Crop Science 42:1611-1614 (2002)
© 2002 Crop Science Society of America
TURFGRASS SCIENCE
Characterization of Soil Microbial Population Dynamics in Newly Constructed Sand-Based Rootzones
Cale A. Bigelow*,a,
Daniel C. Bowmana and
Arthur G. Wollum, IIb
a Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27695-7620
b Dep. of Soil Science North Carolina State Univ., Raleigh, NC 27695-7619
* Corresponding author (Dan_Bowman{at}ncsu.edu)
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ABSTRACT
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Modern sand-based golf course putting greens are constructed for optimum soil physical properties. However, since they are sand based and synthetically prepared, it is often perceived that they support a less numerous and diverse microbial population than comparable native soils. This field study was conducted to monitor the microbial properties of five newly constructed sand-based rootzone mixtures planted to creeping bentgrass (Agrostis stolonifera var. palustris Huds. Farw.) during the first 2 yr of turfgrass establishment. Bacteria, fungi, actinomycetes, and aerobic spore forming (Bacillus spp.) populations were determined on selective media. Nitrifiers and denitrifiers were estimated by a most probable number (MPN) technique. Within the first 6 mo after seeding, bacteria exceeded 108 cfu g-1 dry soil, similar to levels recorded in a mature sand-based putting green. Bacteria were most numerous followed by actinomycetes, fungi, and Bacillus spp., respectively. Temporal changes in microbial populations were observed only in year one. The nitrogen transforming populations were numerically smaller (<104 cfu g-1 dry soil) than total bacteria but followed a similar temporal trend. Rootzone amendments had minimal effects on microbial properties but environmental factors and an actively growing turfgrass root system may have a greater influence on microbial activity.
Abbreviations: cfu, colony forming units • CEC, cation exchange capacities • MPN, most probable number
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INTRODUCTION
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MODERN GOLF GREENS are conventionally constructed with high sand contents (>85%) to minimize compaction under heavy use (USGA, 1993). Although sand confers ideal physical properties for golf rootzones, it is commonly perceived that these rootzones support a numerous microbial community than finer textured native soils because of the lower organic matter and clay contents (Alexander, 1977). Additionally, sand rootzones possess significantly less soil surface area than finer textured soils, which may limit the habitat available for soil microorganisms. An active microbial population performs many beneficial activities such as organic matter decomposition, nutrient availability and recycling, and pathogen suppression (Sylvia et al., 1997). Thus, turfgrasses grown in rootzones containing lower microbial populations may be less healthy and possibly more easily affected by some soil borne turfgrass pathogens like Pythium spp. and Rhizoctonia, resulting in an overall lower quality turfgrass (Hodges, 1990; Couch, 1995).
Mancino et al. (1993) monitored microbial properties of a 5-yr-old, sand-peat moss rootzone and found that microbial populations in the thatch of a sand-based rootzone planted to creeping bentgrass were much greater than the underlying rootzone. They reported that a mature (>5 yr old) putting green supported a relatively large microbial population (>107 cfu g-1 soil), which is similar to populations found in some native soils.
Putting greens require frequent pesticide and fertilizer applications to maintain optimum turfgrass quality. These products have caused inconsistent effects on the soil microbiology found in turfgrass soils. Early research demonstrated commonly used turfgrass fungicides generally have little direct effect on suppression of soil microorganisms (Smiley and Craven, 1979). However, they stated the relationship between fungicides and microorganisms is a complex one and other factors like acidification may have a greater effect than the actual fungicides themselves. Mazur and White (1983), studied the effect of several inorganic N sources on nitrifying and denitrifying populations in two sand-based putting green media and found that both groups were greater in sand amended with 10% (v/v) soil compared with sand amended only with peat moss. Research on a hybrid bermudagrass (Cynodon x transvaalensis Burt.) putting green demonstrated that various natural organic N sources had little effect on microbial populations (Elliott and Des Jardin, 1999). In contrast, other research has documented that the application of a natural organic N product significantly increased fungal and bacterial populations on creeping bentgrass leaves, thatch, and soil compared with several other natural organic and inorganic N sources (Liu et al., 1995).
While these data are informative, most of the research was conducted on relatively mature turfgrass systems. Thus, little documented information exists regarding microbial population dynamics during turfgrass establishment particularly in sand-based rootzones. In addition, a current trend exists to improve the physical and chemical properties of sand rootzone media with porous inorganic amendments rather than peat moss. Some of these inorganic amendments, like clinoptilolite zeolite, have very high cation exchange capacities (>220 cmolc kg-1, Ming and Mumpton, 1989) which may improve nutrient retention in sands. It is reasonable to predict that the increased nutrient status may increase microbial populations in rootzones amended with these materials. Therefore, the objective of this study was to monitor and characterize the soil microbial populations in five different sand-based rootzones amended with inorganic materials or sphagnum peat moss during the first 2 yr of establishment.
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MATERIALS AND METHODS
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This study was conducted at the North Carolina State University Turfgrass Field Laboratory, Raleigh, NC, from October 1997 through November 1999. This study was included as part of a larger study regarding mechanically induced subsurface drainage. Twelve experimental putting greens measuring 18.5 by 3.0 m were constructed in the summer of 1997. Existing native soil (Cecil sandy-loam, Fine, kaolinitic, thermic Typic Kanhapludult) was excavated and greens were constructed according to current guidelines for sand-based construction (USGA, 1993). Each large green was separated from adjacent greens by a compacted zone of native soil, approximately 3 m wide. Within each large green, subplots (3.0 by 3.7 m) were created to evaluate five synthetically prepared sand-based rootzone mixtures representing a variety of moisture and nutrient retention properties (Table 1)
. Water retention of the mixtures ranged from 0.17 to 0.20 cm3 cm-3 and cation exchange capacities (CEC) ranged from 0.8 to 2.7 cmolc kg-1 soil. A detailed description of the amendments and specific construction procedures is presented in Bigelow et al. (2001).
The site was located in full sun, resulting in rapid early morning drying of the turf canopy and consequently an ideal environment for turfgrass growth. Creeping bentgrass (‘L-93’) was seeded at 74 kg ha-1 in October 1997. Following establishment the experimental area was maintained according to standard golf green management practices, except that core cultivation was never conducted. Plots received moderate fertilization, approximately 300-160-100 kg N-P-K ha-1 yr-1 from a complete (N-P-K) fertilizer, with most of the N being in the urea and ammonium forms. Plots were mowed (cutting height, 5 mm) three to four times per week and irrigated to prevent stress. Pesticides including fungicides, insecticides and herbicides were applied on a preventative and as needed basis throughout each growing season (Table 2)
. Soil temperature, at 10 cm below the rootzone surface, was measured continuously throughout the growing season in all sand-peat plots using temperature data logging probes (Onset Computer Corp., Pocasset, MA). Three drainage treatments were arranged in a randomized complete block design and greens receiving gravity drainage only (control) were used for this monitoring experiment.
Rootzone Sampling and Analysis
Soil samples were periodically collected from the center of each plot starting immediately before seeding in October 1997. At each sampling date, eight soil cores (2.5-cm diam. by 10 cm deep) were removed from each plot. When present, the turfgrass leaves and thatch layer (which never exceeded 12 mm) were cut from each core and discarded. For each sampling date and plot, cores were pooled, placed in plastic bags, and transported on ice to the lab. Samples were sieved through a 9-mm screen to remove large roots, and then stored in the dark at 4°C for <24 h or immediately analyzed for microbial properties; all tests were completed within 5 d.
Selective media were used to enumerate a portion of the soil microorganisms present. Culturable bacteria were measured by plating aliquots of soil suspension on 0.1 strength tryptic soy agar (TSA) (Martin, 1975) while Gram-negative organisms were enumerated on TSA plates containing crystal violet (0.004%). Fluorescent Pseudomonas spp. were estimated on NPCC medium (Zuberer, 1994). Culturable fungi were estimated on peptone-glucose-acid agar (Wollum, 1982). Actinomycetes were estimated on starch casein agar (Wellington and Toth, 1994). Pasteurization of dilution tubes (10 min at 85°C) followed by plating aliquots on nutrient agar was used to enumerate Bacillus spp. (Wollum, 1982).
The microtiter MPN procedure outlined by Staley and Griffin (1981) was used to determine ammonium (NH+4) and nitrite (NO-2) oxidizers and denitrifying organisms. Aliquots (50 µL) of the 10-1 to 10-6 dilutions (oxidizers) and 10-2 to 10-7 dilutions (denitrifiers) were used to inoculate the microtiter wells, each of which contained 100 µL of broth. Inoculated microtiter plates were then incubated in the dark at a constant temperature (26°C) for 14 and 21 d for denitrifiers and oxidizers, respectively.
Population values were logarithmically transformed and all data were subjected to analysis of variance (ANOVA) by the SAS statistical package (SAS, 1996). The influence of rootzone composition on microbial properties was determined by conducting ANOVA on pooled measurements across all dates by SAS's GLM procedure. Means for date and rootzone composition effects were separated by Fisher's protected LSD and preplanned orthogonal contrasts where appropriate (Steel et al., 1997).
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RESULTS AND DISCUSSION
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Microbial Community Diversity
There were significant differences (P < 0.05) among sampling dates, rootzone mixtures, and between the microbial groups measured (Tables 3 and 4)
. Although temporal differences for each microbial group were observed, there were no clear effects that could be attributed to sampling date or rootzone mixture (Table 3). Microbial populations increased rapidly from initial values, 106, reaching >108 cfu g-1 dry soil for total bacteria, within the first 6 mo of establishment. These relatively high bacterial numbers are consistent with observations made by other researchers from a 5-yr-old sand-based putting green system (Mancino et al., 1993). The rapid increase in bacterial populations may have been due to growth of indigenous organisms or subsequent introduction via seed, irrigation, fertilizers, and/or wind dispersal. The presence of a young and actively growing turfgrass root system may have provided abundant and easily digestible substrates suitable for proliferation of these soil organisms. In 1998, there was a small but statistically significant decline in total bacteria, Gram-negative bacteria, Pseudomonas spp., and fungi during summer months (Table 3). This decline may have been caused by senescence of the existing turfgrass root system during summer months (Beard and Daniel, 1965, Huang et al., 1998; Bigelow et al., 2001), which would decrease substrate availability. This root system decline was attributed to sustained supraoptimal temperatures, unsuitable for sustaining bentgrass roots (data not shown). In the second year, however, the populations declined slightly but remained relatively constant. This second year response suggests that adequate organic substrates had accumulated in the rootzone mixtures to sustain the microorganisms.
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Table 3. Mean populations of six microbial groupings and nitrogen (N) transforming bacteria during the first 2 yr of establishment as affected by sampling date.
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Table 4. Mean populations of six microbial groupings and nitrogen (N) transforming bacteria in sand-based rootzones averaged across all sampling dates during the first 2 yr of establishment.
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In contrast to the general bacterial populations, the aerobic spore formers, or Bacillus spp., increased slowly and steadily during the first 9 mo (Table 3). However, unlike the highly dynamic general bacterial populations, once established these organisms achieved a relatively constant population with only minor fluctuations. To survive adverse environmental conditions, Bacillus spp. produce extremely drought resistant endospores. These minimal fluctuations of these organisms after establishment is not surprising and is consistent with previous observations in a native agricultural field soil (Kirchner et al., 1993).
Nitrogen Transforming Organisms
Similar to the other microbial populations, the N transforming organisms were also affected by sampling date (Table 3). The initial populations of both NH+4 and NO-2 oxidizers were relatively small, but increased rapidly to >104 within the first 7 mo after seeding. Initially, the NO-2 oxidizing bacteria were somewhat more numerous than the NH+4 oxidizers. However, 6 mo after seeding, NH+4 oxidizers outnumbered the NO-2 oxidizers and this trend continued throughout the remainder of the study.
The denitrifying populations were small, <103, relative to populations observed in some native soils (Mancino and Torello, 1986). This response may be due to the well aerated status of these coarse textured rootzones, which would limit conditions favoring denitrification. Overall, the relative populations of N transforming organisms were on the order of NH+4 oxidizers > NO-2 oxidizers > denitrifiers. Although there were significant differences in the populations of both nitrifying and denitrifying bacteria throughout the study within each rootzone mixture, there was no clear sampling date by rootzone interaction (data not shown).
The overall effect of rootzone amendments was assessed by averaging soil microbial populations across all sampling dates (Table 4). Sand amended with sphagnum peat supported a significantly lower population of NO-2 oxidizers and denitrifiers compared with the other rootzone mixtures and significantly lower NH+4 oxidizers compared with unamended quartz sand (Table 4). This is surprising since sphagnum peat amended rootzones might be expected to sustain larger microbial populations, at least initially because of the presence of an existing organic matter source. It is possible that this response is due partly to a pH effect. Generally, amending sand with sphagnum peat moss results in a relatively low pH. Thus, the lower pH of this peat amended rootzone (pH = 5.3) may have resulted in a more abundant total fungal population becaue of less competition from the bacterial populations (Morton, 1997).
In summary, this study demonstrates that resident or introduced microorganisms increase rapidly in sand-based rootzones during initial turfgrass establishment. Additionally, the microbial diversity and N-transforming organism values demonstrate that these rootzones are capable of sustaining a large and diverse microbial community. Although some fluctuations in populations were observed, soil bacteria reached large numbers and became relatively stable within the first year. Sand rootzone amendments did not appreciably affect microbial populations. Therefore, it seems that microbial population dynamics in these sand-based rootzone mixtures may largely be influenced by the developing turfgrass root system, independent of any rootzone amendment. Consequently, if environmental conditions are not favorable for the root system, the indigenous microbial population may decline, at least temporarily. Future studies focusing on the long-term population dynamics in these rootzone mixtures and the effects of abiotic environmental factors, rootmass cycling, soil aeration status, fertilization practices and changes in rootzone physical properties because of turfgrass–rootzone maturation are warranted. By understanding the factors affecting the rootzone microbial community, perhaps a quantitative measure of the optimum microbial status for sand-based putting green rootzones could be defined. Once defined, strategies to promote certain beneficial groups within the ecosystem could be accomplished.
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NOTES
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Mention of a trademark or a proprietary product does not constitute a guarantee or warranty of the product by N.C. State University or the authors and does not imply its approval to the exclusion of other products that may also be suitable.
Received for publication April 7, 2001.
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REFERENCES
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ABSTRACT
INTRODUCTION
