THE INFLUENCE OF WESTERN JUNIPER DEVELOPMENT ON SOIL NUTRIENT AVAILABILITY

The objective of the research reported here was to assess the effect o f western juniper (Juniperus occidentalis Hook.) invasion and tree agi ng on soil nutrient availability in sagebrush/grass ecosystems of cent ral Oregon. Barley was used as a bioassay test plant to determine avai labilities of N, P, K, and S. The surface 15 cm of soil from beneath j uniper canopies (canopy soil) and intercanopy areas (intercanopy soil) was collected for five age classes of trees ranging from 36 to 160 y. Nutrient availability determined by bioassy in these soils was compar ed to an area where juniper had not invaded. Phosphorus availability o f intercanopy soils was significantly reduced for the two oldest tree classes. This result suggested an alteration of P availability by the lateral root system of western juniper in the intercanopy areas that i s linked to juniper maturity. In canopy soils, juniper did not influen ce N availability. Advancing juniper maturity was associated with incr easing then decreasing P availability with the oldest tree class. The most striking effect of juniper was increased S availability in canopy soils with advancing juniper maturity. Differential responses of N an d S availability in canopy soils may reflect the fact that N mineraliz ation is chiefly a biological process whereas S mineralization is both biological and biochemical. Low inherent availabilities of N and S su ggest that productivity enhancement measures should include fertilizat ion with these elements.