Nitrogen removal in the sediments of the Pearl River Estuary, China: Evidence from the distribution and forms of nitrogen in the sediment cores.

In this study, the spatial distribution and forms of nitrogen in sediment cores collected from the Pearl River Estuary were analyzed. Exchangeable nitrogen (Nex) comprised only a small proportion of total nitrogen (Ntot), with a mean of 3.54% in the sediment cores. NH4+ was the main form of Nex. No obvious change was observed in the vertical content of fixed ammonia (Nfix) in the sediments, and the mean Nfix in all five sediment cores was 141.23 mg·kg-1. The organic nitrogen (Norg), strongly related to organic carbon (Corg), was the main form in Ntot. The dissolved inorganic nitrogen in sediment pore water was much lower than that in estuarine water and no significant variation was observed from upstream to downstream. Our results indicated that most nitrogen deposited on surface sediments from overlying water was rapidly removed by a series of microbial processes, reducing the extent of nitrogen returning to overlying waters.