Spatial distribution of soil organic carbon in an Irish salt marsh (Rogerstown Estuary)

Salt marshes are globally widespread, found on low-lying coastal shores, and are highly effective at long-term carbon storage; thus, they are vital for climate change impact mitigation. Accurate carbon stock estimation requires an understanding of local-scale spatial variability of carbon storage and the facilitating processes. Few studies investigate the cumulative impact of controlling factors on within-site carbon distribution. This study utilises 60 cores from a salt marsh in Turvey Nature Reserve (Rogerstown Estuary), on the Irish east coast, to investigate spatial variability in soil organic carbon (SOC) content, alongside bio-sedimentary, and environmental factors. Mean carbon density (CD) was 11.1 ± 4.2 kg m^-3 at 10-cm depth, ranging from 5.2 to 22 kg m^-3 (423% increase) across the marsh. We recommend that to obtain measurements across the full range of the site, for small sample sizes (n < 20), random sampling should be used (mean difference between the site-wide CD and 'subsample CD' ranged from 0.04 (n = 10) to 0.29 (n = 5) kg m^-3) and marsh edge clustering should be avoided. These results provide the first ever systematic record of local-scale (within ~ 800 m²) SOC and CD variability within an Irish east coast salt marsh and the variation of known influencing factors (including sedimentary and environmental). We also present the first study to systematically provide guidance on capturing marsh-wide SOC and CD most effectively based on limited sampling. The outputs help constrain uncertainties around scaled-up carbon accumulation estimates for regional, national and international inventories.