Influence of Altitude and the Associated Variation of Climate and Vegetation on Selected Soil Properties of Tropical Rainforests Across a Wide Altitudinal Gradient

Abstract

Our objective in this work was to quantify the variation of selected soil properties across a wide altitudinal gradient in tropical rainforests of Sri Lanka and to determine the influence of climate and vegetation on the observed soil variation.  Soil properties were measured in lowland and montane rainforests from 117 to 2235 m above sea level at 0–15 and 15–30 cm depths.  Total soil nitrogen (N), exchangeable potassium (K), organic carbon (SOC), electrical conductivity (EC) and pH showed significant linear increases with increasing altitude.  Soil available phosphorus (P) showed a weak linear decrease with altitude.  Three composite soil variables extracted by Factor Analysis explained 93% of the observed soil variation.  Factor 1, which explained 60% of the total variation, had N, K, SOC and EC loading on it while Factors 2 (25%) and 3 (8%) had P and pH loading respectively on them.  Multiple regression analysis against long-term (1970-2018) mean climate showed that temperature and rainfall had negative relationships with Factor 1 while the diurnal temperature range had a positive relationship.   Factor 2 was positively influenced by temperature but was negatively influenced by rainfall.  Total tree basal area, standing biomass and plot-level vegetation diversity had negative effects on Factors 1 and 2.  Factor 3 was not influenced by climate or vegetation properties.  These results are consistent with the hypothesis that the availability of highly-mobile soil nutrients (N and K) and SOC across altitudinal gradients in tropical rainforests is determined by varying temperature and rainfall controlling the balance between nutrient release via mineralization and decomposition and nutrient removal via leaching and plant uptake.  The observed increases of N, K, EC and SOC with altitude show that lower temperature and rainfall at higher altitudes exert a stronger constraining influence on processes of nutrient removal than on processes of nutrient release.

Keywords: Tropical rainforests, total soil nitrogen, exchangeable potassium, soil organic carbon, soil pH, altitude