Shifts in soil chemical and microbial properties across forest chronosequence on recent volcanic deposits

Afforestation of new unconsolidated volcanic deposits is a practice used to stabilize barren areas and enhance the accumulation of organic matter in the developing soil. Changes in soil carbon (C) and nitrogen (N) pools, including the soluble and microbial fractions, within the first decades since afforestation have been poorly investigated. Therefore the objective of the present study was to investigate how key C and N pools vary in litter and soil of four forests planted on barren volcanic deposits from recent Mount Vesuvius eruptions. We examined three forest stands (40, 70 and 100 years old) afforested with Stone pine (Pinus pinea L.) and a 40-year old forest of Black pine (Pinus nigra Arn.). As a baseline of C and N pools prior to afforestation, data from treeless sites were included in the study. Both the inputs with litter fall and soil C and N stocks increased with forest age in the Stone pine stands. In the mineral soil, C concentration per gram soil dry weight and C:N ratio increased with age from treeless sites to the oldest forest. Microbial biomass C and fungal biomass as a fraction of organic carbon (OC) and respiration per unit OC (an index of organic matter mineralization potential) decreased significantly with stand age. The results suggest that a main driver of C accumulation in the mineral soil is the decline with increasing stand age of the microbial fraction of organic matter and its activity. The comparison between the two pine species revealed that litter production was more abundant in the Black pine than in the even-aged, 40-year-old, Stone pine stand; moreover Black pine litter was more acidic and had a higher stable residue than Stone pine litter. Therefore a different pattern of C sequestration occurs with a higher C stock in the organic layers and a lower C stock in the mineral soil of Black pine compared to Stone pine.