Humic acids as proxies for assessing different Mediterranean forest soils signatures using solid-state CPMAS 13C NMR spectroscopy

Humic acids (HAs) of four representative forest soils profiles from Central Spain (two with different vegetation – pine and oak – but same parent material – granitie, and two with same vegetation – holm oak – but different parent material – granite and limestone) were investigated by solid-state cross polarization with magic angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy. The objectives included the investigation of the impact of different forest properties on HA composition, assessing how the structural characteristics of the HA vary with soil depth, and evaluating the role of HA as surrogates for mapping the different forest soils signatures using structural data derived from 13C NMR spectroscopy. On average, alkyl C is the dominant C constituent (38–48% of the total NMR peak area) in all HA samples, followed by aromatic (12–22%) and O-alkyl C (12–19%), and finally carboxyl C (7.0–10%). The NMR data also indicated that HA composition is likely to be differently affected by the soil physico-chemical properties and type of forest vegetation. The structural characteristics of the HA from soil under oak did not differ broadly downward in the profile, whereas soil HA under pine forest exhibits a somewhat higher recalcitrant nature as a consequence of a higher degree of decomposition. The soil HA from holm oak forests differed from the other two forest soils, exhibiting a progressive decomposition of the alkyl C structures with increasing depth, while the carbohydrate-like indicator (O-alkyl C) is apparently being protected from mineralization in the horizons below the ground level. Overall, these differences in soil HA NMR signatures are an important diagnostic tool for understanding the role of different soil environmental factors on the structural composition of HA from Mediterranean forest soils.