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 five 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 CO₂) decreased in the order natural forest > rubber > eucalypts > teak plantation. The SOC concentration showed significant 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 influenced 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.

土壤有机碳（SOC）是土壤质量和土地利用可持续性的最重要指标。我们在印度喀拉拉邦Kollam区的西南西部高止山脉地区将原生林转变为单一种植的人工林（柚木，桉树和橡胶）后，评估了SOC库并选择了理化土壤变量。从四个选定的土地利用系统的六个位置分别以五个深度增量，以五个深度递增的方式采集土壤样品，深度达50厘米。按照标准方法进行土壤分析。平均SOC浓度范围为41.89至54.09 g kg ^-1（0-10 cm），发现深度随30.11至32.98 g kg ^-1（40-50 cm）而降低。在天然林下（225.34 t ha ^-1）观察到最高的SOC池（0–50 cm）），然后是橡胶（203.48 t ha ^-1），桉树（196.21 t ha ^-1）和柚木种植园（194.61 t ha ^-1）。同样，SOC缓解潜力（相当于CO _2的碳）顺序依次为天然林>橡胶>桉树>柚木人工林。SOC浓度显示天然林和人工林之间存在显着差异。主成分分析（PCA）用于确定主要因素和趋势，以说明数据的可变性。PCA结果表明，在所有处理下，较高的SOC含量及其分数与土壤表层（0–20 cm）密切相关。这项调查的总体结论是，SOC水平受现行土地使用系统的强烈影响，并发现从自然植被转变为单一种植的人工林后会降低。