Pyrolytic descriptors of the effect of parent material, climate conditions and vegetation type on soil biogeochemical quality in Mediterranean soils from Sierra de Cazorla-Segura-Las Villas Natural Park

Abstract

A study was carried out on structural characteristics of the soil humic acid in surface horizons of soils from different geological (mainly calcareous rocks) and ecophysiological characteristics in a Natural Park of great environmental interest under Mediterranean climate in Southeast Spain. The mean annual temperature is 11 ºC, the mean annual rainfall is about 866 mm, the altitude in sites sampled ranged between 700 to over 1780 m.a.s.l., and the slope between 4 and 57%. The vegetation consisted of sclerophyllic (oak) forest, pine forests or dense shrubs (coverage of 75% or more). Analytical pyrolysis was carried out in a Pyrojector (SGE instruments) connected to a GC/MS system Finnigan Trace GC Ultra with a Trace DSQ mass spectrometer and HP-1 capillary column (30 m × 250 ¿m). Humic acid samples from scrubland showed comparatively simple pyrograms, in some cases comparable to those from sclerophyllic oak forest, which is interpreted as an efficient biogeochemical transformation probably responsible for the prevalence of a stable aromatic domain with frequent C¿C intermolecular bonding. Conversely, conifer forests showed large yields and diversity of lignin-derived methoxyphenols accompanied by well-preserved alkyl series suggesting low biodegradation of the humic aliphatic domain. In the case of sclerophyllic forest (evergreen oak and other Quercus species) the comparatively simple pyrograms with dominance of guaiacol and phenol suggest condensed residual lignin (guaiacyl-type) where the low methoxyl content, which is originally associated to lignin evolutionary chemotaxomomic features (guaiacyl lignin in gymnosperms and syringyl lignin in angiosperms) is strongly constrained by their further evolution in soil, where condensed lignins (guaiacyl-type) behaves as comparatively more recalcitrant in soils than lignins with comparatively lower degree of intramolecular cross-linking (syringyl-type). The corresponding ratio, when properly interpreted, may be used as a soil organic matter quality index. The conclusions inferred after application of multivariate data treatments (mainly multidimensional scaling), suggest that the vegetation results a key factor in determining the humification mechanisms, followed by the geological substrate. A series of valuable surrogate descriptors of the factors influencing the humification processes were obtained from the relative abundance of pyrolysis compounds: a) the alteration of the original lignin methoxyphenolic signature (ratio between methoxyl-lacking to methoxyphenol compounds), b) the extent and C-range of the alkyl domain, c) the degree of demethoxylation of the aromatic moiety (syringyl/guaiacyl ratio) which simultaneously inform on the vegetation type (gymnosperms vs. angiosperms) and on the maturity degree of the soil organic matter and, d) the biodiversity indices of the molecular assemblages after analytical pyrolysis, which tend to decrease in comparatively resilient humus formations.