SHADING REDUCES EXPLOITATION OF SOIL NITRATE AND PHOSPHATE BY AGROPYRON DESERTORUM AND ARTEMISIA-TRIDENTATA FROM SOILS WITH PATCHY AND UNIFORM NUTRIENT DISTRIBUTIONS

Shading may both lessen the demand for soil nutrients and also the ene rgy supply for nutrient acquisition. Since root foraging for nutrients in patchy environments can be energy-costly, especially for an immobi le nutrient such as phosphate (P), the effects of shading may be most expected in heterogeneous soils. Plant acquisition of nitrate (N) and phosphate from soils with patchy and uniform nutrient distributions wa s determined in a field study under open sunlight and with shading for two common perennial Great Basin shrub steppe species, Agropyron dese rtorum and Artemisia tridentata. Partial shading in a pattern which ca n occur in shrub steppe vegetation significantly decreased plant N and P acquisition from soils both in the patchy and the uniform nutrient treatments. Artemisia was more affected by the shading than was Agropy ron. Exploitation of the rather immobile P ion by both species was red uced to a much greater degree by the shading in the patchy distributio n treatment than in the uniform nutrient treatment. As expected, plant acquisition of the more mobile N varied little with nutrient distribu tion treatment for both species and the depression of N acquisition by shading was the same in both nutrient distributions. The effects of s hading appeared to have had its primary influence on different compone nts of root foraging in the two species, especially in the nutrient-ri ch patches. For Agropyron shading primarily affected root proliferatio n, as indicated by reduced root density in patches. For Artemisia, sha ding most influenced root physiological uptake capacity and this was m ost pronounced in the nutrient-rich patches. While aboveground competi tion for light may generally reduce nutrient acquisition, the effects appear to be most pronounced if root systems of these steppe species a re foraging for nutrients such as P in spatially heterogeneous soils.