Evaluation of optical techniques for characterising soil organic matter quality in agricultural soils

Soil organic matter (SUM) is one of the main global carbon pools. It is a measure of soil quality as its presence increases carbon sequestration and improves physical and chemical soil properties. The determination and characterisation of humic substances gives essential information of the maturity and stresses of soils as well as of their health. However, the determination of the exact nature and molecular structure of these substances has been proven difficult. Several complex techniques exist to characterise SUM and mineralisation and humification processes. One of the more widely accepted for its accuracy is nuclear magnetic resonance (NMR) spectroscopy. Despite its efficacy, NMR needs significant economic resources, equipment, material and time. Proxy measures like the fluorescence index (FI), cold and hot-water extractable carbon (CWC and HWC) and SUVA-(254) have the potential to characterise SUM and, in combination, provide qualitative and quantitative data of SUM and its processes. Spanish and British agricultural cambisols were used to measure SUM quality and determine whether similarities were found between optical techniques and H-1 NMR results in these two regions with contrasting climatic conditions. High correlations (p < 0.001) were found between the specific aromatic fraction measured with H-1 NMR and SUVA-(254) (Rs = 0.95) and HWC (Rs = 0.90), which could be described using a linear model. A high correlation between FI and the aromatics fraction measured with H-1 NMR (Rs = 0.976) was also observed. In view of our results, optical measures have a potential, in combination, to predict the aromatic fraction of SUM without the need of expensive and time consuming techniques. (C) 2015 Elsevier B.V. All rights reserved.