Major Processes Shaping Mangroves as Inorganic Nitrogen Sources or Sinks: Insights From a Multidisciplinary Study

Abstract

Mangrove wetlands support numerous ecosystem services including nutrient cycling and carbon sequestration and storage (blue carbon). Mangrove sediments may serve as a nitrogen source or sink to the hydrosphere and atmosphere at both regional and global scales. However, major mechanisms controlling the connection between the mangrove and the adjacent tidal creek (nitrogen cycling in sediments and outfluxing) remain unclear. A multidisciplinary study based on intensive investigation, incorporating detailed sediment profiling, multi‐isotopes analysis, sediment incubation, and microbiological identification was conducted in the Yunxiao mangrove reserve and Zhangjiang Estuary in southeast China. Here we show that mineralization and denitrification are major processes shaping mangroves as an ammonium source and nitrate sink. Enrichment of ammonium in pore water (10–40 cm in depth) likely resulted from strong ammonification with limited nitrification in the anaerobic sediments. Denitrification played a key role in nitrate removal from pore waters while producing N2O and N2. Decreasing δ15N‐N2O and associated δ15N:δ18O ratio suggested that most of the outgassing N2O was derived from incomplete denitrification in sediment pore water. Overall, there was a net export of dissolved inorganic nitrogen from mangroves toward the estuary in winter and spring but a net import to mangroves in summer and fall, mainly driven by tidal pumping with seasonal variation of bio‐uptake. These findings highlight the role of mangrove wetlands in regulating nutrient status and carbon budget in coastal areas.

A synthesis of methane emissions from shallow vegetated coastal ecosystems

“…Given the above, plant biomass should, at least in theory, be positively correlated with CH 4 flux. This hypothesis is supported by several individual studies from salt marsh (e.g., Cheng et al, 2007; Hirota et al, 2007; Li, Wang, et al, 2018; Wang et al, 2019) and mangrove ecosystems (Alongi et al, 2008). There is currently no evidence of a similar relationship in seagrass ecosystems.…”

Assessing nutrient dynamics in mangrove porewater and adjacent tidal creek using nitrate dual-stable isotopes: A new approach to challenge the Outwelling Hypothesis?

“…Prior studies assessing nutrient dynamics in other mangrove tidal creeks reported similar patterns (i.e. discharge of N-NH 4 + ), with the only difference being that P-PO 4 3− porewater discharge was sometimes measured (Dittmar, 1999;Gleeson et al, 2013;Maher et al, 2016;Ovalle et al, 1990;Tait et al, 2017;Wang et al, 2019). The tidal creek N-NH 4 + concentrations at low tide (7.9 μM [0; 50.7]) were significantly lower than the porewater concentrations at the mudflat stand (21.1 μM [0.5; 162.4]) and not significantly higher than those of the two vegetation stands (3.0 μM [1.7; 5.3] for Rhizophora; 4.3 μM [1.0; 6.0] for Avicennia) (Table 1).…”

“…Although our study focuses on dissolved inorganic nutrients as they account for the preferential form consumed by primary producers, some studies have shown that dissolved organic nutrients could be assimilated, particularly in nutrient-limited conditions (Glibert et al, 2004;Vonk et al, 2008). Dissolved organic nitrogen and dissolved organic phosphorus concentrations are within the same order of magnitude of the dissolved inorganic fraction in undisturbed mangrove tidal creeks (Dittmar and Lara, 2001b;Gleeson et al, 2013;Tait et al, 2017), and of lower importance in sites with upstream activities generating nutrients input (Wang et al, 2019). Therefore, it is possible that organic nutrients also contribute, to some extent, to the aquatic productivity in mangroves before mineralization.…”

“…Some studies that investigated nutrient exchange in mangroves have been based on complete tidal cycle time series, using concentration variations as a proxy for tidal exchange rather than direct nutrient fluxes (e.g. Leopold et al, 2017;Wang et al, 2019). This underlines the challenge of accounting for elements that are quickly changing in forms and concentrations.…”

“…We tested these hypotheses by combining the use of dual nitrate stable isotopes (δ 15 N NO3 & δ 18 O NO3 ) with particulate δ 13 C and δ 15 N. The dual nitrate stable isotope approach has contributed to identifying nitrate dynamics in various environments including estuaries and coastal waters via the discrimination of natural abundance isotope ratios as indicators of organic matter origin, production, consumption and degradation processes (Archana et al, 2018;Wankel et al, 2009;Xue et al, 2014). However, it has not yet been used directly to document nitrogen cycling in complex intertidal mangrove systems except in one disturbed subtropical mangrove tidal creek (Wang et al, 2019). Conversely, stable isotopes (δ 13 C and δ 15 N) of particulate organic matter have been applied in mangroves to help understand ecosystem functioning including delineation of organic matter and nutrient sources in different pools and transformation processes leading to isotopic fractionation (Bouillon et al, 2008).…”