Soil carbon characterization along the profile of two forest soils under Quercus pyrenaica

Abstract

In light of concerns over climate change and increasing levels of CO2 in the atmosphere, it is of importance to investigate soil organic matter in Mediterranean forests at a profile scale. In-depth studies of the organic fraction are also of interest to improve understanding of carbon balance and to facilitate modelling of carbon fixation in forest soils. This research evaluates the relationships between diverse parameters such as colour, content, and form of soil organic matter (SOM). Two Quercus pyrenica ecosystems with soils classified as inceptisols with a xeric or dry moisture regime, and developed under a Mediterranean climate in Spain, were used to characterize SOM through the complete sequence of layers of the soil profile. The differentiating factor between the two ecosystems was slope gradient. Characterization was done using characteristics of humic substances (HS) as indicators of SOM turnover in inceptisols. Infrared analysis was used to further characterize the humic acids. As soil colour measurements are a tool for soil type classification and soil organic carbon prediction, the relation between HS colour measured by reflection and by transmission was determined in order to establish a relationship between measurement techniques. Infrared analysis and colour provided evidence of a different level of stabilization of HS from both soils, and between the different horizons. Oxidation of humic acids was found to be greater in deeper horizons than in the surface layers. An inverse relationship between HS colour measured by reflection and by transmission was revealed. Both soils showed a clear trend in which horizons presenting lower absorbance numbers showed higher figures of hue and value. A more marked accumulation of humified compounds was found in pedons, (the smallest unit or volume of soil that contains all the soil types), in the less steep slope. This might be explained in terms of the physiographic position affecting infiltration behavior and exposure to runoff.