Influência da topografia na disponibilidade de nitrogênio mineral e na assimilação de nitrato por árvores da floresta de terra firme na Amazônia central

Abstract

On a small scale, relief conditions are not homogeneous in terra firme rainforests of the central Amazonia. Plateaus, slopes and valleys are topographical categories that have peculiarities regarding soil characteristics, creating different environments for growth in vegetation types and biomass. In this work, the availability of mineral nitrogen in forest soil and its acquisition by two native tree species co-existing in the same topographic categories was compared. Scleronema micranthum (Malvaceae) is considered a climax-opportunist specie in forest succession and its leaves usually reach the canopy. Protium hebetatum (Burseraceae) is considered climax in forest succession and it usually occupies understory position. Topography was an influential factor of soil NO3 - and NH4 + availability; however, total N, NO3 - and NR activity in leaves were not related to differences in this mineral N availability. S. micranthum trees showed higher concentrations of leaf N and higher values of NR enzyme activity when compared to P. hebetatum. On the other hand, P. hebetatum trees showed higher NO3 - concentrations in leaves. Differences between the species S. micranthum and P. hebetatum suggest different strategies in the use of N, probably associated to their ecological categories in forest succession and to their distinct positions in the vertical extract of the forest.