HETEROTROPHIC NITROGEN-FIXATION (ACETYLENE-REDUCTION) DURING LEAF-LITTER DECOMPOSITION OF 2 MANGROVE SPECIES FROM SOUTH FLORIDA, USA

Heterotrophic nitrogen-fixation (acetylene reduction) was measured dur ing decomposition (under dark conditions) of Rhizophora mangle L. and Avicennia germinans (L.) Stearn leaf litter. Nitrogen-fixation rates i n leaf litter increased following 24 d incubation, then decreased afte r similar or equal to 44 d for both species. Maximum rates of 66.2 and 64.6 nmol C2H4 g(-1) dry wt h(-1) were reached by R. mangle and A. ge rminans leaf litter, respectively. Higher fixation rates of leaf litte r were associated with an increase in water content and sediment parti cles on leaf surfaces of both species. Rates of nitrogen fixation by d iazotrophs attached to sediment particles were not significantly diffe rent from zero. With additions of D-glucose, ethylene production rates increased by factors of 625-, 34- and 7-fold for sediment, R. mangle and A. germinans leaf litter, respectively, compared to rates prior to enrichment. These organically enhanced rates of nitrogen fixation on leaves could be accounted for by increased activity associated with at tached sediment particles and not the leaf material. Total phenolics [ reported as tannic acid equivalent (TAE) units] decreased nitrogen-fix ation rates when added to D-glucose-enriched sediment at >20 mg TAE l( -1). Phenolic compounds could explain the initial lag in rates of nitr ogen fixation during leaf-litter decomposition of R. mangle (initial c ontent of 110.8 mg TAE g(-1) dry wt), but not of A. germinans (initial content of 23.4 mg TAE g(-1) dry wt). The higher phenolic content and reportedly lower carbon substrate of R. mangle did not result in spec ies-specific differences in either the magnitude or temporal pattern o f nitrogen fixation compared to A. germinans leaf litter. We conclude that the availability of organic substrates leached from the leaf litt er along with colonization by the heterotrophic diazotrophs las indica ted by sediment accumulation) controls nitrogen-fixation rates in a si milar manner in the leaf litter of both species.