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 scries 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 Palcudalf). 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 6, 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 pb 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.