Carbon dioxide exchange rates and plant soil responses to soil loss on shortgrass prairie and sagebrush steppe sites

Abstract

The purpose of this study was to determine the effects of 3 different levels of soil loss on the CO2 exchange rates of 2 rangeland ecosystems in northern Colorado, the shortgrass prairie and the sagebrush steppe. The experimental design was a split-plot, randomized block, with a factorial arrangement of 3 levels of soil removal (0, 11.2, and 22.41 t ha-1). At the shortgrass site, net photosynthesis in western wheatgrass and blue grama was greater (P < 0.10) in the 11.2 and 22.4 t ha-1 soil removal treatments than in the control (0 t ha-1). This response was coupled to a significant (P < 0.10) increase in nitrogen use efficiency (NUE) and a decrease in inter-cellular CO2 concentration (C1). The consistency and similarity of the responses of net photosynthesis, NUE, and C, for these 2 species suggests that it was driven by a common physiological process.

While total soil respiration at the shortgrass site increased significantly (P < 0.10) as soil removal level increased, bare soil respiration did not vary (P > 0.10) among the 3 treatments. Plant respiration rates generally increased as soil removal level increased, but this response was controlled by environmental factors that influenced plant productivity.

The photosynthetic rates of big sagebrush were unaffected (P > 0.10) by the soil removal treatments at the sagebrush site. However, while the net photosynthetic rates of bluebunch wheatgrass did not vary (P > 0.10) between the 0 and 11.2 t ha-1 soil removal levels, they decreased (P < 0.10) at the 22.41 t ha-1 level. The photosynthetic response in bluebunch wheatgrass may have been confounded by the droughty conditions at the sagebrush site. Total, bare soil, and plant respiration rates at the sagebrush site varied little (P > 0.10) among the 3 soil removal treatments.

Results from this study suggest that the effect of soil loss on photosynthetic rates is: 1) the same regardless of the physiology of the plant indicating that the response occurs within a common physiological process; and 2) affects grasses more than Wyoming big sagebrush indicating that where soil loss is severe, this shrub might have a competitive advantage.