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As significant biotic components of aquatic and terrestrial ecosystems, soil animals are multipurpose workers, for example, consumers, litter decomposers, and habitat modifiers, which passively and/or actively disturb the substrate.Bioturbation is defined as biological reworking of soils and sediments through animal activities like burrowing and feeding (Meysman and others 2006). Animal bioturbation and its ecological roles in shaping soil ecosystem processes were first appreciated by Darwin (1881) and were described in great detail in his last book On the Formation ofVegetable Mounds through the Action of Worms with Observations on their Habits. Invertebrate feeding on resources in the sediments evidently affects key processes, such as organic carbon mineralization (Otani and others 2010), nutrient dynamics(Karlson and others 2007; McHenga and Tsuchiya 2008), sulfur and iron cycling (Gribsholt and others 2003; Nielsen and others 2003), sediment texture modification and particle mixing (Paarlberg and others 2005). The altered soil characteristics might further impact microbial activities (Bertics and Ziebis 2009), zooplankton recruitment (Gyllstro¨mand others 2008) and other biotic components(Reinsel 2004; Canepuccia and others 2008), or the spatial heterogeneity might generate niches for smaller organisms (van Nugteren and others 2009).Furthermore, long term functions of bioturbation play important roles in digenetic reaction, and facilitate the development of pristine ecosystems(Herringshaw and Solan 2008). One type of dominant bioturbations in coastal ecosystems is crab burrowing which can transport sediments and modify sediment texture, accelerating ecosystem nutrient cycling.Salt marshes in coastal wetlands are one of themost productive natural ecosystems (Mitsch and Gosselink 1993) and provide considerable ecosystem services for human society (Costanza and others 1997). The decomposition and transformation of primary products from salt marshes provide nutrients for the growth of marsh plants and for export to adjacent ecosystems, subsidizing oceanic productivity (Odum 1980). Sediment biogeochemical processes play important roles in the metabolism and nutrient cycling of salt marshes(Webb and Eyre 2004).Burrowing sesarmid (Grapsidae) and fiddler crabs (Ocypodidae) are the most important macroinvertebrates in many salt marshes (Emmerson1994; Montague 1982). They are often present in large numbers; and their burrowing activities can directly break and transport sediments,decrease the hardness of the soil (Bortolus and Iribarne 1999; Botto and Iribarne 2000; Botto and others 2005), modify microtopography, and increase the density of coarse particles on the soil surface (Warren and Underwood 1986).Crab burrowing also affects soil chemistry and associated microbial processes, increases soil oxygenation,and alters pore water salinity (Fanjul and others 2007). Burrowing crabs significantly affect belowground processes that can impact marsh plants (Bertness 1985; Iribarne and others 1997; Bortolus and Iribarne 1999; Smith and others 2009) in at least three ways. First, crab burrowing increases the passage of liquid and gasbetween the soil and environment (that is, increase drainage), increasing soil oxidation (Katz 1980; Daleo and Iribarne 2009; Weissberger and others 2009) and the decomposition rate of organic debris (Lee 1998; Reinsel 2004; Fanjul andothers 2007). Second, crab burrows can selectively trap sediments that have high organic matter concentrations, finer grain size and low density through the interactions of the burrow opening with tidal water, which can facilitateorganic matter decomposition, which can in turn increase nutrient availability and thus, promote their growth (Iribarne and others 1997, 2000;Botto and others 2006). Third, crab excavation transports soil and nutrients from deep layers to the marsh surface (Fanjul and others 2007, 2008),which might accelerate the turnover of soil and nutrients. Soil properties and plant assemblage characteristics influenced by crab excavation and burrow deposition can in turn affect burrowing processes (Neira and others 2006). Few attempts,however, have been made to examine theseprocesses (but see Botto and Iribarne 2000; Gutierrez and others 2006), and the interactive effects of plant communities and crab burrowing remain largely unexplored. Understanding the relative importance of these processes in controlling energy flow and nutrient transformation will enhance our understanding of the ecological roles of crabs in salt marshes.The objective of this study was to examine the roles of crab burrowing and burrow trapping in sediment turnover, and vertical C and N distributions in a Chinese salt marsh. We specifically examined the following questions. What amounts of soil and C, N nutrients does a crab community vertically transport? How different are the soils of different sources (excavated, deposited, and background) in soil physical and chem
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