Spatial variability of soil elements in two plant communities of NW Spain Antonio Gallardo ⁎ , Rocio Paramá Area de Ecología, Facultad de Biología, Universidad de Vigo, 36310 Vigo, Spain Received 27 January 2006; received in revised form 3 January 2007; accepted 30 January 2007 Abstract 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Grassland; Scrubland; Soil nutrients; Spatial variability; Organic matter; Weathering rate 1. Introduction Variation in soil resources at different scales is likely to have important consequences on both community structure and ecosystem-level processes (Tilman, 1988; Schlesinger et al., 1990; Robertson and Gross, 1994; Ettema and Wardle, 2002). The majority of the studies carried out on natural plant communities revealed that soil organic matter and nutrients are heterogeneously distributed in soils at a spatial scale relevant to plants (Robertson, 1987; Robertson et al., 1988; Jackson and Caldwell, 1993a,b). The mechanisms that cause this spatial pattern are not well known. Robertson et al. (1997) suggested that geomorphologic influences may determine the differences in microtopography and erosion on scales of tens of meters, while influences at the level of the vegetation cover or the rhizosphere may determine the existence of structures at smaller spatial scales. The influence of plant cover on the spatial pattern of soil resources has been described by some authors (Jackson and Caldwell, 1993a,b; Schlesinger et al., 1996; Gallardo et al., 2000). The accumulation of organic matter around individual plants or groups of plants may generate spatial patterns in the mineralization rates and the availability of nutrients cycled through organic matter (such as N). Neverthe- less, other nutrients are cycled through geochemical processes, because important pools exist in primary and secondary soil mineral forms. These elements are released from rock and soil minerals through chemical weathering processes. Local accu- mulation of organic matter may accelerate the weathering rate because root and microbial activity may increase CO 2 partial pressure in soils (Piñol et al., 1995). However, coincident spatial patterns of organic matter accumulation and weathering rate are Geoderma 139 (2007) 199 – 208 www.elsevier.com/locate/geoderma ⁎ Corresponding author. Area de Ecología, Facultad de Ciencias Experi- mentales, Universidad Pablo de Olavide, 41013 Sevilla, Spain. Tel.: +34 954 97 7936; fax: +34 954 34 91 51. E-mail address: agalcor1@upo.es (A. Gallardo). 0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2007.01.022