Seasonal variation in C 2H 2 reduction (N 2-fixation) in the litter layer of eucalypt forests of South-Western Australia

Abstract

Seasonal variation in C 2H 2 reduction by forest litter was determined at four sites in jarrah ( Eucalyptus marginata Donn ex Sm.) and karri ( E. diversicolor F. Muell.) forests in south-western Australia. Rates of C 2H 2 reduction and microbial activity (CO 2 respiration) were measured every 4 weeks for 14 months on 10 intact litter mats at each site. Mean monthly rates of C 2H 2 reduction ranged from 0 to 24 nmol C 2H 2 g −1 litter day −1 in jarrah forest and from 0 to 56 nmol C 2H 2 g −1 litter day −1 in karri forest. Seasonal variation in rates of both C 2H 2 reduction and CO 2 respiration was related to variations in moisture content and temperature of the litter. C 2H 2 reduction and CO 2 respiration were highest during the cool, moist winter and decreased to zero during the warm dry summer. Functions relating C 2H 2 reduction to temperature and moisture content were derived from laboratory incubations of leaf litter under controlled conditions. These were used to develop models of the seasonal variation in N 2 ase activity based on field measurements of C 2H 2 reduction, litter moisture content and temperature. The models demonstrate that moisture content of litter is the factor contributing most to seasonal variation in N 2ase activity in jarrah and karri forests. Approximate amounts of N 2 fixed annually in the litter layer were 38 and 49 mg Nm −2 yr −1 in jarrah forest and 149 and 257 mg Nm −2yr −1 in karri forest. These are within the range of values reported for other forest ecosystems.