DINITROGEN FIXATION (ACETYLENE-REDUCTION) IN PRIMARY SUCCESSION NEAR MOUNT ROBSON, BRITISH-COLUMBIA, CANADA

Three successional plant communities representative of 3 stages of Engelmann spruce Picea engelmannii forest development were sampled from a sequence of recessional moraines located in front of the Robson Glacier. The legume Hedysarum boreale var.mackenzii was the major N-fixing agent on all the moraines. Hedysarum boreale contributed 72% of N input in the pioneer herb stage, 79% in the intermediate Dryas stage, and 88% in the spruce forest stage. Soil microorganisms (eg Gloeocapsa) added 26, 20 and 8% of the N input, respectively. Estimated annual N input from N-fixation decreased c8-fold over the 200 yr forest succession. The input in the pioneer stage was 57.2, in the intermediate stage 41.6, and in the forest stage 7.3 mmol C2H4 m-2 yr-1. A marked decline in N-fixation is characteristic of the late stage in forest succession and may be a mechanism which maintains the steady state in total N accumulation. -from Authors