tropcial rain forest

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns
William H. McDowell, William B. Bowden and Clyde E. Asbury
Biogeochemistry
Vol. 18, No. 2 (1992), pp. 53-75

Abstract: 
Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian ones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN: NH-4+ and NO-3-N) were low in stream water lt 110 mu-g/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO-3-N (550 mu-g/L) and oxygen concentrations were high (5.2 mg/l). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH-4+-N (200-700 mu-g/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 mu-g/L) than at Icacos (600 mu-g/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH-4+ and NO-3- were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appears to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes though the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zones. However, geomorphology appears to strongly modify the importance of specific biotic components.
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