The scale of nutrient heterogeneity around individual plants and its quantification with geostatistics

Authors: Jackson, RB, MM Caldwell

Heterogeneity in the soil is regularly invoked as important for competitive interactions among plants. Variability around individuals is of fundamental importance because current theories of plant competition differ in their treatments of heterogeneity and scale. In this study we use geostatistical techniques to quantify the scale and variability of soil nutrients (N, P, and K) at distances from 10 cm to 10 m at a native sagebrush steppe site, with emphasis on the variability around individual perennial plants. We established a 10 x 12 m grid in the spring of 1991 with points sampled every meter (143 samples). Within this grid we also centered 0.5 x 0.5 m grids around six Artemisia and three Pseudoroegneria plants, sampling at a second scale of 12.5 cm between adjacent points (25 samples around each plant, 225 total samples at this finer scale). Nutrient heterogeneity at the site was quite high, with both ammonium and nitrate varying by 2 to 3 orders of magnitude and P and K close to one order of magnitude within the 10 x 12 m area. Within the 0.5 x 0.5 m subplots, ammonium and nitrate varied by an average factor of 11 and 12, respectively, with smaller variation for P and K. Even at paired distances of 3 cm, nitrate varied by an average factor of 2.8, while P varied by an average factor of only 1.3. Variograms for each nutrient demonstrated increasing autocorrelation, but only at spatial scales of < 1 m. This result implies that an individual pant with roots a metre or so apart likely encounters the same degree of soil variability as found across the entire field plot.


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