Can the flora-based humus functionality index (RxN) predict humus forms in Mediterranean plant communities? A case study in Castelporziano State Natural Reserve

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 survival). 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. Following standard methods, vegetation, humus forms and several soil parameters were surveyed/measured simultaneously 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 forest. 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 significant 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 repandum, Arbutus unedo; and Leontodon tuberosus and Festuca heterophylla. Overall the study confirms the relationship among humus forms, soil parameters (such as organic carbon % and total nitrogen % in the OH horizon), and Ellenberg indicators.