Epiphytic Nitrogen Fixation Associated with Standing Dead Shoots of Smooth Cordgrass, Spartina alterniflora

Abstract

N2 fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N2 fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C2H4 cm−2 h−1 to 14 nmol C2H4 cm−2 h−1, while in the transplanted marsh mean AR rates ranged from 1 nmol C2H4 cm−2 h−1 to 33 nmol C2H4 cm−2 h−1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N2 fixation is estimated to be 2.6 g N m−2 yr−1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m−2 yr−1. These estimates should be added to previous estimates of N2 fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.