European Journal of Soil Science, 2016 doi: 10.1111/ejss.12370 Effect of rock fragments on soil porosity: a laboratory experiment with two physically degraded soils L. Gargiulo a , G. Mele a & F. Terribile b a Institute for Agriculture and Forestry in the Mediterranean – National Research Council, Via Patacca 85, 80056 Ercolano, Italy, and b Department of Agriculture, University of Naples ‘Federico II’, Via Università 100, 80055 Portici, Italy Summary There are still few studies on the role that rock fragments (RFs) have in the change in soil structure based on direct observation of the soil-pore system. Physically degraded soil is of particular interest because small RFs might be considered a factor in its remediation. In our laboratory experiment we mixed fve different proportions of 4 – 8-mm sized RFs with a Luvisol and a Regosol that have poor ability to self-structure and were characterized by a massive structure in the feld. Nine wetting and drying cycles were applied to repacked samples (15 cm in diameter and height) of soil–RF mixtures to facilitate the formation of soil structure. Image analysis was used to quantify development of the pore system at varying RF contents. The physically degraded soils studied in this research initially showed a decrease and then an increase in porosity with increasing amounts of RFs in the soil–RF mixtures. The results indicated that the Regosol responded more than the Luvisol to RF content. Threshold values of RF content at which the mechanism of pore formation prevails over that of pore reduction depended upon the soil type and can be attributed reasonably to small differences in the coeffcient of linear extensibility (COLE). We also identifed a mechanism for the propagation of porosity downwards from the soil surface with increasing RF content, together with a vertical homogenization effect on porosity. Our results contribute to understanding the mechanisms by which small rock fragments embedded in physically degraded topsoil induce changes in the pore system and confrm the potential of rock fragments to protect soil structure in soil susceptible to compaction. Highlights • Physical interaction between rock fragments and fne earth in degraded soil. • Soil porosity examined by combined experimental laboratory approach with image analysis. • Soil porosity frst decreases then increases with increasing content of rock fragments. • Coexistence of two opposing mechanisms: one of porosity reduction and one of formation of new pores. Introduction The role of rock fragments at the soil surface is widely known in terms of the protection they provide from erosion and surface sealing or crusting (Poesen et al., 1999; Cerdà, 2001; Hung et al., 2007; Martinez-Zavala & Jordan, 2008). Furthermore, even when rock fragments are embedded in the soil profle they may prevent physical degradation of the soil. The rock fragments form a skeleton that protects soil structure from compaction (Ravina & Magier, 1984; Rucknagel et al., 2013) and they preserve some essential Correspondence: G. Mele. E-mail: giacomo.mele@cnr.it Received 26 June 2015; revised version accepted 29 June 2016 soil functions such as hydraulic and air conductivity, which are related to greater macroporosity (Sauer & Logsdon, 2002; Verbist et al., 2009). Some experiments have also shown that, because of the benefcial effects of small RFs in maintaining low bulk density and large macroporosity in topsoil (van Wesemael et al., 1995; Ng Cheong et al., 2009), in situ crushing of stones could be considered a better agricultural practice than the removal of stones from soil (Chow et al., 2007; Ng Cheong et al., 2009). The effect of rock fragments on the soil-pore system has been investigated indirectly by laboratory experiments based on mea- surements of water fow (e.g. Urbanek & Shakesby, 2009; Zhou et al., 2009; Zhang et al., 2011) or changes in bulk density or both (e.g. van Wesemael et al., 1995; Fiès et al., 2002). It is assumed that © 2016 British Society of Soil Science 1