DOI: 10.15146/r3k59z
title: Abu Dhabi Blue Carbon project

description: Coastal ecosystems produce and sequester significant amounts of carbon
 (‘blue carbon’), which has been well documented in humid and semi-humid
 regions of temperate and tropical climates but less so in arid regions
 where mangroves, marshes, and seagrasses exist near the limit of their
 tolerance for extreme temperature and salinity. To better understand these
 unique systems, we measured whole-ecosystem carbon stocks (above- and
 belowground biomass and soil) in 58 sites across the United Arab Emirates
 in natural and planted mangroves, salt marshes, seagrass beds, microbial
 mats, and coastal sabkha (inter- and supratidal unvegetated salt flats)
 and were funded by the Abu Dhabi Global Environmental Data Initiative.
description: Plant carbon for mature mangrove trees were measured following
 methodology from Kauffman and Donato (2012). Due to their smaller
 structure, all trees with stems &gt; 3 cm diameter at breast height
 (DBH; 1.3 m in height) within the 7 m plot were measured instead of
 restricting measurement to trees with DBH &gt; 10 cm. Standing dead
 trees and downed woody debris were found at some Gulf of Oman sites and
 pools were quantified appropriately (Kauffman and Donato 2012). In the
 planted mangrove sites, five 2-m radius plots were established at 10-m
 intervals along a 40-m transect. When stands contained individuals
 &lt; 1.3 m tall, we measured the crown diameter and main stem diameter
 at 30-50 cm in height. In the 3, 5, and 10 year old planted mangrove sites
 in Abu al Abyad, trees were planted in an evenly-spaced grid; therefore,
 the plant density was calculated by measuring the average plant spacing
 and main stem and crown diameter of 50-75 trees.Salt
 marsh transect length and plot spacing were the same as with the mature
 mangroves, although the plot radius ranged from 1-4 m depending on plant
 density. The height and elliptical crown area (perpendicular crown widths
 centered on the canopy) were measured on every plant rooted in each
 plot.Biomass for A. marina and A. macrostachyum were
 calculated using allometric equations. Global tree carbon percentages of
 48 and 39% for above- and belowground biomass, respectively, were applied
 (Kauffman and Donato 2012). To examine differences in average annual
 carbon sequestered in planted mangrove trees, we divided total biomass for
 each stand by the number of years since plantation establishment. We
 developed allometric equations for A. macrostachyum aboveground biomass,
 as none previously had been published. Twenty-four plants were collected
 from three sites and measured for crown dimensions and succulent woody
 tissue fresh weight. A subsample of each tissue type from every plant was
 weighed fresh and dry (constant weight at 50°C) to calculate a wet-to-dry
 mass conversion factor for the entire plant. Simple linear regression of
 natural log-transformed oven-dry biomass and plant volume (height x
 elliptical crown area) were calculated, producing two different
 relationships depending on plant size. Tissue samples were analyzed for
 percent carbon and nitrogen; carbon content of woody (n = 12) and
 succulent (n = 10) tissue averaged 45.5 and 34.0%, respectively (SE ± 0.7%
 for both). We used the proportion of 52% woody tissue reported in Neves et
 al. (2010) to calculate carbon content for aboveground biomass as 40.3 ±
 1.4%C. The reported root to shoot ratio of A. macrostachyum measured in
 Portugal (Neves et al. 2010) is likely a conservative estimate of root
 biomass, as soil cores were taken to a depth of 15 cm instead of 20-30 cm
 as in other studies (reviewed in Curcó et al.
 2002).Soil Carbon PoolsAt mangrove
 and salt marsh plots, undisturbed soil samples were collected following
 methodology from Kauffman and Donato (2012) using a 1 m-long gouge auger
 with an open-face, semi-cylindrical chamber of 5.1 cm radius. Soils were
 cored to 3 m or until coarse marine sands or coral rubble representing the
 parent material was encountered. The soil core was divided into depth
 intervals of 0-15, 15-30, 30-50, 50-100, and &gt;100 cm, or until
 refusal. Subsamples collected from center of each interval were analyzed
 for bulk density (dry mass per unit volume) and carbon concentration
 (organic and inorganic). If encountered, unique soil layers were sampled
 separately. The same soil sampling methodology was used within microbial
 mats and coastal sabkha; the number of plots sampled per transect varied
 from 3-6 plots spaced at 20 m intervals along a transect. We determined
 soil carbon stocks following methods outlined in Fourqurean et al.
 (2012a), which are designed to account for soils containing carbonates.
 A variety of soil biogeochemical measurements, soil
 respiration, elevation, and tidal data were collected in selected
 mangroves, salt marsh, microbial mats, and sabkha sampled in 2013;
 mangroves sampled in 2014 were not sampled. Redox potential (Eh) was
 estimated from five replicate platinum-tipped electrodes inserted 10 cm
 into the soil for a period of 1-20 minutes and corrected for the potential
 of the calomel reference electrode by adding 244 mV (Megonigal and
 Rabenhorst 2013). At sites that had a shallow water table, soil pore-water
 was collected from corer boreholes at 5-10 cm below the surface and
 analyzed by the standard methods described in Keller et al. (2009).
 Salinity was determined either by refractometer (values &lt; 160) or
 calculated from [Cl-] (values &gt; 160). pH was measured in the field
 with a portable electrode. Porewater dissolved methane (CH4) was measured
 by headspace equilibration following Keller et al. (2009) and stored in
 evacuated Exetainer vials until analysis by Varian 450-GC gas
 chromatography. Porewater [SO42-] and [Cl-] were determined by Dionex
 ICS-2000 ion chromatography on filter-sterilized (0.22 µm), HCl-acidified
 samples, and the sulfate depletion ratio was calculated per Keller et al.
 (2009). Gas and filtered porewater samples were stored for four weeks
 before analysis, which is well within the capacity of the storage methods
 used. We quantified instantaneous CO2 gas exchange rates as a simple index
 of activity to assist with comparisons across the ecosystems in this
 study. Soil surface CO2 emissions were measured with a LICOR 6400 soil
 respiration analyzer, with a range of 2-18 measurements per site,
 depending on time spent at each site and the rate of soil respiration;
 more measurements were possible at higher respiration rates.