TEMPORAL DYNAMICS OF SOIL SPATIAL HETEROGENEITY IN SAGEBRUSH-WHEATGRASS STEPPE DURING A GROWING-SEASON

Variability in five soil resources essential for plant growth (NH4+, N O3-, P, K and soil moisture) was quantified using univariate, multivar iate and geostatistical techniques in a sagebrush-grass steppe ecosyst em at three times (early April, June and August) during the 1994 growi ng season. Samples were collected every meter in a 10 x 10-m 'macrogri d', every 20 cm within nested 1 x 1-m 'minigrids', and every 3 cm with in additionally nested 15 x 15-cm 'microgrids'. Strong autocorrelation for all variables in the three sample periods was only found over dis tances less than 2 m, indicating that patches of high internal uniform ity in this soil were smaller than 2 m during the growing season. Diff erences in semivariograms between sample periods were most pronounced for NO3-, NH4+ and soil moisture, variables that we consider to primar ily limit plant growth in this system. The distance over which sample points were autocorrelated for NO3-, NH4+ and soil moisture increased from April to June. In contrast P and K, which are relatively more abu ndant at the study site, exhibited relatively constant semivariance pa tterns over the three sample periods. Weak correlation was found betwe en samples collected in the three sample periods for N and soil moistu re indicating that the spatial pattern of these limiting resources cha nged between sample periods. However, P and K had highly significant c orrelations (p < 0.00001) among sample periods, indicating that the di stributional patterns of these relatively more abundant resources rema ined rather constant. There were strong negative correlations between P and K and distance from the base of shrubs for all sample times (p < 0.001), indicating an increase in P and K close to shrubs. Similar st rong negative correlations were not found between distance from the sh rubs and levels of NH4+, NO3-, or soil moisture, nor for any soil vari able and distance from perennial tussock grasses. Changes in patterns of nutrient and soil moisture variability within a growing season sugg est that not only must plants acquire soil resources that vary in time and space, but that they may also have to adjust to different scales of resource patchiness during the season.