Stable isotope profiles of soil organic carbon in forested and grassland landscapes in the Lake Alaotra basin (Madagascar): insights in past vegetation changes

[摘要] The extent to which the central highlands of Madagascarwere once covered by forests is still a matter of debate: whilereconstructing past environments is inherently difficult, the debate isfurther hampered by the fact that the evidence documenting land coverchanges and their effects on carbon and sediment dynamics in Madagascar hashitherto mainly been derived from lake coring studies. Such studies providean integrated view over relatively large areas but do not provideinformation on how land-use change affects hillslopes in terms of carbon andsediment dynamics. Such information would not only be complementary to lakeinventories but may also help to correctly interpret lake sediment data.Carbon stable isotope ratios ( δ 13 C) are particularly usefultracers to study the past dynamics of soil carbon over time spans rangingfrom years to millennia and thus to understand the consequences of land-usechange over such time spans. We analysed soil profiles down to a depth of 2 mfrom pristine forests and grasslands in the Lake Alaotra region in centralMadagascar. Along grassland hillslopes, soil organic carbon (SOC) contentwas low, from 0.4 % to 1.7 % in the top layer, and decreased rapidly to ca.0.2 % below 100 cm depth. The current vegetation predominantly consistsof C 4 grasses ( δ 13 C ∼   −13  ‰), yet topsoil δ 13 C-OC ranges between −23.0  ‰ and −15.8  ‰, and most profiles show a decrease in δ 13 C-OC with depth. This contrasts with our observations in theC 3 -dominated forest profiles, which show a typical profile whereby δ 13 C values increase slightly with depth. Moreover, the SOC stock ofgrasslands was ∼  55.6 % lower than along the forestedhillslopes for the upper 0–30 cm layer. δ 13 C values ingrassland and forest profiles converge to similar values (within 2.0  ±  1.8 ‰) at depths below ∼  80 cm,suggesting that the grasslands in the Lake Alaotra region have indeeddeveloped on soils formerly covered by a tree vegetation dominated by C 3 plants. We also observed that the percent of modern carbon (pMC) of the bulkOC in the top, middle and lower middle positions of grasslands was less than85 % near the surface. This could reflect a combination of (i) the longresidence time of forest OC in the soil, (ii) the slow replacement rate ofgrassland-derived OC (iii) and the substantial erosion of the top positionstowards the valley position of grasslands. At the valley positions undergrassland, the upper 80 cm contains higher amounts of recent grass-derivedOC in comparison to the hillslope positions. This is likely to be related tothe higher productivity of the grassland valleys (due to higher moisture andnutrient availability), and the deposition of OC that was eroded further upslopemay also have contributed. The method we applied, which is based on thelarge difference in δ 13 C values between the two majorphotosynthetic pathways (C 3 and C 4 ) in (sub-)tropical terrestrialenvironments, provides a relatively straightforward approach toquantitatively determine changing vegetation cover, and we advocate for itsbroader application across Madagascar to better understand the island'svegetation history.

[学科分类] 大气科学