Nitrogen Fixation Associated with the New Zealand Mangrove ( Avicennia marina (Forsk.) Vierh. var. resinifera (Forst. f.) Bakh.)

Abstract

Nitrogenase activity in mangrove forests at two locations in the North Island, New Zealand, was measured by acetylene reduction and 15 N 2 uptake. Nitrogenase activity (C 2 H 2 reduction) in surface sediments 0 to 10 mm deep was highly correlated ( r = 0.91, n = 17) with the dry weight of decomposing particulate organic matter in the sediment and was independent of light. The activity was not correlated with the dry weight of roots in the top 10 mm of sediment ( r = −0.01, n = 13). Seasonal and sample variation in acetylene reduction rates ranged from 0.4 to 50.0 μmol of C 2 H 4 m −2 h −1 under air, and acetylene reduction was depressed in anaerobic atmospheres. Nitrogen fixation rates of decomposing leaves from the surface measured by 15 N 2 uptake ranged from 5.1 to 7.8 nmol of N 2 g (dry weight) −1 h −1 , and the mean molar ratio of acetylene reduced to nitrogen fixed was 4.5:1. Anaerobic conditions depressed the nitrogenase activity in decomposing leaves, which was independent of light. Nitrogenase activity was also found to be associated with pneumatophores. This activity was light dependent and was probably attributable to one or more species of Calothrix present as an epiphyte. Rates of activity were generally between 100 and 500 nmol of C 2 H 4 pneumatophore −1 h −1 in summer, but values up to 1,500 nmol of C 2 H 4 pneumatophore −1 h −1 were obtained.