Species effects of Ceanothus velutinus versus Pseudotsuga menziesii, Douglas-fir, on soil phosphorus and nitrogen properties in the Oregon cascades

Many authors have hypothesized that nitrogen-fixing species, as a functional group, would express different controls on soil properties and ecosystem development than non-nitrogen-fixing species. Although nitrogen (N) accretion under nitrogen-fixing tree species has been well studied, the effect of nitrogen-fixing species on other soil nutrients, such as phosphorus (P), has received less attention. We studied differences in soil phosphorus and nitrogen properties beneath Ceanothus velutinus (Ceanothus), a nitrogen-fixing species, and Psuedotsuga menziesii (Douglas-fir), a non-fixing species, in a high elevation successional watershed in the H.J. Andrews Experimental Forest in Oregon. Total P was 20% greater in Douglas-fir soils than Ceanothus soils in surface horizons, but there was no significant difference in deeper soil horizons. Surface soils (5 and 15 cm) under Douglas-fir generally had higher concentrations of specific P fractions than surface soils under Ceanothus, but this difference either disappeared or was not as apparent at greater soil depths (30 and 60 cm). Total nitrogen, and extractable ammonium and nitrate were greater in surface soils under Ceanothus than under Douglas-fir. delta N-15 values of leaves and litter differed between Ceanothus and Douglas-fir (p-value = 0.0001 and 0.03, respectively), but the delta N-15 of bulk soil and KCl extracted nitrate and ammonium did not differ. Soil enzyme activities suggested greater mineralization of organic P (phosphatase activity) under Ceanothus in summer, but not in fall, while no significant differences in general decomposition (P-glucosidase activity) were found in soils between the two species. (C) 2001 Elsevier Science B.V. All rights reserved.