We compared soil properties (bulk density, particle size, organic C, total N and P) and processes (accretion, C, N, P accumulation and denitrification) in two tidal freshwater floodplain forests (tidal forests) of the Altamaha River, an alluvial river (AR) that drains the piedmont region of the southeastern US, and in two tidal forests the Satilla River, a blackwater river (BR) that drains the coastal plain.

Soils of the BR tidal forest were sandier (>80%) and contained less organic matter (10% OM) than tidal forest soils of the AR (50% sand, 40% OM). Bulk density (0-30 cm) was greater and organic C and total N & P were less in BR than AR soils. Rates of soil accretion using ¹³⁷Cs did not differ among tidal forests of BR and AR (0.9-1.1 mm/yr). Soil organic C (39-42 g/m²/yr), N (2.2-2.7 g/m²/yr), P (0.15-0.23 g/m²/yr) and sediment (180-280 g/m²/yr) accumulation also did not differ in tidal forests of the two rivers.

Ambient and potential denitrification were three to four times greater in the tidal forest soils of the AR than BR that is attributed to higher soil organic C and river water NO₃^- concentrations of the AR.

We hypothesize that the sandy soils of the BR tidal forest support greater levels of aerobic decomposition that lead to lower stocks and accumulation of soil organic C and reduced N processing (accumulation in soil and denitrification) relative to tidal forest soils of the AR.