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a Forest Ecology and Management, Univ. of Wisconsin-Madison, 1630 Linden Dr., Madison, WI 53706
b Environmental Horticulture, Univ. of Florida, Fort Lauderdale Research and Education Center, 3205 College Ave., Fort Lauderdale, FL 33314
c Soil and Water Science, Univ. of Florida, Everglades Research and Education Center, 3200 East Palm Beach Road, Belle Glade, FL 33430
d Biological Sciences, Florida Atlantic Univ., 2912 College Ave., Davie, FL 33314
* Corresponding author (ericksonj{at}si.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: MS, mixed-species • SA, St. Augustinegrass
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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While soil and climate conditions in southern Florida are relatively favorable for leaching of applied fertilizers, landscape vegetation and associated maintenance regimes have also been shown to influence nutrient leaching (Hipp et al., 1993; Broschat, 1995; Duwig et al., 2000; Erickson et al., 2001; Brye et al., 2002). St. Augustinegrass, a moderate fertility warm-season turfgrass, is currently the predominant vegetation in Florida residential landscapes. Nitrogen is the nutrient required in the greatest quantity for St. Augustinegrass growth and vigor, yet blended fertilizers are most frequently used on residential landscapes resulting in substantial P and K additions to home lawns (EPA, 1999). As an alternative to extensive turfgrass lawns, the Florida Yards and Neighborhoods (FYN) Program (Best, 1994) has promulgated the use of other landscape materials, such as native woody perennials that might conceivably reduce nutrient leaching in addition to other potential environmental benefits like wildlife habitat and reduced irrigation inputs (Garner et al., 1996). Fertilizer requirements for alternative mixed-species landscapes are variable, but P and K remain important components of any fertilization protocol. These contrasting landscape models have fundamentally different growth habits and maintenance requirements resulting from species functional composition and, thus, potentially differing implications for nutrient leaching from residential land use.
Data on fertilizer N leaching from turfgrass and other residential landscape models are quite extensive (e.g., Snyder et al., 1980; Starr and Deroo, 1981; Snyder et al., 1984; Petrovic, 1990; Broschat, 1995; Erickson et al., 2001). In contrast, relatively few authors have examined P and K leaching losses from residential landscapes (Kelling and Peterson, 1975; Cisar et al., 1990; Gross et al., 1990; Hipp et al., 1993); consequently, our knowledge of P and K leaching is limited compared with N leaching. In fact, we are unaware of any studies in the literature that present P and K leaching losses from residential landscapes (St. Augustinegrass) under subtropical climate conditions in South Florida despite the known importance of P on neighboring Everglades ecosystem structure and function (Davis, 1994; Noe et al., 2001). Ecological consequences of K leaching are not well known, but K availability has been shown to affect species competitiveness (Tilman et al., 1999) and growth responses to soil moisture availability (Sangakkara et al., 2001).
The objectives of this comparative study were therefore to examine temporal dynamics in soil water drainage and P and K leaching losses at the lawn-scale level from contrasting landscape models grown on a sandy soil. Given different fertilizer and water requirements, we tested the hypothesis that a mixed-species landscape model consisting of mostly native plants could reduce P and K leaching compared with a conventional turfgrass monoculture. To address this hypothesis, we designed and implemented a field experiment whereby we collected quantitative leaching data spanning 4 yr from the two contrasting residential landscape models.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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In 1998, eight 9.5- by 5.0-m research plots were constructed with crushed limestone as a foundation that provided a 10% slope for the plots. A 6-mm polyvinyl plastic, which contained the root-zone mix and provided hydrological isolation for each plot was set on the limestone foundation. This allowed for the collection of both surface runoff and subsurface drainage from two contrasting landscape models. A more detailed description of the research facility can be found in Erickson et al. (2001). The 0.75-m deep root-zone mix placed in each of the plots was a mined siliceous sand (Boynton Sand and Gravel, Palm Beach County, Florida) used in landscape development. The sand was analyzed for physical properties by PHD Laboratory, Lake Worth, FL, following USGA published methods (Hummel, 1993). Root-zone mix pH was measured on duplicate samples from each plot in March, 2000.
A completely randomized design with four replications was used for the study. Landscape model was the factor and contained two treatment levels. One treatment consisted of a ‘Floratam’ St. Augustinegrass monoculture (SA) that was routinely fertilized and irrigated when needed to avoid wilt, while the other treatment was a mixed-species (MS) arrangement of ornamental ground covers, woody shrubs, and trees that was fertilized during establishment and irrigated when needed to avoid or recover from wilt. The St. Augustinegrass was installed as a sod and maintained at a height of 7.5 cm. The clippings were removed for the first 6 mo of the study and mulched in situ for the remainder of the study. The alternative mixed-species landscape model consisted of 12 commercially available species: 25 Liriope muscari (Dcne.) Bailey ‘Evergreen Giant’, three Lantana montevedensis (K. Spreng) Briq., three Tripsacum floridana L. ‘Dwarf’, 15 Zamia pumila L., seven Ilex vomitoria Ait. ‘Schellings Dwarf’, five Hamelia patens Jacq. ‘Compacta’, four Galphimia glauca Cav., three Myrcianthes fragrans (Sw.) McVaugh, three Podocarpus macrophyllus (Thunb.) Sweet, one Myrica cerifera (L.) Small, one Tabebuia heterophylla (DC.) Britt., and one Acoelorrhaphe wrightii (Griseb. & H. Wendl.) H. Wendl. Ex Becc. Species were planted together in clumps with smaller vegetation types interspersed among the larger trees and shrubs. Over 50% of the species used were considered native to Florida. The plants were installed from pots with the soil and intact root systems. Species were selected with consideration for functional diversity, hardiness, availability, and aesthetic appeal. For more detailed information regarding the mixed-species vegetation see Table 1 in Erickson et al. (2001). Eucalyptus mulch (The Bushel Stop, Pompano Beach, FL) was uniformly applied and maintained at a depth of 7.5 cm on the mixed-species landscape. Plot establishment (planting) for both treatments occurred at the end of December in 1998.
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Precipitation, Irrigation, and Drainage
Precipitation was recorded daily at the research center and summed monthly. In addition, 30-yr monthly precipitation records from the NCDC for Fort Lauderdale, Florida (26°06' N, 80°12' W) were also used. Irrigation volume applied to each treatment was recorded by a flow meter installed in the irrigation system.
Drainage was collected from a slotted drainage pipe installed below the root zone mix at the lower edge of each plot. It was monitored initially by random manual measurements on each plot and subsequently (from June 1999) by tipping bucket flow gauges (Unidata America-Model 6406H, Lake Oswego, OR) that continuously monitored drainage volume. Drainage was summed hourly and recorded by a datalogger (CR10X, Campbell Scientific, Logan, UT). Periodically, drainage data were lost as a result of tipping bucket malfunctions associated with debris or wire damage. When such malfunctions occurred drainage was then interpolated on the basis of data values before and after the down time or on random point measurements obtained during the interruption. During one 2-wk period in July 2001, the datalogger was down and drainage data were lost on all plots, so we assumed that the root-zone mix was saturated and drainage for all plots was set equal to received precipitation. Surface runoff data were also collected at the onset of the study; however, no significant surface runoff was observed during the first 12 mo of the study despite intense precipitation events. As a result, surface runoff was not considered for the remainder of the study.
Phosphorus and Potassium Leaching
Drainage samples were collected periodically in high-density polyethylene bottles for both P and K analyses. Sampling occurred frequently (multiple times daily) at the beginning of the study and following fertilization and significant rainfall events. Sampling was less frequent (thrice weekly to biweekly) later in the study and during periods of low drainage. The water samples were immediately acidified with sulfuric acid on collection and refrigerated at 4°C until analysis. The samples were analyzed for total P and K by atomic emission spectroscopy (Varian ICP-OES, Varian Inc., Palo Alto, California). Data for P and K concentration were lost for the 4-mo period from October 1999 to January 2000 due to equipment failure. Because of frequent sampling during the first 8 mo, P and K leaching losses (load) were calculated as the product of the daily average nutrient concentration and the measured daily drainage for each plot. For the remainder of the data collection period (37 mo), leaching losses for each plot were determined as the product of the biweekly average nutrient concentration and the respective biweekly drainage.
To explore further the role of landscape vegetation on P and K leaching, estimates of root weight density were determined early in the year (February or March) in 2000, 2001, 2002, and 2004. Roots were extracted from 15-cm soil cores (measured below the thatch and mulch layers) taken from each plot (two cores per plot on the MS treatment, one core on the SA plots). Roots were washed, oven dried to a constant mass at 70°C, weighed, ashed in a muffle furnace (550°C), and reweighed. Root weights are presented as oven dry weight minus ash weight, thus minimizing any potential error associated with residual sand on the washed roots.
Statistical Analyses
Mean treatment drainage and P and K leaching losses are presented from February 1999 to April 2003 for the months where data were available. For statistical comparison, nutrient leaching losses were summed on a plot-by-plot basis over 6-mo intervals (approximately corresponding to annual wet and dry seasons). When data were not available for the entire season, they are presented for the months available. A one-way analysis of variance (ANOVA) was used on the seasonal sums to compare P and K leaching losses between the contrasting landscape models (SAS Institute, 1989).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Precipitation, Irrigation, and Drainage
Annual rainfall varied from the 30-yr mean by –6 to +14% throughout the study with 1860 mm, 1530 mm, 1730 mm, and 1700 mm received in 1999, 2000, 2001, and 2002, respectively (National Climatic Data Center, 2004). During the approximately 45 mo for which P and K leaching data were available, a total of 5665 mm of rainfall was received at the research facility. In addition to rainfall, 1360 mm of supplemental irrigation was applied to the MS treatment, while 1270 mm was used on the SA treatment (Fig. 1)
. The intensity and frequency of rainfall events were greater in the wet season; hence, the majority of irrigation was applied in the dry season, especially at the beginning of the experiment when the vegetation was not well rooted.
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Abundant precipitation and a relatively coarse-textured substrate resulted in substantial quantities of drainage throughout the study (Fig. 2) . Mirroring seasonal patterns of rainfall, drainage tended to be higher during the wet season and lower in the dry season. In total, over the 45-mo data collection period, cumulative drainage did not differ significantly between the treatments (P > 0.05), averaging 3437 mm across the MS plots and 3802 mm across the SA plots (Fig. 3) . Temporally, seasonal drainage was significantly greater on the MS treatment following planting, but was less than the SA treatment throughout the rest of the study, especially in the dry seasons (Table 2). In fact, mean seasonal drainage was significantly (P < 0.1) less on the MS treatment compared with the SA treatment in four of seven seasonal periods since April 2000. Given that seasonal precipitation inputs were generally similar and drainage was often significantly less on the MS treatment indicated that water use was greater by the MS vegetation compared with the St. Augustinegrass and/or evaporation of water from the mulched soil surface was comparatively greater on the MS treatment. This trend appeared to be strengthening over time with growth and expansion of the MS vegetation as larger differences in drainage were seen at the end of the study in both the wet and dry seasons. Thus, contrary to our initial expectations that the mixed-species vegetation could reduce water use and irrigation inputs (Garner et al., 1996), the data supported greater water requirements for the MS landscape model compared with the SA landscape model over time.
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While leaching losses were especially severe early in the study, substantial leaching losses occurred on both landscape models throughout the data collection period. During the time that both landscapes were routinely fertilized, greater nutrient leaching was generally seen from the MS landscape compared with the SA landscape model (Table 2). Of the seven seasonal periods through May 2002, P and K leaching was significantly greater (P 
0.05) on the MS treatment four times, while the quantity of P or K leached from the SA treatment was never significantly greater than the MS treatment. However, when fertilization of the MS vegetation ceased (last fertilization was in December 2001), P and K leaching declined dramatically, especially during the wet season, illustrating the importance of the interaction between fertilization and precipitation on nutrient leaching. In fact, the quantity of P leached was significantly less (P < 0.05) compared with the SA landscape in the 2002 wet season and also in the subsequent dry season. Likewise, K leaching was significantly less on the MS treatment in the 2002 wet season, but not in the subsequent dry season.
Similar to P and K leaching losses, mean biweekly P concentrations in the drainage (from April 2000) varied with landscape model (Fig. 2), averaging 0.9 mg L–1 (maximum of 2.4 mg L–1) on the MS treatment and 0.3 mg L–1 (maximum of 1.6 mg L–1) on the SA treatment. Biweekly K concentrations averaged 15.6 mg L–1 (maximum of 41.1 mg L–1) on the MS treatment and averaged 7.2 mg L–1 (maximum of 21.5 mg L–1) on the SA treatment. While mean nutrient concentrations of P and K in the MS percolate were approximately double that of the SA treatment, both treatments were well above values of approximately 50 µg P L–1 and 3.0 mg K L–1 measured in this study and reported by others for precipitation in southern Florida (e.g., Haag et al., 1996), reflecting substantial P and K contributions from the landscapes. Given P and K concentrations in the percolate from either landscape model, potential ecological impacts (e.g., shifts in species composition) on neighboring hydrologically linked systems, such as the Florida Everglades, remain a concern because of their sensitivity to nutrient additions (McCormick and Stevenson, 1998; Noe et al., 2001).
Factors affecting nutrient leaching are numerous with complicated interactions, but include timing and intensity of precipitation or irrigation, timing, type and quantity of fertilization, and landscape vegetation (Kelling and Peterson, 1975; Petrovic, 1990; Broschat, 1995). We have already illustrated the importance of precipitation and fertilization, but the implications of vegetation for P and K leaching in this study are less clear. Nutrient uptake by plants has been related to root size and density (spatial distribution), soil nutrient concentration, and mobility of the nutrient in the soil (e.g., Caldwell et al., 1985; Clarkson, 1985; Mengel and Steffens, 1985). Ancillary data collected on fine root weight density (upper 15 cm of the root-zone mix) throughout the study revealed a significantly greater (P < 0.01) density of roots in the SA landscape (467 g m–2) compared with the MS landscape (235 g m–2). This evidence in context with fertilizer regime and precipitation inputs may help to further understand the observed P and K leaching. In April 2000, inputs of P and K were increased (by both frequency and intensity) on the MS landscape in response to stunted growth and chlorotic foliage, which resulted in an increase in soil P and K concentrations and presumably an increase in the P and K taken up by the vegetation. The increase in soil P and K concentrations combined with a relatively sparse root distribution on the MS landscape during this period, however, led to greater P and K leaching in absolute terms compared with the SA landscape. Despite overall lower quantities of P and K leaching during this interval, the SA landscape showed similar and in some cases greater P and K leaching as a percentage of applied fertilizer P and K. The performance of the SA landscape with respect to P and K was fairly poor compared with its N uptake ability (Erickson et al., 2001). We concluded, on the basis of the observed leaching data, that comparatively low P and K inputs were adequate for the SA vegetation given its dense rooting, which resulted in relatively low P and K leaching. On the other hand, to supply the MS vegetation demand for P and K, higher inputs of fertilizer P and K were required to compensate for its comparatively sparse rooting, which led to greater P and K leaching.
At the end of the study, we saw a marked improvement in the performance of the MS landscape with respect to P and K leaching because it no longer received fertilizer additions, thereby reducing soil nutrient concentrations and nutrient availability for the vegetation. Assuming that the nutrient capital and its internal cycling were sufficient at the end of the study to sustain the MS vegetation without further fertilizer additions, one might reasonably expect the MS landscape model to reduce P and K leaching substantially over time. However, some regimen of future P and K fertilizer additions will probably be necessary for growth or even to maintain aesthetic vegetation, as nutrients are lost from the MS landscape through leaching and other means over time. As a result, the long-term implications for adopting either landscape model on P and K leaching remain in question.
In this study, we tested the hypothesis that a landscape model including many native trees and shrubs could reduce P and K leaching from residential land use compared with a more traditional turfgrass monoculture. The combination of intense precipitation, a coarse-textured substrate, and routinely fertilized residential landscapes posed a relatively high risk for P and K leaching regardless of landscape model. Still, we found that landscape model differentially affected the actual P and K leaching observed. During the period of data collection following planting, the quantity of P and K leached from MS landscape was greater, even though less P and K was applied, and much of the MS vegetation began to show signs of nutrient deficiencies (e.g., stunted growth and chlorotic and curled leaves). In April 2000, an increase in the frequency and quantity of P and K applied to the MS vegetation was implemented to promote growth and address nutrient deficiencies in the MS vegetation. Subsequent data collected during this period of routine fertilizations on both landscapes showed that the SA landscape model still reduced absolute leaching losses of P and K compared with the MS model, probably because of a greater root weight density and lower P and K inputs. In contrast, after fertilization was no longer applied to the MS landscape, it leached comparatively less P and K. Thus, minimizing environmental impacts associated with residential land use is complex. Strategies for minimizing P and K leaching do not appear to be as simple as including or excluding vegetation types but rather consideration of species, species assemblage, fertilizer protocols during establishment and post-establishment, and water use is needed.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication April 6, 2004.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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b), and total porosity represent means (± s.e.) that are based on samples collected from each plot before plant installation (n = 8).
