The effect of saltwater intrusion on the spatial inaccessibility of carbon in coastal agricultural fields

As sea-levels continue to rise, coastal ecosystems are vulnerable to saltwater intrusion (SWI). Saltwater intrusion is the landward movement of sea salts that can force ecosystem shifts and change soil biogeochemistry. The storage of soil organic matter (SOM) can be disrupted by a change in plant community and microbial activity, which can be induced by SWI. Specifically, in coastal farmlands, we expect SWI to drive changes in carbon (C) storage. Transitioning agricultural fields have the potential to become a C sink as SWI advances inland because of reduced agricultural management and increased plant communities. Farmers face a tough decision - continue farming as usual or abandon their fields and let nature take over. Our objectives are to: 1) understand how C is stored in agricultural fields undergoing SWI; 2) describe the C storage potential of agricultural soils as they become more saline due to SWI. To determine the effect of SWI on soil C storage, we collected soils (to ~140 cm) along a transect from the edge of a salt-intruded field to the center. Soils were segmented by depth and separated into different aggregate size classes to determine the spatial inaccessibility of C. Total organic C pools increase dramatically across the salinity gradient (P<0.05). Significantly more C is stored at the edge of salt-damaged fields (P<0.05) and more C is found in macroaggregates (>250 μm) and microaggregates (53-250 μm). Our results show C is stabilized and protected in agricultural soil transitioning to a tidal wetland. Protecting carbon in coastal systems means we are protecting healthy coastal ecosystems that provide benefits to communities through storm protection, nursery habitats for fisheries, and recreational uses. This research is some of the first in identifying the C storage potential of these transitioning lands, in order to inform conservation strategies and agricultural subsidy policies for climate change adaptation and mitigation.