Independence of Savanna Grasses from Soil Organic Matter for Their Nitrogen Supply

In Lamto savanna (Cote d'Ivoire), grass δ 1 5 N (≈‐1.3%) is much lower (has a smaller absolute value) than soil organic matter δ 1 5 N (≈‐1.3%). In order to understand such a discrepancy, we have analyzed 1 5 N 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 dinitrogren fixation, and decomposition of plant litter. The first source (negative δ 1 5 N) only contributes ≈7% of nitrogen requirements, as does the second (δ 1 5 N≈+7%) due to a very low humus mineralization rate. The third source (δ 1 5 N = 0) contributes up to 17%, due to nonsymbiotic N 2 fixation by microorganisms associated with grasses, legumes being almost absent from the savanna. All these processes cannot account for the low δ 1 5 N of grasses, suggesting that most of the assimilated nitrogen originates from the decay of root material (δ 1 5 N≈‐1.1%).