Temporal dynamics of soil spatial heterogeneity in sagebrush-wheatgrass steppe during a growing season

Variability in five soil resources essential for plant growth ($NH_4^ + $, $NO_3^ - $, P, K and soil moisture) was quantified using univariate, multivariate and geostatistical techniques in a sagebrush-grass steppe ecosystem at three times (early April, June and August) during the 1994 growing season. Samples were collected every meter in a 10 × 10-m 'macrogrid', every 20 cm within nested 1 × 1-m 'minigrids', and every 3 cm within additionally nested 15 × 15-cm 'microgrids'. Strong autocorrelation for all variables in the three sample periods was only found over distances less than 2 m, indicating that patches of high internal uniformity in this soil were smaller than 2 m during the growing season. Differences in semivariograms between sample periods were most pronounced for $NO_3^ - $, $NH_4^ + $ and soil moisture, variables that we consider to primarily limit plant growth in this system. The distance over which sample points were autocorrelated for $NO_3^ - $, $NH_4^ + $ and soil moisture increased from April to June. In contrast P and K, which are relatively more abundant at the study site, exhibited relatively constant semivariance patterns over the three sample periods. Weak correlation was found between samples collected in the three sample periods for N and soil moisture indicating that the spatial pattern of these limiting resources changed between sample periods. However, P and K had highly significant correlations (p < 0.00001) among sample periods, indicating that the distributional patterns of these relatively more abundant resources remained 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 strong negative correlations were not found between distance from the shrubs and levels of $NH_4^ + $, $NO_3^ - $, or soil moisture, nor for any soil variable and distance from perennial tussock grasses. Changes in patterns of nutrient and soil moisture variability within a growing season suggest 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.