Spectral Analysis of Tillage‐Induced Differences in Soil Spatial Variability

Tillage research has traditionally focused on mean effects; few studies have compared treatments in terms of their spatial variability. We applied several methods of time‐space series analysis to investigate the spatial variability of gravimetric water content (w), volumetric water content (θ), bulk density (ρ b ) and total C (TC) in the upper 10 cm of long‐term conventional‐till (CT) and no‐till (NT) soil management practices at Lexington, KY. The soil was a Maury silt loam (fine, mixed, semiactive, mesic Typic Paleudalf). Replicate transects were established in untracked interrows parallel to the direction of tillage in the CT practice. Each transect was 48.5 m long with 107 equally spaced sampling points. Soil spatial variability was higher under NT than under CT. Spectral analyses of variance identified significant differences between tillage treatments at frequencies <0.02 cycles m −1 for all soil properties, and at 0.18 cycles m −1 for w, 0.13, 0.31 and 0.48 cycles m −1 for θ, and 0.22, 0.70 and 0.90 cycles m −1 for ρ b Spatial variations in water content and ρ b appeared to be related to the distribution of TC. Coherency analysis indicated relationships were strongest at frequencies <0.09 cycles m −1 For NT the relationship between w and TC was also significant at higher frequencies (1.01–1.05 cycles m −1 ). Gravimetric water content increased as TC increased, while θ and ρ b decreased. Lagged relations for w versus TC were more frequent in CT than NT, possibly due to soil translocation during tillage operations. The opposite was true for θ versus TC and ρ b versus TC, suggesting that soil aggregates form at some distance from sites of carbon deposition under NT.