Spatial Variation in Soil CO2 Flux and Testing for Temporal Stability
Soil CO2 flux will be measured at positions on transects that extend through the same three study sites that contain the eddy covariance towers and autochambers There are significant variations in topography across theses sites with elevations changes as great as 35 rn and slopes sometimes exceeding 15 degrees within a single pasture. Multispectral images collected over the study area in 2003 (courtesy of Dr. Kevin Price, University of Kansas) showed distinct patterns of vegetative cover that corresponded to topographic position and soil characteristics. Each sampling transect for the soil measurements will be about 500-600 rn in length and pass through the source area of the eddy covariance tower. Initially data will be collected at 20-m intervals along the transect, resulting in up to 30 sampling locations. A survey grade GPS system (Trimble AgGPS® 214) will be used to lay out the transects and determine the slope, aspect, and elevation of each sampling location. Once initial data are available, the length scale of variation can be quantified and we may change the distance between sampling points. Ultimately the final length and sampling interval of the transects will be determined by time required for taking a soil CO2 flux measurement. We would like to be able to sample a complete transact in less than 2 hours because transect sites will need to be sample twice over 4-hour period. The reasoning for this protocol is discussed in following section . Soil-surface CO2 will be measured at each sampling location using a portable Li-Cor 8100 non-steady state chamber system (Li-Cor, Lincoln, NE). The chamber is 10.3 cm m in diameter with a total volume of 955 cc. Flux is estimated from the rate change in CO2 in the headspace. Soil collars made from PVC will be permanently installed at each sampling site on the transect to insure that the same point is sampled on each measurement day. Measurement will consist of placing the chamber on the collar and collecting data until CO2 concentrations in the chamber are at least 75 ppm above ambient. Flux will be approximated from the CO2 vs time curves. Both quadratic and non-linear least squares methods will be used to correct data for increasing head space concentrations and make unbiased estimates of d CO2/dt at the start of the measurement. During each handheld measurement, the operator also will measure soil temperature at 10 cm with a handheld reader. A thermocouple probe will be permanently installed at each site within 0.25 m of the sampling collar. At selected sites, soil temperature and water content will be measured at 10 cm using dual probe heat capacity sensors . Every two to four weeks during the growing season, the height of the canopy will be measured at each soil-collar site (visual obstruction pole method) and the sampling locations will he imaged using a multispectral digital camera (Tetracam, Chatsworth, CA). Images will be collected at 1.5 m above the surface using a portable tripod. The Tetracam images can be used to compute vegetation indices (e.g., NDVI, SAVI) and make approximations of percent cover. Aerial images of the soil-respiration transect and the tower footprints will be collected on a monthly basis with the Tetracam and a 5.1 MP digital camera (Olympus C-5060 Wide Zoom). Co-PI, Dr. Pat Coyne, has experience evaluating the Tetracam data and will process the images using Idrisi and ARCIINFO software. During the second and third years of the study, LAI and leaf tip angle will be measured at each sampling location using a L12000 plant canopy analyzer. Previous research conducted on the same tallgrass prairie during the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment or FIFE, showed that the LI2000 could measure LAI to with 15 %. Collection of LAI will allow approximation of intercepted PAR at each sampling location. This will be a valuable covariate given that soil respiration is partially governed by canopy photosynthesis. Statistical analysis will include tests for temporal stability of spatial patterns both within and between sequential sampling dates. Spearman's rank correlation and a relative differencing technique will be used. A difficulty associated with conducting the temporal stability analysis is that soil respiration follows a strong diurnal pattern and the measurements along the transect will occur at different times during the day. Thus, to compare readings, data will need to be adjusted to predict CO2 from the sample sites at a given moment in time (e.g., 11:00 LST). To accomplish this task we will read each transect twice on a given day and then interpolate to a target time between the two measurements. For example, 30 points on a transect might be measured between 0900 and 1030 LST then again between 1130 and 1300 LST. Both cycles should be completed prior to the time of maximum flux (approximately 1400 LST). The rate change in flux with time is normally increasing in a very linear manner during this morning period (see figure in following section) which will make it possible to approximate the flux at all 30 points at 1100 LST. The hourly autochamber data collected during the same period will be used to verify that fluxes are increasing in a linear fashion. More complex interpolative schemes are possible, but simple linear interpolation is a good starting point. Sudden changes in weather conditions during the measurement cycle would disallow interpolation in this manner. A second opportunity to sample a transect twice will be available in the afternoon following the time of maximum (e.g., after 1500 LST) when fluxes are decreasing. Our goal will be to use this sampling protocol at each tower site (1 ungrazed and 2 grazed) on a weekly basis between May and August.