Macrophytes and crabs affect nitrogen transformations in salt marshes of the Yangtze River Estuary

Nitrogen (N) 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 N 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 and gross nitrate consumption were determined using an ex situ incubation approach. Gross N mineralization was significantly lower in vegetated marshes than in the bare flat, probably because dissolved organic carbon (the main energy source for microbes) was lower in vegetated marshes. Less dissolved organic carbon in vegetated marshes may be attributable to high microbial demand for labile carbon relative to the supply of labile carbon in the rhizosphere. The presence of crabs increased the GAI:GNM ratio, indicating that crabs may increase the soil microbial demand for ammonium. Ammonium immobilization competed with nitrification for ammonium, and limited nitrification in crab-present plots. Furthermore, the lower nitrate production by nitrifies in crab-present plots resulted in lower rates of nitrate consumption. These results highlight the role of labile carbon and N in mediating the effects of macrophyte and soil fauna on N transformations.