Tree or soil? Factors influencing humus form differentiation in Italian forests

•238 Italian forest sites from the BioSoil database have been investigated.•Environmental factors of humus forms were analyzed by boosted regression trees (BRTs).•Interactions between tree species and other environmental factors were discussed.•Soil nutritional status, conditioned by parent material, was found to be fundamental.

We aim to investigate the occurrence of forest humus forms (Moder, Amphi and Mull) in relation to environmental factors describing parent material, climate and tree species. Boosted regression trees (BRTs) were applied as modeling tool to analyze data of 238 plots of the BioSoil database covering the whole Italian forest territory. Though predictive ability was not very high, especially for the Amphi form, we could gain significant insight into factors controlling humus form differentiation. In the BRT analysis, the diversity of tree species was the most important predictor for Moder and Mull models and specific plant effects were evidenced. However, our results showed that the geographic distribution of Italian forest species was influenced by soil and climate conditions, partly explaining the high weight of tree species as factor. The importance of the soil nutritional status, due to parent material properties, in driving humus form differentiation was stated, highlighting the key role played by pH and calcium content, with the hitherto understated importance of phosphorus. This study further clarified the functioning of the still poorly understood Amphi form. Reduced effective soil volume (EfVol) combined with seasonality appeared to constrain pedofauna activity in otherwise favorable and nutrient rich systems, favoring the evolution of Amphi instead of Mull forms.