Seasonal Patterns of Root Production with Water and Nitrogen Additions Across Three Dryland Ecosystems

Root production is known to contribute at least 50% of total net primary production in dryland ecosystems, yet few studies have addressed seasonal dynamics of root production or the belowground response to altered resource availability. We aimed to identify how root production varies across three dryland ecosystems dominated by different plant functional types: the shortgrass steppe, dominated by C4 perennial bunchgrasses, the northern mixed-grass prairie, co-dominated by perennial C3 and C4 grasses, and the sagebrush steppe, dominated by a perennial shrub with an understory of C3 and C4 grasses and forbs. We also sought to determine how root production changes throughout a growing season for these three systems, and how it responds to increased water and nitrogen availability. We used root ingrowth cores to monitor root production in 6-week intervals between June 1 and October 5, 2017, and with water and nitrogen additions in shortgrass steppe, mixed-grass prairie, and sagebrush steppe in the western Great Plains of the United States. We found that the northern mixed-grass prairie had the highest total root production during our sampling period, and that seasonal patterns of root growth varied significantly across the dryland types, with root growth greatest during the middle of the growing season at the sagebrush steppe and greatest during the end of the growing season at the northern mixed-grass prairie. As expected, belowground root production was about half aboveground production in all sites. Root production responded less to water and nitrogen addition than did aboveground production.