Acetylene Reduction in an Avicennia marina Community in Southern Australia

Abstract

Rates of C₂H₂ reduction associated with decomposing leaf litter of Avicennia marina (white mangrove) reached 469 nmol C₂H₄ g^-1 h^-1 in air after 21 days of decomposition. Maximum rates of C₂H₂ reduction measured in sea water were 328 nmol C₂H₄ g-1 h-1 for leaf litter confined in mesh bags and 352 nmol C₂H₄ g^-1 h^-1 for unconfined leaf litter; the rates in sea water were always, and often substantially, lower than rates in air. Rates of decomposition and C₂H₂ reduction were less in leaf litter confined in mesh bags than in unconfined litter. Decomposing dead fibrous roots and live roots showed average rates of C₂H₂ reduction in interstitial water of only 9.3 and 3.5 nmol C₂H₄ g^-1 h^-1, respectively. During decomposition of leaf litter, the concentration of nitrogen increased; 41-64% of this increase was estimated to be the direct result of N₂ fixation. No change in nitrogen concentrations occurred in decomposing dead roots or in live roots. N₂ fixation associated with decomposing leaf litter and dead roots, and with live roots and surficial sediments, is thought to supply about 40% of the nitrogen required annually by the Avicennia marina trees. This estimate is relatively low in comparison with data for other marine wetlands, and it follows from the low rates of C₂H₂ reduction associated with the root system and the sediments.