Vol:.(1234567890) Tropical Ecology (2021) 62:126–138 https://doi.org/10.1007/s42965-020-00137-y 1 3 RESEARCH ARTICLE Soil organic carbon pool under selected tree plantations in the Southern Western Ghats of Kerala, India Vijo Thomas Kurien 1,2  · Elvin Thomas 1  · S. Prasanth Narayanan 3  · A. P. Thomas 3 Received: 21 May 2020 / Revised: 2 December 2020 / Accepted: 13 December 2020 / Published online: 13 January 2021 © International Society for Tropical Ecology 2021 Abstract Soil organic carbon (SOC) is the most important indicator of soil quality and land-use sustainability. We evaluated the SOC pool and selected physico-chemical soil variables after conversion of native forest to monoculture tree plantations (teak, eucalypts and rubber) in the Southern Western Ghats region of Kollam district, Kerala, India. Soil samples were collected up to a depth of 50 cm in fve depth increments from six locations each from the four selected land-use systems. Soil analyses were carried out as per standard methods. Average SOC concentration ranged from 41.89 to 54.09 g kg −1 (0–10 cm), and found decreased with depth up to 30.11 to 32.98 g kg −1 (40–50 cm). The highest SOC pool (0–50 cm) was observed under natural forest (225.34 t ha −1 ), followed by rubber (203.48 t ha −1 ), eucalypts (196.21 t ha −1 ) and teak plantation (194.61 t ha −1 ). Similarly, the SOC mitigation potential (carbon equivalent to CO 2 ) decreased in the order natural forest > rubber > euca- lypts > teak plantation. The SOC concentration showed signifcant variation between natural forest and tree plantations. Principal component analysis (PCA) was used to determine the major factors and trends to illustrate the variability in the data. PCA results suggest that higher SOC contents and its fractions were closely linked to the surface layers (0–20 cm) of soils under all treatments. Overall conclusion from this investigation is that SOC levels are strongly infuenced by the prevailing land-use systems and found to decrease after conversion from natural vegetation to monoculture tree plantations. The information presented herein on the depth distribution of the SOC fractions and trends in aggregate carbon content may improve our knowledge and help in framing soil carbon prediction models in this region. Keywords Carbon fractions · Land-use systems · Soil carbon dynamics · SOC stock · Soil aggregates · Typic plinthohumults Introduction Soil organic matter (SOM) and associated soil organic car- bon (SOC) plays an important role in the management of soil fertility (Rasool et al. 2008), soil quality (Franzlueb- bers et al. 1995; Haynes 2005), soil structure and carbon dynamics (Lal 2002; Beldini et al. 2009; de Moraes Sá et al. 2018). The global soil carbon pool is estimated at 2500 peta- grams of carbon (Pg C; 10 15  g or billion tons of carbon), of which about 1550 is SOC, and the remaining 950 is soil inorganic carbon (SIC) to a depth of 1-m which is about 1.8 times more carbon than in the atmosphere and 2.3–3.3 times more than what is held in the terrestrial vegetation (Kurien et al. 2019). This soil carbon pool plays a crucial role in global carbon cycle and associated climate change. As part of advances in climate change mitigation through Reducing Emissions from Deforestation and forest Degrada- tion (REDD+), greater importance has been put on assess- ing above ground carbon than below ground carbon (Cole and Ewel 2006; Harris et al. 2008; Návar 2009; Cerbu et al. 2011). India has already agreed under Bali Action Plan to all the elements of REDD+ in 13th meeting of the Confer- ence of parties (COP 13) at Bali (Wani et al. 2014). In this context SOC estimates in the present study are signifcantly important from an Indian context in relation to huge data gaps for this region which pose a great challenge to collect information on carbon stocks in both natural and managed International Society for Tropical Ecology * Vijo Thomas Kurien vijokurien@gmail.com 1 School of Environmental Sciences, Mahatma Gandhi University, Kottayam, India 2 Department of Zoology, CMS College Kottayam (Autonomous), Kottayam, India 3 Advanced Centre of Environmental Studies and Sustainable Development, Mahatma Gandhi University, Kottayam, India