More rain, less often: Extreme precipitation events and the impact on carbon cycling in the grasslands of Rocky Flats NWR, Colorado

Abstract:

The grasslands of the U.S. Great Plains vary greatly year-to-year in aboveground productivity due in large part to rainfall patterns. The timing and magnitude of productivity is further influenced by the composition of plants that are C₃ (cool season) or C₄ (warm season) photosynthetic types. This region is forecasted to undergo seasonal shifts in precipitation, with large rain events separated by longer dry intervals, and much uncertainty remains about the impact that this will have on carbon cycling in these grasslands. Furthermore, past observations may be inadequate for estimating how arid and semi-arid grasslands will respond to more extreme precipitation patterns. Our study at Rocky Flats National Wildlife Refuge (NWR) contrasts above and belowground fluxes of water and CO₂ between a C₃ dominated, reclaimed grassland and a nearby C₄ dominated, native tallgrass prairie. Each grassland is instrumented with an eddy covariance tower, soil moisture and temperature arrays, and soil gas wells measuring soil CO₂ concentrations at 5, 10, and 15 cm depths. Leaf area index (LAI) and biomass were obtained through destructive sampling about every 2 weeks throughout the growing season. Measurements of water and carbon cycling include the growing seasons for 2011-2014, with special focus on the late growing season and highly saturated conditions following the extreme precipitation events of September 2013. Cumulative rainfall measurements at the study sites for the period September 9-15, 2013 were approximately 200mm, and soil moisture increased from 15% to over 50% (completely saturated) during the same period. Preliminary results in the C₃ dominated grassland during this event showed a rapid increase in the CO₂ concentration gradient, with the soil CO₂ concentration at the 15cm depth increasing from 1339 ppm to 4246 ppm in less than 1 hour. Yet increasing soil water content caused a decrease in diffusion of CO₂, with efflux going from 0.1 mgC/m²/hr to zero during the same period. LAI and biomass along with NEE measurements indicate that aboveground productivity increased in the C₃ dominated grassland in October 2013, while the C₄ dominated tallgrass prairie did not increase production at this time.