COASTAL FOREST ECOSYSTEM NEAR ROME Can the flora-based humus functionality index (RxN) predict humus forms in Mediterranean plant communities? A case study in Castelporziano State Natural Reserve Cristina De Nicola • Anna Testi • Roberto Crosti • Giuliano Fanelli Received: 31 May 2014 / Accepted: 23 December 2014 Ó Accademia Nazionale dei Lincei 2015 Abstract Humus is the fraction of the topsoil strongly influenced by organic matter, corresponding to the sequence of organic and the related underlying organo- mineral horizons. It is functional both for carbon storage and for the plant recruitment process (including seed storage, germination, roots development and seedling sur- vival). In the years 2011–2012, 32 plots were established in Castelporziano State Natural Reserve as part of a pilot study on the relationships between soil and plants. Fol- lowing standard methods, vegetation, humus forms and several soil parameters were surveyed/measured simulta- neously in all the plots. The main aim of the study was to explore for the first time the application of the humus functionality index (RxN), derived from the Ellenberg flora-based eco-indication model, in a Mediterranean for- est. In the Reserve three main humus forms were detected: Mull, Moder and Amphi. The distribution frequency of Ellenberg soil ecological indicator values such as R-soil reaction and N-soil nutrients, and to a lesser extent F-soil moisture, varied among plots with different humus forms. The index RxN was most powerful in emphasizing sig- nificant differences among the humus forms. Moreover, species were also differently distributed among the three humus forms: Ranunculus lanuginosus, Stellaria media and Tanacetum parthenium were exclusive to the Mull, while Moder and Amphi forms had a set of preferential species, respectively: Erica arborea, Carex flacca, Cyclamen rep- andum, Arbutus unedo; and Leontodon tuberosus and Festuca heterophylla. Overall the study confirms the rela- tionship among humus forms, soil parameters (such as organic carbon % and total nitrogen % in the OH horizon), and Ellenberg indicators. Keywords Humus forms Á Soil Ellenberg indicators Á RxN index Á Organic carbon Á Total nitrogen 1 Introduction Humus forms (HFs), the fraction of the topsoil strongly influenced by climate, organic matter and soil organisms, are indicators of the functionality status of an ecosystem (Topo- liantz and Ponge 2000), particularly for sites affected by long- term disturbance of the herbaceous layer (Klinka et al. 1990). Humus forms are considered to be indicators of ecosystem cycling and nutrient management strategies (Ponge 2003); they are critical to floristic composition and vegetation structure because they are functional for the plant recruitment process from seeds (storage and germination) to seedling (roots development and inoculation), providing moisture and nutrients for plant survival and growth (Crosti 2011). The study of humus forms represents a tool for the characterization of ecosystems and biotic communities; humus forms may be indicative for environmental changes since they evolve together with the entire ecosystem (Ponge 2003; De Nicola et al. 2013). On the other hand, it is not easy to gather knowledge on dynamic ecosystem processes via the interaction between This peer-reviewed article is a result of the multidisciplinary project coordinated by the ‘‘Accademia Nazionale delle Scienze detta dei XL’’, Rome, Italy, in the area of the Presidential Estate of Castelporziano near Rome. C. De Nicola Á A. Testi (&) Á G. Fanelli Department of Environmental Biology, University Sapienza, P.le A. Moro 5, 00185 Rome, Italy e-mail: anna.testi@uniroma1.it R. Crosti ISPRA, IV Dip. STS Palermo, Palermo, Italy 123 Rend. Fis. Acc. Lincei DOI 10.1007/s12210-014-0375-0