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

摘要

Nitrogen fixation rates were estimated by the ~15N isotope dilution method for Ceanothus velutinus and Purshia tridentata in the understory of central Oregon ponderosa pine forests. Field rate were measured in small pole, large pole, and sawtimber-sized pine stands using two shrubs (Arctostaphylos patula, Ribes cereum) and one graminoid (Carex rossii) as non-fixing reference species. Shrub cover ranged from 23 to 59% for Ceanothus and from 9 to 15% for Purshia. Foliage samples were collected monthly following two applications of ~15N ammonium sulfate (5 kg N ha~(-1) at 10 atom% ~15N) to compare N uptake patterns and determine the fraction of N derived from fixation (Ndff). Several violations of the underlying assumption of the isotope dilution method that N Fixing and reference plants have access to similar pools of soil ~15N were identified. These included non-uniform distribution of ~15N in the soil profile and dissimilarities in rooting volumes, N uptake patterns, 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 ~15N uptake by Ceanothus and Purshia evidently negated the requirement that N fixing and reference plants have comparable spatial and temporal access to soil ~15N. Both Ceanothus and Purshia were highly effective, obtaining more than 80% of their N from fixation regardless of pine stand, shrub cover, of 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 ecosystems in the western United states, yet is sufficient to replace N losses from 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 deposition in central Oregon.