Carbon and nitrogen losses through root exudation by Agropyron cristatum, A smithii and Bouteloua gracilis

Abstract

C and N released in root exudates throughout a growing season were estimated in Bouteloua gracilis and Agropyron smithii (dominant species in the shortgrass steppe ecosystem) and A. cristatum (dominant species in a 40-yr-old crested wheatgrass ecosystem). The release of C and N exudate was measured with or without the presence of a rhizosphere microflora. These values were used to estimate the potential contribution of root exudates to the N cycle of both ecosystems. Total C released through root exudates by A. cristatum, A. smithii and vB. gracilis was estimated at 8, 17 and 15%, respectively, of C fixed. The contributions of root exudates to the N cycle were estimated to be 11 g N m −2 in the shortgrass steppe ecosystem and 6 g N m −2 in the crested wheatgrass ecosystem. The presence of rhizosphere microorganisms resulted in a significant increase in root exudates of A. cristatum and A. smithii. A. cristatum and A. smithii plants grown under axenic conditions released 60 and 17%, respectively, of the C released via root exudates by inoculated plants. No differences between inoculated and non-inoculated plants were found in vB. gracilis, which indicated that it may be inherently insensitive to the presence of a rhizosphere microflora. These data suggested that an introduced plant species may be markedly different from native species in the shortgrass steppe in terms of exudate releases. These differences may have a significant influence upon the N cycle dynamics of both ecosystems.