Soil C and N sequestration in organic and mineral layers of two coeval forest stands implanted on pyroclastic material (Mount Vesuvius, South Italy)

Tree species influence organic matter sequestration in forest soil. We explored the pattern of soil organic carbon (SOC) and soil organic nitrogen (SON) sequestration in two 40-year-old, adjacent forest stands, one with Black pine (Pinus nigra Arn.) and one with Black locust (Robinia pseudoacacia L.). We compared the patterns of annual carbon (C) and nitrogen (N) sequestration in the organic and mineral soil layers of the two forests with literature data for < 100 years old deciduous and coniferous forests from different climatic zones. Carbon and N inputs, with aboveground litter fall, were significantly higher in the Black pine than in the Black locust stand. In the soil profile of the Black pine stand, the amount of SOC was significantly higher and that of SON significantly lower than in the Black locust stand. The average annual sequestration rates were 41.3 and 75.0 g C m-2, and 2.9 and 2.6 g N m-2, respectively, in the Black locust and the Black pine stands. In both forests most of SOC and SON was stored in the organic layers (OL and OF+OH). In the mineral soil layers the amount of SOC was similar for the two stands whereas the amount of SON was significantly higher in the Black locust stand (23.7 vs 14.6 g N m-2). In all soil layers, the concentrations of water soluble organic matter (WSOM) were higher in the Black locust stand compared to that with Black pine. Manganese accumulated in the OF+OH layer of the Black pine stand. The results suggest different sequestration patterns, with higher SOC sequestration rates in the Black pine stand and higher SON sequestration rates in the Black locust stand. Comparison with literature data suggests this to be a generality in C and N sequestration patterns for coniferous and deciduous forests.