Soil Easily-Extractable Glomalin (EEG): molecular characterization.

Abstract

Easily Extractable Glomalin (EEG) is an operational term applied to a soil organic fraction supposed to contain mainly glomalin-related soil protein (GRSP) a glycoprotein abundantly found in soils that are produced in the hyphae and spores of arbuscular mycorrhizal fungi (AMF). This fraction is a significant component of SOM which effectively has multiple ecological functions including the facilitation of soil aggregate formation and its stability and resilience to degradation contributing to SOC storage. Given its potential as soil perturbation indicator here a detailed molecular characterization "fingerprint" was performed using analytical pyrolysis (Py-GC/MS) of the EEG fraction extracted from a Mediterranean soil beneath different plant covers (pine and shrubs) affected and unaffected by forest fire and at different times. 60 Soils (16) under pine and shrub covers from Gorga (NE Alicante Spain) were studied. Surface soil (2.5 cm depth; A horizon) samples were taken immediately after a forest fire that occurred in July 2011 and 4 8 and 12 months after the fire. EEG was extracted and measured using the Bradford assay and the lyophilized extracts analysed by PyGC/MS. A total of 139 compounds were identified and grouped according to their probable biogenic origin. Py-GC/MS did not show any clear differences between samples. Only minor differences could be detected between samples under pine and shrub covers regardless of the effect of fire. This fact indicates that the EEG is a structurally stable soil organic fraction very homogeneous and highly resistant in soil if temperatures remain below 200-250ºC. The main compounds found were polysaccharides (PS); polyphenols (LIG); proteins and polypeptides (PRO); non-specific aromatic compounds (ARO); hydro-aromatics (HAR); lipids (LIP); polycyclic aromatic hydrocarbons (PAH) and terpenes (TER). Surface density plots (also known as 3D van Krevelen diagrams) were built up from average values of pyrolytic products. The spatial distribution of these compounds supports similar pyrolytic patterns for the main factors studied: vegetation and fire. The results provide insight into the role of EEG in soils under high-level perturbations such as fire events. Due to their chemical similarity with humic acids EEG may play a relevant role in soil carbon storage