Spatial variability of soil elements in two plant communities of NW Spain

The belowground resources in terrestrial plant communities are frequently heterogeneously distributed. However, the scale or extent to which this distribution occurs, the differences in distribution in various communities, and the mechanisms underlying this spatial pattern are poorly understood. We examined the variability and the spatial dependence of soil elements in two soil types differentiated by the degree of soil profile development and covered respectively by a scrubland (less developed soil) and a grassland plant communities. Our aim was to test whether differences in variability and spatial structure exist between (1) essential and nonessential nutrients for plants, (2) elements in organic and mineral reservoirs, and (3) the different degrees of soil development. To answer these questions, we described the variability and spatial structure of 16 elements (C, N, P, S, Ca, K, Mg, Fe, Mn, Zn, Si, Al, Na, Ti, Rb, and Ba), the organic matter content, and the Si to (Al + Fe) ratio (as a weathering rate index) in the top 10 cm of the two contrasting soils high in organic matter. The coefficient of variation and geostatistics were used to describe soil variability. In both plant communities, the coefficient of variation was higher for total C, N, P, S, Ca, and organic matter than for the other elements and the Si to (Al + Fe) ratio. In the less developed soil (scrubland), only the most variable nutrients (C, N, P, and S) and two micronutrients (Mn and Zn) showed spatial dependence. In the grassland, all elements showed a spatial structure, including nonessential elements for plants. Our results showed that the concentration of elements retained by biological mechanisms are more variable than elements retained by geochemical mechanisms, irrespective of whether the element is essential or nonessential for plants. The spatial dependence of elements retained in mineral pools increased with soil development.