Assessing stock and thresholds detection of soil organic carbon and nitrogen along an altitude gradient in an east Africa mountain ecosystem

Abstract

It is well established altitude driven agro-ecological factors play a key role to influence soil organic carbon (SOC) and total nitrogen (TN) stocks in tropical mountain ecosystems. Land cover transformation is chiefly responsible for soil nutrient stock changes, with the magnitude of stock changes being a crucial concern for most of east African mountain ecosystems. With biophysical heterogeneity defined by localized micro-climates, spatial soil nutrient stocks patterns and their detection thresholds remain poorly understood. This study describes SOC and TN stocks within a research transect in the Taita hills, Southeast Kenya, using three forms of stratification: altitude, soil types and land cover categories. Results show a linear and positive relationship between altitude and either carbon (R2=0.30; P-value<0.05) or nitrogen (R2=0.35; P-value<0.05) stocks, but varied within land cover and altitude gradations. Cambisols had 30% higher average SOC stock relative to Ferrasols and contained about half the stock observed for forest dominated Umbrisols. Average total nitrogen stocks in Cambisols and Ferrasols were comparable (0.4kgm2) in contrast to Umbrisols (0.9kgm2). Altitude and soil temperature explained 63% soil C and 62% total soil in Ferrasols. Altitude, soil temperature and water filled pore space explained >80% SOC and TN variation in Umbrisols. The absolute minimum detectable difference for carbon and nitrogen stocks varied at the low and high altitude categories of the study transect. Biophysical soils, land cover and altitude delineations play a crucial role to determine spatial soil nutrient patterns in east African mountain ecosystems and should be considered in the design of sustainable nutrient management practices.