Soil Biogeochemical Processes across a Lateral Toposequence in an Old‐Growth Floodplain Forest

Abstract

Floodplain forests are important landscapes that retain and transform nutrients derived from watershed sources. Although research has been conducted regarding biogeochemical processes in floodplains, few studies have been in old‐growth bottomland forests. Therefore, the goal of this study was to quantify how landscape variability and environmental factors influence floodplain biogeochemistry in old‐growth ecosystems of Congaree National Park (CONG), SC. During a 2‐yr period, we measured biogeochemical indices along an alluvial soil toposequence that included seasonal variation in microbial biomass and net N mineralization. Soil microbial biomass was relatively consistent throughout the study and displayed few relationships to environmental variables. Net N mineralization showed distinct seasonal variations ranging from 37 ± 9 to 412 ± 73 g N ha−1 d−1 during cool winter and warm summer months, respectively. Nitrate‐N was the major product of net N mineralization in all seasons, and only hydric soils in low‐lying topographic positions had appreciable net NH4–N production (>15% total N mineralized), which typically occurred after spring flooding. These results suggest that the intrinsic biogeochemical processes within the old‐growth bottomland forests of CONG differ from those of aggrading floodplain forests of the southeastern United States, especially with regard to production of excess NO3–N.