Program

Nitrogen, as essential nutrients of the growth of the marine organisms in the ocean，plays an important role in biogeochemical cycle. Nitrification, the oxidation of ammonium (NH₄⁺) to nitrite (NO₂^-) and to nitrate (NO₃^-)，is a significant part of the marine nitrogen cycle.  In the last few decades, there has been a growing attention paid on the nitrification rate. However, the vertical distribution and rates of NH₄⁺ and NO₂^- oxidation in South China Sea (SCS) are not clear so far. We performed shipboard incubations with ¹⁵N tracers to characterize the depth distribution of nitrification in the northern SCS. Our finding show that the rates of NH₄⁺ and NO₂^- oxidation are extremely low above the depth of chlorophyll-a maxima layer. With the increase of depth, both of them increased significantly, moreover, the rate of NO₂^- increased more significantly, indicating that the NH₄⁺ and NO₂^- oxidation in surface water are inhibited by the light and nitrite oxidation bacteria is more sensitive than ammonia oxidation microorganisms. The rates of NH₄⁺ and NO₂^- oxidation showed distinct subsurface maxima, and positively correlated with concentration, respectively. The rate of NO₂^- oxidation is higher than NH₄⁺ oxidation in the coupling layer of nitrite concentration maxima and the rate of NH₄⁺ oxidation maxima, suggesting that there is other nitrite source addition except for NH₄⁺ oxidation. We deduced that the phytoplankton release is a potential source due to the high biomass of phytoplankton in this layer. Because of the lack of substrates and decrease of nitrifier, the NH₄⁺ and NO₂^- oxidation rates decreased rapidly with the depth increased below the the euphotic layer. Generally, NO₂^- oxidation rate is several times higher than NH₄⁺ oxidation rate. The results support the notion that NH₄⁺ oxidation is the limiting step of nitrification.