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Carbon Dioxide and Ammonia Exchange in the Trachypogon Savannas of the Orinoco Llanos

J. J. SAN JOSÉ, R. MONTES and N. NIKONOVA-CRESPO
Annals of Botany
Vol. 68, No. 4 (October 1991), pp. 321-328
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/42758464
Page Count: 8
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Carbon Dioxide and Ammonia Exchange in the Trachypogon Savannas of the Orinoco Llanos
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Abstract

Atmospheric carbon dioxide (CO₂) and ammonia (NH₃) exchanges were determined in the Trachypogon savannas of the Orinoco Llanos using the energy balance approach. Total dry mass and separate dry mass values of plant parts were used for a growth analysis of the community and for measurements of nitrogen content. During the growth period, the net assimilation (Pw) ranged from 0·102 to 0·127 MJ m⁻² d⁻¹ (6·6-7·9 g dry mass m⁻² d⁻¹). These figures were similar to mean crop growth rate measured using the mass balance approach (2·8-6·9 g dry mass m⁻² d⁻¹). Analysis of the daily trend of the CO₂ assimilated by the community showed a low total energy conversion of net photosynthesis (ϵn = 0·7) compared with the values reported for tropical grasses. During the dry season, the community conserved 71 % of the maximum N accumulated during the previous wet season. Sixty-eight per cent of the community nitrogen content was lost as volatile NH₃ from the community during the reproductive period. Results suggested that the predominant net NH₃ efflux from the vegetation was determined by the low concentration of NH₃ in the atmosphere (≤ 1·8 μg m⁻³) and the compensation concentration point. However, N losses were balanced by annual nitrogen input to the community from precipitation and biological fixation. Thus, a redistribution rather than a loss of nitrogen seems to be occurring in the ecosystem.

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