Evaluation of a non‐destructive acetylene reduction assay of nitrogen fixation for pasture legumes grown in pots

Plants of white, red, alsike, broad red, and subterranean clovers, lucerne, and lotus were grown in pots with adequate moisture at day/night temperatures of 15°c/10°c and 23°c/10°c, and under moisture stress at 23°c/10°c. At weekly intervals during 5‐ and 6‐week growth periods successive non‐destructive acetylene‐reduction (AR) assays were performed by incubating potted plants in 51 plastic containers for 2 h with 200 ml acetylene. N 2 fixation over the same periods was calculated from chemical analysis of plant tissue. AR and N 2 fixation rates were closely related within species and treatments, but the molar ratio of C 2 H 2 reduced: N 2 fixed differed between clovers (mean ratio 2.8: 1), lucerne (3.4: 1), and lotus (4.3: 1). Moisture stress increased the AR: N 2 fixed ratio, but temperature had no consistent effect. Repeated assays had no effect on herbage DM accumulation and herbage %N.