GEOSTATISTICAL PATTERNS OF SOIL HETEROGENEITY AROUND INDIVIDUAL PERENNIAL PLANTS

1 Univariate, multivariate, and geostatistical techniques were used to quantify the scale and degree of soil variability around individual perennial sagebrush (Artemisia tridentata ssp. vaseyana) and bluebunch wheatgrass (Pseudoroegneria spicata ssp. spicata) plants. This variability was then compared to that found across the larger sagebrush-steppe site. Samples were taken every metre in a 10-m x 12-m grid and every 12.5 cm in nine nested 0.5-m x 0.5-m grids containing at least one Artemisia shrub or Pseudoroegneria tussock (362 total samples). The 11 soil properties measured were organic matter, pH, water content, live root mass, microbial respiration, net N mineralization, nitrification potential, and soil-extractable ammonium, nitrate, phosphate, and potassium. 2 There was considerable biological variation in many of the properties measured. Soil organic matter varied from 1.3% to 7.4% within the 10-m x 12-m area and pH varied by as much as 1.3 pH units among samples less than 50 cm apart. Samples showing strong negative N mineralization (N immobilization) were only 12.5 cm from samples showing strong positive N mineralization. 3 Spearman rank-correlation coefficients between pairs of the 11 soil parameters showed a number of strong, positive associations between some variables (e.g. phosphate and potassium, r(s) = 0.64), but not others (e.g. ammonium and organic matter, r(s) = 0.013). 4 Semivariograms for soil organic matter and pH showed strong spatial autocorrelation at distances of less than 1 m, and both showed a high spatial dependence of c. 90%. A combined index of soil fertility (incorporating information on soil ammonium, nitrate, phosphate, and potassium) also showed strong autocorrelation at scales of less than 1 metre. None of the microbial processes analysed (net N mineralization, nitrification potential, or microbial respiration) showed any significant autocorrelation, even at the finest measurement scale of 12.5 cm. 5 Kriged (interpolated) contour plots of soil organic matter, phosphate, and potassium showed strong spatial patterning associated with the tussock grasses, but less consistent patterning for the sagebrush plants. From the degree and scale of variability seen in this study and previously, we conclude that root plasticity and active foraging in a heterogeneous soil environment are likely to be important to the nutrient balance of many plants.