Spatial Variability in Soil Ion Exchange Chemistry in a Granitic Upland Catchment

Abstract

Advances in quantifying the spatial variability of soil properties made for agricultural soils are not being mirrored for naturally structured upland soils. The objectives of this study were to determine the degree of spatial variability and variance structure of cation exchange chemistry in a granitic, heather moorland site (Northeast Scotland). Two 20 by 20 m soil plots, a Typic Placaquod and a Typic Humaquept, were sampled at O and B horizon depths at 104 locations in a regular grid overlayed with cluster (Placaquod) and transects (Humaquept) patterns. Soils were analyzed for pH and exchangeable cation, physical, and hydraulic properties. Results showed strongly significant vertical differences in exchange chemistry between surface organic and mineral horizons at either site for all chemical properties. Strong lateral variability was also apparent within the plots. Coefficients of variation (CV) were 18 to 52% for O horizon chemical properties, with similar variability between sites. In B horizons CV values were 26 to 119%, the highest associated with Humaquept chemical properties. Compared with the Placaquod the Humaquept had lower mean pH, but higher mean concentrations of exchangeable Ca and Mg in both horizons. Geostatistical analyses highlighted a generally strong degree of spatial dependence to Placaquod properties, particularly in the organic horizon where correlation ranges were greater. By contrast, the majority of properties for the Humaquept showed random, pure nugget variance indicating no spatial correlations at this scale of observation. It is postulated that seasonal water logging of the Humaquept may explain some differences in the exchange chemistry between sites. These differences in chemistry and in the spatial patterns of variability have implications not only for modeling the role of such soils in controlling the hydrochemical environment of the uplands, but also for the design of field soil sampling strategies for accurately quantifying soil properties.