20 and 50 cm would be needed. More studies are necessary to investigate changes in salinity at greater soil depths and whether or not groundwater or low lying aquifers are affected by turfgrass irrigation with saline water.
Visual quality of the warm-season turfgrasses studied was neither affected by type of irrigation system nor by the quality of the irrigation water and generally responded only to seasonal changes. Lower summer quality in 2005 may be attributed to a reduced coverage compared to 2006 or 2007. Plots were established in 2004 with some grasses having not reached full coverage by the end of the 2004 growing season (Johnson, 2007).
Among all cultivars studied, seashore paspalum exhibited the best visual quality during all three growing seasons, while saltgrass had the lowest quality (Table 6). Our findings support those of Duncan and Carrow (2000), Berndt (2007), and Lee et al. (2005) who reported a high salinity tolerance in seashore paspalum and consequently a high turf quality in saline soils. Low quality ratings for inland saltgrass could not be attributed to salt stress, as water quality had no effect on turf performance in our study (Table 5). Moreover, the salt tolerance of saltgrasses has been demonstrated in past studies (Pasternak et al., 1993; Marcum et al., 2005). Both studies used dry matter yield or relative live shoot and root weight as indicators of salt tolerance but not visual quality as applied in our study. Saltgrass exhibits a low stand density and light green color regardless of the level of salinity applied (Pessarakli et al., 2009), which resulted in a lower visual quality compared to other grasses used in our study. Schiavon et al. (2011) also reported low turf quality on inland saltgrass plots when compared to other warm-season turfgrasses. In spite of a low visual quality during summer and fall, inland saltgrass showed higher quality in spring due to an early spring green up (Table 7).
The salinity tolerance of zoysiagrass has been ranked as similar to that of seashore paspalum (Duncan et al., 2009; Harivandi et al., 2008) or in the range of bermudagrass (Carrow and Duncan, 1998); but a broad range exists among varieties (Marcum, 1999b). Qian et al. (2000) identified De Anza as the cultivar with the best ability to concentrate and exclude Na+ from the shoots, resulting in greater tolerance of high salinity levels than other cultivars. In our study, mean summer and fall quality of De Anza was lower than most other grasses tested (Table 6), but rated higher for sprinkler than drip irrigated plots. Riviera bermudagrass was the only bermudagrass cultivar that exhibited higher summer quality under sprinkler irrigation than under drip irrigation. Irrigation system had no effect on summer quality for all other bermudagrasses. Our findings are in part supported by Gibeault et al. (1985) who found drip irrigated zoysiagrass and seashore paspalum lower in quality than sprinkler irrigated and no effect of irrigation system on bermudagrass quality. More research is needed to understand how differences in canopy temperature between sprinkler and drip irrigation affect summer turf quality of warm-season grasses. Sprinkler irrigation has been shown to cool the canopy and may provide a more favorable microclimate for grasses that are less heat tolerant which could result in higher turf quality.
Among the four cultivars of bermudagrass, Princess 77 exhibited highest summer and fall quality (Table 6). In a comparison of salinity tolerances of 35 bermudagrasses, Marcum and Pessarakli (2006) reported similar EC thresholds for 50% growth reduction in Princess 77 and Riviera. These results, coupled with our findings that neither EC nor Na content accurately predicted summer quality of Princess 77 and Riviera suggest that factors other than irrigation water quality may be responsible for the differences in turf quality we observed.
When comparing the effect of irrigation systems on turf quality, seven out of nine grasses showed no difference in quality during the first 2 yr and eight out of nine grasses did not differ in fall quality for all 3 yr. These findings are supported by Choi and Suarez-Rey (2003), who reported that visual quality of bermudagrass was not affected by type of irrigation system used when recycled water was applied. Devitt and Miller (1988) have demonstrated that bermudagrass can be grown with acceptable quality using irrigation water with EC levels as high as 6.0 dS m−1. In summer of 2007, four of the nine grasses showed reduced quality on drip irrigated plots compared to sprinkler irrigated ones. The reasons for this drop in quality remain unclear and did not lead to a reduction in fall quality. Further investigations are necessary to explore whether or not short-term clogging occurs in emitterless porous pipes and affects water application and turf quality. Contrary to our findings, Schiavon et al. (2011) reported a steady and high performance of subsurface drip irrigated warm-season turfgrasses over a 4-yr period, when potable water was applied by means of emitters from point sources.
One of the limitations of growing warm-season turfgrasses in the transition zone is the long dormancy period during which turfgrasses have reduced or no color for up to 5 mo. Any treatment that could reduce this dormancy period would help improve the acceptability of warm-season grasses in transition zone climates. Water quality did not affect spring green-up but in 2005 and 2006 drip irrigated plots showed earlier green-up than sprinkler irrigated plots (Table 7). Faster green up of turfgrasses under drip irrigation could be a result of higher night canopy temperatures due to a lack of cooling from irrigation water applied by aboveground sprinkler heads. However, additional research is needed to investigate potential differences in canopy temperature between the two irrigation systems.
The significant but weak correlation we observed between visual turfgrass quality and NDVI was consistent with findings of Ghali (2011), Haendel and Wissemeier (2008), and Schiavon et al. (2011). Our results show a wide spread of NDVI values for each visual quality rating value, which has also been reported by Bunderson et al. (2009), Haendel and Wissemeier (2008), and Schiavon et al. (2011). Ghali (2011) suggested that such a weak correlation is the result of comparing discrete (quality ratings) to continuous variables (spectral reflectance values). Schiavon et al. (2011) concluded that the weak correlation between visual quality and NDVI could be due to differences in color and canopy structure between species that are detected by spectral reflectance but are not as noticeable when visually assessing plots. If NDVI measurements are to replace visual and subjective quality ratings, more research is needed to investigate this variability.
Our results indicate that most of the warm-season grasses included in this study can be maintained at an acceptable quality level when irrigated with saline water from a subsurface drip system. Salinity levels in our irrigation water were higher than those found in recycled water currently used in the Southwest to irrigate lawn and turf areas and long-term exposure to salinity levels used in our study are considered deleterious to plant growth and soil structure. Nevertheless, our results indicate that over the course of the 3-yr study warm-season turfgrasses maintained acceptable quality and were not affected by these soil salinities when these high levels were reached in a cyclic pattern followed by leaching. To determine the long-term viability of using saline waters for irrigation, more research is needed to assess the ability of soils and plants to withstand continued salt accumulation, but to also determine any detrimental effects on aquifers and groundwater.