DOI: 10.5285/72962722-bd56-4b3e-b397-90ceec6c821e
title: Seasonal denitrification rates and vegetation dynamics from Warton Bank and Old Hall saltmarshes, England, September 2024-January 2025

description: In two saltmarshes, Old Hall in the Blackwater (southeast England) and Warton Bank in the Ribble (northwest England), we took vegetation and soil samples every six to eight weeks from August/September 2024 to January 2025 to characterise seasonal denitrification and vegetation dynamics. The data includes, vegetation species and diversity; seawater and porewater samples (NO3-, NH4+, PO43-); seawater ion concentrations (Cl-, Na2+, K+, Br-, Mg3+ and Ca2+); moisture and organic matter content in the sediment. 

A key ecosystem service in coastal systems is the remediation of nutrient pollution through sediment burial, vegetative uptake and microbial processing. Denitrification is a facultative anaerobic process where microbial activity transforms nitrate (NO3-), which in high concentrations can be environmentally harmful, into the environmentally benign dinitrogen gas (N2). Denitrification's magnitude is considered particularly important in saltmarsh systems compared to other habitats, although an intermediate product, nitrous oxide (N2O), can also be given off and contribute to climate change.
description: In two saltmarshes, Old Hall in the Blackwater (southeast England) and Warton Bank in the Ribble (northwest England), we sampled for vegetation and soil every six to eight weeks from August/September 2024 to January 2025 to characterise seasonal denitrification and vegetation dynamics. 

1 - Field methods
We sampled three distinct elevation-related vegetation zones, aiming for pioneer to low, low-mid and upper saltmarsh communities to the extent that was practicable. 
In each vegetation zone identified in the field, we surveyed six 1 x 1 m quadrats at random. Once the quadrat had been placed we recorded percentage cover of bare ground, litter, and live vegetation. In general, vegetation was identified to genus level although clearly identifiable species were recorded separately. For those species with greater than or equal to 15 % cover, height was also recorded, on five randomly selected individuals per species. 
Within the 1 x 1m quadrats, we chose 2 locations of approximately the same aboveground plant community composition and biomass (estimated by eye) for saltmarsh denitrification core collection. We used sediment extracts from these cores, after the denitrification assays described in the Laboratory analyses section, for characterisation of organic matter content and bulk density measures.   
We extracted porewater samples from one excavated core hole using samplers of approximately 5 cm length, inserted at three depths - 5, 10 and 15 cm. Each sampler was attached to a syringe to ensure a vacuum and draw for the porewater. Having waited at least 20 minutes, we collected individual samples, with the aim of having at least 2 ml of porewater solution per depth. 
Tide water samples were sourced from a tidal creek / main estuary channel location, approximately 1 to 2 hours either side of high water. These samples were used to characterise the nutrient environment of the saltmarsh, and we used this information when running denitrification samples through the Wetland Hydroperiod Simulator (see 2) Laboratory analyses section).

2 - Laboratory analysis
2.a - Denitrification Dynamics: The Wetland Hydroperiod Simulator Approach
Denitrification rates from the whole core samples were analysed using the custom-built Wetland Hydroperiod Simulator. To calculate actual denitrification for one sample, two cores were taken per quadrat, one to undergo acetylene (C2H2) inhibition and one to act as a control. 

2.b - Denitrification Dynamics: Gas Analysis
Gas samples were analysed by gas chromatography using a Varian model 450 gas chromatograph (GC) instrument, equipped with an electron capture detector (ECD) for N2O. 

2.c - Denitrification Dynamics: Potential Denitrification Rates were calculated using equations (see Supporting documentation)

2.d - Porewater and Seawater Samples
Seawater and porewater samples were analysed using colorimetric based methods. Nitrate (NO3-) was measured using a Vanadium reduction followed by a Griess reaction. Ammonium (NH4+) was measured using a buffered indophenol method. Phosphate (PO43-) was measured using the molybdenum blue method. Other seawater ion concentrations (Cl-, Na2+, K+, Br-, Mg3+ and Ca2+) were measured using a Metrohm 850 Professional IC. For anions, 20 ul of sample was injected onto a Metrosep A Supp 5 - 150/4.0 column using an 8mM Na2CO3, 0.2mM NaHCO3 eluent. For cations, 20 ul of sample was injected onto a Metrosep C 4 - 250/4.0 column using a 5mM HNO3/1mM Oxalic acid eluent. 

2.e - Organic Matter content and Bulk Density - The Loss on Ignition (LoI) method was employed to determine the percentage of moisture and organic matter content in the sediment. 
	- Plant data processing - The percentage of area covered by live vegetation or algae in each quadrat was added to have a "vegetated area cover". Shannon diversity index was calculated for each quadrat in the R software (R Core Team 2025) using function diversity from R package vegan (Oksanen et al. 2001).