Microbial activity and organic matter composition in Mediterranean humus forms

- Different biogeochemical parameters were analysed in Mediterranean humus forms.
- Enzyme activities clearly discriminated among humus forms.
- Changes in SOM composition among humus forms were evident using Py-GC.
- A clear gradient from mull macro to moder humus forms was observed.

The aim of this study was to characterise humus forms, intended as organic profile morphologies, and the associated soil chemical, biochemical (β-glucosidase, phosphatase and urease activities) and organic matter (SOM) properties in Mediterranean forest soils. Samples were collected from the organic layers (OF + OH) and the mineral topsoil (M01) in 31 Quercus spp. sites. To highlight the differences in soil organic matter (SOM) composition among humus forms, pyrolysis-gas chromatography (Py-GC) was used and the ratios of specific pyrolysis products were calculated. The main results achieved were that humus forms can be clearly separated according to enzyme activities and SOM pyrolytic fragments of the first mineral layer (M01). Humus forms showed a progression from mull macro, which was the richest in soil nutrients and with the highest enzyme activities, across amphi macro and amphi meso to an opposite end represented by mull meso and moder. This trend was also observed in the qualitative changes of SOM composition. In particular, the high ratios of pyrrole to phenol suggested that in macrostructured humus forms soil organic matter is more degraded, whereas high values of acetic acid and of the aliphatic to aromatic compounds ratio in moder and mull meso forms, indicate accumulation of fresh and biodegradable material. This study points to the high potential of humus forms as indicators of forest soil processes.