Microscale Spatial Variability of Redox Potential in Surface Soil

Redox potential (Eh) is one of the most important chemical properties that control a variety of chemical or biochemical processes in soil ecosystem. Conventional Eh measurements that assume soil Eh as a random, independent variable provide a means for representing the unsampled neighborhood. However, the assumption is inappropriate if spatial dependence of the soil Eh occurs among the samples. This study was conducted to examine microscale spatial dependence and variability of Eh in surface soil using both classic and spatial variance analyses. Four undisturbed soil cores, 13 cm in depth and 10 cm in diameter, were taken from the Ap horizon of Mexico silt loam (fine montmorillonitic, mesic Udollic Ochraqualfs). Soil Eh was measured in situ at 1-cm horizontal and vertical intervals by the platinum microelectrode system and the spatial-dependent structure determined by semivariogram function. The Eh variability in the vertical intervals was higher than that in the horizontal intervals, indicating a higher degree of the vertical heterogeneity. All cores showed the spatial dependence of soil Eh with either spherical or exponential function, but only one core exhibited the range of spatial dependence. Spatially dependent variances accounted for a large portion of the sample total variance, which might be attributed to the Eh trends over soil depth. This study demonstrated that spatial dependence of soil Eh does exist in microscale, and the extent or degree of the spatial variability varies with sampling location and scale, which may affect the extent of a means for representing unsampled neighborhood. Characterization of microscale spatial variability would be a useful tool in revealing or investigating the microscale heterogeneous interactions among soil constituents, plants, and environment.