Nitrogen Fixation Occurring in Sediments: Contribution to the Nitrogen Budget of Lake Taihu, China

Abstract

Nitrogen fixation (N2 fixation) in benthic sediment contributes a certain amount of bioavailable nitrogen (N) to aquatic systems. However, data on sediment N2 fixation have been poorly documented and the contribution of sediment N2 fixation to the N budget in lakes has rarely been studied. In this study, N2 fixation rates in sediments of Lake Taihu were measured periodically using the traditional acetylene reduction assay. Results showed that the highest sediment N2 fixation rate occurred in winter in Meiliang Bay, with a value of 258 nmol N·kg−1·hr−1, and the lowest rates occurred in all seasons at some sites, with values near zero. East Taihu Bay, Meiliang Bay, the southwestern coastal area, and Zhushan Bay were found to be hotspots for sediment N2 fixation activity. The annual N2 fixation rate and volume in sediments of Lake Taihu were 0.083 g N·m−2·year−1 and 195 t N, respectively. The N inputs via fixation were much lower than those from river inflow, sediment release, and atmospheric deposition. This study reveals that N2 fixation in sediments contributes only a minor part to the overall N budget, while it compensates for about 1.8% of sediment N2 loss via denitrification. The minor contribution of sediment N2 fixation to the overall N budget in Lake Taihu may be closely linked to sediment characteristics.