Tuesday, November 14, 2006 - 11:20 AM

Determining Optimal Sensor Depth for Gradient-Based Flux Estimates from Measured Subsurface Co2 Concentrations.

Scott Jones1, Raghuveer Vinukollu1, and Janis Boettinger2. (1) Utah State University, Dept. Plants, Soils and Climate, 4820 Old Main Hill, Logan, UT 84322-4820, (2) Utah State University, PSB, 4820 Old Main Hill, Logan, UT 84322-4820

Terrestrial carbon dynamics that contribute to the buildup of atmospheric CO2 are being monitored and studied to better understand the role of soil in sequestering this greenhouse gas. The Earth’s land surface is a significant area over which CO2 is stored as organic carbon and emitted as a byproduct of soil chemical reactions in addition to plant and microbial respiration. Novel methods for determining CO2 flux from soil include a gradient-based approach which relies on subsurface concentration measurement and the assumption of a linear pressure gradient driving CO2 flux at the surface. Our objective was to identify optimal sensor depths where a linear gradient is maintained between sensor and soil surface under three different cropping systems (i.e., 3-rooting depths) in a calcareous Mollisol. An array of water content, temperature and CO2 sensors were installed at different depths to monitor changes in CO2 at each location over a period of several days in each cropped plot. Sensors were placed through the wall of a 30-cm diameter pipe previously installed vertically in turf, pasture and alfalfa plots (10 x 10 m). Carbon dioxide flux estimations were compared to measurements from a LI-COR CO2 soil chamber. Maximum measured concentrations increased with increasing plant rooting depth and therefore optimal sensor placement depth also increased for deeper rooted plants. The optimal sensor placement depths approximating the linear gradient-based CO2 flux calculation extended from near the surface down to 10- or 20-cm depending on rooting depth.