Effects of Climate Change on Soil Organic Matter C and H Isotope Composition in a Mediterranean Savannah (Dehesa): An Assessment Using Py-CSIA

Abstract

Dehesas are Mediterranean agro-sylvo-pastoral systems sensitive to climate change. Extreme climate conditions forecasted for Mediterranean areas may change soil C turnover, which is of relevance for soil biogeochemistry modeling. The effect of climate change on soil organic matter (SOM) is investigated in a field experiment mimicking environmental conditions of global change scenarios (soil temperature increase, +2-3 °C, W; rainfall exclusion, 30%, D; a combination of both, W+D). Pyrolysis-compound-specific isotope analysis (Py-CSIA) is used for C and H isotope characterization of SOM compounds and to forecast trends exerted by the induced climate shift. After 2.5 years, significant δ¹³C and δ²H isotopic enrichments were detected. Observed short- and mid-chain n-alkane δ¹³C shifts point to an increased microbial SOM reworking in the W treatment; a ²H enrichment of up to 40‰ of lignin methoxyphenols was found when combining W+D treatments under the tree canopy, probably related to H fractionation due to increased soil water evapotranspiration. Our findings indicate that the effect of the tree canopy drives SOM dynamics in dehesas and that, in the short term, foreseen climate change scenarios will exert changes in the SOM dynamics comprising the biogeochemical C and H cycles.

Keywords: Mediterranean soil; analytical pyrolysis; biomarkers; climate change; δ13C; δ2H.

Figure 1

Distribution of δ¹³C Py-CSIA values (average expressed as ‰; n = 3) of (a) open and (b) tree habitat, for the different biogenic compounds identified by pyrolysis. Abbreviations: N, nitrogen compounds; ARO, aromatics from unknown origin; PS, polysaccharides; LH, p-hydroxyphenyl lignin units; LG, guayacil lignin units; LS, syringyl lignin units; FA, fatty acids; FAME, fatty acid methyl ester; ALK, n-alkanes. Error bars indicate standard errors. Treatments with the same letters indicate no significant differences between different composting times for the same biogenic group. Asterisks indicate significant differences between habitats.

Figure 2

Distribution of δ²H Py-CSIA values (average expressed as ‰; n = 3) of (a) open and (b) tree habitat, for the different groups identified by pyrolysis, attributed to nonexchangeable biogenic groups. LH: p-hydroxyphenyl lignin units; LG: guayacil lignin units; LS: syringyl lignin units; FA: fatty acids; FAME: fatty acid methyl-ester; ALK: n-alkanes. Error bars indicate standard error. Treatments with the same letters indicate no significant differences between different composting times for the same biogenic group. “*” indicate significant differences between habitats. LS excluding δ²H values of propynylsyringol.

Figure 3

Distribution of compound-specific δ²H and δ¹³C n-alkanes data from soils under the two different factors. The size of the circles for the different samples denotes the standard error of isotopic values between individual n-alkanes and climatic treatments. The shape of the forms represents the two different habitats.

Figure 4

Changes in δ¹³C and δ²H isotope composition (expressed as Δ) relative to the control in three climatic treatments. Difference = (Treatment – Control)/Control. Data points represents means (n = 3 ± standard error). Letters represent significant differences (Scheirer–Ray–Hare Test, p < 0.05) in the same compounds between climatic treatments. Asterisks indicate values out of graphical representation. Abbreviations: LH-1, vinylphenol; LG-1, guaiacol; LG-2, methylguaiacol; LG-3, vinylguaiacol; LG-4, vanillin; LG-5, eugenol; LG-6, acetoguaiacone; LS-1, syringol: LS-2, propynylsyringol; LS-3, methoxyeugenol; LS-4, acetosyringone.