Determination of C and N Concentrations
To determine TN and TC concentrations, dried soil samples were ground to powder in a mortar to pass through a 100-mesh sieve. C and N analyses were performed on a FlashEA 1112 Series NC Analyzer(Italy). Organic C concentrations were also determined using the NC Analyzer after inorganic C as CO2 was removed by adding 1:1 HCl and ovendried to a constant weight.Soil NO3-N concentration was determined by KCl extraction colorimetry of fresh soil samples,whereas NH4-N concentrations were determined by indophenol blue colorimetry on KCl extraction(Liu 1996). Soil grain size was analyzed using a particle size analyzer (Mastersizer 2000, Malvern Instruments, England). Salinity was measured using a Metler SevenEasy conductivity meter.Statistical Analyses Three-way ANOVA was used to test the effects of burrow or burrow mimic size, duration of high tide(days since the first day of high tide) and habitat type (Phragmites, Spartina marshes and mudflats) on the amounts of soil excavated or deposited, deposition efficiency and soil net transport to the ground surface. In this study the soil net transported was calculated as the excavated soil minus deposited soil, collected per day. Burrow mimic trapping, that is soil deposition, is defined as the process in which surface soil and detritus are selectively deposited into burrow mimics through the interaction of burrow mimic openings and tidal water, and burrow mimic deposition is characterized by high organic content, fine grain size and low-density sediments that are easily moved by water flow and transported into crab burrow mimics. Two-way ANOVA was used to test the effects of habitat type and soil source (background surface soil, background subsurface soil, soil excavated or deposited into burrow mimics) on soil
RESULTS
Crab Excavation and Burrow Mimic Deposition of Soil Four parameters were used to characterize the soil excavating–depositing processes driven by crabs,that is, excavation, deposition, deposition efficiency,and net transport. The deposition efficiency was the amount of deposited soil per unit volume of burrow; and the net transport was the difference between the amount of soil excavated by crabs and that deposited into burrow. Habitat type, duration of high tide, and burrow or burrow mimic diameter all had significant effects on the four parameters. Soils excavated, deposited into burrow mimics, and net transported to the surface increased with the increasing diameter of burrows or burrow mimics, but deposition efficiency decreased with increasing burrow or burrow mimic diameter (Figure 1). The four parameters all increased with increasing duration of high tide (Figure 1). Because crab excavation exceeded burrow mimic deposition, excavating–depositing processes resulted in net transport of soil to the marsh surface. Furthermore, the values of these parameters for mudflats were significantly higher than those for Phragmites and Spartina marshes. The soil excavated by crabs was positively correlated with burrow diameter (Figure 2A) and soil deposited into burrow mimics (Figure 2B) in both mudflats and Phragmites and Spartina marshes, and the regression slopes for mudflats were significantly greater than those for Phragmites and Spartina marshes.