Farming carbon: The link between saltwater intrusion and carbon storage in coastal agricultural fields

Abstract

As sea levels continue to rise, coastal agroecosystems have become more vulnerable to saltwater intrusion (SWI). Saltwater intrusion is the landward movement of sea salts, which can alter carbon (C) storage. Transitioning agricultural fields have the potential to become C sinks as SWI advances inland and turns farms to marshes. The objectives of our study were to (1) quantify the size of C pools along a salinity transect; (2) determine how soil C is occluded through levels of physical protection in soil aggregates; (3) examine relationships between indicators of SWI and soil C measures in coastal farm fields. We collected soils (to a depth of ~140 cm) along a transect from the edge of a salt-damaged field (high salinity and frequently inundated) to the center (low salinity and drier). We measured bulk soil C pools and the amount of C stored in large macroaggregates, small macroaggregates, microaggregates, and silt+clay size classes. Soil C pools were largest in the edge of field and ditch bank soils compared to the center of the field (P = 0.01). In ditch bank soils, most of the C was stored in large macroaggregates; however, in the center of the field, most C was stored in silt+clays. We suggest that frequent wetting events may lead to increased C storage along the edges of farm fields both as a result of suppressed decomposition rates and increased clay flocculation leading to macroaggregate formation. Further, a vegetation shift from annual, shallow-rooted crops to perennial, deeply rooted marsh grasses along field edges likely contributed to higher soil C levels as well as increased aggregation. An improved understanding of soil C dynamics can help inform management strategies that support farm and environmental wellbeing as our coastlines change.