Nitrogen fixing activity in warty lenticellate tree barks

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Abstract
Detached lenticellate warty bark of Inga laurina was first shown to develop acetylene reducing activity (ARA) (18 nmol C2H2red·g-1 (F.W.)·hr-1) after a few days' incubation with ambient air containing 0.10 atm C2H2. Similar N2ase development was found to occur with warty barks of many other plant species including 12 leguminous and 9 non-leguminous trees growing in wet tropical and temperate forests. Among them higher activities (>7 nmol C2H2 red·g-1 (F.W.)·hr-1) were recorded in Inga laurina, Cynometra ramiflora, Cassia siamea, Robinia pseudo-acacia, Albizia julibrissin in the leguminous plants, and Ilex crenata, Ilex pedunculosa, Rhizophora mucronata, Bruguiera gymnorrhiza and Mallotus japonicus in the non-leguminous plants. The development of ARA was often accelerated under lower pO2 (0.05 atm) and pC2H2 (0.05 atm). Complete replacement of O2 with N2 or Ar in the incubation atmosphere resulted in full suppression of the development of N2ase activity. When ARA measurement was made with intact barks of natural stands (Robinia pseudo-acacia, Ilex pedunculosa and Mallotus japonicus), it was found that all of these intact barks were reducing C2H2 linearly without any time lag. The activities recorded were 4.6–11.5 nmol C2H2 red·g-1 (F.W.)·hr-1 corresponding to 1.19–1.27 nmol C2H2 red·cm-2hr-1. These values roughly coincided with ARA of detached warty barks of the same plant species. The present results suggest that in situ N2-fixation in tree barks of the forests would amount to several 10 kg·ha-1·year-1 in temperate and tropical wet forests