Rapid Water Flow and Transport of Inorganic and Organic Nitrogen in a Highly Aggregated Tropical Soil

SOILS in the humid tropics are known for their low retention capacity of nutrients and high susceptibility to leaching (Van Wambeke, 1992). Nitrogen is particularly susceptible to leaching losses since mineralization and nitrification both proceed very rapidly under humid tropical conditions and because nitrate is very mobile in most soils. In central Amazonian Oxisols, a large accumulation of nitrate in the subsoil (defined here as 0.1–2 m) was found under annual cropping systems (Melgar et al., 1992; Cahn et al., 1993) as well as under tree cropping systems and primary forests (Schroth et al., 1999a) despite their high subsoil root activity (Lehmann et al., 2001b). This result suggests that water and N percolation is not only very rapid at the topsoil but is also more rapid in the subsoil than previously known. Direct evidence for a rapid N leaching into the subsoil has not been provided to date. This is important for understanding the biogeochemistry of terrestrial ecosystems as it relates to forest (Laurance et al., 1999) and crop productivity RAPID WATER FLOW AND TRANSPORT OF INORGANIC AND ORGANIC NITROGEN IN A HIGHLY AGGREGATED TROPICAL SOIL