Humus forms in a Mediterranean area (Castelporziano Reserve, Rome, Italy): classification, functioning and organic carbon storage

This work reports the results of an investigation on humus forms in a Mediterranean LTER (Long-Term Ecosystem Research) site. The study tests the effectiveness of the recent European humus forms classification (Zanella et al. 2011b), based on morpho-genetic characteristics of diagnostic organic and organo-mineral horizons. Furthermore it reveals how humus forms, at the level of diagnostic horizons, are related to vegetation/soil type and carbon storage capacity of the soil. The humus forms were surveyed in different substrates, soil types, and vegetation units of a Mediterranean forest ecosystem in Central Italy (Castelporziano Reserve). Thickness, organic carbon (OC) and total nitrogen (Ntot) contents in organic and organo-mineral horizons, pH and texture in the organo-mineral horizon were measured and statistically analysed. The three main humus forms (Mull, Moder and Amphi) covering the soil of the Reserve appeared well separated on the PCA diagram. Thickness, OC and Ntot showed a high correlation with axes of PCA. Kruskal–Wallis tests revealed differences between Mull, Moder and Amphi: 1) the thickness of the A horizon was larger in Amphi; 2) OC in the A horizon was less in Moder; 3) Mull had less OC in organic horizons than Moder and Amphi; and 4) Mull and Amphi had twice OC of Moder in bulked organic and organo-mineral horizons. Two main ecological gradients, from neutral/aerated Mull to acid/anaerobic-water saturated logged Moder on one side and from fresh Mull to dry Amphi on the other, explain the distribution of humus forms in the Reserve. The first gradient concerns circular belts around periodically filled small basins and is peculiar to the more or less impermeable plain of Castelporziano; on the contrary, the second progressive transition is very common in Mediterranean forests. The European humus forms classification allows to better understand the organic matter cycle at the level of soil bio-structures even in this relatively small Mediterranean forest. New diagnostic horizons are proposed and necessary for surveying humus forms perturbed by wild boars. The crucial role played by the here investigated Amphi forms could increase with the prospect of climate change.