Document: HEA-3-59-92

Soil N turnover, N fixation and Ceanothus cordulatus patches in a mixed-conifer forest.

ERICKSON, H.E.*

Universidad Metropolitana, San Juan, PR 00928 USA ¹

Abstract:
Distinct patches of conifers, shrubs, and open areas characterize mixed-conifer forests in the central Sierra Nevada Mountains. Shrub patches dominated by Ceanothus cordulatus may exhibit high rates of soil nitrogen (N) turnover, presumably due to increased N input from symbiotic fixation. I measured net N mineralization in three patch types and estimated N fixation by Ceanothus in an old-growth, mixed-conifer forest at the Teakettle Experimental Forest. Ten-day aerobic laboratory incubations of O and A (0-15 cm) horizon soils from 54 patches designated as either conifer, Ceanothus or open canopy were used to estimate net N mineralization. N fixation by Ceanothus was estimated using the ¹⁵N natural abundance method. Soil N turnover was high under Ceanothus compared with other patch types. For example, N was mineralized in the O horizon under Ceanothus (54 ug N. g soil^-1.10 d^-1) and open patches (18 ug N. g soil^-1.10 d^-1) but was immobilized under conifer patches (-17 ug N. g soil^-1.10 d^-1). Similarly, A horizon net N mineralization was greater under Ceanothus (2.1 ug N. g soil^-1.10 d^-1) than under open (1.2 ug N. g soil^-1.10 d^-1) or conifer (0.6 ug N. g soil^-1.10 d^-1) patches. Foliar del ¹⁵N for Ceanothus (-1.0 ) differed by 4 from a reference species, suggesting that Ceanothus obtains much of its N from atmospheric sources. Environmental change affecting dominance by Ceanothus or rates of N-fixation may govern future patterns of soil N cycling and N availability in mixed-conifer forests.

Keywords: mixed-conifer forests, N cycling, net N mineralization, N fixation

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.