Fragmented mangroves are generally ignored in N2O flux studies. Here we report observations over the course of a year from the Mangalavanam coastal wetland in Southern India. The wetland is a fragmented mangrove stand close to a large urban centre with high anthropogenic nitrogen inputs. The study found the wetland was a net source of N2O to the atmosphere with fluxes ranging between 17.5 to 117.9 µmol m−2 day−1 which equated to high N2O saturations of between 697 and 1794%. The average dissolved inorganic nitrogen inputs (80.1 ± 18.1 µmol L−1) and N2O emissions (59.2 ± 30.0 µmol m−2 day−1) were highest during the monsoon season when the rainfall and associated river water inputs and terrestrial runoff were highest. The variation in N2O dynamics was shown to be driven by the changes in rainfall, water column depth, salinity, dissolved oxygen, carbon, and substrate nitrogen. The study suggests that fragmented/minor mangrove ecosystems subject to high human nutrient inputs may be a significant component of the global N2O budget.

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It is imperative to understand the carbon (C) and nitrogen (N) pools in tropical mangrove systems considering the diminished C storage potential and increasing nutrient loading in a global context due to rising anthropogenic disturbances. Here, we report that the Mangalavanam Coastal Wetland (MCW), a tropical mangrove stand, is a C-rich environment, with high sediment total organic carbon (70.35 ± 23.84 mg/g) and the δ 13 C (−26.56 ± 0.65‰) signatures indicate that the major source of carbon in the study area is from mangrove litter. The total nitrogen concentration in the sediment (3.78 ± 0.18 mg/g) was high despite the low N environment of mangrove ecosystems where the mangrove leaf litter was a major contributor to C. The sediment δ 15 N (7.70 ± 0.23‰) signatures indicated that the influence of anthropogenic N discharges and the contribution of marine algae were the primary cause of N enrichment. The mean δ 13 C and δ 15 N signatures of the mature mangrove leaves of the MCW were −28.66 ± 1.04‰ and 5.93 ± 0.65‰, respectively. The mangrove dweller crab Parasesarma plicatum was observed to have enriched δ 13 C (−23.78 ± 0.33‰) and δ 15 N (6.54 ± 0.35‰) signatures, indicative of a mixed dietary source. This study provides the impetus for ongoing conservation efforts to limit the loss of carbon storage and protect against the impact of anthropogenic nitrogen loadings on lowering the carbon and nitrogen emission potential for this system, which can have implications for other tropical mangrove systems as well.