Management Practice Effects on Surface Total Carbon: Differences in Spatial Variability Patterns

Lack of information about the spatial variability of soil C in different management systems limits accurate extrapolation of C sequestration findings to large scales. The objectives of this study were to: (i) describe and quantify variability of total C in three management systems, chisel-plow (CT) and no-till (NT) with conventional chemical inputs and a chisel-plow organic management practice with cover crops (CT-cover) 15 yr after conversion from conventional management; (ii) assess the strengths of spatial correlation in the three studied systems; and (iii) evaluate contributions of topography and texture to the overall total C variability and its spatial components. The data were collected at 12 60 by 60 m plots at the Long Term Ecological Research site, Kellogg Biological Station, MI. The data consisted of elevation measurements taken on a 2 by 5 m grid and a total of 1160 measurements of total C, sand, silt, and clay contents taken from the 0to 5-cm depth. Overall variability of total C in NTwasmore than four times greater than in CT, and in CT-cover the variability was more than two times greater than CT. Spatial correlation of total C was the strongest in NT, followed byCT-cover, and then by CT. Stronger spatial structures in NT and CT-cover were found to form in response to topographical and texture gradients. Effects of texture were largely associated with topographical effects; however, even when topography was controlled for, texture still substantially contributed to explaining total C variability. THERE is a growing need for more accurate assessments of soil C, including more accurate estimations of sizes of existing soil C pools, their vulnerability to change, and the impact of the changes on atmospheric CO2. The release of soil C due to intensive agriculture is an historically significant source of atmospheric CO2 loading (Wilson, 1978) and the potential for agricultural soils to regain some of this lost C with conversion to conservation tillage practices, including no-till and management systems with cover crops, is likely to become a modest but potentially important part of the U.S. and global CO2 stabilization portfolio (Caldeira et al., 2004, Council for Agricultural Science and Technology, 2004). Despite a large body of recent research on comparisons between different types of agricultural management practices for soil C storage (e.g., Franzluebbers, 2004), accurate estimates of soil C gains and losses on large scales remain problematic. One of the components contributing to the lowaccuracy is a large spatial variability in soil C cycling processes that occur across diverse landscapes, and a lack of accurate means to quantify this variability (Lal et al., 1995). Even though the spatial variability of soil C within individual agricultural fields has been assessed in a number of studies (Robertson et al., 1993, 1997; Cambardella et al., 1994; Robertson et al., 1997; Bergstrom et al., 2001), there is only limited information on how conversion from a conventional to a no-till management system or conversion from a conventional chemical to an organic management system influences spatial variability of soil C. Gathering such information will greatly aid the efforts to accurately upscale C sequestration findings from experimental plots to fieldor watershed-scale levels. Conversion from conventional tillage to no-till results in changes in soil physical and hydraulic properties, including bulk density, saturated hydraulic conductivity, and soil water retention (Hill and Cruse, 1985; Hill, 1990; Zhai et al., 1990; Dı́az-Zorita et al., 2004), thus affecting water flow and solute and material transport across the landscape. These changes are expected to result in changing patterns of organic C accumulation and decomposition in no-till compared with the land that remained in conventional tillage. One of the main driving forces behind the changes is removal of the soil mixing by tillage that has a homogenizing effect on all topsoil processes. Therefore, in areas with relatively flat topography and negligible erosion, topsoil distribution of total C can be expected tobemorevariable in long-termno-tillmanagement systems than in intensively tilled systems. For example, Perfect and Caron (2002) have observed lower variability of total C in conventional tillage than in no-till systems on an Alfisol with 1 to 3% terrain slope. There is a lack of quantitative information, however, on the factors contributing to the increase in variability and on the site-specific soil and environmental characteristics contributing to differences in the factor effects. We hypothesize that not only the overall variability of total C will be greater in no-till but also the spatial correlation in the total C distribution will be stronger. That is, more pronounced patterns of high and low C values will be found in no-till than in conventional tillage systems. Since topography and texture are among the most important factors affecting soil C (Parton et al., 1987; Hook and Burke, 2000), we hypothesize that a stronger spatial correlation is largely related to topographical A.N. Kravchenko and X. Hao, Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824-1325; G.P. Robertson, W.K. Kellogg Biological Station and Dep. of Crop and Soil Sciences, Michigan State Univ., Hickory Corners, MI 49060-9516; and D.G. Bullock, Dep. of Crop Sciences, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801. Received 2 Mar. 2006. *Corresponding author