PNNL

Abstract

Coastal soils along the terrestrial aquatic interface (TAI) are exposed to surface flooding of varying frequency, duration and salinity. These hydrological disturbances impact biogeochemical processing of carbon (C) and other nutrients in TAI soils, but the underlying mechanisms remain poorly understood. This is partially due to historical focus on upland or wetland endmembers, but not their interface. Here, our objectives were to determine how coastal TAI soils varied (a) at regional scales and (b) across transects from upland forest to wetland. We generated and analyzed data from 20 soil analytes describing carbon and other macronutrients, micronutrients, mineralogy, and physicochemical parameters across six sites in two contrasting regions — Western Lake Erie, a freshwater lacustrine system, and Chesapeake Bay, a saltwater estuarine system. We found that regional-scale patterns of soil biogeochemistry differed strongly, with Erie soils exhibiting clear gradients between upland, transition, and wetland zones, whereas in the Chesapeake Bay, transitional soils were more similar to upland soils. Spatial variability among soils was strongly driven by Na, TS, and Cl in both regions. However, Ca and Fe were strong drivers among the Erie soils, versus Al and P in the Chesapeake. These patterns reflect underlying geologic and environmental differences between the two regions. Notably, we found that coastal transitions may not be mid points between upland and wetland TAI endmembers with several analytes showing non-monotonic trends along the gradient. The non-monotonic trend of C, P, and Fe along transect positions make these three analytes key candidates for further analysis to improve understanding and model predictions in these systems. These results suggest that soil responses may act as precursors to larger ecosystem disturbance, and that our data-driven approach generates new mechanistic understanding of ecosystem dynamics.