Responses of Salt Marsh Plant Rhizosphere Diazotroph Assemblages to Drought

Drought has many consequences in the tidally dominated southeastern Spartina sp. salt marshes of the US; emergent vegetation dieback, changes in sediment chemistry and obvious changes in the landscape. These coastal systems tend to be highly productive, yet many salt marshes are also nitrogen limited and depend on plant associated diazotrophs as their source of ‘new’ nitrogen. A 4-year study of the rhizosphere diazotroph assemblages associated with 5 distinct plant zones in one such salt marsh included a period of greatly restricted tidal inundation and precipitation, as well as two incidences of drought (June-July 2004, and May 2007). DGGE of nifH PCR amplicons from rhizosphere samples, principal components analysis of the resulting banding patterns, and unconstrained ordination analysis of taxonomic data and environmental parameters were conducted. Diazotroph assemblages were organized into 5 distinct groups (R2 = 0.41; p value <0.001), whose presence varied with the environmental conditions of the marsh. Diazotroph assemblage group presence differed during and after the drought event, indicating that persistent diazotrophs maintain populations that provide new nitrogen for vegetation during the periods of drought.