Above- to Belowground Vegetation Biomass Ratio in Temperate North-East Atlantic Saltmarshes Increases Strongly with Soil Nitrogen Gradient

Saltmarshes provide a broad range of ecosystem services, but have been degraded globally and are further threatened by sea-level rise. Changes in biomass partitioning between above- and belowground components of plant community biomass as a consequence of nutrient enrichment can impact those services profoundly. However, the evidence for such impacts is largely confined to North America, leaving a gap in global understanding of coastal ecosystem resilience. Using field surveys across 15 saltmarshes along the south and east coasts of Ireland, we tested whether aboveground (AG) and belowground (BG) plant biomass allocation across all plant assemblages combined and within individual plant assemblages is related to soil nutrients. AG biomass and AG/BG biomass ratio were positively related, and BG biomass was negatively related to NOx-, both across all assemblages combined and within individual assemblages. AG biomass and AG/BG ratio were also positively related to NH4+ within individual assemblages, but not across all of them combined. The relationships between biomass and labile P were weaker, and their direction varied across the whole P gradient and among individual assemblages. These findings demonstrate broad, community-wide links between biomass allocation and soil nutrients in saltmarshes. The up to 6.6-fold increase in AG/BG biomass ratio along the N gradient, coupled with differential responses among functionally contrasting plants, could have significant implications for multiple saltmarsh functions such as energy fixation for consumers, wave attenuation, sediment binding and carbon storage. Improved understanding of the underlying causalities would help promoting saltmarsh resilience.