SGS-LTER Transect Study - Organic Carbon in Soils across Toposequences on the Central Plains Experimental Range, Nunn, Colorado, USA 1983-1984

CPER SOC across Toposequences - Pedons and their corresponding topography were described along an 8 km transect oriented normal to the major drainages of the CPER. A total of 140 pedons representing 23 toposequences and 7 plains segments were characterized. Sampling sites were selected within toposequences according to slope position (summit, shoulder, backslope, footslope, toeslope) and within plains segments at approximate 100 m intervals. Pedons were described and sampled by genetic horizon according to the standards of the National Cooperative Soil Survey. Analyses, conducted at Colorado State University, included particle size and organic C. Bulk density was estimated empirically according to: Rawls, W.J. 1983. Estimating soil bulk density form particle size analysis and organic matter content. Soil Sci 135: 123-125. Organic C accumulation was measured along an 8 km transect at a site in the semiarid shortgrass steppe of northeastern Colorado. Specific objectives of the study were to (I) measure the quantity and distribution of organic C across toposequences, (ii) test the hypothesis that a disproportionate amount of soil organic C resides in the lowlands (as defined herein), and (iii) assess the role of geomorphic history as a determinant of contemporary rates of biogeochemical organic C transformations. Results of the study showed the surface (A) horizon organic C concentration did not vary systematically among slope positions of a given toposequence. Similarly, the mass of organic C within the surface meter of soil often did not increase with decreasing elevation across a toposequence. Mass of organic C was found to range from 2.5 kg m^-3 on terrace escarpments to 10.7 kg m^-3 on sandy uplands. The mass of organic C, as calculated to the BCk horizon, was highest in the lowlands. The mass of buried organic C, as calculated uniformly for a 50 cm thickness of material, represents 17% of the total organic C estimate for the site. In spite of buried soils, lowlands did not contain a disproportionate amount of total landscape organic C.