Soil response to canopy position and feral pig disturbance beneath Quercus agrifolia on Santa Cruz Island, California

We assessed the effects of canopy position and feral pig disturbance on soil moisture, pH, and inorganic nitrogen concentrations beneath canopies of Quercus agrifolia and surrounding grasslands on Santa Cruz Island, California. Percent soil moisture, pH, nitrate-N, and total inorganic nitrogen varied significantly with canopy position. Soil moisture decreased, and pH increased along transects from the subcanopy trunk position to the open grassland outside the canopy. Nitrate and total nitrogen increased from the trunk position to midcanopy and then decreased from midcanopy to the open grassland position. The ratio of nitrate to ammonium varied with canopy position, even after controlling for the effects of soil moisture and pH, both of which strongly influence rates of nitrogen mineralization. Temporally, soil pH, soil moisture, nitrate-N, and total carbon all varied significantly between drought and non-drought years. Soils from fenced exclosures that removed feral pig disturbance did not exhibit significant differences in any soil property compared to control plots where pig access was permitted. The results of our analysis suggest that the primary factors influencing spatial and temporal variability in oak woodland soils on Santa Cruz Island were subcanopy position and climate variability, respectively. Biotic disturbance by feral pigs did not cause a significant cause in soil properties over the spatial and temporal domains examined in this study. © 2000 Elsevier Science B.V. All rights reserved.