Nitrification, the bacterial oxidation of ammonium into nitrate, is known to be highly variable in time and space in various soils, including tropical soils (Lodhi 1977; Schimel & Parton 1986; Wedin & Tilman 1990; Lensi et al. 1991; Zak & Grigal 1991; Ross, Luizão & Luizão 1992; Davidson & Hackler 1994; De Boer, Klein Gunnewiek & Parkinson 1996). Lack of nitrification is generally linked to low nutrient availability and is supposed to improve nitrogen conservation by suppressing denitrification and limiting mineral nitrogen leaching (Huber et al. 1977; Jordan, Todd & Escalante 1979; Vitousek et al. 1979; Keeney 1986; Robertson 1989). Wet tropical savanna soils are typical examples of ecosystems characterized by low nutrient availability and low nitrification capacities. Paradoxically, some of them can support high plant productivity, which suggests efficient and conservative nutrient dynamics (Hopkins 1966; Egunjobi 1974; Menaut & César 1979; Abbadie, Mariotti & Menaut 1992).
The Lamto area, Côte d’Ivoire, West Africa, is representative of wet savanna ecosystems where nitrogen has been shown to be the major factor limiting primary production (Bate 1981). Previous studies have suggested that the potential of nitrification in this area was related to the specific composition of the plant cover (Lensi, Domenach & Abbadie 1992). In Lamto, the grass cover is generally dominated by two perennial tufted Graminaceae (Poaceae) species, either Loudetia simplex or Hyparrhenia diplandra (Menaut & César 1979). The soils from the sites dominated by L. simplex exhibit a substantial potential of nitrification while those dominated by H. diplandra generally exhibit extremely low potential of nitrification (Lensi, Domenach & Abbadie 1992). Within the H. diplandra-dominated sites, the potential of nitrification was even significantly lower under than between the H. diplandra tussocks. These results may be compared with previous data from Munro (1966), Meiklejohn (1962, 1968) and Rice & Pancholy (1973) showing a depressive effect of many grasses on the activity of nitrifiers, especially those belonging to the Andropogoneae group. Recently, the general character of this depressive effect was questioned by Le Roux et al. (1995) who fortuitously discovered a site dominated by H. diplandra exhibiting high rates of nitrate accumulation in soil.
The aim of the present work was to answer the following questions: (1) does the potential of nitrification significantly differ between the site reported by Le Roux et al. (1995) and a standard area where nitrification rate is known to be extremely low; (2) are these differences stable over time? To answer the first question, the average and variance of the potential of nitrification were compared between the two model sites. A potential nitrification assay was used in order to evaluate the long-term ability of micro-organisms to oxidize ammonium. It was assumed that the potential activity is not affected by short-term environmental variations. To answer the second question, two sets of H. diplandra grown from seeds sampled in the two sites were cultivated in greenhouse under identical conditions (in particular with non-limiting nitrate supplies). The potential nitrate reductase activity (NRA) was measured in roots and leaves hypothesizing that a difference of plant ability to reduce nitrate should result from a permanent difference in nitrate availability between the soils from the two sites.