Combining Experimental Warming and Environmental Gradients to Elucidate Continuous and Interacting Drivers of Soil Response to Warming
Elaine Pegoraro* (ElainePegoraro@lbl.gov), Cristina Castanha, Sigrid Dengel, Margaret Torn
Climate and Ecosystem Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA.
Grassland ecosystems comprise 40% of the global ice-free land surface area and store up to a third of terrestrial carbon. Unlike most ecosystems, nearly all the carbon in grasslands is stored belowground, making the response and management of their soil carbon cycling crucial in a warmer world. Experimental warming in grasslands stimulates soil carbon fluxes, but the response magnitude varies significantly among studies. Ecosystem responses to warming can be difficult to assess due to large or unmeasured spatial and temporal heterogeneity and limited numbers of field replicates (Kreyling et al. 2018). To address some of these issues, scientists are setting up a new whole-soil warming experiment in a California coastal grassland in Point Reyes, where they will heat the top meter of soil by 3°C and 6°C relative to control plots. Soil will be warmed using 22 heating rods in a 4 m diameter circle and a central rod all installed 2.4 m deep in the soil, plus three circular surface heating cables buried at 5 cm at radii 0.45, 1.0, and 1.75 m. Extensive multidisciplinary site characterization was conducted to better understand the spatial heterogeneity of the site. The small elevation gradient has three distinct zones (e.g., upslope, midslope, and downslope) that are shaped by vegetation, hydrological, and pedological properties. Twenty-four strategically placed plots grouped into eight blocks span the three zones along this gradient. Soil carbon dioxide flux varies spatially and temporally across the gradient. In late winter and early spring, when plant activity is low and soil moisture is at its maximum, soil flux is lower downslope relative to the other zones, due to highly saturated soils. As the growing season progresses, soil flux increases across the site reflecting enhanced root respiration from higher plant productivity. As the soil becomes less saturated downslope, soil flux rates more than double relative to the upslope and midslope zones, where normalized difference vegetation index and soil moisture values are lower and plants senescence earlier in the season. This research will capitalize on the vegetation and soil moisture gradient at the site (1) to better elucidate interacting environmental drivers and their response to whole-soil warming and (2) to improve the overall prediction success given the high spatial heterogeneity, which is common in ecological experiments.
Kreyling, J., et al. 2018. “To Replicate, or not to Replicate—That Is the Question: How to Tackle Nonlinear Responses in Ecological Experiments,” Ecology Letters 21(11), 1629–1638. DOI:10.1111/ele.13134.