Estimating temporal changes in soil carbon stocks at ecoregional scale in Madagascar using remote-sensing

Soil ! organic ! carbon ! (SOC) ! plays ! an ! important ! role ! in! climate ! change ! regulation ! notably ! through ! release ! of CO2! following ! land ! use ! change ! such ! a! deforestation, ! but ! data ! on ! stock! change ! levels ! are ! lacking. ! This! study aims! to ! empirically ! assess ! SOC ! stocks ! change ! between ! 1991 ! and! 2011 ! at! the ! landscape ! scale ! using ! easyto-access ! spatially-explicit ! environmental ! factors. ! The ! study ! area ! was ! located ! in! southeast ! Madagascar, in ! a! region ! that ! exhibits ! very ! high ! rate ! of! deforestation ! and ! which ! is! characterized ! by ! both ! humid ! and ! dry climates.! We ! estimated ! SOC ! stock ! on! 0.1! ha ! plots ! for! 95 ! different ! locations ! in ! a ! 43,000 ! ha ! reference ! area covering ! both ! dry ! and ! humid ! conditions ! and ! representing ! different ! land ! cover ! including ! natural ! forest, cropland, ! pasture ! and ! fallows. ! We ! used ! the ! Random ! Forest ! algorithm ! to! find ! out ! the! environmental ! factors explaining ! the ! spatial ! distribution ! of ! SOC. ! We ! then ! predicted ! SOC ! stocks ! for! two ! soil! layers ! at ! 30 ! cm ! and 100 ! cm ! over! a! wider ! area ! of ! 395,000 ! ha.! By ! changing ! the ! soil ! and ! vegetation ! indices ! derived ! from ! remote sensing ! images ! we ! were ! able ! to ! produce ! SOC ! maps ! for! 1991 ! and ! 2011. ! Those ! estimates ! and ! their! related uncertainties ! where ! combined ! in! a! post-processing ! step ! to! map ! estimates ! of! significant ! SOC ! variations and ! we ! finally ! compared ! the! SOC ! change ! map ! with ! published ! deforestation ! maps. ! Results ! show ! that ! the geologic ! variables, ! precipitation, ! temperature, ! and! soil-vegetation ! status ! were ! strong ! predictors ! of ! SOC distribution ! at! regional ! scale. ! We! estimated ! an! average ! net ! loss ! of ! 10.7% ! and ! 5.2% ! for ! the ! 30 ! cm ! and ! the 100 ! cm ! layers ! respectively ! for! deforested ! areas! in ! the! humid ! area. ! Our ! results ! also ! suggest ! that ! these losses ! occur ! within ! the ! first ! five ! years ! following ! deforestation. ! No ! significant ! variations ! were ! observed for ! the ! dry ! region. ! This ! study ! provides ! new ! solutions ! and ! knowledge ! for ! a! better ! integration ! of! soil ! threats and ! opportunities ! in ! land ! management ! policies. ©! 2016 ! Published ! by ! Elsevier ! B.V.