Acetylene Reduction (N 2 -Fixation) by Jarrah Forest Legumes Following Fire and Phosphate Application

Abstract

(1) Rates of acetylene reduction were determined for nodules attached to the roots of seedlings of the native legumes Acacia pulchella, Kennedia prostrate and K. coccinea grown in glasshouse experiments and in the field. (2) In the glasshouse, application of phosphorus to nitrogen-free yellow sand and to surface soil from three forest sites significantly increased yields, the weight of nodules per plant and the rate of acetylene reduction per plant for these native legumes. The legumes did not respond to application of molybdenum or cobalt to the forest soils. (3) In nitrogen-free yellow sand the maximum rates of acetylene reduction for A. pulchella and K. coccinea were approximately 260 nmol and 190 nmol C2H2 reduced per gram fresh weight of nodules per minute respectively. (4) Plants sampled from forest sites in the first season following fire gave mean rates of acetylene reduction of 173, 164 and 159 nmol C2H2 per gram fresh weight of nodules per minute for A. pulchella, K. prostrate and K. coccinea respectively. Acetylene reducing activity occurred from early winter to late spring and the variability between forest sites was small. Molybdenum concentrations in the nodules varied from 1 to 42 Pg Mo g-' dry weight of nodule and did not appear to be related to the rate of acetylene reduction. Application of phosphorus to legumes in the field increased yield and rates of acetylene reduction. However, the legumes did not respond to molybdenum applications. (5) These experiments show that a considerable proportion of the nitrogen in young legumes regenerating after fires may be from biological fixation of N2 and that rates of N2 fixation and growth can be enhanced by phosphorus application. (6) Further studies of N2 fixation by mature legumes and of the distribution of legumes throughout jarrah forests are required to allow quantitative estimates of nitrogen inputs to the ecosystem.