Soil organic carbon fractions across soil depths vary among key tropical vegetation types

Analyzing the patterns of soil organic carbon (SOC) fractions is important for enhancing C sequestration and mitigating global warming. However, studies on SOC fractions and nutrient availabilities across multiple forest types within the same ecological zone (EZ) are limited. This study aimed to investigate how SOC fractions vary across tropical vegetation types and how soil nutrient availability mediates these effects. A total of 1224 soil samples were collected from two different depths: surface (0–15 cm) and sub-surface (15–30 cm), across six vegetation types; montane forest (MF), sub-montane forest (SMF), moist monsoon forest (MMF), open and sparse forest (OSF), grassland (GL) and forest plantation (FP) located in Knuckles Forest region (KFR), Sri Lanka. Total OC (TOC), microbial biomass C (MBC), KMnO4 oxidizable C (LC), water soluble C (WSC), and soil nutrient availabilities (i.e., N, P, K, Ca, and Mg), soil pH, moisture content and electrical conductivity (EC) were quantified using standard chemical procedures. Results indicated that MF showed the highest SOC (4.52%), LC (0.071%), and WSC (0.047%) in the upper soil layer, while FP showed the lowest values (SOC 2.70%, LC 0.055%, WSC 0.014%). In contrast, MBC was highest in SMF and lowest in MF. The results revealed that vegetation types significantly and differently impact SOC fractions in both soil layers, depending on the characteristics of the vegetation types (i.e., cover and diversity), even within the same EZ. The estimated SOC values will be valuable for enhancing the SOC pool through the restoration of degraded soils, agroforestry, and plantations.