Abstract
Mangrove forests are considered to be a highly productive ecosystem, but they are also generally nitrogen (N)-limited. Thus, soil N2 fixation can be important for the stability of both mangrove ecosystem functions and upland N supply. This study evaluates the N2 fixation activity and composition of relevant microorganisms in two coastal mangrove forests—the Guandu mangrove in an upstream estuary and the Bali mangrove in a downstream estuary—using the acetylene reduction method, real-time polymerase chain reaction, and next-generation sequencing. The results demonstrated that ambient nitrogenase activity was higher in downstream mangrove forests (13.2–15.6 nmol h−1 g−1 soil) than in upstream mangrove forests (0.2–1.4 nmol h−1 g−1 soil). However, both the maximum potential nitrogenase activity and nitrogenase gene (nifH gene) copy number were found to be higher in the upstream than in the downstream mangrove forests, implying that the nitrogenase activity and diazotrophic abundance may not necessarily be positively correlated. In addition, amended MoO4 (which inhibits the activity of sulfate-reducing bacteria in N2-fixation) yielded low nitrogenase activity, and sulfate-reducing bacteria made up 20–50% of the relative diazotrophic abundance in the mangrove forests, indicating that these bacteria might be the major active diazotrophs in this environment.
Highlights
Coastal ecosystems have long been considered to have high primary production and biodiversity, but they are generally limited in nutrients such as nitrogen (N) and phosphorus (P) [1,2]
The functional nifH gene in diazotrophs was used to examine diazotrophic communities in mangrove soils [15,17]. The results of these analyses demonstrated that diazotrophs such as Azotobacter, Azospirillum, and Vibrio were dominant in mangrove soils [18], while others have found genera such as Desulfuromonas and Pseudomonas to be the most important N2-fixing microorganisms in this system [19]
With the amended glucose solution, the maximum potential activities increased 10–40 and 3–4-fold in the Guandu and Bali mangrove soils, respectively, compared to their ambient potential activities, and the rates decreased with increasing soil depth (Figure 3)
Summary
Coastal ecosystems have long been considered to have high primary production and biodiversity, but they are generally limited in nutrients such as nitrogen (N) and phosphorus (P) [1,2]. Studies have demonstrated that the microbial N2 fixation rate can be high in coastal ecosystems. The mangrove forest is an important coastal ecosystems in tropical and sub-tropical regions and occupies more than 16.4 million hectares worldwide [4]. It provides many ecosystem services, such as wildlife habitats, blue carbon (C) storage, and storm water dissipation [5]. Studies have estimated that microbial N2 fixation may support 40%–60% of the N demand of the mangrove ecosystems [6]. Microbial N2 fixation rates can vary widely from 0 to 10.1 mmol N m−2 d−1, depending on the mangrove forest’s geographic location [7,8,9,10]
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