Formation of thatch and mat layers is one of the major problems in management of modern turfgrass golf greens. Thatch is a layer of organic matter that accumulates between the soil and green turfgrass and contains both living and dead plant tissues intermingled tightly with each other. Thatch consists of stolons, rhizomes, roots, crown tissue, leaf sheaths, and blades (Engel, 1954; Roberts and Bredakis, 1960). The mat layer is generally below the thatch layer and is distinguished from thatch by the presence of sand or soil intermingled with thatch as a result of cultural practices like core aeration and topdressing (McCarty, 2005). A small amount of organic matter reduces surface hardness, moderates soil temperature extremes, increases the resilience, and improves wear tolerance of the turfgrass surface (Beard, 1973); however, excessive thatch and mat layers are undesirable in turfgrass.
High organic matter accumulation in the form of thatch-mat causes problems such as decreased movement of oxygen through the thatch or mat zone, decreased saturated hydraulic conductivity, and excessive water retention (Carrow, 2003; Hartwiger, 2004; McCarty et al., 2007). These primary problems may further lead to secondary problems like wet wilt, soft surface, black layer, limited rooting, and extra- and intracellular freezing damage (Beard, 1973; Carrow, 2004; O’Brien and Hartwiger, 2003). Although structured organic matter, present in live underground plant tissues, is thought to have no adverse effect on the soil physical properties, rapid root death that results in dead gelatinous organic matter swells in the presence of water during decomposition and plugs the soil macropores (air-filled pores), causing low oxygen levels in the root zones (Carrow, 2004; O’Brien and Hartwiger, 2003). Excessive accumulation of organic matter causes anaerobic conditions, further reducing the rate of organic matter decomposition (McCoy, 1992). Grasses also generally produce more adventitious roots (surface roots) during anaerobic conditions, again further increasing organic matter content (Carrow, 2004).
Thatch management techniques such as core aeration, vertical mowing, grooming, and topdressing are currently the most effective strategies to manage thatch-mat buildup but have shown contrasting results (Barton et al., 2009; Carrow et al., 1987; Dunn et al., 1981; McCarty et al., 2005; McWhirter and Ward, 1976; Weston and Dunn, 1985; White and Dickens, 1984). These cultural practices are intensive in terms of cost, energy, and labor and have adverse effects on turfgrass quality (Barton et al., 2009; Landreth et al., 2008; McCarty et al., 2007). Several non-destructive thatch control studies using glucose, cellulase solutions (Ledeboer and Skogley, 1967), and commercial inocula containing various microorganisms were ineffective in reducing the amount of thatch (McCarty et al., 2005; Murdoch and Barr, 1976). Reduction in cellulose content and total oxidizable organic matter of bermudagrass (Cynodon dactylon L.) and centipedegrass (Eremochloa ophiuroides) (Sartain and Volk, 1984) and weight loss of bermudagrass pellets, St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze], and zoysiagrass (Zoysia japonica Stued., ‘Meyer’) stolons (Martin and Dale, 1980) was observed when inoculated with different wood-decaying fungi under controlled greenhouse and laboratory conditions. However, field inoculation experiments on bermudagrass showed no thatch degradation (Martin and Dale, 1980).
The formation of the thatch-mat layer is the result of a greater rate of organic matter accumulation than degradation (Beard, 1973). Most microbial degradation mechanisms are restricted by the presence of lignin, a plant cell wall constituent. The slow decomposition of soil lignin has long been recognized (Kirk and Farrell, 1987). Lignin limits the accessibility of microbial degraders to more biodegradable plant materials such as cellulose and hemicelluloses (Ledeboer and Skogley, 1967). Lignin is formed in plants by oxidative coupling of monolignols of three primary hydroxycinnamyl alcohols: p-coumaryl, coniferyl, and sinapyl alcohols. The corresponding lignin monomers are known as p-hydroxy phenyl, guaiacyl, and syringyl units, respectively (Wong, 2009). Lignification is achieved by crosslinking of monomers with a growing polymer through polymer–polymer coupling. Based on the random coupling theory, several models of lignin molecular structure have been proposed but these models do not imply any particular sequence of monomeric units in the lignin macromolecule (Chen and Sarkanen, 2003; Davin and Lewis, 2003).
Natural degradation of lignin occurs in the environment by certain white-rot fungi, which solubilize and mineralize lignin with the help of lignolytic enzymes (Kirk et al., 1975, 1976). White-rot fungi preferentially attack lignin more than cellulose or hemicellulose in the wood tissue (Blanchette, 1984; Mester et al., 2004). This process of selective delignification exposes cellulosic materials for further bacterial degradation in the environment (Otjen and Blanchette, 1987). The presence of naturally occurring (guaiacol) and synthetic (1-hydroxybenzotriazole) chemicals, known as mediators, have been shown to enhance the activity of the lignolytic enzyme laccase (Kang et al., 2002; Roper et al., 1995). Because lignin content in thatch layer is higher than that of live grass tissues, the thatch layer in turfgrass species with high lignin content is more resistant to microbial decomposition (Beard, 1973; Ledeboer and Skogley, 1967).
We hypothesize that the use of lignin-degrading enzymes such as fungal laccases can effectively reduce the rate of thatch layer accumulation in golf greens. The objectives of our study were: 1) to determine if degradation of soil organic matter can be enhanced by laccase application; 2) to determine if addition of guaiacol enhances laccase efficacy in organic matter decomposition; and 3) to determine if application of laccase enzyme and guaiacol adversely affect turf quality.
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