Carbon dioxide and ammonia exchange in the Trachypogon savannas of the orinoco llanos

Abstract

Atmospheric carbon dioxide (CO2) and ammonia (NH2) 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 (P40) 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 (ε× = 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-2) 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. © 1991 Annals of Botany Company.