INDEPENDENCE OF SAVANNA GRASSES FROM SOIL ORGANIC-MATTER FOR THEIR NITROGEN SUPPLY

In Lamto savanna (Cote d'Ivoire), grass delta-N-15 (almost-equal-to -1.3 parts per thousand) is much lower (has a smaller absolute value) than soil organic matter delta-N-15 (almost-equal-to +5 parts per thousand). In order to understand such a discrepancy, we have analyzed N-15 natural concentrations in the four major sources of mineral nitrogen that can meet the annual requirements of plants: bulk precipitation, mineralization of humified soil organic matter, atmospheric dinitrogen fixation, and decomposition of plant litter. The first source (negative delta-N-15) only contributes almost-equal-to 7% of nitrogen requirements, as does the second (delta-N-15 almost-equal-to +7 parts per thousand) due to a very low humus mineralization rate. The third source (delta-N-15 = 0) contributes up to 17%, due to nonsymbiotic N2 fixation by microorganisms associated with grasses, legumes being almost absent from the savanna. All these processes cannot account for the low delta-N-15 of grasses, suggesting that most of the assimilated nitrogen originates from the decay of root material (delta-N-15 almost-equal-to -1.1 parts per thousand).