Nitrogen distribution and forms in sediment cores of Daya Bay, China

Abstract

Sediment was the hotspot of microbial nitrogen sink, affecting the biogeochemical cycle of nitrogen in aquatic ecosystems. However, the distribution and forms of nitrogen in coastal bay subsurface sediments are poorly understood, limiting our knowledge about nitrogen distribution characteristics in subsurface sediments and the implications for nitrogen sinks. Here, the spatial distribution of different forms of nitrogen was examined in the sediment cores of Daya Bay (DYB). The results showed that exchangeable nitrogen (Nex) with an average concentration of 13.14 ± 5.99 mg kg−1, mainly consisted of NO3− and NH4+, occupied only 1.88 ± 0.97% in the total nitrogen (Ntot) in the sediment cores. As the dominant nitrogen form, organic nitrogen (Norg) accounted for 50.52-94.29% in Ntot and displayed a similar decreasing trend with Ntot in sediment cores. No evident change was discovered in the vertical content of fixed ammonia (Nfix) in sediment cores, and the mean concentration of Nfix in the four sediment cores was 97.14 ± 29.08 mg kg−1. These results suggested that Nfix and Norg were the stable recalcitrant nitrogen stocks in the sediment cores, and the labile nitrogen input from anthropogenic activities was transformed by microbial processes, thus reducing the nitrogen pollution efficiently in DYB.