Stable isotope profiles of soil organic carbon in forested and grassland landscapes in the Lake Alaotra basin (Madagascar): insights in past vegetation changes
Authors: Razanamahandry Vao Fenotiana , Dewaele Marjolein , Govers Gerard , Brosens Liesa , Campforts Benjamin , Jacobs Liesbet , Razafimbelo Tantely , Rafolisy Tovonarivo , Bouillon Steven 
Published: January 2022

Abstract. The extent to which the central highlands of Madagascar were once covered by forests is still a matter of debate: while reconstructing past environments is inherently difficult, the debate is further hampered by the fact that the evidence documenting land cover changes and their effects on carbon and sediment dynamics in Madagascar has hitherto mainly been derived from lake coring studies. Such studies provide an integrated view over relatively large areas but do not provide information on how land use change affects hillslopes in terms of carbon and sediment dynamics. Such information would not only be complementary to lake inventories but may also help to correctly interpret lake sediment data. Carbon stable isotope ratios (δ13C) are particularly useful tracers to study the past dynamics of soil carbon over timespans ranging from years to centuries and thus to understand the consequences of land-use change over such timespans. We analyzed soil profiles down to a depth of 2 m from pristine forests and grasslands in the Lake Alaotra region in central Madagascar. Along grassland hillslopes, SOC content was extremely low, from 0.4 to 1.8 % in the top layer, and decreased rapidly to ca. 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ~−13 ‰), yet soil δ13C-OC range between −25.9 and −16.6 ‰, and most profiles show a decrease in δ13C-OC with depth. This contrasts with our observations in the C3-dominated forest profiles, which show a typical enrichment in 13C with depth. Moreover, the SOC stock of grasslands was ~55.6 % lower than along the forested hillslopes for the upper 0–30 cm layer. δ13C values in grassland 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 indeed developed on soils formerly covered by a tree vegetation dominated by C3 plants. We also observed that the percent of modern carbon (pMC) of the bulk OC in the top, middle and lower middle positions of grasslands was less than 85 % near the surface. This could reflect a combination of (i) the long residence time of forest OC in the soil, (ii) the slow replacement rate of grassland-derived OC, (iii) and the substantial erosion of the top positions towards the valley position of grasslands. In the valley positions of our grassland hillslopes, the topsoil OC has a higher δ13C value and consists of younger carbon (pMC > 100 %), suggesting a recent expansion of grass vegetation. Our approach, based on the large difference in δ13C values between the two major photosynthetic pathways (C3 and C4) in (sub)tropical terrestrial environments, provides a relatively straightforward method to quantitatively determine changing vegetation cover. We advocate for its broader application across Madagascar to better understand the islands’ vegetation history.