Suitability and use of the N-15-isotope dilution method to estimate nitrogen fixation by actinorhizal shrubs

Nitrogen fixation rates were estimated by the N-15-isotope dilution method for Ceanothus velutinus and Purshia tridentata in the understory of central Oregon ponderosa pine forests. Field rates were measured in small pole-, l arge pole-, and sawtimber-sized pine stands using two shrubs (Arctostaphylo s patula, Ribes cereum) and one graminoid (Carex rossii) as non-fixing refe rence species. Shrub cover ranged from 23 to 59% for Ceanothus and from 9 t o 15% for Purshia. Foliage samples were collected monthly following two app lications of N-15 ammonium sulfate (5 kg N ha(-1) at 10 atom% N-15) to comp are N uptake patterns and determine the fraction of N derived from fixation (Ndff). Several violations of the underlying assumption of the isotope dil ution method that N fixing and reference plants have access to similar pool s of soil N-15 were identified. These included non-uniform distribution of N-15 in the soil profile and dissimilarities in rooting volumes, N uptake p atterns, and plant ages between the reference and N fixing plants. Despite these compromises, there were only minor differences in percentage of Ndff when calculated independently using each reference species. Low N-15 uptake by Ceanothus and Purshia evidently negated the requirement that N fixing a nd reference plants have comparable spatial and temporal access to soil N-1 5. Both Ceanothus and Purshia were highly effective, obtaining more than 80 % of their N from fixation regardless of pine stand, shrub cover, or shrub age. Annual N fixation by Ceanothus ranged from 4 to 15 kg ha(-1) year(-1). This rate is lower than previously reported for Ceanothus in other ecosyst ems in the western United States, yet is sufficient to replace N losses fro m disturbances such as prescribed fire. Purshia fixed considerably less N, ca. 1 kg ha(-1) year(-1), roughly equivalent to the rate of atmospheric N d eposition in central Oregon. (C) 2000 Elsevier Science B.V. All rights rese rved.