Carbon Mineralization in Tropical Montane Forests Soils in Sri Lanka

Abstract

The change of soil carbon (C) mineralization of tropical montane forests (TMFs) across elevation remains unclear. The objective of the present study was to determine the variation of soil Cmineralization along an elevation gradient in TMFs in Sri Lanka. Soil samples were collected from four permanent sampling plots (PSPs): 1042 (Sinharaja-Enasalwatte-1/ENS1), 1065 (Sinharaja-
Enasalwatte-2/ENS2), 2080 (Pidurutalagala/PTG) and 2132 m asl (Horton Plains/HNP). Each sampling plot was 1 ha in size and 17 near-surface representative soil samples were collected up to 0.25 m depth. Four replicates from each PSP were used for the study. Evolution of CO2 was measured in a 60-day soil laboratory incubation study by trapping in 1 N NaOH and titrating a 20
mL aliquot with a standardized 1 N HCl. An average temperature of ~25° C was maintained throughout experiment. Basic soil physico-chemical properties (pH, electrical conductivity (EC), redox potential (Eh), cation exchange capacity (CEC), organic carbon (OC) content, volumetric water content (VWC), bulk density and porosity) were determined. All data were subjected to
analysis of variance (one way ANOVA) at 95% confident level and mean separation was conducted following Tukey‘s Test. Results showed a significant (P<0.05) effect of elevation on the CO2 emission rate, cumulative CO2 emission and, above mentioned soil properties. The highest and the lowest average CO2 emission rates; 67.6±9.7 and 28.8±2.2 mg C kg-1 soil day-1, respectively,
were determined in PTG and ENS1. PTG showed the highest cumulative CO2 emission (4,071 mg C kg-1 soil), followed by HNP, ENS2 and ENS1: 3,799, 2,091 and 1,733 mg C kg-1 soil, respectively. In mean comparison between cumulative CO2 emission and elevation, upper montane (above 2080m) forests (PTG, HNP) and lower montane (1042–1668 m) forests (ENS1, ENS2) were distinctly separated into two different groups. We found a significant (P<0.05) positive correlation between cumulative CO2 emission and pH, EC, soil porosity, CEC and soil OC content. The cumulative CO2 emission had a significant (P<0.05) negative correlation with bulk density. In conclusion, the soil physicochemical properties of selected PSPs also showed variation across the elevational gradient. The present findings suggested that the attitudinal variation of CO2 evolution is partially supported
by the variation of physicochemical properties of selected TMFs. But further studies are required to confirm the relationship between CO2 evolution and soil physicochemical properties of the TMFs studied.

Keywords: Incubation, Elevation, CO2 emission, Horton Plains, Sinharaja, Enasalwatte, Piduruthalagala