VI 



V 



IIIJV 



TC 



Figure 16, Diagrammatic scheme of the relationship of bottomland hardwood zones to 

 soil types on a large alluvial river floodplain (Congaree Swamp National Monument, SC). 

 Three sources of water are indicated: vertical arrows, rainfall; horizontal solid 

 arrows, normal cyclic flooding from river; and dashed arrows, periodic but irregular 

 side flooding by "hill freshets" from a tributary stream. Orders and suborders of 

 recent soil classification are given in parentheses (Soil Conservation Service 1975): 

 D (Zone II), Dorovan muck (Histosol, typic medisaprist) ; C (Zone V) Congaree silt loam 

 (Entisol, typic udifluvent); TC (Zone III, IV) Taw Caw silty clay loam (Inceptisol, 

 fluvaquentic dystrochrept) ; CH (Zone II) along distributary, Chastain loam (Inceptisol, 

 typic haplaquept); VI, upland; I, river. 



within the plot. Hay (1977) noted that 

 blackwater floodplain soils only 1 m (3 

 ft) apart varied in radiocesium levels by 

 as much as 190%. 



Mineral Subsidy 



Both inorganic sediments and nutri- 

 ents are deposited on the floodplain dur- 

 ing overbank Tiooding, although average 

 sediment deposits are so thin as to be 

 unnoticeable. The fates of these materials 

 vary. Residence time and biotic utiliza- 

 tion remain key questions. Some sediments 

 may reside in the floodplain long enough 

 to be mineralized by weathering. As flood 

 waters subside, leaves in swamp pools be- 

 come coated with silt and clay which may 

 be trapped by the biotic slime. Other 

 sediments are redistributed by scour dur- 

 ing flooding. Mineral nutrients may be 

 transported or trapped adsorbed to sedi- 

 ment particles (Delaune et al. 1976). 

 Nutrients are incorporated into tissues of 

 the biotic community and into sediments in 

 response to vegetative growth and decay 

 cycles (see Chapter 6). The shallow root 

 systems of many floodplain trees (Figure 

 17) enable them to take advantage of this 

 importtd mineral subsidy. 



Nutrients, notably nitrogen, also are 

 conserved and recycled on the floodplain 

 (Brinson et al. 1981). Inorganic nitrogen 

 (N), especially ammonium, is immobilized 



by heterotrophic microorganisms in leaf 

 litter in the fall, held for several 

 months, then mineralized and absorbed by 

 filamentous algal mats in winter and early 

 spring. It is then released by the dying 

 algae and absorbed by trees and shrubs at 

 leaf-out, with little or no net release 

 into the water from autumn leaf fall until 

 tree growth in the spring. This tight 

 nutrient recycling offsets potential loss 

 by flooding or leaching. 



Sediment inputs to alluvial streams 

 include a high proportion of fine-grained 

 clays and silt from Piedmont runoff (Table 

 6). These aro deposited during the rela- 

 tively long residence time of water in 

 Zone IV backswamps and sloughs. Coinci- 

 dent with clay deposition is the deposi- 

 tion of various materials adsorbed to the 

 clay particles, including nutrient ions, 

 metal ions, and pesticides. 



Soil Oxygen Conditions 



Over the course of a year, floodplain 

 soils may vary from being completely 

 oxygen-depleted to being as saturated with 

 oxygen as upland soils. Because gas ex- 

 change is curtailed drastically in water- 

 logged soils (Ponnamperuma 1972) and bio- 

 logical respiration of the soil microbes 

 rapidly depletes the available oxygen, 

 inundated or wet saturated floodplain 

 soils become anaerobic for extended per- 



27 



