CONTINGENT EFFECTS OF PLANT-SPECIES ON SOILS ALONG A REGIONAL MOISTURE GRADIENT IN THE GREAT-PLAINS

The central grassland region of the United States encompasses major gr adients in temperature and precipitation that determine the distributi on of plant life forms, which in turn may influence key ecosystem proc esses such as nutrient cycling and soil organic matter dynamics. One s uch gradient is the threefold increase in precipitation from the easte rn Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountai ns, to the tallgrass prairie in eastern Kansas. We investigated the re lative roles of plant species and plant cover in influencing soil C an d N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more impor tant than plant species in explaining variability in soil properties a t the dry site, the Central Plains Experimental Range in northeastern Colorado. However, plant species explained relatively more of the vari ability in soil properties than did plant cover at the two wetter site s, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced va riation in soil C and N, than did the dry site, which had distinct pat ches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition . Aboveground tissue C:N was better correlated with net nitrogen miner alization rates at the wet sites than the dry site. Thus, tissue chemi stry appears to exert more control on nitrogen dynamics at the wet tha n the dry sites. The results suggest that plant species traits that ar e relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patte rns, productivity) reflect environmental limitations as well as specie s' physiological potentials. Furthermore, a dominant environmental dri ver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling.