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Journal Article

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

Luc Abbadie, André Mariotti and Jean-Claude Menaut
Ecology
Vol. 73, No. 2 (Apr., 1992), pp. 608-613
Published by: Wiley on behalf of the Ecological Society of America
DOI: 10.2307/1940766
Stable URL: http://www.jstor.org/stable/1940766
Page Count: 6
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Independence of Savanna Grasses from Soil Organic Matter for Their Nitrogen Supply
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Abstract

In Lamto savanna (Cote d'Ivoire), grass δ^1^5N (≈ -1.3%) is much lower (has a smaller absolute value) than soil organic matter δ^1^5N (≈ +5%). In order to understand such a discrepancy, we have analyzed ^1^5N 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^1^5N) only contributes ≈ 7% of nitrogen requirements, as does the second (δ^1^5N ≈ +7%) due to a very low humus mineralization rate. The third source (δ^1^5N = 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^1^5N of grasses, suggesting that most of the assimilated nitrogen originates from the decay of root material (δ^1^5N ≈ -1.1%).

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