Soil organic matter and carbon sequestration in forest stands on Mount Vesuvius

Abstract

Soil organic matter in forest ecosystems represent an important C pool. Litter fall is the largest source of organic material and nutrients for the organic and mineral layers in forest ecosystems. The chemical composition of plant litter has a large influence on soil microbial communities and is one of the main factors affecting litter decay rates and the dynamics of SOM. Introduction of alien species, in forest ecosystems, may have a potential allelopathic effect on other tree species and allelopathic chemical compounds produced by alien tree species might inhibit soil microbial activity with consequent effects on storage rates of SOM and ecosystem nutrient cycling. Until know little attention has been paid to this topic that is worthy of investigation because alien species are largely used for afforestation of poor soils or easily invade bare soils. Forest stands at Mount Vesuvius are a convenient study area to investigate C sequestration in soil. Chronosequences of tree plantation occur on volcanic substrate of know age; moreover different species of trees have been used for afforestation including the alien invasive species Black locust (Robinia pseudoacacia L.). Thus it is possible to study carbon sequestration in the soil as related to the age of the stand and to type of tree cover. In the two coeval (36 yrs old) stands on lapillus of the last eruption of Vesuvius, with a different tree cover (Black locust and Corsican pine, Pinus nigra Arn. var. corsicana), litter fall (years 2006–2008) is higher in the Corsican pine than in the Black locust stand (P < 0.05). Total Litter Fall over Basal area (TLF/BA) is 90.7 Kg/m2 for Black locust vs 210.2 Kg/m2 for Corsican pine and the higher litter fall amount of Corsican pine corresponds to the higher biomass. In the organic soil layers amounts of organic C are higher (P < 0.05) in Corsican pine as compared to Black locust stand (g/m2 = 2701.9 vs 1636.4), while in the mineral layers organic C amounts are lightly higher (but not statistically different) in Black locust than in Corsican pine soil (136 vs 116 g/m2). 13C CPMAS NMR data show that the aromaticity degree of humus is higher for Black locust than for Corsican pine (% Ar = 9.54 vs 7.08). In the Stone pine cronosequence, litter fall (2006–2008) increases gradually from the younger stand (36 yrs old) to the oldest one (96 yrs old) and is positively related to tree basal area (BA); Total Litter Fall over Basal Area (TLF/BA) also increases with stand age (123 to 145 Kg/m2). Moreover chemical analyses of organic and mineral soil layers clearly indicate the accumulation of C with stand age (g/m2= 1053.9 for 36 yrs old stand vs 2523.5 for 66 yrs old one and vs 3065.2 for 96 yrs old one). The results dealing with the black locust stand allow to conclude that Black locust litter: i) inhibit soil microorganisms activity, ii) produces a recalcitrant residue, rich of aromatic compounds, including 4–hydroxyacetophenon (indentified by 1H NMR), known for its allelopathic potential. This may lead to high accumulation of organic matter in the soil and thus to high C sequestration. The comparison of all pine stands shows that the trend of litter fall and C sequestration, increases from the younger to the older stand.