Relationships between humus profiles and C cycling, first results from a Mediterranean pine forest

Pine species used for land restoration in the Mediterranean basin since the late 19th century are nowadays showing symptoms of degradation. Forest degradation may reduce the forest potential to act as a C sink, or even increase the C losses through greenhouse gas (GHG) emissions. Microbial processes strongly affect SOM decomposition and therefore C fluxes. Since the first and most important modifications of microbial processes occur within humus profiles, humus characteristics and structure might affect microbial processes and GHG emissions. The study was carried out to relate C cycling in a degraded pine forest with the morphology and characteristics of the humus profiles.The study area was located near Florence (Central Italy) in a reforest area planted with Black pine, Brutia pine, and Cypress. In spring 2016, 9 humus profiles were described, sampled and classified, and CO2 and CH4 fluxeswere measured. Humus profiles were analyzed for bulk density, particle size, N, C, lime content and pH. Bio macroaggregates of the first mineral horizon were separated by moist sieving in three size classes ( <1 mm, 1-4 mm, >4 mm) for the humus classification. Statistical relationships were checked by a Spearman test.Eumacroamphi was the main humus form, while Dysmull and Pachiamphi were less frequent. There were significant correlations between CH4 fluxes and both the thickness of the transition from organic to inorganic horizons, and the percentage of aggregates in different size classes. CO2 emission did not provide significant correlations with humus features. These first results suggest that the activity of methanotrophic bacteria responsible for CH4 uptake might be lower where the thickness of the transition between the organic and mineral horizons is larger.