Abstract
It is important to assess the mineralisation of soil organic carbon (SOC) to predict the short-term response of biosphere carbon reservoirs to changing environmental conditions. We investigated the labile (easily-mineralisable) SOC in volcanic soils, where the bioavailability of SOC is typically affected by physico-chemical stabilisation mechanisms that are characteristic of these soils. Ten soils were selected that represent the most typical soil types (mainly Andosols) and natural habitats (xerophytic scrubland, laurel forest and pine forest) in the Canary Islands, a volcanic archipelago. Over two years we measured several physico-chemical SOC fractions with different degrees of bioavailability: water-soluble carbon in fresh soil samples (WSC) and in the saturated extract (WSC<sub>se</sub>), hot water-extractable carbon (HWC), potassium sulphate-extractable carbon (PSC), microbial biomass carbon (MBC), particulate organic carbon (POC), humic substances carbon (HSC), and total organic carbon (TOC), and performed CO<sub>2</sub> emission incubation assays. We related these measurements to the potential C inputs of plant litter and roots and to the activity of certain hydrolytic enzymes (CM-cellulase, β-D-glucosidase, and dehydrogenase) that are involved in carbon turnover. In vitro carbon mineralisation measurements from short assays (ten days) were fitted with simple first-order kinetics to investigate SOC. This procedure was simple and allowed us to obtain estimates both for potentially mineralisable SOC and for the heterogeneity of the substrates that were consumed during incubation. The investigated volcanic soils had large labile SOC concentrations in which simple carbohydrates predominated and that were mainly derived from roots and aboveground non-woody residues. Among the analysed physico-chemical SOC fractions, HWC (3.1 g kg<sup>-1</sup> on average at 0-30 cm depth in Andosols) was the most correlated with C<sub>0</sub> (1.2 g kg<sup>-1</sup>) and therefore best represents potentially mineralisable SOC. PSC (0.77 g kg<sup>-1</sup>), which represents an SOC pool of low bioavailability, was protected by its adsorption to allophane in silandic Andosols.
Highlights
Gypsisols are extremely scarce in the world (IUSS 2007), making up less than 0.1% of European soils (EC 2005)
Soil pH is basic for all the horizons due to the presence of CaCO3, except the 2Az horizon in toe-slope, which is very basic because of sodium and magnesium carbonates (Table 2)
We find some iron oxy-hydroxide nodules, which are considered relict from a past seasonal soil flooding because they are found inside soil aggregates (Figures 5 and 7)
Summary
Gypsisols are extremely scarce in the world (IUSS 2007), making up less than 0.1% of European soils (EC 2005). From the viewpoint of soil moisture retention, some authors report very low values (Herrero 2005), while others report the opposite. This can be attributed to different gypsum particle size and to the degree of mixing of the gypsum infilling with the groundmass (Poch et al 1998)
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