The Effects of Drought Stress on Belowground Feedbacks of Bromus tectorum (cheatgrass)

Entire landscapes of sagebrush steppe are being displaced by the invasive annual grass Bromus tectorum (cheatgrass). One way we understand invasion of B. tectorum is through the grass-fire cycle, a positive feedback in which annual grass invasion increases fire occurrence and loss of native cover, resulting in increased grass cover following fire. Our research investigates another potential feedback mechanism that may drive B. tectorum invasion where root exudation increases nitrogen (N) availability. We hypothesize that drought stress triggers root exudation in B. tectorum, which stimulates microbial activity in the rhizosphere. Increased microbial activity leads to increased decomposition of soil organic matter and therefore N mineralization, resulting in increased grass cover. To test this hypothesis and how it may be influenced by local adaptation, we grew 60 plants from two populations of B. tectorum (Washoe County, NV and Siskiyou County, CA) in a greenhouse. 2 months following germination, we implemented a drought treatment for 50% of the plants in each population. Plants assigned a drought treatment underwent 4 weeks of progressive drying and remained at 33% water holding capacity for 2 weeks before we hydroponically extracted root exudates from all plants. Soil samples taken from each plant were treated with root exudates from all possible combinations of population and drought treatment. These treated soils were used in a respiration and N mineralization incubation experiment. The two populations showed meaningful differences prior to the drought treatment: germination rates and seedling biomass were higher in individuals from Washoe county. Our incubations showed that A) all planted soils had increased microbial biomass compared to pre-treatment soils, B) soils treated with root exudates from droughted plants had increased N mineralization rates compared to soils treated with exudates from watered controls, and C) respiration rates in soils with root exudates were higher than untreated soils. Preliminary results suggest that drought induced root exudation is associated with higher N availability, which may indicate a critical below-ground feedback that influences Bromus tectorum invasion.