Can Topographical and Yield Data Substantially Improve Total Soil Carbon Mapping by Regression Kriging?

Accurate mapping of total soil C on the field scale is essential for evaluating efforts to sequester soil C and for providing individual producers with information on C sequestration potentials of their fields. Data on easily measured secondary variables that are strongly related to soil C are believed to be helpful in improving mapping accuracy. The objective of this study was to assess improvement in mapping accuracy due to dense topographical and long-term yield monitoring information. Approximately 1200 total C measurements at the 0to 5-cm depth along with topographical and 7-yr crop grain yield data were collected at twelve 60by 60-m plots at the Long-Term Ecological Research Site in Michigan. Total C was found to be significantly related to topography and 7-yr average standardized yield in all studied plots, with regression R .0.5 in approximately half of the plots. Accounting for either topographical or yield information in regression kriging, however, produced onlymodest (,10%) improvement inmapping accuracy compared with ordinary kriging. Plots with promisingly strong relationships of total C with topography or yield were also found to be the ones where spatial distributions of total C were highly continuous, thus leading to no advantages in using regression kriging. The results indicated that under soil and topographical conditions similar to those of this study, dense topographical data or dense longterm yield data might not lead to substantial improvement in C map-