Soil carbon and nitrogen changes under Douglas-fir with and without red alder

We sampled pure Douglas-fir (DF) [Pseudotsuga menziesii (Mirb.) Franco] and mixed red alder (Alnus rubra Bong.)(RA) and DF (RA/DF) stands in 1980 and in 1999 to investigate the influence of RA on soil C and N pools. In RA/DF plots with 25% RA, the soil N pool to a 45-cm depth increased significantly (P < 0.05) by 190 g N m^-2, corresponding to 10 g N m^-2 yr^-1 accretion. The average between treatment soil N difference in 1999 was 166 g m^-2, representing N accretion of 8.7 g m^-2 yr^-1. In pure DF plots, the soil N pool remained nearly constant. Resin N mineralization in RA/DF plots was about ten fold greater than on pure DF plots, but the enhanced resin N availability did not affect DF foliar N concentration. Temporal plot pairing was necessary within this landscape with high spatial variability to detect significant changes in soil N pools, and only large effects, such as N addition by RA, could be identified with statistical significance. Minimum detectable difference (MDD) estimates for mean total soil C differences in RA/DF plots showed that it would require about 30 more years of C accretion to detect differences at P < 0.05. Conversely, total soil N accretion in RA/DF plots was 28% greater than the MDD after 19 yr.