Keywords: SOM quality Hot-water extractable carbon Cold-water extractable carbon Fluorescence index EEM SUVA-254 1 H NMR Aromatic fraction A B S T R A C T Soil organic matter (SOM) 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 SOM 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 hotwater extractable carbon (CWC and HWC) and SUVA-254 have the potential to characterise SOM and, in combination, provide qualitative and quantitative data of SOM and its processes. Spanish and British agricultural cambisols were used to measure SOM quality and determine whether similarities were found between optical techniques and 1 H NMR results in these two regions with contrasting climatic conditions. High correlations (p < 0.001) were found between the specific aromatic fraction measured with 1 H 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 1 H NMR (Rs = À0.976) was also observed. In view of our results, optical measures have a potential, in combination, to predict the aromatic fraction of SOM without the need of expensive and time consuming techniques.

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This study analyses soil organic carbon (SOC) and hot-water extractable carbon, both measures of soil quality, under different land management-(i) conventional tillage (CT); (ii) CT plus the addition of oil mill waste alperujo (A); (iii) CT plus the addition of oil mill waste olive leaves (L); (iv) no tillage with chipped pruned branches (NT 1 ); and (v) no tillage with chipped pruned branches and weeds (NT 2 )-in a typical Mediterranean agricultural area: the olive groves of Andalusia, southern Spain. SOC values in CT, A, NT 1 and NT 2 decreased with depth, but in NT 2 , the surface horizon (0-5 cm) had higher values than the other treatments, 47% more than the average values in the other three soils. In L, SOC also decreased with depth, although there was an increase of 88·5% from the first (0-10 cm) to the second horizon (10-16 cm). Total SOC stock values were very similar under A (101·9 Mg ha À1 ), CT (101·7 Mg ha À1 ), NT 1 (105·8 Mg ha À1 ) and NT 2 (111·3 Mg ha À1 , if we consider the same depth of the others). However, SOC under L was significantly higher (p < 0·05) at 250·2 Mg ha À1 . Hot-water extractable carbon decreased with depth in A, CT and NT 1 . NT 2 and L followed the same pattern as the other management types but with a higher value in the surface horizon (2·3 and 4·9 mg g À1 , respectively). Overall, our results indicate that application of oil mill waste olive leaves under CT (L) is a good management practice to improve SOC and reduce waste.