Acetylene reduction (N2-fixation) by jarrah forest legumes following fire and phosphate application.
In glasshouse experiments, applications of phosphorus to nitrogen-free yellow sand and to surface soil from three forest sites significantly increased plant dry matter yields, the weight of nodules per plant and the rate of acetylene reduction per plant in seedlings of the native Australian legumes Acacia pulchella, Kennedia prostrata and K. coccinea. The legumes did not respond to applied molybdenum or cobalt in the forest soils. 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, and occurred at 40 and 70 days from emergence. In plants sampled from forest sites in the first season following fire mean rates of acetylene reduction were 173, 164 and 159 nmol C2H2 per gram fresh weight of nodules per minute for A. pulchella, K. prostrata and K. coccinea, respectively. Acetylene reduction 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 mu g Mo g-1 dry weight of nodule, and were not related to the rate of acetylene reduction. Application of phosphorus to legumes in the field increased plant yields and rates of acetylene reduction. The legumes did not respond to molybdenum applications.