Nitrogen cycling is important in determining ecosystem primary productivity and the succession of plant communities in coastal marshes. In order to examine the effects of biotic disturbances (i.e. vegetation change and crab burrowing) on nitrogen transformations, we conducted a field experiment in which crabs were either removed or left intact in three marsh types (bare flat, Phragmites australis marsh and Spartina alterniflora marsh) at Chongming Dongtan in the Yangtze River Estuary. The potential rates of soil gross nitrogen mineralization (GNM), gross ammonium immobilization (GAI), gross nitrification (GN) and gross nitrate consumption (GNC) were determined using ex situ incubation.
Results/Conclusions
GNM was significantly lower in vegetated marshes than in the bare flat, probably because dissolved organic carbon (DOC, the main energy source for microbes) was lower in vegetated marshes. Lower DOC in vegetated marshes may be attributable to high microbial demand for labile carbon relative to the supply of DOC in the rhizosphere. The presence of crabs increased the GAI:GNM ratio, indicating that crabs may increase the soil microbial demand for NH4-N. GAI competed with GN for ammonium, and limited the GN in crab-present plots. Furthermore, the lower nitrate production by nitrifies in crab-present plots resulted in lower rates of GNC. These results highlight the role of labile carbon and nitrogen in mediating the effects of macrophyte and soil fauna on nitrogen transformations.