Dissolved inorganic nitrogen in a tropical estuary in Malaysia: transport and transformation

Shan Jiang,Jing Zhang,Edwin Sien Aun Sia,Aazani Mujahid,Moritz Müller,Guosen Zhang,Mohd Fakharuddin Muhamad,Kun Zhu,Tim Rixen,Ying Wu,Faddrine Holt Ajon Jang,Jie Jin

doi:10.5194/bg-16-2821-2019

Abstract

Abstract. Dissolved inorganic nitrogen (DIN), including nitrate, nitrite and ammonium, frequently acts as the limitation for primary productivity. Our study focused on the transport and transformation of DIN in a tropical estuary, i.e., the Rajang River estuary, in Borneo, Malaysia. Three cruises were conducted in August 2016 and February–March and September 2017, covering both dry and wet seasons. Before entering the coastal delta, decomposition of the terrestrial organic matter and the subsequent soil leaching was assumed to be the main source of DIN in the river water. In the estuary, decomposition of dissolved organic nitrogen was an additional DIN source, which markedly increased DIN concentrations in August 2016 (dry season). In the wet season (February 2017), ammonium concentrations showed a relatively conservative distribution during the mixing, and the nitrate addition was weak. La Niña events induced high precipitations and discharge rates, decreased reaction intensities of ammonification and nitrification. Hence similar distribution patterns of DIN species in the estuary were found in September 2017 (end of the dry season). The magnitude of riverine DIN flux varied between 77.2 and 101.5 t N d−1, which might be an important support for the coastal primary productivity.

Highlights

Nitrogen (N) is an essential element for life
In the light of the seasonality, the mean oxygen undersaturation degree (OUD) at the fresh river water and the estuary in three tributaries in August 2016 was significantly higher than the values from the remaining two cruises
The Dissolved inorganic nitrogen (DIN) concentration in the river water varied between seasons in the Rajang, mainly resulting from the decomposition of terrestrial organic matter

Summary

Introduction

Nitrogen (N) is an essential element for life. The concentration of N may significantly influence species composition and diversity in terrestrial, freshwater and ocean ecosystems (Vitousek et al, 1997). Dissolved inorganic nitrogen (DIN), including nitrate (NO−3 ), ammonium (NH+4 ) and nitrite (NO−2 ), can be assimilated by terrestrial plants, algae and bacterial communities (Seitzinger et al, 2002). Due to the high solubility, DIN can be transported among ecosystems as a part of the hydrologic cycle (Galloway et al, 2004). DIN is assumed to be the most active component in the N cycle, and the transport of DIN among ecosystems is a hotspot in biogeochemical research on a global scale (Gruber and Galloway, 2008)

Methods

Results

Discussion

Conclusion