Saturday, 15 July 2006

Effects of Topography and Texture on Spatial Variability of Total C in Different Management Systems.

Alexandra.N Kravchenko, Senthil K. Subramanian, Xinmei Hao, G. Phillip Robertson, and Alvin J.M. Smucker. Michigan State Univ, 520A PSSB, Crop & Soil Sciences Dept, East Lansing, MI 48824

Lack of information regarding the effects of topography and texture on the spatial variability of soil carbon (C) limits efforts to accurately up-scale C sequestration findings from field experiments to large scales. In this study we describe and quantify topographical and texture effects on total C spatial variability in three management systems: chisel-plow tillage and no-till management systems with conventional chemical inputs and a chisel-plow tillage system with zero chemical inputs and leguminous cover crops. Data on total C, sand and silt contents at 0-5 cm depths, and topographical measurements were collected at four 60x60 m plots of each management system at the Long Term Ecological Research site, Kellogg Biological Station, MI. Approximately 100 soil samples and more than 300 elevation measurements were collected at each plot. As expected, topography explained a significant portion of total C variability in all three studied management systems. Multiple regression models for total C with main topographical characteristics, such as relative elevation, terrain slope, terrain curvature, and aspect, had R2 values ranging from 0.11 to 0.74. Effects of sand and silt contents on total C were also highly significant, even when topography was controlled for. High partial regression coefficients for sand and silt that ranged from 0.11 to 0.42 indicated that still a substantial portion of total C variability can not be explained using topographical information alone. Topographical effects significantly interacted with effects of management systems. Regression slopes for relative elevation were significantly less than zero in all four plots of no-till management (p<0.05) and in all four plots of zero input management systems (p<0.1), however, they were not significantly different from zero in all four plots of the chisel-plow management. Regression slopes for terrain curvature were significantly greater than zero in three no-till plots and two zero input plots, indicating that, when other topographical variables were controlled for, higher C values corresponded to convex surfaces while lower C values corresponded to concave surfaces in soils of these management systems. Effect of topography on total C was found to be spatially continuous extending over 20-30 m, thus reflecting the dominant role that topography plays in a spatially continuous process of water and material redistribution across a landscape. Influences of texture when topography was controlled for appeared to be limited in space to distances of 0-5 m. Spatial effects of topography were reflected in larger spatial correlation ranges and smaller nugget to sill ratios of the total C variograms from no-till and zero input management systems as compared to those of the chisel-plow tillage management. These results suggest that no-till system and zero input system management induced differences in soil C processes are differentially affected by topography and soil texture and vary across a landscape as a function of, among other factors, topographical and texture characteristics. Accurate large-scale evaluations of management effects on C sequestration, thus, can not be achieved without conducting prior assessment of or without making educated assumptions regarding the differences in C sequestration processes' rates and directions produced by topography and soil texture.

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